Plant Ecology

Plant Ecology .

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Plant Ecology

Virendra Batra

Oxford Book Company Jaipur, India

ISBN: 978-81-89473-65-5 First Published 2009

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All Rights are Reserved. No part ofthis publication may be reproduced, stored in a retrieval system. or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, scanning or otherwise, without the prior written permiss'ion of the copyright owner. Responsibility for the facts stated, opinions expressed, conclusions reached and plagiarism, if any. m this volume is entirely that of the Author, according to whom the matter encompassed in this book has been originally created/edited and resemblance with any such publication may be incidental. The Publisher bears no responsibility for them, whatsoever.

Preface Ecology involves the biological study of relationships of organisms to their environment and to one another. Plants, a primary unit of ecological processes, are involved in constant interaction with the environment they inhabit, depending upon natural resources like sunlight, air and water for their nourishment, pr9viding oxygen and food for other organisms, and being involved in cyclical processes to maintain ecological and environment balance. As the biosphere faces constant threats in the light of global warming and pollution, it becomes even more pertinent . upon us to comprehend the role plants play in sustaining life through their ecological niche. The present book has been aimed as an introductory manual for botany students on the subject of plant ecology. It seeks to delineate the concepts, principles, processes and facts associated with the functioning of plants and their interaction with other organisms, highlighting the ways in which they lend support to maintain a stable ecological system. In addition to stressing upon current discoveries and breakthroughs in the field, the book provides space for understanding the role of plants in applied ecology, especially in the management and preservation of natural resources and environments. Virendra Batra

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Contents Preface 1. Introduction 2. Biosphere and Plant Vegetation

v 1 15

3. Impact of Physical Environment on Plant Growth 37 4. Ecological Evolution of Plants

81

5. Ecology of Fungi

107

6. Ecology of Nonvascular Plants

125

7. Ecology of Seed Plants

153

8. Plant Community and Ecosystem Dynamics

179

9. Ecology of Weeds and Invasive Plants

197

10. Phage Ecology and Plants

227

11. Ecology of Plant Diseases

249

12. Plant Ecology and Climate Change

275

Bibliography Index

297 301

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1 Introduction Plants are a major group of life forms and include familiar organisms such as trees, herbs, bushes, grasses, vines, ferns, mosses, and green algae. About 350,000 species of plants, defined as seed plants, bryophytes, ferns and fern allies, are estimated to exist currently. As of 2004, some 287,655 species had been identified, of which 258,650 are flowering and 15,000 bryophytes. Green plants, sometimes called metaphytes, obtain most of their energy from sunlight via a process called photosynthesis. Aristotle divided all living things between plants and animals. In Linnaeus' system, these became the Kingdoms Vegetabilia and Animalia. Since then, it has become clear that the Plantae as originally defined included several unrelated groups, and the fungi and several groups of algae were removed to new kingdoms. However, these are still often considered plants in many contexts, both technical and popular. When the name Plantae or plants is applied to a specific taxon, it is usually referring to one of three concepts. From smallest to largest in inclusiveness, these three groupings are: Land plants, also known as Embryophyta or Metaphyta. Green plants, also known as Viridiplantae, Viridiphyta or Chlor-obionta - cOlnprise the above Embryophytes, Charophyta (Le., primitive stoneworts), and Chlorophyta (Le., green algae such as sea lettuce).

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Plant Ecology

Archaeplastida, also known as Plantae sensu lato, Plastida or Primoplantae, comprises the green plants above, as well as Rhodophyta (red algae) and Glaucophyta. As the broadest plant clade, this comprises most of the eukaryotes that eons ago acquired their chloroplasts directly by engulfing cyanobacteria. Informally, other creatures that carry out photosynthesis are called plants as well, but they do not constitute a formal taxon and represent species that are not closely related to true plants. There are around 375,000 species of plants, and each year more are found and described by science. ALGAE

Most algae are no longer classified within the Kingdom Plantae. The algae comprise several different groups of organisms that produce energy through photosynthesis, each of which arose independently from separate nonphotosynthetic ancestors. Most conspicuous among the algae are the seaweeds, multicellular algae that may roughly resemble terrestrial plants, but are classified among the green, red, and brown algae. Each of these algal groups also includes various microscopic and single-celled organisms. Only two groups of algae are considered close relatives of land plants (embryophytes). The first of these groups is the Charophyta (desmids and stoneworts), from which the embryophytes developed. The sister group to the combined embryophytes and charophytes is the other group of green algae, and this more inclusive group is collectively referred to as the green plants or Viridiplantae. The Kingdom Plantae is often taken to mean this monophyletic grouping. With a few exceptions among the green algae, all such forms have cell walls containing cellulose, have chloroplasts containing chlorophylls a and b, and store food in the form of starch. They undergo closed mitosis without

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Introduction

3

centrioles, and typically have mitochondria with flat cristae. The chloroplasts of green plants are surrounded by two membranes, suggesting they originated directly from endosymbiotic cyanobacteria. The same is true of two additional groups of algae: the Rhodophyta (red algae) and Glaucophyta. All three groups together are generally believed to have a common origin, and so are classified together in the taxon Archaeplastida. In contrast, most other algae have chloroplasts with three or four surrounding membranes. They are not close relatives of the green plants, presumably acquiring chloroplasts separately from ingested or symbiotic green and red algae. FUNGI

Fungi are no longer considered to be plants, though they were previously included in the plant kingdom. Unlike embryophytes and algae, fungi are not photosyntheoc, but are saprotrophs: obtaining food by breaking down and absorbing surrounding materials. Fungi are not plants, but were historically treated as closely related to plants, and were considered to be in the purview of botanists. It has long been recognised that fungi are evolutionarily closer to animals than to plants, but they still are covered more in depth in introductory botany courses and are not necessarily touched upon in introductory zoology courses. Most fungi are formed by microscopic structures called hyphae, which mayor may not be divided into cells but contain eukaryotic nuclei. Fruiting bodies, of which mushrooms are most familiar, are the reproductive structures of fungi. They are not related to any of the photosynthetic groups, but are close relatives of animals. Therefore, the fungi are in a kingdom of their own. PLANT DIVERSITY

About 350,000 species of plants, defined as seed plants, bryophytes, ferns and fern allies, are estimated to exist

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Plant Ecology

currently. As of 2004, some 287,655 species had been identified, of which 258,650 are flowering plants, 16,000 bryophytes, 11,000 ferns and 8,000 green algae. Embryophytes

Most familiar are the multicellular land plants, called embryophytes. They include the vascular plants, plants with full systems of leaves, stems, and roots. They also include a few of their close relatives, often called bryophytes, of which mosses and liverworts are the most common. All of these plants have eukaryotic cells with cell walls composed of cellulose, and most obtain their energy through photosynthesis, using light and carbon dioxide to synthesize food. About three hundred plant species do not photosynthesize but are parasites on other species of photosynthetic plants. Plants are distinguished from green algae, which represent a mode of photosynthetic life similar to the kind modem plants are believed to have evolved from, by having specialised reproductive organs protected by non-reproductive tissues. Bryophytes first appeared during the early Palaeozoic. They can only survive where moisture is available for significant periods, although some species are desiccation tolerant. Most species of bryophyte remain small throughout their life-cycle. This involves an alternation between two generations: a haplOid stage, called the gametophyte, and a diploid stage, called the sporophyte. The sporophyte is short-lived and remains dependent on its parent gametophyte. Vascular plants first appeared during the Silurian period, and by the Devonian had diversified and spread into many different land environments. They have a number of adaptations that allowed them to overcome the limitations of the bryophytes. These include a cuticle

Introduction

5

resistant to desiccation, and vascular tissues which transport water throughout the organism. In most the sporophyte acts as a separate individual, while the \ gametophyte remains small. The first primitive seed plants, Pteridosperms (seed ferns) and Cordaites, both groups now extinct, appeared in the late Devonian and diversified through the Carboniferous, with further evolution through the Permian and Triassic periods. In these the gametophyte stage is completely reduced, and the sporophyte begins life inside an enclosure called a seed, which develops while on the parent plant, and with fertilisation by means of pollen grains. Whereas other vascular plants, such as ferns, reproduce by means of spores and so need moisture to develop, some seed plants can survive and reproduce in extremely arid conditions. Early seed plants are referred to as gymnosperms (naked seeds), as the seed embryo is not enclosed in a protective structure at pollination, with the pollen landing directly on the embryo. Four surviving groups remain widespread now, particularly the conifers, whicJ:t are dominant trees in several biomes. The angiosperlns, comprising the flowering plants, were the last major group of plants to appear, emerging from within the· gymnosperms during the Jurassic and diversifying rapidly during the Cretaceous. These differ in that the seed embryo (angiosperm) is enclosed, so the pollen has to grow a tube to penetrate the protective seed coat; they are the predominant group of flora in most biomes today. Fossils

Plant fossils include roots, wood, leaves.! seeds, fruit, pollen, spores, phytoliths, and amber (the fossilised resin produced by some plants). Fossil land plants are recorded in terrestrial, lacustrine, fluvial and nearshore marine sediments. Pollen, spores and algae (dinoflagellates and

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Plant Ecology

acritarchs) are used for dating sedimentary rock sequences. The remains of fossil plants are not as common as fossil animals, although plant fossils are locally abundant in many regions worldwide. The earliest fossils clearly assignable to Kingdom Plantae are fossil green algae from the Cambrian. These fossils resemble calcified multicellular members of the Dasycladales. Earlier Precambrian fossils are known which resemble single-cell green algae, but definitive identity with that group of algae is uncertain. The oldest known trace fossils of embryophytes date from the Ordovician, though such fossils are fragmentary. By the Silurian, fossils of whole plants are preserved, including the lycophyte Baragwanathia longifolia. From the Devonian, detailed fossils of rhyniophytes have been found. Early fossils of these ancient plants show the individual cells within the plant tissue. The Devonian period also saw the evolution of what many believe to be the first modern tree, Archaeopteris. This fern-like tree combined a woody trunk with the fronds of a fern, but produced no seeds. The Coal Measures are a major source of Palaeozoic plant fossils, with many groups of plants in existence at this time. The spoil heaps of coal mines are the best places to ~ollect; coal itself is the remains of fossilised plants, though structural.detail of the plant fossils is rarely visible in coal. kt the Fossil Forest at Victoria Park in Glasgow, Scotland, the stumps of Lepidodendron trees are found in their origina~ growth positions. The fossilized remains of conifer and angiosperm roots, stems and brancl1.es may be locally abundant in lake and inshore sedimentary rocks from the Mesozoic and Caenozoic eras. Sequoia and its allies, magnolia, oak, and palms are often found. Petrified wood is common in some parts of the world, and is most frequently found in arid or desert areas where

Introduction

7

it is more readily exposed by erosion. Petrified wood is often heavily silicified (the organic material replaced ~y silicon dioxide), and the impregnated tissue is often preserved in fine detail. Such specimens may be cut and polished using lapidary equipment. Fossil forests of petrified wood have been found in all continents. Fossils of seed ferns such as Glossopteris are widely distributed throughout several continents of the southern hemisphere, a fact that gave support to Alfred Wegener's early ideas regarding Continental drift theory. PLANT GROWIH

Most of the solid material in a plant is taken from the atmosphere. Through a process known as photosynthesis, plants use the energy in sunlight to convert carbon dioxide from the atmosphere into simple sugars. These sugars are then used as building blocks and form the main structural component of the plant. Plants rely on soil primarily for support and water (in quantitative terms), but also obtain nitrogen, phosphorus and other crucial elemental nutrients. - For the majority of plants to grow successfully they also require oxygen in the atmosphere and around their roots for respiration. However, a few specialised vascular plants, such as Mangroves, can grow with their roots in anoxic conditions. Factors Affecting Growth

The genotype of a plant affects its growth, for example selected varieties of wheat grow rapidly, maturing within 110 days, whereas others, in the same environmental conditions, grow more slowly and mature within 155 days. Growth is also determined by environmental factors, such as temperature, available water, available light, and available nutrients in the soil. Any change in the availability of these external conditions will be reflected in

8

Plant Ecology

the plants growth. Biotic factors (living organisms) also affect plant growth. flants compete with other plants for space, water, light and nutrients. Plants can be so crowded that no single individual makes normal growth. Many plaIi.ts rely on birds and insects to effect pollination. Grazing animals may affect vegetation. Soil fertility is influenced by the activity of bacteria and fungi. Bacteria, fungi, viruses, nematodes and insects can parasitise plants. Some plant roots require an association with fungi to maintain normal activity (mycorrhizal association). Simple plants like algae may have short life spans as individuals, but their populations are commonly seasonal. Other plants may be organised according to their seasonal growth pattern: Annual: live and reproduce within one growing season. B,iennial: live for two growing seasons; usually reproduce in second year. Perennial: live for many growing seasons; continue to reproduce once mature. Among the vascular plants, perennials include both evergreens that keep their leaves the entire year, and deciduous plants which lose their leaves for some part of it. In temperate and boreal climates, they generally lose their leaves during the winter; many tropical plants lose their leaves during the dry season. The growth rate of plants is extremely variable. Some mosses grow less than 0.001 mm/h, while most trees grow 0.025-0.250 mm/h. Some climbing species, such as kudzu, which do not need to produce thick supportive tissue, may grow up to 12.5 mm/h. Plants protect themselves from frost

Introduction

9

and dehydration stress with antifreeze proteins, heat-shock proteins and sugars. LEA (Late Embryogenesis Abundant) protein expression is induced by stresses and protects other proteins from aggregation as a result of desiccation and freezing. Vascular plants differ from other plants in that they transport nutrients between different parts through specialised structures, called xylem and phloem. They also have roots for taking up water and minerals. The xylem moves water and minerals from the root to the rest of the plant, and the phloem provides the roots with sugars and other nutrient produced by the leaves. ECOLOGICAL RELATIONSHIPS

The photosynthesis conducted by land plants and algae is the ultimate source of energy and organic material in nearly all ecosystems. Photosynthesis radically changed the composition of the early Earth's atmosphere, which as a result is now 21% oxygen. Animals and most other organisms are aerobic, relying on oxygen; those that do not are confined to relatively rare anaerobic environments. Plants are the primary producers in most terrestrial ecosystems and form the basis of the food web in those ecosystems. Many animals rely on plants for shelter as well as oxygen and food. Land plants are key components of the water cycle and several other biogeochemical cycles. Some plants have coevolved with nitrogen fixing bacteria, making plants an important part of the nitrogen cycle. Plant roots play an essential role in soil development and prevention of soil erosion. Plants are distributed worldwide in varying numbers. While they inhabit a multitude of biomes and ecoregions, few can be found beyond the tundras at the northernmost regions of continental shelves. At the southern extremes, plants have adapted tenaciously to the prevailing

10 condi~ions.

Plant Ecology

Plants are often the dominant physical and structural component of habitats where they occur. Many of the Earth's biomes are named for the type of vegetation because plants are the dominant organisms in those biomes, such as grasslands and forests. Numerous animals have coevolved with plants. Many animals pollinate flowers in exchange for food in the form of pollen or nectar. Many animals disperse seeds, often by eating fruit and passing the seeds in their feces. Myrmecophytes are plants that have coevolved with ants. The plant provides a home, and sometimes food, for the ants. In exchange, the ants defend the plant from herbivores and sometimes competing plants. Ant wastes provide organic fertiliser. The majority of plant species have various kinds of fungi associated with their root systems in a kind of mutualistic symbiosis known as mycorrhiza. The fungi help the plants gain water and mineral nutrients from the soil, while the plant gives the fungi carbohydrates manufactured in photosynthesis. Some plants serve as homes for endophytic fungi that protect the plant from herbivores by producing toxins. The fungal endophyte, Neotyphodium coenophialum, in tall fescue does tremendous economic damage to the cattle industrys. Various .forms of parasitism are also fairly common among plants, from the semi-parasitic mistletoe that merely takes some nutrients from its host, but still has photosynthetic leaves, to the fully parasitic broomrape and toothwort that acquire all their nutrients through connections to the roots of other plants, and so have no chlorophyll. Some plants, known as myco-heterotrophs, parasitize mycorrhizal fungi, and hence act as epiparasites on other plants. Many plants are epiphytes, meaning they grow on other plants, usually trees, without parasitizing them. Epiphytes may indirectly harm their host plant by intercepting

Introduction

11

mineral nutrients and light that the host would otherwise receive. The weight of large numbers of epiphytes may break tree limbs. Many orchids, bromeliads, ferns and mosses often grow as epiphytes. Bromeliad epiphytes accumulate water in leafaxils to form phytotelmata, complex aquatic food webs. A few plants are carnivorous, such as the Venus flytrap and sundew. They trap small animals and digest them to obtain mineral nutrients, .;?Specially nitrogen. IMPORTANCE OF PLANTS

The study of plant uses by people is termed economic botany or ethnobotany. They are often used as synonyms but some consider economic botany to focus mainly on uses .)f modern cultivated plants, while ethnobotany studies uses of indigenous plants by native peoples. Human cultivation of plants is part of agriculture, which is the basis .)f human civilisation. Plant agriculture is subdivided intu agronomy, horticulture and forestry. Virtually all human nutrition depends on land plants directly or indirectly. Much of human nutrition depends on cereals, especially maize or corn, wheat and rice or other staple crops such as potato, cassava, and legumes. Other parts from plants that are eaten include fruits, vegetables, nuts, herbs, spices and edible flowers. Beverages from plants include coffee, tea, wine, beer and alcohol. Sugar is obtained mainly from sugar cane and sugar beet. Cooking oils and margarine come from corn, soybean, canola, safflower, sunflower, olive and others. Food additives include gum arabic, guar gum, locust bean gum, starch and

pectin. Wood is used for buildings, furniture, paper, cardboard, musical instruments and sports equipment. Cloth is often made from cotton, flax or synthetic fibers derived from cellulose, such as rayon and acetate. Renewable fuels from plants include firewood, peat and many other biofuels.

12

Plant Ecology

Medicines derived from plants include aspirin, taxol, morphine, quinine, reserpine, colchicine, digitalis and vincristine. There are hundreds of herbal supplements such as ginkgo, Echinacea, feverfew, and Saint John's wort. Pesticides derived from plants include nicotine, rotenone, strychnine and pyrethrins. Drugs obtained from plants include opium, cocaine and marijuana. Poisons from plants include ricin, hemlock and curare. Plants are the source of many natural products such as fibers, essential oils, dyes, pigments, waxes, tannins, latex, gums, resins, alkaloids, amber and cork. Products derived from plants include soaps, paints, shampoos, perfumes, cosmetics, turpentine, rubber, varnish, lubricants, linoleum, plastics, inks, chewing gum and hemp rope. Plants are also a primary source of basic chemicals for the industrial synthesis of a vast array of organic chemicals. These chemicals are used in a vast variety of studies and experiments. Thousands of plant species are cultivated to beautify the human environment as well as to provide shade, modify temperatures, reduce windspeed, abate noise, provide privacy and prevent soil erosion. People use cut flowers, dried flowers and house plants indoors. Outdoors, they use lawngrasses, shade trees, ornamental trees, shrubs, vines, herbaceous perennials and bedding plants. Images of plants are often used in art, architecture, humor, language and photography and on textiles, money, stamps, flags and coats of arms. Living plant art forms include topiary, bonsai, ikebana and espalier. Ornamental plants have sometimes changed the course of history, as in tulipomania. Plants are the basis of a multibillion dollar per year tourism industry which includes travel to arboretums, botanical gardens, historic gardens, national parks, tulip festivals, rainforests, forests with colorful autumn leaves and the National Cherry Blossom Festival. Venus fly trap, sensitive plant and resurrection plant are examples of plants sold as novelties.

Introduction

13

Tree rings are an important method of dating in archeology and serve as a record of past climates. Basic biological research has often been done with plants, such as the pea plants used to derive Gregor Mendel's laws of genetics. Space stations or space colonies may one day rely on plants for life support. Plants are used as national and state emblems, including state trees and state flowers. Ancient trees are revered and many are famous. Numerous world records are held by plants. Plants are often used as memorials, gifts and to mark special occasions such as births, deaths, weddings and holidays. Plants figure prominently in mythology, religion and literature. The field of ethnobotany studies plant use by indigenous cultures which helps to conserve endangered species as well as discover new medicinal plants. Gardening is the most popular leisure activity in the U.S. Working with plants or horticulture therapy is beneficial for rehabilitating people with disabilities. Certain plants contain psychotropic chemicals which are extracted and' ingested, including tobacco, cannabis and opium. Weeds are plants that grow where people do not want them. People have spread plants beyond their native ranges and some of these introduced plants become invasive, damaging existing ecosystems by displacing native species. Invasive plants cause billions of dollars in crop losses annually by displacing crop plants, they increase the cost of production and the use of chemical means to control them affects the environment. Plants may cause harm to people. Plants that produce windblown pollen invoke allergic reactions in people who suffer from hay fever. A wide variety of plants are poisonous. Several plants cause skin irritations when touched, such as poison ivy. Certain plants contain psychotropic chemicals, which are extracted and ingested or smoked, including tobacco, cannabis (marijuana), cocaine and opium, causing damage to health or even

14

Plant Ecology

death. Both illegal and legal drugs derived from plants have negative effects on the economy, affecting worker productivity and law enforcement costs. Some plants cause allergic reactions in people and animals when ingested, while other plants cause food intolerances that negatively affect health. REFERENCES

Evans, L. T. (1998). Feeding the Ten Billion - Plants and Population Growth. Cambridge University Press. Paperback, 247 pages. Kenrick, Paul & Crane, Peter R. The Origin and Early Diversification of Land Plants: A Cladistic Study. Washington, D. c.: Smithsonian Institution Press. 1997. Raven, Peter H., Evert, Ray F., & Eichhorn, Susan E. Biology of Plants (7th _ ed.). New York: W. H. Freeman and Company. 2005. Taylor, Thomas N. & Taylor, Edith L. The Biology und Evolution of Fossil Plants. Englewood Cliffs, NJ: Prentice Hall. 1993.

2 Biosphere and Plant Vegetation Plants occur in almost every conceivable habitat on Earth -submerged on lake bottoms, exposed on windsweptmountain tops, hidden within polar rocks, or perched perilously on branches in the rain forest canopy. They can be microscopic or enormous like sequoias and eucalypts that may tower more than a hundred meters tall. Their flowers may span nearly a meter across or extend the height of a human, and be almost any color of the rainbow. Plants comprise more than 99 percent of all the Earth's living matter. The history of the biosphere is largely the history of the origin and diversification of plants. Without plants, conditions on Earth-including temperature, types of rocks, the composition of the atmosphere, and even the chemical composition of the oceans-would be vastly different. While plant ecology is generally defined as lithe study of relationships between plants and the environment," plants do not, as this definition implies, merely inhabit environments. Plants also modify the environments, and they may even control them. PLANTS AND BIOSPHERE

The word biosphere, which refers to that relatively thin layer on the surface of the Earth within which life exists, is now rather familiar to students. Yet the concept, according to Hutchi.nson, was introduced into science

16

Plant Ecology

rather casually by the Australian geologist, Eduard Suess in 1875. The idea was largely overlooked until the Russian mineralogist, Vladimir Vernadsky, published La Biosphe're in 1929. The word has now attained a general usage and significance that Vernadsky probably could not have imagined. The biosphere has conditions that are rare in the universe as a whole-liquid water in substantial quantities, an external energy source, and temperatures at which there are interfaces between solid, liquid, and gaseous forms of water. Liquid water exists under a rather narrow range of conditions of temperature and pressure. It was once abundant present on Mars and may still occur beneath the ice on Jupiter's moon Europa. New information is continually emerging from interplanetary space probes. At one end of the galactic temperature gradient there are temperatures of trillions of degrees inside stars, and, at the other end, there are conditions near absolute zero in the vastness of space. Neither extreme provides the conditions where biological chemistry, at least as humans understand itl, can occur. ENERGY FLOW

For life to exist, energy flow is required. Such a requirement is met when a planet is situated near enough to a star for sufficient energy released by solar fusion to pass the planet before dissipating into outer space. This is the case for our particular planet, situated near a star we know as the Sun. While it is not known how often life occurs, it may not be infrequent, given the enormous size of the universe our own galaxy has some 100 billion suns, and there now appears to be convincing evidence that some of these suns have their own solar systems. This provides many opportunities for other possible planets to be affected by flowing energy. Proximity to a source of solar energy is essential for life because that energy flow, by itself,

Biosphere and Plant Vegetation

17

organises matter. Life, at least as it is presently understood, is matter that has been organised by energy flow. Morowitz has examined the relationships among energy flow, thermodynamics, and life asserting that in order to properly understand life, one must look at the relationship between physical laws and biological systems. He demonstrates that flowing energy can create complexity out of simplicity. Once the requirement for energy is met, life then requires resources. This begs the question of what those early resources might have been. One way to answer such a question is to ask what conditions would have existed in the early Earth's atmosphere before there was life, since the early atmosphere would likely have been one source of resources for the precursors of living cells. Determining what the early atmosphere was like, however, requires considerable detective work. It seems that this atmosphere would have come, in part, from volcanic outgassings. For clues about its composition one can measure the current composition of volcanic gases. The early atmosphere would likely have been composed of water, carbon dioxide, and sulfur. It was an atmosphere rather different from that of today. Yet, some billions of years later, these basic molecules remain as the principal constituents of cellulose, the dominant structural molecule of plants, and the most abundant molecule in the biosphere. Morowitz presents thermodynamic calculations il'lustrating how energy flow stimulates chemical interactions and creates molecules with higher potential energy. Morowitz demonstrates mathematically that, with energy flow and simple mixtures of gases, increasingly complex molecules are formed. For example, a gaseous mixture of ~arbon, hydrogen, nitrogen, and oxygen at 500 "C yields mo!:>'tly water and CO2 with smaller amounts of other molecules, such as methane and ethane, which have higher potential energy. The latter molecules are less likely

18

Plant Ecology

to form because they are larger and therefore more energy is required to create them. As energy flows through the molecular system, however, the energy distribution shifts upward toward more and more complicated molecules. Morowitz postulates that energy flow through the early atmosphere yielded similar results: starting off with simple low energy molecules such as water, CO2, and nitrogen, more complex molecules were produced. The production of molecules was driven by the external energy source, which on Earth is the Sun. While some authors suggest that the origin of life by such means contradicts the second law of thermodynamics, what they fail to appreciate is that the second law applies to closed systems. The biosphere is an open system where, so long as energy flow occurs, organisation will increase. Another important physical condition of the early environment on Earth was the abundance of water. It is not surprising that water is still a major constituent of the bodies of living organisms. Given the probable temperatures on Earth at that time, water would be evaporating from some areas, condensing in the atmosphere, and then falling as rain. As it flowed back into the sea, water would dissolve elements from the rockselements that would rise in concentration as water evaporated from the ocean again. These elements could interact in solution, and concentrate in locations where seawater was evaporating most rapidly. Of course, while energy flow tends to produce larger and more complex molecules, there is a natural countervailing tendency-complex molecules will also tend to fall apart into simpler molecules. But here is the crucial point-some molecules will be more stable than others. These stable ones will tend to persist and accumulate. They will steadily become more common than those other molecules that are unstable. It does not require any great scientific insight to appreciate this, nor does it

Biosphere and Plant Vegetation

19

require us to imagine any sort of magical complexity or life force-this process is simply a logical consequence of what we mean by the terms "stable" and "unstable". Nothing lasts forever. Some things fall apart quickly, some things fall apart slowly. So long as both kinds of things are being steadily built by energy flowi, the long-lived ones will tend to become more common than the short-lived ones. It is so very simple-yet note that even at the chemical level, long before there is anything that one might be tempted to call life, there is a crude p~ocess of natural selection. Some things are surviving longer than others, and hence are becoming more common. Ammonia and methane are two such molecules that likely accumulated in.the Earth's early atmosphere. Once a reservoir of larger and more stable molecules forms, these molecules can in turn interact with each other, yielding molecules with greater complexity and higher levels of potential energy. Like the simpler molecules, these more complex molecules will have varying degrees of stability. Again, molecules that are unstable will fall apart and those that are stable will accumulate. Imagine this process continuing, with increasingly complex molecules forming as a consequence of external energy flow. In this simple scenario, there is ongoing natural selection for stability and persistence, even at the molecular level (Figure 1). Such ideas are based upon thermodynamic calculations, simple chemistry, and logic. Experimental work nicely complements them. In an early experiment, Miller and Urey set up a simple atmospheric system with a hydrological cycle. Water was evaporated and then cooled and condensed while sealed within glass tubes. Miller and Urey' then let the hydrological cycle run, created electrical sparks to simulate lightning, and found that primitive amino acids formed.

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Plant Ecology

,

, I

\

\

•

I

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I

•

Figure 1. Solar energy creates high-energy molecules out of simpler lowenergy molecules.

This classic piece of work was done in the early 1950s, and it is worth emphasizing that it was done by a graduate student. Miller was fishing around for a research project to do for graduate work and had already tried one project that did not work. Then he and his advisor heard a seminar about early conditions on Earth that stimulated them to try their experiment. This single study led to a large series of experiments wherein researchers created all manner of artificial atmospheres and utilised different types of energy flow to explore what kinds of molecules could be produced. One could ask what factors might allow complex molecules to further increase in stability and further accumulate. Such factors would likely include:

Biosphere and Plant Vegetation

21

protective walls, the direct use of sources of energy such as sunlight, a,nd . the ability to form larger aggregations to buffer against short-term periods of unsuitable conditions. Consciousness would be another step, but this is not a step that plants have taken. In The Selfish Gene, Dawkins argues that consciousness can be thought of as the ability to develop predictive models for future events. For example, if an organism knows that certain conditions are likely to bring winter, then it can store up fo('-:i. Such ideas will not be explored further here, but Dawkins does raise other issues, one of them being the way in which molecules that copy themselves will proliferate. Let us try to mentally reconstruct the circumstances on Earth some 4 billion years ago. Pools of increasingly complex molecules are accumulating as water evaporates and energy flow stimulates chemical interactions. Molecules that are stable are accumulating, those that are unstable are falling apart. Now consider the possibility of replication. Anymolecule that tends to create copies of itself will accumulate more rapidly than other molecules. Dawkins suggests that the occurrence of such replicators was a critical event in the origin of life. Although he uses the word "replication," "reproduction" is the analogous biological term. From this perspective, then, molecular stability is survival, and molecular replication is reproduction. Thus, in a very basic and non-living molecular system, it is possible to find the sorts of ecological and evolutionary processes that occur in whole organisms. Further, one can also find larger ecological processes such as competition and predation. Margulis and Sagan describe the circumstances on Earth at this time: The ponds, lakes andwarmshallowseas of the early Earth, exposed as they were to cycles of heat and cold, ultraviolet light and darkness, evaporation and rain, harbored their chemical ingredients through the gamut of energy states.

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Plant Ecology

Combinations of molecules formed, broke up, and reformed, their molecular links forged by the constant energy input of sunlight. As the Earth's various microenvironments settled into more stable states, more complex molecule chains formed, and remained intact for longer periods. By connecting to itself five times, for example, hydrogen cyanide (HCN), molecule created in interstellar space and a deadly poison to modem oxygen-breathing life, becomes adenine (HsCsNs), the main part of one of the universal nucleotides which make up DNA, RNA and ATP.

a

ORIGIN OF BIOSPHERE

Life began during the first billion years of an Earth history which is 4.5 billion years old. The illustration depicts an early Earth in which volcanoes, a gray, lifeless ocean, and a turbulent atmosphere dominated the landscape. Vigorous chemical activity is represented by the heavy clouds, which were fed by volcanoes and penetrated both by lightning discharges and solar radiation. The ocean received organic matter from the land and the atmosphere, as well as from infalling meteorites and comets. Here, substances such as water, carbon dioxide, methane, and hydrogen cyanide formed key molecules such as sugars, amino acids, and nuc1eotides. Such molecules are the building blocks of proteins and nucleic acids, compounds ubiquitous to all living organisms. A critical early triumph was the development of RNA and DNA molecules, which directed biological processes and preserved life's "operation instructions" for future generations. RNA and DNA are depicted in the illustration, first as fragmets and then as fully assembled helices. These helices formed some of the living threads, as shown in the illustration, however, other threads derived from planetary processe~$uch as ocean chemistry a~d volcanic activity. This evolving bundle of threads thus arose from a variety of sources, illustrating that the origin of life was triggered not only by special molecules such as RNA or DNA, but

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also by the chemical and physical properties of the Earth's primitive environments. Most of life's history involVled the biochemical evolution of single-celled microorganisms. We find individual fossilized microbes in rocks 3.5 billion years 'old,. yet we can conclusively identify multicelled fossils only in rocks younger than 1 billion years. The oldest microbial communities often constructed layered mound-shaped deposits called stromatolites, whose structures suggest that those organisms sought light and were therefore photosynthetic. These early stromatolites grew along ancient seacoasts and endured harsh sunlight as well as episodic ~etting and drying by tides. Thus it appears that, even as early as 3.5 billion years ago, microorganisms had become remarkably durable and sophisticated. Many important events mark the interval between 1 and 3 billion years ago. Smaller volcanic terrains were joined by larger, more stable granitic continents. Life learned how to release oxygen from water, and it populated the newly expanded continental shelf regions. The illustration depicts these events, both in the abundant mound-shaped stromatolites along the shoreline and in the greater variety of filamentous and spherical microbes in the foreground. Finally, between 1 and 2 billion years ago, the eukaryotic cells with their complex system of organells and membranes d~veloped and began to experiment with multicelled body structures. PLANT VEGETATION

Vegetation is a general term for the plant life of a region; it refers to the ground cover life forms, structure, spatial extent or any other specific botanical or geographic characteristics. It is broader than the term flora which refers exclusively to species composition. Perhaps the clos~s~ synonym is plant community, but vegetation can, and often does, refer to a wider range of spatial scales. Primeval

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Plant Ecology

redwood forests, coastal mangrove stands, sphagnum bogs, desert soil crusts, roadside weed patches, wheat fields, cultivated gardens and lawns; are all encompassed by the term vegetation. Vegetation supports critical functions in the biosphere, at all possible spatial scales. First, vegetation regulates the flow of numerous biogeochemical cycles, most critically those of water, carbon, and nitrogen; it is also of great importance in local and global energy balances. Such cycles are important not only for global patterns of vegetation but also for those of climate. Second, vegetation strongly affects soil characteristics, including soil volume, chemistry and texture, which feed back to affect various vegetational characteristics, including productivity and structure. Third, vegetation serves as wildlife habitat and the energy source for the vast array of animal species on the planet. Vegetation is also critically important to the world economy, particularly in the use of fossil fuels as an energy source, but also in the global production of food, wood, fuel and other materials. Perhaps most importantly, and often overlooked, global vegetation has been the primary source of oxygen in the atmosphere, enabling the aerobic metabolism systems to evolve and persist. Lastly, vegetation is psychologically important to humans, who evolved in direct contact with, and dependence on, vegetation, for food, shelter, and medicine. Vegetation Classification

Much of the work on vegetation classification comes from European and North American ecologists, and they have fundamentally different approaches. In North America, vegetation types are based on a combination of the following criteria: climate pattern, plant habit, phenology and/ or growth form, and dominant species. In the current US standard (adopted by the Federal Geographic Data Committee (FGDC), and originally developed by UNESCO

Biosphere and Plant Vegetation

25

and The Nature Conservancy), the classification is hierarchical and incorporates the non-floristic criteria into the upper (most general) five levels and limited floristic criteria only into the lower (most specific) two levels. In Europe, classification often relies much more heavily, sometimes entirely, on floristic (species) composition alone, without explicit reference to climate, phenology or growth forms. It often emphasizes indicator or diagnostic species which separate one type from another. In the FGDC standard, the hierarchy lev~ls, from most general to most specific, are: system, class, subclass, group, formation, alliance, and association. The lowest level, or association, is thus the most precisely defined, and incorporates the names of the dominant one to three (usually two) species of the type. Structure

A primary characteristic of vegetation is its threedimensional structure, sometimes referred to as its physiognomy, or architecture. Most people have an understanding of this idea through their familiarity with terms like "jungle", "woods", "prairie" or "meadow"; these terms conjure up a mental image of what such vegetation looks like. So, meadows are grassy and open, tropical rainforests are dense, tall and dark, savannas have trees dotting a grass-covered landscape, etc. Obviously, a forest has a very different structure than a desert or a backyard lawn. Vegetation ecologists discriminate structure at much more detailed levels than this, but the principle is the same. Thus, different types of forests can have very different structures; tropical rainforests are very different from boreal conifer forests, both of which differ from temperate deciduous forests. Native grasslands in South Dakota, Arizona, and Indiana are visibly different from each other, low elevation chaparral differs from that at high elevations, etc.

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Plant Ecology

Structure is determined by an interacting combination of environmental and historical factors, and species composition. It is characterised primarily by the horizontal and vertical distributions of plant biomass, particularly foliage biomass. Horizontal distributions refer to the pattern of spacing of plant stems on the ground. Plants can be very uniformly spaced, as in a tree plantation, or very non-uniformly spaced, as in many forests in rocky, mountainous terrain, where areas of high and low tree density alternate dependi~g on the spatial pattern of soil and climatic variables. Three broad categories of spacing are recognised: uniform, random and clumped. These correspond directly to the expected variation in the distance between randomly chosen locations and the closest plant to such locations. Vertical distributions of biomass are determined by the inherent productivity of an area, the height potential of the dominant species, and the presence/absence of shade tolerant species in the flora. Communities with high productivities and in which at least one shade tolerant tree species is present, have high levels of biomass because of their high foliage densities throughout a large vertical distance. Although this discussion centers on biomass, it is difficult to measure in practice. Ecologists thus often measure a surrogate, plant cover, which is defined as the percentage of the ground surface area that has plant biomass vertically above it. If the vertical distribution of the foliage is broken into defined height layers, cover can be estimated for each layer, and the total cover value can therefore be over 100; otherwise the values range from zero to 100. The measure is designed to be a rough, but useful, approximation of biomass. In some vegetation types, the underground distribution of biomass can also discriminate different types. Thus a sod-forming grassland has a more continuous and

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connected root system, while a bunchgrass community's is much less so, with more open spaces between plants (though often not as drastic as the openings or spacings in the above-ground part of the community, since root systems are generally less constrained in their horizontal growth patterns than are shoots). However, below-ground architecture is so much more time-consuming to measure, that vegetation structure is almost always described in relationship to the above-ground parts of the community. Dynamism in Vegetation

Like all biological systems,_plant communities are temporally and spatially dynamic; they change at all possible scales. Dynamism in vegetation is defined primarily as changes in either or both of species composition and vegetation structure. Temporal Dynamics

Temporally, a large number of processes or events can cause change, but for sake of simplicity they can be categorised roughly as either abrupt or gradual. Abrupt changes are generally referred to as disturbances; these include things like wildfires, high winds, landslides, floods, avalanches and the like. Their causes are usually external to the community-they are natural processes occurring independently of the natural processes of the community. Such events can change vegetation structure and species composition very quickly and for long time periods, and they can do so over large areas. Very few ecosystems are without some type of disturbance as a regular and recurring part of the long term system dynamic. Fire and wind disturbances are particularly common throughout many vegetation types worldwide. Fire is particularly potent because of its ability to destroy not only living plants, but also the spores and seeds representing the

28

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Plant Ecology

potential next generation, and because of fire's impact on faunal populations and soil characteristics. Temporal change at a slower pace is ubiquitous; it Icomprises the field of ecological succession. Succession is / the relatively gradual change in structure and composition that arises as the vegetation itself modifies various environmental variables, including light, water and nutrient levels over time. These modifications change the suite of species most adapted to grow, survive and reproduce in an area, causing floristic changes. These floristic changes contribute to structural changes that are already inherent in plant growth even in the absence of species changes, causing slow and broadly predictable changes in the vegetation. Succession can be interrupted at any time by disturbance, setting the system either back to a previous state, or off on another trajectory altogether. Because of this, successional processes mayor may not lead to some static, final state. Moreover, accurately predicting the characteristics of such a state, even if it does arise, is not always possible. In short, vegetative communities are subject to many and unpredictable variables that limit predictability. Spatial Dynamics

As a general rule, the larger an area under consideration, the more likely the vegetation will be heterogeneous across it. Two main factors are at work. First, the temporal dynamics of disturbance and succession are increasingly unlikely to be in synchrony across any area as the size of that area increases. That is, different areas will be at different developmental stages due to different local histories, particularly their times since last major disturbance. This fact interacts with inherent environmental variability, which is also a function of area. Environmental variabi11ty constrains the suite of species

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that can occupy a given area, and the two factors together interact to create a mosaic of vegetation conditions across the landscape. Only in agricultural or horticultural systems does vegetation ever approach perfect uniformity. In natural systems, there is always heterogeneity, although its scale and intensity will vary widely. A natural grassland may seem relatively homogeneous when compared to the same area of partially burned forest, but highly diverse and heterogeneous when compared to the wheat field next to it. Global Vegetation Patterns

At regional and global scales there is predictability of certain vegetation characteristics, especially physiognomic ones, which are related to the predictability in certain environmental characteristics. Much of the variation in these global patterns is directly explainable by corresponding patterns of temperature and precipitation. These two factors are highly interactive in their effect on plant growth, and their relationship to each other throughout the year is critical. Such relationships are shown graphically in climate diagrams. By graphing the long term monthly averages. of the two variables against each other, an idea is given as to whether or not precipitation occurs during the warm season, when it is most useful, and consequently the type of vegetation to be expected. For example, two locations may have the same average annual precipitation and temperature, but if the relative timing of the precipitation and seasonal warmth are very different, so will their vegetation structure and growth and development processes be. Scientific Study on Vegetation

Vegetation scientists study the causes of the patterns and processes observed in vegetation at various scales of space and time. Of partiCular interest and importance are

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Plant Ecology

questions of the relative roles of climate, soil, topography, and history on vegetation characteristics, including both species composition and structure. Such questions are often large scale, and so cannot easily be addressed by experimentation in a meaningful way. Observational studies supplemented by knowledge of botany, paleobotany, ecology, soil science etc, are thus the rule in vegetation science. Vegetation science has its origins in the work of botanists and/or naturalists of the 18th century, or earlier in some cases. Many of these were world travelers on exploratory voyages in the Age of Exploration, and their work was a synthetic combination of botany and geography that today we would call plant biogeography. Little was known about worldwide floristic or vegetation patterns at the time, and almost nothing about what determined them, so much of the work involved collecting, categorising, and naming plant specimens. Little or no theoretical work occurred until the 19th century. The most productive of the early naturalists was Alexander von' Humboldt, who collected 60,000 plant specimens on a five year voyage to South and Central America from 1799 to 1804. Humboldt was one of the first to document thecorrespondence between climate and vegetation patterns, in his massive, life-long work Voyage to the Equinoctial Regions of the New Continent, which he wrote with Aime Bonpland, the botanist who accompanied him. Humboldt also described vegetation in physiogonmic terms rather than just taxonomically. His work presaged intensive work on environment-vegetation relationships that continues to this day. The beginnings of vegetation study as we know it today began in Europe and Russia in the late 19th century, particularly under Jozef Paczoski, a Pole, and Leonty Ramensky, a Russian. Together they were much ahead of their time, introducing or elaborating on almost all topics-

Biosphere and Plant Vege~tion

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germane to the field today, well before they were so in the west. These topics included plant community analysis, or phytosociology, gradient analysis, succession, and topics in plant ecophysiology and functional ecology. Due to language and/or political reasons, much of their work was unknown to much of the world, especially the Englishspeaking world, until well into the 20th century. In the United States, Henry Cowles and Frederic Clements developed ideas of plant succession in the early 1900s. Clements is famous for his now discredited view of the plant community as a superorganism. He argued that, just as all organ systems in an individual must work together for the body to function well, and which develop in concert with each other as the individual matures, so the individual species in a plant community also develop and cooperate in a very tightly coordinated and synergistic way, pushing the plant community towards a defined and predictable end state. Although Clements did a great deal of work on North American vegetation, his devotion to the superorganism theory has hurt his reputation, as much work since then by numerous researchers has shown the idea to lack empirical support. In contrast to Clements, several ecologists have since demonstrated the validity of the individualistic hypothesis, which asserts that plant communities are simply the sum of a suite of species reacting individually to the environment, and co-occurring in time and space. Ramensky initiated this idea in Russia, and in 1926, Henry pleason developed it in a paper in the, 'Pnited States. Gleason's ideas were categorically rejected for 'many years, so powerful was the influence of Clementsian ideas. However, in the 1950s and 60s, a series of well-designed studies by Robert Whittaker provided strong evidence for Gleason's arguments, and against those of Clements. Whittaker, considered one of the brightest and most productive of American plant ecologists, was a developer

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Plant Ecology

and proponent of gradient analysis, in which the abundances of individual species are measured against quantifiable environmental variables or their wellcorrelated surrogates. In studies in three very different montane ecosystems, Whittaker demonstrated strongly that species respond primarily to the environment, and not necessarily in any coordination with other, co-occurring species. Other work, particularly in paleobotany, has lent support to this view at larger temporal and spatial scales. Since the 1960s, much research into vegetation has revolved around topics in funCtional ecology. In a functional framework, taxonomic botany is relatively less important; investigations center around morphological, anatomical and physiological classifications of species, with the aim of predicting how particular groups thereof will respond to various environmental variables. The underlying basis for this approach is the observation that, due to convergent evolution and adaptive radiation, there is often not a strong relationship between phylogenetic relatedness and environmental adaptations, especially at higher levels of the phylogenetic taxonomy, and at large spatial scales. Functional classifications arguably began in the 1930s with Raunkiaer's division of plants into groups based on the location of their apical meristems relative to the ground surface. Functional classifications are crucial in modeling vegetation-environment interactions, which has been a leading topic in vegetation ecology for the last 30 or more years. Currently, there is a strong drive to model local, regional and global vegetation changes in response to global climate change, particularly changes in temperature, precipitation and disturbance regimes. Functional :lassifications such as the examples above, which attempt to categorise all plant species into a very small number of groups, are unlikely to be effective for the wide variety of different modeling purposes that exist or will exist.

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It is generally recognised that simple, all-purpose classifications will likely have to be replaced with more detailed and function-specific classifications for the modeling purpose at hand. This will require much better understanding of the physiology, anatomy, and developmental biology than currently exists, for a great number of species, even if only the dominant species in most vegetation types are considered. ECOLOGICAL SUCCESSION

Ecological succession, a fundamental concept in ecology, refers to more-or-Iess predictable and orderly changes in the composition or structure of an ecological community. Succession may be initiated either by formation of new, unoccupied habitat or by some form of disturbance of an existing community. Succession that begins in areas where no soil is initially present is called primary succession, whereas succession that begins in areas where soil is already present is called secondary succession. The trajectory of ecological change can be influenced by site conditions, by the interactions of the species present, and by more stochastic factors such as availability of colonists or seeds, or weather conditions at the time of disturbance. Some of these factors contribute to predictability of successional dynamics; others add more probabilistic elements. In general, communities in early succession will be dominated by fast-growing, welldispersed species. As succession proceeds, these species will tend to be replaced by more competitive species. Trends in ecosystem and community properties in succession have been suggested, but few appear to be general. For example, species diversity almost necessarily increases during early succession as new species arrive, but may decline in later succession as competition eliminates opportunistic species and leads to dominance by locally superior competitors. Net Primary Productivity, biomass,

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Plant Ecology

and trophic level properties all show-variable patterns over succession, depending on the p~rticular system and site. Ecological succession was formerly seen as having a stable end-stage called the climax, sometimes referred to as the 'potential vegetation' of a site, shaped primarily by the local cli:i.nate. This idea has been largely abandoned by modem ecologists in favor of nonequilibrium ideas of how ecosystems function. Most natural ecosystems experience disturbance at a rate that makes a "climax" community unattainable. Climate change often occurs at a rate and frequency sufficient to prevent arrival at a climax state. Additions to available species pools through range expansions and introductions can also continually reshape communities. Many species are specialised to exploit disturbances. In forests of northeastern North America trees such as Betula alleghaniensis and Prunus serotina are particularly welladapted to exploit large gaps in forest canopies, but are intolerant of shade and are eventually replaced by other species in the absence of disturbances that create such gaps. The development of some ecosystem attributes, such as pedogenesis and nutrient cycles, are both influenced by community properties, and, in turn, influence further community development. This process may occur only over centuries or millennia. Coupled with the stochastic nature of disturbance events and other long-term changes, such dynamics make it doubtful whether the 'climax' concept ever applies or is particularly useful in considering actual vegetation. The idea of ecological succession goes back to the 19th Century. The French naturalist Adolphe Dureau de la Malle was the first to make use of the word succession about the vegetation development after forest clear-felling. In 1860 Henry David Thoreau read an address called "The Succession of Forest Trees" in which he described successIon in an Oak-Pine forest.

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Henry Chandler Cowles, at the University of Chicago, developed a more formal concept of succession. Inspired by the studies of Danish dunes done by Eugen Warming, Cowles studied vegetation development sand dunes on the shores of Lake Michigan. He recognised that vegetation on sand-dunes of different ages might be interpreted as different stages of a general trend of vegetation development on dunes, and used his observations to propose a particular sequence and process of primary succession. His paper, "The ecological relations of the vegetation of the sand dunes of Lake Michigan" in 1899 in the Botanical Gazette is one of the classic publications in the history of the field of ecology. Understanding of succession was long dominated by theories of Frederic Clements, a contemporary of Cowles, who held that successional sequences of communities, were highly predictable and culminated in a climatically determined stable climax. Clements and his followers developed a complex taxonomy of communities and successional pathways. A contrasting view, the Gleasonian framework, is more complex, with three items: invoking interactions between the physical environment, population-level interactions between species, and disturbance regimes, in determining the composition and spatial distribution of species. It differs most fundamentally from the Clementsian view in suggesting a much greater role of chance factors and in denying the existence of coherent, sharply bounded community types. Gleason's ideas, first published in the early 20th century, were more consistent with Cowles' thinking, and were ultimately largely vindicated. However, they were largely ignored from their publication until the 1960s. About Frederic Clements' distinction between primary succession and secondary succession, Cowles wrote:

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Plant Ecology

This classification seems not to be of fundamental value, since it separates such closely related phenomena as those of erosion and deposition, and it places together such unlike things as human agencies and the subsidence of land.

Beginning with the work of Robert Whittaker and John Curtis in the 1950s and 1960s, models of succession have gradually changed and become more complex. In modem times, among North American ecologists, less stress has been placed on the idea of a single climax vegetation, and more study has gone into the role of contingency in the actual development of communities. REFERENCES

Archibold, O. W. Ecology of World Vegetation. New York: Springer Publishing, 1994. Barbour, M. G. and W. D. Billings (eds). North American Terrestrial Vegetation. Cambridge: Cambridge University Press, 1999. Barbour, M.G, J.H. Burk, and W.D. Pitts. "Terrestrial Plant Ecology". Menlo Park: Benjamin Cummings, 1987. Breckle, S-W. WaIter's Vegetation of the Earth. New York: Springer Publishing, 2002. Burrows, C. J. Processes of Vegetation Change. Oxford: Routledge Press, 1990.

Feldmeyer-Christie, (et.al). Modern Approaches In Vegetation Monitoring. Budapest: Akademiai Kiado, 2005. Gleason, H.A. 1926. The individualistic concept of the plant association. Bulletin of the Torrey Botanical Club, 53:1-20. Grime, J.P. 1987. Plant strategies and vegetation processes. Wiley Interscience, New York NY. Kabat, P., et al. (eds). Vegetation, Water, Humans and the Climate: A New Perspective on an Interactive System. Heidelberg: Springer-Verlag 2004.

Macarthur, R.H. and E.O. Wilson. The theory of Island Biogeography. Princeton: Princeton University Press. 1967 Mueller-Dombois, D., and H. Ellenberg. Aims and Methods of Vegetation Ecology. The Blackburn Press, 2003. Van Der Maarel, E. Vegetation Ecology. Oxford: Blackwell Publishers, 2004.

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Vankat, J. L. The Natural Vegetation of North America. Krieger Publishing Co., 1992.

3 Impact of Physical Environment on Plant Growth This chapter describes the physical environment (soil, light, temperature, humidity, wind) about plants and how the physical environment affects the physiological status plants and how plants affect their physical environment. SOIL AND PLANTS

Soils physically support plants, and act as reser oirs for the water and nutrients needed by plants. Soils are complex mixtures of mineral particles of various shapes and sizes; living and dead organic materials including microorganisms, roots, and plant and animal residues; air; and water. In the soil, physical chemical, and biological reactions occur constantly and are closely interrelated. The physical form of the soil plays a large role in influencing the nature of biological and chemical reactions. Optimum plant growth depends as much on a favorable physical environment as it does on what we call soil fertility. The discussion of soil physical characteristics begins with the sizes (texture) and arrangements (structure) of individual soil particles. These two characteristics intimately affect the pore space between the particles. The pore space is important as the conveyor of water, dissolved mineral nutrients, and air, as well as for providing space in which rots can grow. Soil color is discussed because it

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Plant Ecology

often provides information about the chemical makeup or status of drainage in the soil. Finally, it is important to consider the whole soil mass, and how it changes with depth below the surface. Soil Texture

Soil texture is a term which describes the mixture of different sizes of mineral particles. The mineral particles, originally from solid rock, assumed their present form because of physical and chemical processes called weathering. At some stage in the weathering process, mineral particles became a favorable medium for plant growth, that is, they were able to provide storage of water, air, and mineral nutrients, as well as space in which roots could grow. Organic matter then accumulated near the soil surface due to the decomposition of plant residues. Generally, organic matter further improved the properties of the soil as an environment for plant growth. Soil texture relates primarily to particles smaller than 2 millimeters in diameter-sand, silt, and clay-since these are the particles most active in soil processes which support plant growth. Coarser particles, gravel and stones, are either inert or detrimental to plant cultivation. Sand, the coarsest of the active particles, feels gritty when rubbed. Sandy soils usually have rapid water infiltration and good aeration but low water holding and nutrient storage capacity. However, there is a considerable range in these properties within the sand fraction. Silt, the intermediate size, feels smooth when dry, and slippery but not sticky when moist. Because the smaller particle size promotes smaller pore spaces between particles, silty soils have a slower water intake rate but a higher water holding capacity than sandy soils. A few soils are very high in silt. These are difficult for storage because they often lack aggregation. This results in high density and

Impact of Physical Environment on Plant Growth

39

a pore size too small for suitable water percolation and aeration. Nevertheless, silt is an essential component of the medium textured, versatile soil called loam. Clay, the finest size fraction, gives the soils a sticky or plastic feel. Clay exhibits some unusual properties, unexpected if it were merely composed of smaller particles or the same minerals that make up sand and silt. Clay is largely composed of a different set of minerals, called secondary minerals. These are weathering products of the primary minerals-quartz, feldspar, and mica-of which sand and silt are largely composed. One unusual property of clay is its attraction (called adsorption) for positive ions, such as calcium, magnesium, potassium, ammonium, and others. Because of this adsorption, the clay in as quantities of the plantions. On the other, negative plant nutrient ions such as nitrate, phosphate, and sulfate are repelled by clay particles, and can only be stored for plant use to the extent that they occur dissolved in the water held in soil pores. Clay has a very high affinity for water, partly because of its small particle size and partly because the aforementioned positive ions associated with clay also attract water. Montmorillonite clay, the type found in many soils, swells greatly when wetted, and shrinks-leaving wide cracks when dry. While soils high in day are difficult to manage because of their great strength and sticky nature, an intermediate amount of clay in a soil improves its capacity to hold water and plant nutrient ions. The swelling and shrinking of clay also helps form favorable structure in medium textured soils. One useful and often used grouping of soil texhires includes the following three categories: Coarse-textured soils-Sands, loamy sands, and some sandy loams. Medium-textured soils-Loams, sandy loams, silt loams, and some sandy day loams and clay loams.

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Plant Ecology

Fine-textured soils ---Clays, sandy clays, silty clays, and some sandy clay loams, silty clay loams, and clay loams. With experience, the texture of a soil can be felt and determined fairly simply by rubbing moist soil between thumb and forefinger, and noticing its characteristics-how it ribbons or is pushed out into a thin strip-how it hangs together, and how sticky, smooth, or gritty it is. Soil Structure

Soil structure refers to the arrangement of soil particles. Sand, silt, and clay seldom occur as separate units in the soil; rather, they combine into aggregates held together by small binding forces of clay and organic matter. The size and form of aggregation is known as the structure of the soil. Soil structure is one of the more important physical characteristics of soil, yet perhaps the least understood. Plant growth is strongly influenced by soil structure. Soil structure affects movement of water, air, and roots through the soil. Soil structural aggregate may vary from a fraction of an inch to several inches in diameter; may be approximately spherical, elongated, or platelike; and may be held together strongly or weakly. A granular structure provides an ideal environment for plant roots, and is particularly helpful for establishing plants from seeds or transplants. The larger pores between the granular aggregates are continuous, and roots may penetrate them with ease. Water drains readily through this soil, yet moisture is held back sufficiently in the aggregates to supply root needs. Granular structure occurs in loam soils and in some clay soils near the surface. One of the good things about clay is its promotion of granular structure in medium textured soils. A greater organic matter content also results

Impact of Physical Environment on Plant Growth

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in better granular structure of a soil. Sandy soils are low in both organic matter and clay, and aggregation is very weak to nonexistent. The structure is called single-grained; such a soil drains well but doe not retain much moisture. Single-grained soils require more frequent irrigation and fertilisation for plant roots to thrive. Prismatic and blocky structures most often occur as the result of shrinking and cracking of clay loams and clay soil layers (called horizons) upon drying. The large cracks that are visible at the surface of dry clay soils may occasionally extend to three feet or more in depth. The elongated chunks of soil between these vertical cracks are called prisms. The lower portions of the prisms often have horizontal cracks intersecting the vertical ones so that more or less equidimensional blocky structure results. Prismatic or blocky aggregates may vary considerably in size but are always coarser than those of granular structure. The aggregates swell when wet and fit together so tightly that water drains through them rather slowly. Plant roots may follow cracks downward but do not usually penetrate to the centers of prismatic or blocky aggregates. Thus, the roots may not have access to a significant portion of the water and nutrients in these soils. Platy structure refers to the occurrence of thin layers of soil stacked on top of one another. These most often occur when silty soil materials are deposited in thin layers by stream overflow. The discontinuities caused by this minute layering may interrupt the movement of water, air, and roots into the soil. Artificial platy structure may be caused by repeated compression of soils in faim roadways. Many medium textured soils do not have well defined structural aggregates. This is true because of a much lower organic matter content than most midwestern soils. If particles are weakly bound together in the whole soil mass, soils are said to have a massive structure. If open and porous, these soils may still provide a favorable root

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Plant Ecology

environment. Many massive soils, however, are dense and nonporous providing only slow water and air movement. Compact, massive layers occur naturally in the subsoils of some old terrace soils, but farming activity has caused similar compaction near the surface of many cultivated soils that originally had granular or single-grained structure. Intensive cultivation usually results in some breakdown of the natural soil structure. Forces holding soil particles together in aggregates may not be strong enough to resist the crushing effect of heavy equipment, or the shearing effect resulting from working the soil at too high a moisture content. Excessive traffic over the land results in a compact soil mass in which large pores have collapsed due to crushing of the granules. In the absence of large pores, water penetration becomes very slow. The small pores, still present, may fill slowly with water after irrigation, and drain even more slowly because water is held strongly by particle surfaces. This has two serious effects. Water movement to lower depths is very slow; and little or no airspace is left in the compacted soil. Feeder roots of most crops will die if deprived of air for only a few hours. The more dense layers resulting from man-made soil compaction usually show up within the surface foot of soil. However, compression by tractor wheels and tillage equipment may cause some compaction as deep as two feet below the soil surface. Regardless of soil permeability beneath the compact layer, water cannot percolate or infiltrate faster than the limiting rate set by the compacted layer. Compaction can develop in almost all soils, although some soils seem more susceptible than others. Preventing Soil Structure Breakdown

Although some breakdown of structure within the surface font may be inevitable where land is intensively cultivated,

Impact of Physical Environment on Plant Growth

43

and understanding of soil texture and structure enables the cultivator to apply solid cultural practices with a minimum of structural breakdown. Structural breakdown is easier to prevent than to cure. The following recommendations will help prevent structural breakdown. Plow and cultivate soil at an intermediate moishire content-not to wet, not too dry. It is especially important to avoid recompaction of freshly plowed or loosened soil. The less tillage after loosening the better. Make tractor and implement tracks on the smallest amount of land possible and use the same tracks for all operations. Harvest and spray when the soil is as dry as possible, within the limitations of weather and timely schedule of operation. Rejuvenating Good Soil Structure

If compaction is severe, there is some possibility of rejuvenating structure. The method used for such rejuvenation will depend on the crop and the soil. The factors favoring formation of granular structure are: Wetting and drying of soils ca use swelling and shrinking, resulting in improved aggregation. Bacterial decomposition of plant residues produces gums that help band soil particles together. Planting fibrous rooted cover crops, particularly grasses, helps to push soil particles together and makes aggregates with continuous pore spaces between them. The effect of swelling and shrinking on improving granulation is particularly noticeable with medium and fine textured soils in fall plowed fields left rough through winter. To be most effective, the compacted layers should be brought to the surface by deep plowing. If compacted

44

Plant Ecology

layers come up in large chunks or slabs, they will be able to undergo swelling and shrinking in three dimensions due to alternating wetting and drying. By spring the soil should be in much better physical condition. Incorporating crop residues should be included as a management ,practice whenever possible in field or vegetable crop production. Although it is difficult to build up the percentage of soil organic matter because of rapid decomposition in the hot regions, regular additions of crop wastes can only have a beneficial effect on maintaining or improving soil structure. Cover crops or permanent sod in orchards and vineyards can provide some structural improvement is these plant roots can penetrate the compacted layer. Often, however, the root penetration of the cover or sod crop itself is restricted by the compaction, so it may be advisable to break up the compaction mechanically before planting the cover or sod. Soil Color

Soil color is obvious and easily determine is one of the most useful characteristics in class~fication and identification. Determination of soil color with a Munsell color chart provides a standard method of describing solid. Although color has no direct influence on the functioning or productivity of the soil, a great deal may be inferred about a soil from its color. A few broad generalisations may be made about soils of different colors. Gray and brown soils form the largest group of soils. They are moderately low in organic matter but include some of the most productive alluvial soils. Gray soils of the eastside, formed from granite alluvium, tend to be coarse to medium textured. The brown soils of the westside formed from sedimentary alluvium, tend to be medium to fine textured. In all areas within each group, there is a wide variation in productivity and other characteristics.

Impact of Physical Environment on Plant Growth

45

Black soils are relatively high in organic matter but the amount may vary from less than 5 percent (mineral soils) to more than 50 percent (peats and mucks). Black soils formed under poorly drained conditions and are either peaty or clayey in texture, but may with good management, be highly productive for field and vegetable crops. In upland or coastal areas, black soils with strong granular structure have formed under native grassland, on fine textured parent materials, and cool climates. Red soils are generally older soils that have undergone intensive weathering. In valleys, red soils occur on terraces or bench lands much older than the soils of the recent alluvial fans. These older soils often have restrictive clay pans or hardpans in the subsoil. In the mountains, red soils occur in the lower timber zone where a combination of high winter rainfall and warm summer temperatures prevail. Red soils are often deficient in phosphorous, zinc, and sulfur, in addition to nitrogen. White or light gray soils are usually sandy or calcareous (contain lime). In sandy soils, look for possible waterholding and nutrient problems; in calcareous soils, iron deficiency may be a problem to some crops and ornamental plants, but particularly to orchard crops. Blue of blue-gray layers are usually found in poorly aerated subsoils where organic matter is decomposing anaerobically (without air). Often, such soils have a sewerlike odor. These soils contain gases and dissolved materials toxic to plant roots. Extensive aeration is necessary to restore these soils to a condition suitable for plant growth. Soil Depth

Soil depth is important to the management of plant growth. The deeper the soil, the greater the totai water and nutrient storage capacity available to plants. Soil depth can be observed in roadcuts, stream banks, or by digging holes. A soil auger is useful where exposed cuts are unavailable.

Plant Ecology

Holes are normally dug at least 5 feet deep unless hard rock or hardpan is encountered. In making soil surveys, the soil is investigated to a depth of 5 to 6 feet. ,In special cases, investigation to a greater depth, possibly 10 to 20 feet, may be desirable, particularly where salty layers or a fluctuating water table may damage deep rooted crops. Root and water penetration through a soil are altered by layers having a distinctly different texture from the layers above or below. If a sub-soil layer has a noticeable increase in clay, water may accumulate above this layer, and roots may be injured because of poor aeration. This condition is often called waterlogging. Very sandy or gravely layers can also interrupt the normal downward penetration of roots or percolation of water. For example, water does not drain freely from a loamy layer into a sandy or gravely layer until the loamy layer becomes saturated for some depth above the coarser layer. When drainage has ceased, a saturated layer remaining just above the textural change will have an adverse effect on roots. The lingering saturated zone remains because particle-to-particle flow of water is poor from the loamy layer into sand or gravel. Very dense, unfractured rocklike layers (hardpan) sometimes occur in older alluvial soils on relatively flat terraces. These cemented hardpans are impervious to both water and roots. Winter rainfall accumulates above the hardpan but cannot soak through it. Unless the hardpan is shattered and drainage is improved, native grasses or crops grow very poorly on the shallow root zone left as water slowly evaporates from saturated soil. Many of the soils in the uplands rest on hard rock. The density, as well as the degree of fracture of the rock, is quite variable. As a rule, the rock under the soil is more dense in the lower foothills than in the mountainous areas. The density and degree of fracture of the rock are important to moisture storage, drainage, and runoff. A dense,

Impact of Physical Environment on Plant Growth

47

nonfractured, hard rock does not allow water to drain readily from the soil above, nor does the rock store water. A highly fractured rock stores water and allows soil drainiige. In a soil underlain with fractured rocks, forest tree roots may extract water to a depth of more than 20 feet. The term effective root depth has been used to describe that portion of the soil favorable for roots. In an alluvial soil, with no noticeable stratification, effective root depth may be more than five feet; in a claypan soil it may be as little as 12 inches, or the depth of soil above the clay layer. Thus, to determine soil depth, it is necessary to determine which layers in the soil will be restrictive to root and water penetration. PLANT -WATER RELATIONSHIPS

Water is essential in the plant environment for a number of reasons. Water transports minerals through the soil to the roots where they are absorbed by the plant. Water is also the principal medium for the chemical and biochemical processes that support plant metabolism. Under pressure within plant cells, water provides physical support for plants. It also acts as a solvent for dissolved sugars and minerals transported throughout the plant. In addition, evaporation within intercellular spaces provides the cooling mechanism that allows plants to maintain the favorable temperatures necessary for metabolic processes. Water is transported throughout plants almost continuously. There is a constant movement of water from the soil to the roots, from the roots into the various parts of the plant, then into the leaves where it is released into the ahnosphere as water vapor through the stomata (small openings in the leaf surfaces). This process is called transpiration. Combined with evaporation from the soil and wet plant surfaces the total water loss to the ahnosphere is called evapotranspiration.

Plant Ecology

48

One of the openings (stoma) is shown on the leaf cross section in Figure 1.

~\

ls:v VII..

Figure 1. Leaf cross section

Guard cells which are found on both sides of the stoma control its opening and closing (Figure 2). Stomata can be found on one (typically underside) or both sides of a leaf depending Qll plant species.

Figure 2. Stoma and guard cells

\ Well-watered plants maintain their shape due to the internal pressure in plant cells {turgor pressure}. This pressure is also necessary for plant cell expansion and

Impact of Physical Environment on Plant Growth

49

consequently for plant growth. Loss of this pressure due to insufficient water supply can be noticed as plant wilting. The schematic effects of water stress on plant growth are presented in Figure 3 . The major economic consequence of insufficient water in agricultural crops is yield reduction. When too little water is available in the root zone, the plant will reduce the amount of water lost through transpiration by partial or total stomatal closure. This results in decreased photosynthesis since the CO2 required for this process enters the plant through the stomata. Decreased photosynthesis reduces biomass production and results in decreased yields. WA'TIIR DIFlCrr

LJJS.&~ 10_=,_ ••

Figure 3. Schematic effects of water stress on plant growth

The role of soil in the soil-plant-etmosphere continuum is unique. It has been demonstrated that soil is not essential

so

Plant Ecology

for plant growth and indeed plants can be grown hydroponically (in a liquid culture). However, usually plants are grown in the soil and soil properties directly affect the availability of water and nutrients to plants. Soil water affects plant growth directly through its controlling effect on plant water status and indirectly through its effect on aeration, temperature, and nutrient transport, uptake and transformation. The understanding of these properties is helpful in good irrigation design and management. The soil system is composed of three major components: solid particles (minerals and organic matter), water with various dissolved chemicals, and air. The percentage of these components varies greatly with soil texture and structure. An active root system requires a delicate balance between the three soil components; but the balance between the liquid and gas phases is most critical, since it regulates root activity and plant growth process. The amount of soil water is usually measured in terms of water content as percentage by volume or mass, or as soil water potential. Water content does not necessarily describe the availability of the water to the plants, nor indicates, how the water moves within the soil profile. The only information provided by water content is the relative amount of water in the soil. Soil water potential, which is defined as the energy required to remove water from the soil, does not directly give the amount of water present in the root zone either. Therefore, soil water content and soil water potential should both be considered when dealing with plant growth and irrigation. The soil water content and soil water potential are related to each other, and the soil water characteristic curve provides a graphical representation of this r~lationship (Figure 4). The nature of the soil characteristic curve depends on the physical properties of the soil namely, texture and structure. Soil texture refers to the distribution of the soil

Impact of Physical Environment on Plant Growth

S1

particle sizes. The mineral particles of soil have a wide range of sizes classified as sand, silt, and clay. The proportion of each of these particles in the soil determines its texture.

.... Figure 4. Graphical representation of soil water content-soil water potential relationship

All mineral soils are classified depending on their texture. Every soil can be placed in a particular soil group using a soil textural triangle presented in Figure 5 . For example a soil with 60% sand and 10% clay separates is classified as a Sandy loam. In addition almost all soils contain some organic material, particularly in the top layer. This organic material, together with the fine soil particles, contributes to aggregate formation which results in the improvement of the soil structure. Soil structure refers to the arrangement of soil particles into certain patterns. The structural pattern, the extent of aggregation, and the amount and nature of the pore space describe the structure of the particular soil. No

Plant Ecology

S2

structure is usually present in sandy soils, however the presence of the organic matter can improve tho Tu"'ter holding capacity of the soil. The size, shape, and arrangement of the soil particles and the associated voids (pores) determine the ability of a soil to retain water. It is important to realize that large pores in the soil can conduct more water more rapidly than fine pores. In addition, removing water from large pores is easier and requires less energy than removing water from smaller pores.

Figure 5. Soil textural triangle

Sandy soils consist mainly of large mineral particles with very small percentages of clay, silt, and organic matter. In sandy soils there are many more large pores than in clayey soils. In addition the total ,,:olume of pores in sandy soils

Impact of Physical Environment on Plant Growth

53

is significantly smaller than in clayey soils (30 to 40% for sandy soils as compared to 40 to 60% for clayey soils). As a result, much less water can be stored in sandy soil than in the clayey soil. It is also important to realize that a significant number of the pores in sandy soils are large enough to drain within the first 24 hours due to gravity and this portion of water is lost from the system before plants can use it. To study soil-water-plant relationships it is convenient to subdivide soil water into water available to the plant and water unavailable to the plant. After the soil has been saturated with water one can observe a vertical, downward movement of water due to gravity. In Florida soils, this drainage process happens quickly. Usually 24 hours is sufficient to remove most of the gravitational water in sandy soils. The exact time depends on the soil type; the drainage of the gravitational water generally takes a little longer for clayey soils. Most gravitational water moves out of the root zone too rapidly to be used by the plants. The remaining water is stored under tension in the various size pores. The smaller the pore the greater the tension and the more energy required to remove its water. As a result plants have the ability to remove water only from the certain size pores. The removal of water from very small pores requires too much energy and consequently, this water is not available to the plant. There is also some water which is very closely bound to soil particles. This water is called hygroscopic water. It is also very difficult to remove, and is not available to the plants. The range of water available to plants is between field capacity (FC) and the permanent wilting point (PWP). The soil is at field capacity when all the gravitational water has been drained and a vertical movement of water due to gravity is negligible. Further water removal for most of the soils will require at least 7 kPa (7 cbars) tension. The permanent wilting point is defined as the point where there

Plant Ecology

54

is no more water available to the plant. The permanent wilting poi,ht depends on plant variety, but is usually around 1500 kPa (15 bars). This means that in order for plants to remove water from the soil, it must exert a tension of more than 1500 kPa (IS,bars). This is the limit for most plants and beyond .. they experience permanent wilting. It is easy to see that soils which hold significant amounts of water at tension in the range plants are able to exert (up to 1500 kPa (15 bars) of tension) will provide better water supply for plant growth (Figure 6).

t1lis

DMlIIAGII

WMIIR

DIW.....

W.,..

UJllLI.AIIY WA1'D IAVAILAIII..,

OAPlIJ.AIIIY WAla

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pwp

WATER

HYUOSCOPtC WATUt

CLAY

SAND

Figure 6. Water supply for plant growth

The pores in sandy soils are generally large and a significant percentage drain under the force of gravity in the first few hours after a rain. This water is lost from the root zone to deep percolation. What remains is used very quickly and the state of PWP can be reached in only a few days.

Impact of Physical Environment on Plant Growth

55

PLANTS AND LIGHT

Plants have three basic responses or reactions to light. They are photosynthesis, phototropism and photoperiodism. Photosynthesis is, of course, the process on which all life on earth depends. Radiant energy from the sun is converted into chemical energy. The energy is stored in chemical bonds in sugars like glucose and fructose. Phototropism is the plant's movement in response to light. All of us have seen the houseplant that leans toward the window. That is phototropism. Growth hormones are produced which cause the stem cells on the side away from the light to multiply causing the stem to tilt. The leaves are then closer to the light source and aligned to intercept the most light. The most interesting response is photoperiodism. This is the plant's reaction to dark and is controlled by the phytochrome pigment in the leaves. The pigment shifts between two forms based on whether it'receives more red or far red light. The reaction controls several different plant reactions including seed germination, stem elongation, dormancy, and blooming in day length sensitive plants. Some seeds are also light sensitive. Germination is controlled by the reaction in the phytochrome pigment. Many lettuce varieties must have light to germinate. Lettuce is packaged and distributed in foil packets to prevent sprouting before planting. Most weed seeds are in this category. Have you noticed how every time you till the soil more weeds shoot up? Weed seeds lie dormant in the soil for years waiting for you to stir up the soil so they get enough light to germinate. Phytochrome also controls lengthening or elongation of stems. Leggy plants in low light are one example. The light reaction in phytochrome also guides the germinating seedling stem through the soil toward light. The last photoperiod response is stimulation of dormancy. Several

56

Plant Ecology

things trigger dormancy, but a major one is the shortening day length. This is critical when we move plants out of the area where they evolved. For example, a sugar maple grown in the north but from southern seed will not become dormant early enough to escape winter cold injury. Therefore it is important to buy perennial plants from seed sources at similar latitudes to our own. Photosynthesis

Photosynthesis is the conversion of light energy into chemical energy by living organisms. The raw materials are carbon dioxide and water; the energy source is sunlight; and the end-products are oxygen and (energy rich) carbohydrates, for example sucrose, glucose and starch. This process is arguably the most important biochemical pathway, since nearly all life on Earth either directly or indirectly depends on it. It is a complex process occurring in higher plants, phytoplankton, algae, as well as bacteria such as cyanobacteria. Photosynthetic organisms are also referred to as photoautotrophs. Photosynthesis uses light energy and carbon dioxide to make triose phospates (G3P). G3P is generally considered the prime end-product of photosynthesis. It can be used as an immediate food nutrient, or combined and rearranged to form disaccharide sugars, such as sucrose, which can be transported to other cells, or stored as insoluble polysaccharides such as starch. Photosynthesis occurs in two stages. In the first phase, light-dependent reactions or photosynthetic reactions (also called the Light reactions) capture the energy of light and use it to make high-energy molecules. During the second phase, the light-independent reactions (also called the Calvin-Benson Cycle, and formerly. known as the Dark Reactions) use the high-energy molecules to capture carbon dioxide (C02 ) and make the precursors of carbohydrates.

Ini.pact of Physical Environment on Plant Growth

57

In the light reactions, one molecule of the pigment chlorophyll absorbs one photon and loses one electron. This electron is passed to a modified form of chlorophyll called pheophytin, which passes the electron to a quinone molecule, allowing the start of a flow of electrons down an electron transport chain that leads to the ultimate reduction of NADP into NADPH. In addition, it serves to create a proton gradient across the chloroplast membrane; its dissipation is used by ATP Synthase for the concomitant synthesis 6f ATP. The chlorophyll molecule regains the lost electron by taking one from a water molecule through a process called photolysis, that releases oxygen gas. In the Light-independent or dark reactions the enzyme RuBisCO captures CO2 from the atmosphere and in a process that requires the newly-formed NADPH, called the Calvin-Benson cycle releases three-carbon sugars, which are later combined to form sucrose and starch. Photosynthesis may simply be defined as the conversion of light energy into chemical energy by living organisms. It is affected by its surroundings and the rate of photosynthesis is affected by the concentration of carbon dioxide, the intensity of light, and the temperature. Most plants are photoautotrophs, which means that they are able to synthesize food directly from inorganic compounds using light energy - for example from the sun, instead of eating other organisms or relying on nutrients derived from them. This is distinct from chemoautotrophs that do not depend on light energy, but use energy from inorganic compounds.

The energy for photosynthesis ultimately comes from absorbed photons and involves a reducing agent, which is water in the case of plants, releasing oxygen as a waste

58

Plant Ecology

,produc~, The

light energy is converted to chemical energy (knowr..;;\ts light-dependent reactions), in the form of ATP and N~.DPH, -which are used for synthetic reactions in \' pho~Qa\ltotrophs. The overall equation for the lightdependent reactions under the conditions of non-cydic electron flow in green plants is: 2~0

+ 2NADP+ + 2ADP + 2Pi + light + 2ATP + 02

~

2NADPH + 2H+

Most notably, plants use the chemical energy to fix carbon dioxide into carbohydrates ann other organic compounds through light-independent reactions. The overall equation for carbon fixation (sometimes referred to as carbon reduction) in green plants is: 3C02 + 9ATP + 6NADPH + 6 H+ ~ C3HP3-phosphate 9ADP + BPi + 6NADP+ + 3 Hp

-t

To be more specific, carbon fixation produces an intermediate product, which is then converted to the final carbohydrate products. The carbon skeletons produced by photosynthesis are then variously used to form other organic compounds, such as the building material cellulose, as precursors for lipid and amino acid biosynthesis, or as a fuel in cellular respiration. The latter occurs not only in plants but also in animals when the energy from plants gets passed through a food chain. Organisms dependent on photosynthetic and chemosynthetic organisms are called heterotrophs. In general outline, cellular respiration is the opposite of photosynthesis: Glucose and other compounds are oxidized to produce carbon dioxide, water, and chemical energy. However, the two processes take place through a different sequence of chemical reactions and in different cellular compartments.

Impact of Physical Environment on Plant Growth

S9

Plants absorb light primarily using the pigment chlorophyll, which is the reason that most plants have a green color. The function of chlorophyll is often supported by other accessory pigments such as carotenes and xanthophylls. Both chlorophyll and accessory pigments are contained in organelles (compartments within the cell) called chloroplasts. Although all cells in the green parts of a plant have chloroplasts, most of the energy is captured in the leaves. The cells in the interior tissues of a leaf, called the mesophyll, can contain between 450,000 and 800,000 chloroplasts for every square millimeter of leaf. The surface of the leaf is uniformly coated with a water-resistant waxy cuticle that protects the leaf from excessive evaporation of water and decreases the absorption of ultraviolet or blue light to reduce heating. The transparent epidermis layer allows light to pass through to the palisade mesophyll cells where most of the photosynthesis takes place. Plants convert light into chemical energy with a maximum photosynthetic efficiency of approximately 6%. By comparison solar panels convert light into electric energy at a photosynthetic efficiency of approximately 1020%. Actual plant's photosynthetic efficiency varies with the frequency of the light being converted, light intensity, temperature and proportion of CO2 in atmosphere. Algae come in multiple forms from multicellular organisms like kelp, to microscopic, single-cell organisms. Although they are not as complex as land plants, the biochemical process of photosynthesis is the same. Very much like plants, algae have chloroplasts and chlorophyll, but various accessory pigments are present in some algae such as phycocyanin, carotenes, and xanthophylls in green algae and phycoerythrin in red algae (rhodophytes), resulting in a wide variety of colors. Brown algae and diatoms contain fucoxanthol as their primary pigment. All algae produce oxygen, and many are

60

Plant Ecology

autotrophic. However, some are heterotrophic, relying on materials produced by other organisms. For example, in coral reefs, there is a mutualistic relationship between zooxanthellae and the coral polyps. Photosynthetic bacteria do not have chloroplasts (or any membrane-bound organelles). Instead, photosynthesis takes place directly within the cell. Cyanobacteria contain thylakoid membranes very similar to those in chloroplasts and are the only prokaryotes that perform oxygengenerating photosynthesis. In fact, chloroplasts are now considered to have evolved from an endosymbiotic bacterium, which was also an ancestor of and later gave rise to cyanobacterium. The other photosynthetic bacteria have a variety of different pigments, called bacteriochlorophylls, and do not produce oxygen. Some bacteria, such as Chromatium, oxidize hydrogen sulfide instead of water for photosynthesis, producing sulfur as waste. Evolution of Photosynthetic Systems

The ability to convert light energy to chemical energy confers a significant evolutionary advantage to living organisms. Early photosynthetic systems, such as those from green and purple sulfur and green and purple nonsulfur bacteria, are thought to have been anoxygenic, using various molecules as electron donors. Green and purple sulfur bacteria are thought to have used hydrogen and sulfur as an electron donor. Green nonsulfur bacteria used various amino and other organic acids. Purple nonsulfur bacteria used a variety of nonspecific organic molecules. The use of these molecules is consistent with the geological evidence that the atmosphere was highly reduced at that time. Oxygen in the atmosphere exists due to the evolution of oxygenic photosynthesis, sometimes referred to as the

Impact of Physical Environment on Plant Growth

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oxygen catastrophe. Geological evidence suggests that oxygenic photosynthesis, such as that in cyanobacteria, became important during the Paleoproterozoic era around 2 billion years ago. Modem photosynthesis in plants and most photosynthetic prokaryotes is oxygenic. Oxygenic photosynthesis uses water as an electron donor which is oxidized into molecular oxygen by the absorption of a photon by the photosynthetic reaction center. In plants the process of photosynthesis occurs in organelles called chloroplasts. Chloroplasts have many similarities with photosynthetic bacteria including a circular chromosome, prokaryotic-type ribosomes, and similar proteins in the photosynthetic reaction center. The endosymbiotic theory suggests that photosynthetic bacteria were acquired (by endocytosis or gene fusion) by early eukaryotic cells to form the first plant cells. In other words, chloroplasts may simply be primitive photosynthetic bacteria adapted to life inside plant cells, whereas plants themselves have not actually evolved photosynthetic processes on their own. Another example of this can be found in complex plants and animals, including humans, whose cells depend upon mitochondria as their energy source; mitochondria are thought to have evolved from endosymbiotic bacteria, related to modem Rickettsia bacteria. Both chloroplasts and mitochondria actually have their own DNA, separate from the nuclear DNA of their animal or plant host cells. This ~ontention is supported by the finding that the marine molluscs Elysia viridis and Elysia chlorotica seem to maintain a symbiotic relationship with chloroplasts frl'JIn algae with similar RDA structures that they encounter. However, they do not transfer these chloroplasts to the next generations. The biochemical capacity to use water as the source for electrons in photosynthesis evolved once, in a coIIU1l.on

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Plant Ecology

ancestor of extant cyanobacteria. The geological record indicates that this transforming event took place early in our planet's history, at least 2450-2320 million years ago (Ma), and possibly much earlier. Geobiological interpretation of Archean (>2500 Ma) sedimentary rocks remains a challenge; available evidence indicates that life existed 3500 Ma, hut the question of when oxygenic photosynthesis evolved continues to engender debate and research. A clear paleontological window on cyanobacterial evolution opened about 2000 Ma, revealing an already-diverse biota of blue-greens. Cyanobacteria remained principal primary producers throughout the Proterozoic Eon (2500-543 Ma), in part because the redox structure of the oceans favored photautotrophs capable of nitrogen fixation. Green algae joined blue-greens as major primary producers on. continental shelves near the end of the Proterozoic, but only with the Mesozoic (251-65 Ma) radiations of dinoflagellates, coccolithophorids, and diatoms did primary production in marine shelf waters take modern form. Cyanobacteria remain critical to marine ecosystems as primary producers in oceanic gyres, as agents of biological nitrogen fixation, and, in modified form, as the plastids of marine algae. Carbon Fixation

The fixation or reduction of carbon dioxide is a lightindependent process in which carbon dioxide combines with a five-carbon sugar, ribulose 1,5-bisphosphate (RuBP), to yield two molecules of a three-carbon compound, glycerate 3-phosphate (GP), also known as 3phosphoglycerate (PGA). GP, in the presence of ATP and NADPH from the light-dependent stages, is reduced to glyceraldehyde 3-phosphate (G3P). This product is also referred to as 3-phosphoglyceraldehyde (PGAL) or even as triose phosphate.

Impact of Physical Environment on Plant Growth

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Triose is a 3-carbon sugar. Most (5 out of 6 molecules) of the G3P produced is used to regenerate RuBP so the process can continue. The lout of 6 molecules of the triose phosphates not "recycled" often condense to form hexose phosphates, which ultimately yield sucrose, starch .md cellulose. The sugars produced during carbon metabolif, 11 yield carbon skeletons that can be used for other metaboli\reactions like the production of amino acids and lipids. C4, C3 and CAM

In hot and dry conditions, plants will close their stomata

to prevent loss of water. Under these conditions, oxygen gas, produced by the light reactions of photosynthesis, will concentrate in the leaves causing photorespiration to occur. Some plants have evolved mechanisms to increase the CO2 concentration in the leaves under these conditions. C4 plants capture carbon dioxide using an enzyme called PEP Carboxylase that adds carbon dioxide to the three carbon molecule Phosphoenolpyruvate (PEP) creating the 4-carbon molecule oxaloacetic acid. Plants without this enzyme are called C3 plants because the primary carboxylation reaction produces the three-carbon sugar 3-phosphoglycerate directly in the Calvin-Benson Cycle. When oxygen levels rise in the leaf, C4 plants reverse the reaction to release carbon dioxide thus preventing photorespiration. By preventing photorespiration, C4 plants can produce more sugar than C3 plants in conditions of strong light and high temperature. Many important crop plants are C4 plants including maize, sorghum, sugarcane, and millet. Xerophytes such as cacti and most succulents also can use PEP Carboxylase to capture carbon dioxide in a process called Crassulacean acid metabolism (CAM). They store the CO2 in different molecules than the C4 plants (mostly they store it in the form of malic acid via carboxylation of phosphoenolpyruvate to oxaloacetate, which is then

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Plant Ecology

reduced to malate). Nevertheless, C4 plants capture the CO2 in one type of cell tissue (mesophyll) and then transfer it to another type of tissue (bundle sheath cells) so that carbon fixation may occur via the Calvin cycle. They also have a different leaf anatomy than C4 plants. They grab the CO2 at night, when their stomata are open, and they release it into the leaves during the day to increase their photosynthetic rate. C4 metabolism physically separates CO2 fixation from the Calvin cycle, while CAM metabolism temporally separates CO2 fixation from the Calvin cycle. Phototropism

Phototropism is directional growth in which the direction of growth is determined by the direction of the light source. Phototropism is most often observed in plants, but can also occur in other organisms such as fungi. Phototropism is one of the many plant tropiSms or movements which respond to external stimuli. Growth towards a light source is a positive phototropism, while growth away from light is called negative phototropism. Most plant shoots exhibit positive phototropism, while roots usually exhibit negative phototropism, although gravitropism may playa larger role in root behavior and growth. Some vine shoot tips exhibit negative phototropism, which allows them to grow towards dark, solid objects and climb them. Phototropism in plants such as Arabidopsis thaliana is regulated by blue light receptors called phototropins. Other photosensitive receptors in plants include phytochromes that sense red light and cryptochromes that sense blue light. Different orgOans of the plant may exhibit different phototropic reactions to different wavelengths of light. Stem tips exhibit positive phototropic reactions to blue light, while root tips exhibit negative phototropic reactions to blue light. Both root tips and most stem tips exhibit positive 'phototropism to red light.

Impact of Physical Environment on Plant Growth

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Phototropism is enabled by auxins. Auxins are plant hormones that have many functions. In this respect, auxins are responsible for expelling H+ ions (creating proton pumps) which decreases pH in the cells on the dark side of the plant. This acidification of the cell wall region activates enzymes known as expansins which break bonds in the cell wall structure, making the cell walls less rigid. In addition, the acidic environment causes disruption of hydrogen bonds in the cellulose that makes up the cell wall. The decrease in cell wall strength causes cells to swell, exerting the mechanical pressure that drives phototropic movement. Phototropism relates to photosynthesis. Photoperiodism

Photoperiodicity is the physiological reaction of organisms to the length of day or night. It occurs in plants and animals. Many flowering plants use a photoreceptor protein, such as phytochrome or cryptochrome, to sense seasonal changes in day length, which they take as signals to flower. Broadly, flowering plants can be classified as long day plants, short day plants, or day neutral plants. Long day plants are plants that flower when the day is longer than a critical length (i.e. the night is shorter than a critical length). These plants generally flower in the spring or early summer, as days are getting longer. Short day plants are plants that flower when the day is shorter than a critical length, or the night is longer than a critical length. These plants generally flower in late summer or fall, as days are getting shorter. It is actually the night length rather than day length that controls flowering, so flowering in a long day plant is triggered by a short night (which of course will mean it also sees a long day). Conversely, short day plants will flower when nights get longer than a critical length. This is known by using night break experiments. Fcx example, a short day

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plant (long night) will not flower It a pwse (say 5 minutes) of artificial light is shone on the plant during the middle of the night. This generally does not occur from natural light such as moonlight, lightning, fire flies, etc, since the light from these sources is not sufficiently strong to trigger the response. Day- neutral plants do not initiate flowering based on photoperiodism i.e. they can flower regardless of the night length; some may use temperature (vernalization) instead. Quantitative long day or short day plants will have their flowering advanced or retarded by short or long days, but will eventually flower in sub-optimal day lengths. Again, temperature is likely to also influence flowering time in these plants. Modem biologists believe that it is the coincidence of the active forms of phytochrome or cryptochrome, created by light during the daytime, with the rhythms of the circadian clock that allows plants to measure the length of the night. Other instances of photoperiodism in plants include the growth of stems or roots during certain seasons, or the loss of leaves. TEMPERATURE EFFECI'S ON PLANT

Sometimes temperatures are used in connection with day length to manipulate the flowering of plants. Chrysanthemums will flower for a longer period of time if daylight temperatures are 50°F. The Christmas cactus forms flowers as a result of short days and low temperatures. Temperatures alone also influence flowering. Daffodils are forced to flower by putting bulbs in cold storage in October at 35 to 40°F. The cold temperature allows the bulb to mature. The bulbs are transferred to the greenhouse in midwinter where growth begins. The flowers are then ready for cutting in 3 to 4 weeks. Thermoperiod refers to daily temperature change. Plants produce maximum growth when exposed to a day

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temperature that is about 10 to 15°F higher than the night temperature. This allows the plant to photosynthesize (build -up) and respire (break down) during an optimum daytime temperature, and to curtail the rate of respiration during a cooler night. High temperatures cause increased respiration, sometimes above the rate of photosynthesis. This means that the products of photosynthesis are being used more rapidly than they are being produced. For growth to occur, photosynthesis must be greater than respiration. Low temperatures can result in poor growth. Photosynthesis is slowed down at low temperatures. Since photosynthesis is slowed, growth is slowed, and this results in lower yields. Not all plants grow best in the same temperature range. For example, snapdragons grow best when night time temperatures are 55°F, while the poinsettia grows best at 62°F. Florist cyclamen ,does well under very cool conditions, while many bedding plants grow best at a higher temperature. Buds of many plants require exposure to a certain number of days below a critical temperature (chilling hours) before they will resume growth in the spring. Peaches are a prime example; most cultivars require 700 to 1,000 hours below 45°F and above 32°F before they break their rest period and begin growth. This time period varies for different plants. The flower buds of forsythia require a relatively short rest period and will grow at the first sign of warm wea!her. During dormancy, buds can withstand very low temperatures, but after the rest period is satisfied, buds become more susceptible to weather conditions, and can be damaged easily by cold temperatures or frost. PLANTS AND HUMIDITY

Humidity is a measure of the amount of water that air will hold. The water is usually in the form of invisible droplets. At 100 percent humidity the air cannot hold any more

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water. The highest humidity often occurs on hot days, creating a "muggy" feeling. Fog occurs when the air is saturated and the invisible water now becomes visible. Humidity is measured relative to temperature and is called relative humidity (RH). The measurement is taken this way because humidity and temperature are directly related: the warmer the air, the more water it can hold. Humidity in the Home If warm air holds more moisture than cold, then why is it so dry inside the house in winter? Remember that the furnace is taking dry outside air and warming it. If no water is added to this outside air, then it will still be dry. You can increase the humidity inside to ,a certain extent by adding water to the air. Warm, moist air is always being lost from the house, and cold dry air is always being brought in, so high humidity in the entire house is not possible. When warm, moist air comes in contact with a cold, dry surface, the water in the air condenses. This is very common on windows, and is an indication that the humidity inside the house is higher than outside. If the inside walls of the house are cooler than the air inside the house, water can condense on the walls, and can cause wallpaper to come unstuck, but don't rely on this as a means of stripping wallpaper. HUmidity is important to plants because it partly controls the moisture loss from the plant. The leaves of plants· have tiny pores in them called stomata. Carbon dioxide enters the plants through these pores; oxygen and water leave through them. The humidity inside a plant is close to 100%. A plant growing in a dry room will lose moisture because water always moves from high to low humidity. When the difference in humidity is large, the loss of moisture from the plant is rapid and severe.

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Most houseplants prefer a humidity of about 60%. Cacti, succulents and plants native to desert environments tolerate much lower humidity (30-35%), but prefer not to drop below 20%. House plants that are native to tropical rain forests require much higher humidity, 90% for example, and thus pose problems for most home owners. Plants that require a very high humidity are best grown in terrariums or closed containers where it is possible to regulate the humidity. Under very humid environments, fungal diseases can spread. This seldom happens during winter, but can be a problem in fall when the temperature is cool and the humidity is high. Mildew on plant leaves is an indication of excess humidity and lack of ventilation. Plants that prefer a more humid environment, but that are forced to grow in a dry environment will commonly suffer damage to younger leaves and to leaf tips. New leaves and leaf tips are the area of the plant with the most actively growing cells, and these cells are the most susceptible to dry air. Older leaves that are fully formed may be shed as a result of lack of humidity, but they will not be deformed or damaged by the dry air. Plants stressed in this way very frequently shed flower buds, or flowers die soon after opening. There are a number of ways that a home owner can increase the humidity in the room or around the plants. Humidifiers-Using a humidifier is by far the most effective way to increase humity. Humidifiers that attach directly to the furnace will increase the humidity throughout the house. Portable humidifiers can be used to increase the humidity in one or more rooms. Changing locations-Bathrooms and kitchens, if they are sunny, often have a higher humidity than other areas of the home, and may be more suitable for house plants requiring extra humidity.

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Double potting-Take a small potted plant or a number of small plants, and put them in a larger pot. Fill the area underneath and around the small pots with peat moss. Keep the peat moss constantly moist. As water evaporates from the peat moss, it increases the humidity around the plants. Make sure the large pot has a tray underneath to catch excess moisture from the peat moss. A similar approach is to place a house plant in a basket lined with moist peat moss. Pebble trays-Fill a large plant saucer with pebbles or stones. Place a number of small pots (or a large pot) on top of the stones. To assure that the pots do not contact the water, you may wish to place them on saucers which sit on the pebbles. Now fill the larg~ plant saucer with water up to the level of the pebbles. Make sure the saucer with pebbles is large enough to be effective - the larger the surface area of pebbles, the more effective the method will be. Totems for climbing plants-Take chicken wire and roll it into a totem (tube). Fill the tub with peat moss. Anchor the tube in the plant pot and then wind the climbing plant around the tube. Keep the peat moss inside the tub moist. Do not worry if the plant forms roots into the totem, but if this occurs make sure the totem is kept evenly moist. Grouping-Moisture loss from one plant can benefit the plant next to it. Try and group plants with similar watering requirements together, and keep them close to each other. The closer together they are, the more effective the method. Misting-This is the least effective but often the most used method. Misting plants with tepid water will result in a layer of water on the leaves, which will reduce the transpiration of water from the leaves. However, soon after misting, the water will evaporate, and once this occurs, the air is once again low in

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humidity. If plants are misted too often or too much, however, fungal growth and tissue rotting may result. Plants with hairy leaves cannot be misted, for leaf spotting wi1llikely occur as a result. EFFECT OF WIND ON PLANT GROWTH

One dictionary defined wind as "air moving hOrizontally." It is not quite correct because some winds blow up and some down. Such a definition gives no idea of the diversity of winds; wind is moving air; that may carry water, dust, ice, sand and chemicals. Above all wind has energy that can be transferred to sailing ships and windmills and can be used by plants in ways essential to their growth and development. Different is the "wet monsoon" wind of India from the dry hot "Mistral" blowing into France from the Sahara, from the "foehn" blowing from the European Alps or from the "Squamish" winds blowing into the fiords of the Be coast from the Interior Plateau. To the ancients, wind was mysterious. Where did it come from and where did it go? In Scandinavian mythology, Thor was the god of the north wind and the god of battle and tempest. Aeolius was the ancient Greek god of the four winds. Today wind has lost much of what the ancients found so intangible but we still feel the "poetic" or "archaic" wind as somewhat different from the winds of science, the winds of air mass analysis, the jet stream or the solar wind. When unicellular plants first established on land, wind may have played a role in their dispersal. Once they evolved as multicellular and erect, they have affected and been affected by wind speed and direction. Fossil evidence suggests that by the Silurian, 500 to 700 million years ago, leaf-like structures with stomata and semi-permeability had evolved and that the sporophyte generation of the seed plants was large (and the gametophyte generation diminished) and a conducting system with root and stem

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had evolved. Why the bryophytes with a relatively large leafy gametophyte and a small sporophyte with only rhizoids and a poorly developed conducting system should remain short statured, hugging the ground as the mosses and liverworts do today is not dear from the fossil record. Wind played a role in the evolution of the root as a holdfast and uptake organ for water and soil nutrients, and in evolution of the stem to support leaf and flower. The vascular system provided effective transport between root and shoot to meet the needs of the expanding photosynthetic surfaces. Does wind continue to playa role as one of many factors in the evolu-tion of plants? The internal and external architecture of leaves seem to affect the type of damage caused by high wind. Often the parallelveined monocot leaves are torn to strips reticulateveined dicot leaves are bent at the petiole or broken at the stem insertion. Wind, Flex Plants and Brittle Plants

Broken cottonwoods silhouetted against a Fraser Valley sky attest to the brittleness of old branches in wind and ice storms while nearby the closely related Lombardy poplars gracefully sway unbroken in the wind. The American, Robert Frost in his famous poem, "Birches" recalls from boyhood swinging up and down on young and bending birch trees. Older birch trees are brittle. Zea mays (Indian corn) plants in the windy areas produce brace roots, whereas other cereals, grasses and many forbs, such as Epilobium angustifolium (fireweed) flatten and lodge in windstorms, but when young become erect again. Grasses have special structures termed pulvini on stem (culm) just below nodes that assist lodged grasses to become erect again. Young arborescent species, such as Alnus sinuata (slide alder) that have been flattened by wind, avalanche or the weight of

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snowpacks can often become erect again. Astonishingly the wind helps to untangle and separate members and to assist in the process of re-erection. There is a special charm in the flexible shoots of weeping willows or birches. Landscapers often place them on the margins of lakes and streams providing a curtain of swaying limbs that create a dynamic view across the water. Wind and Aquatic Plants

Most aquatic plants are flexible. The energy balances between physical support and transport in land plants that are provided by lignin and cellulose are changed because the water provides much of the physical support for stem and leaf. Some aquatics are submerged and largely out of the direct impact of the buffeting effects of wind and wave but others such as water lilies and pond weeds produce foliage on the water surface or near the surface. The surface foliage of aquatics may serve to calm small wind-driven small wave motion. In the shallow bays of lake margins and in ponds the long petioles and "split" fronds of water lilies or the ribbon-like leaves of the pondweeds absorb the energy of the waves. Other emergents such as the bulrush (Scirpus spp.) rely on the round or triangular cross-section of their rigid stems to absorb most of the buffeting of wind and waves. Flexibility is only one of several factors important in the ecology of emergent aquatics; for example, the timing of the emerging first floating leaves of Zizania palustris (annual wild rice) and the water regime and depth appears to confine its geographical range to the shallow waters of the Great Lakes region. Once past the critical flex-leaf stage, stiff culms arise above the water surface. The timing of the leaf emergence must match the rise in the spring water levels; if the levels are too high, the first floating leaves die and the emergent wave resistant culms are not produced. Mankind has, from ancient times used natural fencing of

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trees and shrubs to reduce the impact of wind on dwellings and on gardens. The use of flexible whips (branches) by coppicing, pleaching and inosculation deserves mentionc,oppicing to grow and harvest whips, pleaching to interweave whips to make a fence, which when dry becomes strong, and ino~ulating to link living whips from one tree or shrub to another. In various ways and to varying degrees and times almost all plants flex in wind. Windfall and Breakage

Violent wind can cut into a forest like a scythe cutting grass. The ecological effects of blowdown, windthrow and breakage of trees by wind differ somewhat. Blowdown leaves patches of tangled trees that are often discernable at a distance. Windthrow of individual trees allows light penetration of the forest canopy and the sun flecks serve to enhance understorey vegetation and seedling growth on decaying "nurse 10gs".In addition, windthrow mixes litter with mineral soil. Windfall and breakage leave stumps and sloughing bark accumulating at the base of boles. Insects and fungi provide cavities for nesting birds and foocl for woodpeckers as they decay attack the stumps. Decayed stumps leave mounds of organic matter on the forest floor cr~ating microhabitat that may favour western hemlock seedlings while nearby mineral soil favours Douglas fir seedlings. Wind, rain, frost, snow and ice all contribute to the 'throughfall', of organic materials shed by woody plants. This rain builds the duff and litter (the mor and the mull) on the mineral soil surface. Throughfall occurs in all seasons but varies in amount. The rain of organic matter includes pollen, bud scales, diseased needles and leaves, twigs, insect frass, bark dust, flower parts and whole catkins, and debris from epiphytes (lichens, mistletoe, etc). Wind serves to clean trees and shrubs of old unproductive or diseased materials, removing shade and increasing

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exposure to the sun's radiant energy. The throughfall from winter storms, including twigs and branches from the high crowns falls on a snowy surface and provides food for wintering deer, elk, moose and even native sheep. The twigs are nutrient rich but may also be toxic. Much has been written about the decomposition of throughfall and litter, but very little about its function in nature or about the roles of wind. As if defying gales and age, trees and shrubs standing on promontories, rocky shores and sand dunes are often dwarfed and have scraggy limbs and twisted boles. Trained by wind their foliage is flagged and tattered. Bonsai art from Japan and China reflects these forms of woody plants. So do the trees in the paintings of the Canadian "Group of Seven" and the photos by Ansel Adams. Bonsai artists in practice pinch leaf and twig to dwarf a tree or shrub and achieve a wind-tattered form. In nature, cold or dry wind, sometimes does the pinching, the pinches remove and kill new twigs and leaves, greatly reducing the opportunities to build carbohydrate food reserves. Sometimes loss of leaf and twig occurs as a result of abrasion by sand or ice or snow crystals and on marine shores by salty spray. Limited mineral nutrient and water supplies can only come from root penetration of cracks in the rocky cliffs or deep root penetration of sand dunes. Desiccation is often a reason for dwarfing but it is likely not that alone. Throughfall and Summer Drought

Dry hot wind and cold soil contribute to water stress in plants. Water stresses evidenced in summer leaf fall often escape the attention of photographers, naturalists and the urbanized public. Plants may pay a high energetic price by losing green foliage in summer. Aided by wind, leaf abscission quickly reduces transpiration and water loss. Older and less photosynthetically active leaves drop first. The impact of needle and leaf losses on flower and fruit

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production and on carbohydrate reserve food in twig, bole and root may continue for several years following summer drought. Leaf fall in summer often goes unnoticed because there is no associated colour and nutrient withdrawal as is characteristic of autumn leaf fall. Hot dry summer winds contribute to other responses to water stress. Even pubescent plants such as Antennaria parvifolia (pussytoes) behave as natural hygrometers when leaves curl during the day reducing transpiration and uncurl by night or during a humid breeze as they regain turgor. Other plants, such as the Balsamorrhiza sagittata (balsam root), draw their crowns into the soil, as the leaves curl, dry, become brittle and are soon shredded by wind and the trampling of animals. Surface, Roughness and Wind Breaks

Even low-growing rosettes and mats reduce wind velocity and create turbulence. Tall trees influence wind velocity in their lee for up to one hundred times their height. In dense forest, wind velocity on the ground may be greatly reduced while the forest canopy is buffeted. The boles of spaced trees create eddies manifested in winter by the development of snow cirques which in turn result in variations in snow depth, snow melt and soil moisture. Snow cirques provide a distinctive micro habitat for plants and animals. In spring snow patches may linger and summer soil moisture vary over relatively short distances as a result of variations caused by uneven surfaces, drifting snow, and living snow fences that influence the winds of winter. Land plants evolved in the presence of fires and lightning. Fire is nearly universal. It is unique in that it generates its own wind in complex ways with some special ecological responses. Its destructive, sanitizing or renewing actions make it both an ecological enemy and friend.

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Wind and Dispersal of Plants

The ancients and many aboriginal peoples had some understanding of the roles of wind pollination and pollen dispersal. In the western world appreciation of wind function was limited un~il in the 1700's when several studies on pollination were publiShed. By the mid 1800's, Victorians such as Charles Darwin had established that pollination was a noteworthy service in nature. Th.g fossil record of wind and insect pollination dates back to the mid-Paleozoic time. Today many more plant species are wind pollin-ated than insect pollinated, although public attention is directed more towards pollination by insects and other animals and to the colourful co-evolution of flowers and-insects. Many plant species take advantage of both wind and animal pollination. Wind is reliable and wind is everywhere but pollen must be produced in prodigious quantities ana. demands the' allocation of much of a plant's energy. The direct transfer of pollen from flower to flower by insect or other animal agent is less energy demanding, but insect populations may be limited by the temperature at which insects fly and by their distribution. Air-dispersed pollen falls by gravity to receptive surfaces in a seemingly haphazard process, but plants have evolved some efficiencies to aid the basic process. Rate of fall varies, as does its buoyancy. Some pollen is winged or variously patterned on the surface. Plant grouping and the timing of foliage production both can modify and even direct pollen fall. As sufferers from pollen allergies know, the release of pollen is temperature and humidity controlled; as a result, pollen release is effi-cient and timely. Receptive surfaces and structures, such as the cones of conifers or the foliage of jojoba, facilitate the sequestering of airborne pollen.

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Feathery stigmas like those of grasses "pluck" pollen from passing zephyrs; other stigmas are sticky and hold grains until germination. The modern literature on pollination received new impetus and direction in the 1960s with the work of Faegri and van der Pijl . Spores from bryophytes and pteridophytes as well as those from fungi and lichens deserve direct conSideration as do bacteria, but reviews of this very large literature do not apparently exist, although the case studies are scattered through the journals of microbiology, plant pathology and other disciplines. Wind may not be as effective a dispersal agent of seeds, fruits and associated plant parts as\it is with the generally smaller propagules of some fungi, lichens and bryophytes, some of which may be distributed worldwide. Dispersal is governed to some extent by the nature of major reproductive structures such as pods or inflorescence: for example, in Sporobdus cryptandrus (sand dropseed grass), where the' whole mature inflorescence with seed (fruits) is abscised in a leafy sheath, the seed is shaken loose as wind blows the severed structure over grass and dune. Whole tumbleweed plants, such as Sisymbrium atissimum (tumbling mustard), mature, break at soil level and tumble in the wind across grass prairie and drop seed from siliques either intermittently as they tumble or when at a fence or other barrier. Many conifers, elms, maples and many other plants have winged propagules, which, borne by wind, may be delivered several hundred metres from their parent plant. Wind may assist establishment of seed that falls into soil, cracks and crannies, because the wings or other appendages are hygroscopic and respond to changes in wind humidity and temperature to lengthen and contract thus driving the seed further into the subsoil. Watch the behaviour of the awns of Stipa spp. (needlegrass) in a light 'breeze. Wihd can carry parachute "seeds" like those of Tragopogon spp. (yellow salsify) to elevations of several

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hundred metres and over low mountains. Balloon fruits such as those of Physalis peruviana (ground cherry), a common garden weed, may not travel so far in the wind but can effectively move from urban lot to urban lot. Effect of Wind on Leaf Surfaces

The thin layE9 of air immediately adjacent to leaf surfaces does not behave in quite the same way as ambient air; it is usefully termed the "boundary layer". Boundary layer air is not quite motionless because it is held close to the leaf surface. It may be only a millimetre or two thick, a distance that varies from species to species and with the nature of the leaf surface. Leaf form (flat or needle-like), pubescence, venation, location next to upper or lower leaf surface, and the number and placement of stomata all affect the boundary layer. Gas exchange occurs at and through the semi-permeable protective cuticle and the boundary layer. This is also where leaf aroma and fragrance are generated, and where palatability and the flavour of foliage to all herbivores and omnivores are perceived. The wind distributes the greenhouse gases, carbon dioxide and methane, and water vapour, which contribute to the haze that develops over tropical rainforests and over some temperate latitude skies on hot summer days. The chemistry and physics of the boundary layer are difficult to study, but its gas composition is very complex and the number of chemicals in one fragrance may alone exceeded four hundred. Chemistry and physics aside, the astonishing diversity of leaf surfaces and variety of leaves is telling a story of immense importance to life on earth and also of a long evolutionary history of intimacy of plant and wind. REFERENCES

Bouma J., R.B. Brown, and P.S.c. Rao. 1982. "Basics of Soil-Water Relationships -Part I. Soil as a Porous Medium." Soil Science Fact Sheet SL-37. Florida Cooperative Extension Service. IFAS. Gainesville, FL.

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de Villiers, M. Windswept: the story of wind and weather. Toronto: McClelland and Stewart 2006. Dickenson, C.H. and Pugh, G.J.F. (editors) Biology of Plant Litter Decomposition. London and New York: Academic Press. 1974. Paergi, K. and van der Pijl, L. The Principles oj Pollination Ecology. Oxford, UK: Pergamon Press Ltd. 1966. Merva G.E. Physioengineering principles. 1975. The AVI Publishing Company, Inc. Westport, CT, Phillipson, J. Ecological Energenics. London and Beede!>. William Clowes and Sons. 1966. Pielou, E.C. The Energy of Nature. Chicago: The University Of Chicago Press. 200l. Ridley, H.N. The dispersal of plants throughout the world. Ashford (UK): L. Reeve. 1930. Thomas B. and Vince-Prue D., Photoperiodism in plants, Academic Press, 1997.

4 Ecological Evolution of Plants Plants are multicellular photosynthetic organisms that are believed to have evolved from green algae. Both groups have chlorophylls a and band betacarotene as their photosynthetic pigments, both store reserve food as starch, and both have cellulose containing cell walls. Perhaps the evolution of plants began on land, when algae are left high and dry between tides. When they stayed ashore, they adapted to the new open-air lifestyle with great success. To survive in air, they thickened their walls, thus staying wet on the inside while the air dried them on the outside. Fungi, a kingdom including molds, mushrooms, and yeasts, also now appear on shores, where they practice the ability to digest organic food with excreted enzymes before consuming it. Some fungi cooperate with algae to form a single-organism partnership called lichens. Plant cells are more complex than those of either animals or fungi because they have both mitochontria and chloroplasts. The first real plants to evolve are mosses, often found in close association with fungi. From the time they invent a cooperative lifestyle as lichens until today, plants and fungi have a very close association. It may even be that the first plants are a genetic fusion of the two. Ninety percent of plants have fungi called mycorrhizal (root fungus) living in special co-evolved compartments inside their roots or in the soil, intertwined with roots, making

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food for each other according to specialty and recycling wastes. Plants have evolved through a number of grades, from the earliest algal mats, to bryophytes, lycopods, ferns and gymnosperms to the complex angiosperms of today. While the simple plants continue to thrive, especially in the environments in which they evolved, each new grade of organisation has eventually become more "successful" than its predecessors by most measures. Further, most cladistic analyses suggest that each more complex group arose from the most complex group at the time. Evidence suggests that an algal scum formed on the land 1200 million years ago, but it was not until the Ordovician period, around 500 million years ago, that land plants appeared. These begun to diversify in the late Silurian period, around 420 million years ago, and the fruits of their diversification are displayed in re.markable detail in an early Devonian fossil assemblage known as the Rhynie chert. This chert preserved early plants in cellular detail, petrified in volcanic springs. By the middle of the Devonian period most of the features recognised in plants today are present, including roots, leaves and seeds. By the late Devonian, plants had reached a degree of sophistication that allowed them to form forests of tall trees. Evolutionary innovation continued after the Devonian period. Most plant groups were relatively unscathed by the Permo-Triassic extinction event, although the structures of communities changed. This may have set the scene for the evolution of flowering plants in the Triassic, which exploded the Cretaceous and Tertiary. The latest major group of plants to evolve were the grasses, which became important in the mid Tertiary, from arolmd 40 million years ago. The grasses, as well as many other groups, evolved new mechanisms of metabolism to survive the low CO2 and warm, dry conditions of the tropics over the last 10 million years.

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Land plants evolved from chlorophyte algae, perhaps as early as 510 million years ago; their closest living relatives are the charophytes, specifically Charales. Assuming that the Charales' habit has changed little since the divergen~e of lineages, this means that the land plants evolved from a branched, filamentous, haplontic alga, dwelling in shallow, fresh water, perhaps at the edge of desiccating pools. Plants weren't the first photosynthesisers on land, though: consideration of weathering rates suggests that organisms were already living on the land 1200 million years ago. These organisms were probably small and simple, forming little more than an "algal scum". The first evidence of plants on land comes from trilete spores, from the mid-Ordovician The microstructure of the earliest spores resembles that of modern liverwort spores, suggesting they share an equivalent grade of organisation. Trilete spores are the progeny of spore tetrads. These consist of four identical, connected spores, produced when a single cell undergoes meiosis. Spore tetrads are borne by all land plants, and some algae. Depending exactly when the tetrad splits, each of the four spores may bear a "trilete mark", a Y-shape, reflecting the points at which each cell was squashed up against its neighbours. However, in order for this to happen, the spore walls must be sturdy and resistant at an early stage. This resistance is closely associated with having a desiccationresistant outer wall-a trait only of use when spores have to survive out of water. Indeed, even those embryophytes that have returned to the water lack a resistant wall, thus don't bear trilete marks. A close examination of algal spores shows that none have trilete spores, either because their walls are not resistant enough, or in those rare cases where it is, the spores disperse before they are squashed enough to develop the mark, or don't fit into a tetrahedral tetrad.

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The earliest megafossils of land plants were thalloid organisms, which dwelt in fluvial wetlands and are found to have covered most of an early Silurian flood plain. They could only survive when the land was waterlogged, edit Once plants had reached the land, th~re were two approaches to desiccation, The bryophytes avoid it or give in to it, restricting their ranges to moist settings, or drying out and putting their metabolism "on hold" until more water arrives. Tracheophytes resist desiccation. They all bear a waterproof outer cuticle layer wherever they are exposed to air, to reduce water loss-but since a total covering would cut them off from CO2 in the atmosphere, they rapidly evolved stomata-small openings to allow gas exchange. Tracheophytes also developed vascular tissue to aid in the movement of water within the organisms, and moved away from a gametophyte dominated life cycle. The establishment of a land-based fauna permitted the accumulation of oxygen in the atmosphere as never before, as the new hoardes of land plants pumped it out as a waste product. When this concentration rose above 13%, it permitted the possibility of wildfire. This is first recorded in the early Silurian fossil record by charcoalified plant fossils. Apart from a controversial gap in the Late Devonian, charcoal is present ever since. Charcoalification is an important taphonomic mode. Wildfire drives off the volatile compounds, leaving only a shell of pure carbon. This is not a viable food source for herbivores or detritovores, so is prone to preservation; it is also robust, so can withstand pressure and display exquisite, sometimes sub-cellular, detail. CHANGING LIFE CYCLES

All multicellular plants have a life cycle comprising two phases (often confusingly referred to as "generations"). One is termed the gametophyte, has a single set of chromosomes (denoted In), and produces gametes (sperm and eggs). The

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other is termed the sporophyte, has paired chromosomes (denoted 2n), and produces spores. The two phases may be identical, or phenomenally different. The overwhelming pattern in plant evolution is for a reduction of the gametophytic phase, and the increase in sporophyte dominance. The algal ancestors to land plants were almost certainly haplobiontic, being haploid for all their life cycles, with a unicellular zygote providing the 2n stage. All land plants (i.e. embryophytes) are diplobionticthat. is, both the haploid and diploid stages are multicellular. There are two competing theories to explain the appearance of a diplobiontic lifecyc1e. The interpolation theory (also known as the antithetic or intercalary theory) holds that the sporophyte phase was a fundamentally new invention, caused by the mitotic division of a freshly germinated zygote, continuing until meiosis produces spores. This theory implies that the first sporophytes would bear a very different morphology to the gametophyte, on which they would have been dependant. This seems to fit well with what we know of the bryophytes, in which a vegetative thalloid gametophyte is parasitised by simple sporophytes, which often comprise no more than a sporangium on a stalk. Increasing complexity of the ancestrally simple sporophyte, including the eventual acquisition of photosynthetic cells, would free it from its dependence on a sporophyte, as we see in some hornworts, and eventually result in the sporophyte developing organs and vascular tissue, and becoming the dominant phase, as in the tracheophytes. This theory may be supported by observations that smaller Cooksonia individuals must have been supported by a gametophyte generation. The observed appearance of larger axial sizes, with room for photosynthetic tissue and thus self-sustainability, provides a possible route for the development of a self-sufficient sporophyte phase.

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The alternative hypothesis is termed the transformation theory (or homologous theory). This posits that the sporophyte appeared suddenly by a delay in the occurrence of meiosis after the zygote germinated. Since the same genetic material would be employed, the haploid and diploid phases would look the same. This explains the behaviour of some algae, which produce alternating phases of identical sporophytes and gametophytes. Subsequent .adaption to the desiccating land environment, which makes sexual reproduction difficult, would result in the simplification of the sexually active gametophyte. and elaboration of the sporophyte phase to better disperse the waterproof spores. The tissue of sporophytes and gametophytes preserved in the Rhynie chert is of similar complexity, which is taken to support this hypothesis. WATER TRANSPORT

In order to photosynthesise, plants must uptake CO2 from

the atmosphere. However, this comes at a price: while stomata are open to allow CO 2 to enter, water can evaporate: Water is lost much faster than CO2 is absorbed, so plants need to replace it, and have developed systems to transport water from the moist soil to the site of photosynthesis. Early plants sucked water between the walls of their cells, then evolved the ability to control water loss (and CO2 aquisition) through the use of stomata. Specialised water transport tissues soon evolved in the form of hydroids, tracheids, then secondary xylem, followed by an endodermis and ultimately vessels. The high CO2 levels of the Silu-Devonian, when early plants were colonising land, meant that the need for water was relatively low in the early days; as CO2 was withdrawn from the atmosphere by plants, and stored in coal, so more water was lost in its .capture, and more elegant transport mechanisms evolved. As water transport mechanisms, and waterproof cuticles, evolved, plants could survive without

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being continually covered by a film of water. This transition from poikilohydry to homoiohydry opened up new potential for colonisation. Plants were then faced with a balance, between transporting water as efficiently as possible and preventing transporting vessels to implode and cavitate. During the Silurian, CO2 was readily available, so little water needed expending to acquire it. By the end of the Carboniferous, when COo levels had lowered to something approaching today's, around 17 times more water was lost per unit of CO2 uptake. However, even in these "easy" early days, water was at a premium, and had to be transported to parts of the plant from the wet soil to avoid desiccation. This early water transport took advantage of the cohesiontension mechanism inherent in water. Water has a tendency to diffuse to areas that are drier, and this process is exacberated when water can be wicked along a fabric with small spaces. In small passages, such as that between the plant cell walls (or in tracheids), a column of water behaves like steel -when molecules evaporate from one end, they literally pull the molecules behind them along the channels. Therefore transpiration alone provided the driving force for water transport in early plants. However, without dedicated transport vessels, the cohesion-tension mechanism cannot transport water more than about 2cm, severely limiting the size of the earliest plants. This process demands a steady supply of water from one end, to maintain the chains; to avoid exhausing it, plants developed a waterproof cuticle; early cuticle may not have had pores but did not cover the entire plant surface, so that gas exchange could continue. However, dehydration at times was inevitable; early plants cope with this by having a lot of water stored between their cell walls, and when it comes to it sticking out the tough times by putting life "on hold" until more water is supplied.

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Fcolll~)

In order to be free from the constraints of small size and constant moisture that the parenchymatiC' transport system inflicted, plants needed a more efficient water transport system. During the early {'ilurian, they developed specialized cells, which were\{ignified (or bore similar chemical compounds) to avoid implosion; this process coincided with cell death, allowing their innards to be emptied and water to be passed through them. These wider, dead, empty cells were a million times more conductive than the inter-cell method, giving the potential for transport over longer distances, and higher CO2 diffusion rates. The first macrofossils to bear water-transport tubes in situ are the early Devonian pretracheophytes Aglaophyton and Horneophyton, which have structures very similar to the hydroids of modern mosses. Plants continued to innovate new ways of reducing the resistance to flow within their cells, thereby increasing the efficiency of their water transport. Bands on the walls of tubes, in fact apparent from the early Silurian onwards, are an early improvisation to aid the easy flow of water. Banded tubes, as well as tubes with pitted ornamentation on their walls, were lignified and, when they form single celled conduits, are considered to be tracheids. These, the "next generation" of transport cell design, have a more rigid structure than hydroids, allowing them to cope with higher levels of water pressure, Tracheids may have a single evolutionary origin, possibly within the hornworts, uniting all tracheophytes. Water transport requires regulation, and dynamic control is provided by stomata. By adjusting the amount of gas exchange, they can restrict the amount of water lost through transpiration. This is an important role where water supply is not constant, and indeed stomata appear to have evolved before tracheids, being present in the nonvascular hornworts.

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An endodermis probably evolved during the SiluDevonian, but the first fossil evidence for such a structure is Carboniferous. This structure in the roots covers the water transport tissue and regulates ion exchange (and prevents unwanted pathogens etc. from entering the water transport system). The endodermis can also provide an upwards pressure, forcing water out of the roots when transpiration is not enough of a driver. Once plants had evolved this level of controlled water transport, they were truly homoiohydric, able to extract water from their environment through root-like organs rather than relying on a film of surface moisture, enabling them to grow to much greater size. As a result of their independence from their surroundings, they lost their ability to survive desiccation-a costly trait to retain. During the Devonian, maximum xylem diameter increased with ~ime, with the minimum diameter remaining pretty constant. By the middle Devonian, the tracheid diameter of some plant lineages had plateaued. Wider tracheids allow water to be transported faster, but the overall transport rate depends also on the overall crosssectional area of the xylem bundle itself. The increase in vascular bundle thickness further seems to correlate with the width of plant axes, and plant height; it is also closely related to the appearance of leaves and increased stomatal density, both of which would increase the demand for water. While wider tracheids with robust walls make it possible to achieve higher water transport pressures, this increases the problem of cavitation. Cavitation occurs when a bubble of air forms within a vessel, breaking the bonds between chains of water molecules and preventing them from pulling more water up with their cohesive tension. A tracheid, once cavitated, cannot have its embolism removed and return to service. Therefore it is well worth plants' while to avoid cavitation occurring. For this reason, pits in

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tracheid walls have very small diameters, to prevent air entering and allowing bubbles to nucleate. Freeze-thaw cycles are a major cause of cavitation. Damage to a tracheid's wall almost inevitably leads to air leaking in and cavitation, hence the importance of many tracheids working in parallel. Cavitation is hard to avoid, but once it has occurred plants have a range of mechanisms to contain the damage. Small pits link adjacent conduits to al10w fluid to flow between them, but not air--although ironically these pits, which prevent the spread of embolisms, are also a major cause of them. These pitted surfaces further reduce the flow of water through the xylem by as much as 30%. Conifers, by the Jurassic, developed an ingenious improvement, using valve-like structures to isolate cavitated elements. These torus-margo structures have a blob floating in the middle of a donut; when one side depressurises the blob is sucked into the torus and blocks further flow. Growing to height also employed another trait of tracheids-the support offered by their lignified walls. Defunct tracheids were retained to form a strong, woody stem, produced in most instances by a secondary xylem. However, in early plants, tracheids were too mechanically vulnerable, and retained a central position, with a layer of tough sclerenchyma on the outer rim of the stems. Even when tracheids do take a structural role, they are supported by sclerenchymatic tissue, Tracheids end with walls, which impose a great deal of resistance on flow; vessel members have perforated end walls, and are arranged in series to operate as if they were one continuous vessel. The function of end walls, which were the default state in the Devonian, was probably to avoid embolisms. An embolism is where an air bubble is created in a tracheid. This may happen as a result of freezing, or by gases dissolving out of solution. Once an embolism is formed, it usually cannot be removed; the

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affected cell cannot pull water up, and is rendered useless. End walls excluded, the tracheids of prevascular plants were able to operate under the same hydraulic conductivity as those of the first vascular plant, Cooksonia.The size of tracheids is limited as they comprise a single cell; this limits their length, which in turn limits their maximum useful diameter to 80 pm. Conductivity grows with the fourth power of diameter, so increased diameter has huge rewards; vessel elements, consisting of a number of cells, joined at their ends, overcame this limit and allowed larger tubes to form, reaching diameters of up to 500 pm, and lengths of up to 10 m. Vessels first evolved during the dry, low CO2 periods of the late Permian, in the horsetails, ferns and Selaginellales independently, and later appeared in the mid Cretaceous in angiosperms and gnetophytes. Vessels allow the same cross-sectional area of wood to transport around a hundred times more water than tracheids! This allowed plants to fill more of their stems with structural fibres, and also opened a new niche to vines, which could transport water without being as thick as the tree they grew on. Despite these advantages, tracheid-based wood is a lot lighter, thus cheaper to make, as vessels need to be much more reinforced to avoid cavitation. EVOLUTION OF LEAVES

Leaves today are, in almost all instances, an adaptation to increase the amount of sunlight that can be captured for photosynthesis. Leaves certainly evolved more than once, and probably originated as spiny outgrowths to protect early plants from herbivory. The rhyniophytes of the Rhynie chert comprised nothing more than slender, unornamented axes. The early to middle Devonian trimerophytes, therefore, are the first evidence we have of anything that could be considered leafy. This group of vascular plants are recognisable by

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their masses of terminal sporangia, which adorn the ends of axes which may bifurcate or trifurcate. Some organisms, such as Psilophyton bore enations. These are small, spiny outgrowths of the stem, lacking their own vascular supply. Around the same time, the zosterophyllophytes were becoming important. This group is recognisable by their kidney-shaped sporangia, which grew on short lateral branches close to the main axes. They sometimes branched in a distinctive H-shape. The majority of this group bore pronounced spines on their axes. However, none of these had a vascular trace, and the first evidence of vascularised enations occurs in the Rhynie genus Asteroxylon. The spines of Asteroxylon had a primitive vasuclar supply-at the very least, leaf traces could be seen departing from the central protostele towards each individual "leaf". A fossil known as Baragwanathia appears in the fossil record slightly earlier, in the late Silurian. In this organism, these leaf traces continue into the leaf to form their midvein. One theory, the "enation theory", holds that the leaves developed by outgrowths of the protostele connecting with existing enations, but it is also possible that microphylls evolved by a branching axis forming "webbing". Asteroxylon and Baragwanathia are widely regarded as primitive lycopods. The lycopods are still extant today, familiar as the quillwort Isoetes and the club mosses. Lycopods bear distinctive microphylls-Ieaves with a single vascular trace. Microphylls could grow to some size-the Lepidodendrales boasted microphylls over a meter in length-but almost all just bear the one vascular bundle. The branching pattern of megaphyll veins may belie their origin as webbed, dichotomising branches. The more familiar leaves, megaphylls, are thought to have separate origins-indeed, they appeared four times independently, in the ferns, horsetails, progymnosperms, and seed plants. They appear to have originated from dichotomising branches, which first overlapped one

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another, and eventually developed "webbing" and evolved into gradually more leaf-like structures. So megaphyUs, by this "teleome theory", are composed of a group of webbed branches-hence the "leaf gap" left where the leaf's vascular bundle leaves that of the main branch resembles two axes splitting. In each of the four groups to evolve megaphylls, their leaves first evolved during the late Devonian to early Carboniferous, diversifying rapidly until the designs settled down in the mid Carboniferous. The cessation of further diversification can be attributed to developmental constraints, but why did it take so long for leaves to evolve in the first place? Plants had been on the land for at least 50 million years before megaphylls became significant. However, smal1, rare mesophylls are known from the early Devonian genus Eophyllophytonso development could not have been a barrier to their appearance. The best explanation so far incorporates observations that atmospheric CO2 was declining rapidly during this time-falling by around 90% during the Devonian. This corresponded with an increase in stomatal density by 100 times. Stomata allow water to evaporate from leaves, which causes them to curve. It appears that the low stomatal density in the early Devonian meant that evaporation was limited, and leaves would overheat if they grew to any size. The stomatal density could not increase, as the primitive steles and limited root systems would not be able to supply water quickly enough to match the rate of transpiration. Clearly, leaves are not always beneficial, as illustrated by the frequent occurrence of secondary loss of leaves, famously exemplified by cacti and the "whisk fern" Psilotum. Secondary evolution can also disguise the true evolutionary origin of some leaves. Some genera of ferns display complex leaves which are attached to the pseudostele by an outgrowth of the vascular bundle,

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leaving no leaf gap. Further, horsetail (Equisetum) leaves bear only a single vein, and appear for all the world to be microphyllous; however, in the light of the fossil record and molecular evidence, we conclude that their forbears bore leaves with complex venation, and the current state is a result of secondary simplification. Deciduous trees deal with another disadvantage to having leaves. The popular belief that plants shed their leaves when the days get too short is misguided; evergreens prospered in the Arctic circle during the most recent greenhouse earth. The generally accepted reason for shedding leaves during winter is to cope with the weather-the force of wind and weight of snow are much more comfortably weathered without leaves to increase surface area. Seasonal leaf loss has evolved independently several times and is exhibited in the ginkgoales, gymnosperms and angiosperms. Leaf loss may also have arisen as a response to pressure from insects; it may have been less costly to lose leaves entirely during the winter or dry season than to continue investing resources in their repair. EVOLUTION OF TREES

The early Devonian landscape was devoid of vegetation taller than waist height. Without the evolution of a robust vascular system, taller heights could not be obtained. There was, however, a constant evolutionary pressure to attain greater J;1.eight. The most obvious advantage is the harvestihg of more sunlight for photosynthesis-by overshadowing competitors-but a further advantage is present in spore distribution, as spores (and, later, seeds) can be blown greater distances if they start higher. This may be demonstrated by Prototaxites, thought to be a late Silurian fungus reaching eight metres in height. In order to attain arborescence, early plants needed to develop woody tissue that would act as both support and

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water transport. To understand wood, we must know a little of vascular behaviour. The stele of plants undergoing "secondary growth" is surrounded by the vascular cambium, a ring of cells which produces more xylem (on the inside) and phloem (on the outside). Since xylem cells comprise dead, lignified tissue, subsequent rings of xylem are added to those already present, forming wood. The first plants to develop this secondary growth, and a woody habit, were apparently the ferns, and as early as the middle Devonian one species, Wattieza, had already reached heights of 8 m and a tree-like habit. Other clades did not take long to develop a tree-like stature; the late Devonian Archaeopteris, a precursor to gymnosperms which evolved from the trimerophytes, reached 30 m in height. These progymnosperms were the first plants to develop true wood, grown from a bifacial cambium, of which the first appearance is in the mid Devonian Rellimia. True wood is only thought to have evolved once, giving rise to the concept of a "lignophyte" clade. These Archaeopteris forests were soon supplemented by lycopods, in the form of lepidodendrales, which topped Sam in height and 2m across at the base. These lycopods rose to dominate late Devonian and Carboniferous coal deposits. Lepidodendrales differ from modern trees in exhibiting determinate growth: after building up a reserve of nutrients at a low height, the plants would "bolt" to a genetically determined height, branch at that level, spread their spores and die. They consisted of "cheap" wood to allow their rapid growth, with at least half of their stems comprising a pith-filled cavity. Their wood was also generated by a unifacial vascular cambium-it did not produce new phloem, meaning that the trunks could not grow wider over time. The horsetail Calamites was next on the scene, appearing in the Carboniferous. Unlike the modern

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horsetail Equisetum, Calamites had a unifacial vascular cambium, allowing them to develop wood and grow to heights in excess of 10 m. They also branched multiple times. While the form of early trees was similar to that of todays', the groups containing all modem trees had yet to evolve. The dominant groups today are the gymnosperms, which include the coniferous trees, and the angiosperms, which contain all fruiting and flowering trees. It was long thought that the angiosperms arose from within the gymnosperms, but recent molecular evidence suggests that their living representatives form two distinct groups. It must be noted that the molecular data has yet to be fully reconciled with morphological data, but it is becoming accepted that the morphological support for paraphyly is not especially strong. This would lead to the conclusion that both groups arose from within the pteridosperms, probably as early as the Permian. The angiosperms and their ancestors played a very small role until they diversified during the Cretaceous. They started out as small, damp-loving organisms in the understory, and have been diversifying ever since the midCretaceous, to become the dominant member of non-boreal forests today. EVOLUTION OF ROOTS

Roots are important to plants for two main reasons: Firstly, they provide anchorage to the substrate; more importantly, they provide a source of water and nutrients from the soil. Roots allowed plants to grow taller and faster. The onset of roots also had effects on a global scale. By disturbing the soil, and promoting its acidification (by taking up nutrients such as nitrate and phosphate), they enabled it to weather more deeply, promoting the drawdown of CO2 with huge implications for climate. These effects may have been so profound they led to a mass

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extinction. But how and when did roots evolve in the first place? While there are traces of root-like impressions in fossil soils in the late Silurian, body fossils show the earliest plants to be devoid of roots. Many had tendrils which sprawled alonp; or beneath the ground, with upright axes or thalli dotted here and there, and some even had nonphotosynthetic subterranean branches which lacked stomata. The distinction between root and specialised branch is developmental; true roots follow a different developmental trajectory to stems. Further, roots differ in their branching pattern, and in possession of a root cap. So while "SiluDevonian plants such as Rhynia and Horneophyton possessed the physiological equivalent of roots, rootsdefined as organs differentiated from stems-did not arrive until later. Unfortunately, roots are rarely preserved in the fossil record, and our understanding of their evolutionary origin is sparse. Rhizoids-small structures performing the same role as roots, usually a a cell in diameter-probably evolved very early, perhaps even before plants colonised the land; they are recognised in the Characeae, an algal sister group to land plants. That said, rhizoids probably evolved more than once; the rhizines of lichens, for example, perform a similar role. Even some animals have root-like structures! More advanced structures are common in the Rhynie chert, and many other fossils of comparable early Devonian age bear structures that look like, and acted like, roots. The rhyniophytes bore fine rhizoids, and the trimerophytes and herbaceous lycopods of the chert bore root-like structure penetrating a few centimetres into the soil. However, none of these fossils display all the features borne by modem roots. Roots and root-like structures became increasingly more common and deeper penetrating during the Devonian period, with lycopod trees forming roots around 20 cm long during the Eifelian and Givetian. These were

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joined by progymnosperms, which rooted up to about a metre deep, during the ensuing Frasnian stage. True gymnosperms and zygopterid ferns also formed shallow rooting systems during the Famennian period. The rhizomorphs of the lycopods provide a slightly approach to rooting. They were equivalent to stems, with organs equivalent to leaves performing the role of rootlets. A similar construction is observed in the extant lycopod Isoetes, and this appears to be evidence that roots evolved independently at least twice, in the lycophytes and other plants. A vascular system is indispensable to a rooted plants, as non-photosynthesising roots need a supply of sugars, and a vascular system is required to transport water and nutrients from the roots to the rest of the plant. These plants are little more advanced than their Silurian forbears, without a dedicated root system; however, the flat-lying axes can be clearly seen to have growths similar to the rhizoids of bryophytes today. By the mid-to-Iate Devonian, most groups of plants had independently developed a rooting system of some nature. As roots became larger, they could support larger trees, and the soil was weathered to a greater depth. This deeper weathering had effects not only on the aforementioned drawdown of CO2, but also opened up new habitats for colonisation by fungi and animals. Roots today have developed to the physical limits. They penetrate many metres of soil to tap the water table. The narrowest roots are a mere 40 pm in diameter, and could not physically transport water if they were any narrower. The earliest fossil roots recovered, by contrast, narrowed ,from 3 mm to under 700 pm in diameter; of course, taphonomy is the ultimate control of what thickness we can see. The efficiency of many plants' roots is increased via a symbiotic relationship with a fungal partner. The most

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common are arbuscular mycorrhizae (AM), literally "treelike fungal roots". These comprise fungi which invade some root cells, filling the cell membrane with their hyphae. They feed on the plant's sugars, but return nutrients generated Dr extracted from the soil, which the plant would otherwise have no access to. This symbiosis appears to have evolved early in plant history. AM are found in all plant groups, and 80% of extant vascular plants, suggesting an early ancestry; a "plant"fungus symbiosis may even have been the step that enabled them to colonise the land, and indeed AM are abundant in the Rhynie chert; the association occurred even before there were true roots to colonise, and is has even been suggested that roots evolved in order to provide a more comfortable habitat for mycorrhizal fungi. EVOLUTION OF SEEDS

Early land plants reproduced in the fashion of ferns: spores germinated into small gametophytes, which produced sperm. These would swim across moist soils to find the female organs on the same or another gametophyte, where they would fuse with an ovule to produce an embryo, which would germinate into a sporophyte. This mode of reproduction restricted early plants to damp environments, moist enough that the sperm could swim to their destination. Therefore, early land plants were constrained to the lowlands, near shores and streams. The development of heterospory freed them from this constraint. Heterosporic organisms, as their name suggests, bear spores of two sizes-microspores and megaspores. These would germinate to form microgametophytes and megagametophytes, respectively. This system paved the way for seeds: taken to the extreme, the megasporangia could bear only a single megaspore tetrad, and to complete the transition to true seeds, three of the megaspores in the original tetrad cold be aborted, leaving one megaspore per megasporangium.

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The transition to seeds continued with this megaspore being "boxed in" to its sporangium while it germiates. Then, the megagametophyte is contained within a waterproof integuement, which forms the bulk of the seed. The microgametophyte-a pollen grain which has germinated from a microspore-is employed for dispersal, only releasing its desiccation-prone sperm when it reaches a receptive microgametophyte. Lycopods go a fair way down the path to seeds without ever crossing the threshold. Fossil lycopod megaspores reaching 1 cm in diameter, and surrounded by vegitative tissue, are known-these even germinate into a megagametophyte in situ. However, they fall short of being seeds, since the nucellus, an inner spore-covering layer, does not completely enclose the spore. A very small sJit remains, meaning that the seed is still exposed to the atmosphere. This has two consequences-firstly, it means it is not fully resistant to desiccation, and secondly, sperm do not have to "burrow" to access the archegonia of the megaspore. The first "spermatophytes"-that is, the first plants to bear true seeds-were progymnosperms called pteridosperms: literally, "seed ferns". They ranged from trees to small, rambling shrubs; like most early progymnosperms, they were woody plants with fern-like foliage. They all bore ovules, but no cones, fruit or similar. While it is difficult to track the early evolution of seeds, we can trace the lineage of the seed ferns from the simple trimerophytes through homosporous Aneruophytes. This seed model is shared by basically all gymnosperms ("naked seeds"), most of which encase their seeds in a woody or fleshy cone, but none of which fully enclose their seeds. The angiosperms ("vessel seeds") are the only group to fully enclose the seed, in a carpel. Fully enclosed seeds opened up a new pathway for plants to follow: that of seed dormancy. The embryo,

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completely isolated from the external atmosphere and hence protected from desiccation, could survive some years of draught before germinating. Gymnosperm seeds from the late Carboniferous have been found to contain embryos, suggesting a lengthy gap between fertilisation and germination. This period is associated with the entry into a greenhouse earth period, with an associated increase in aridity. This suggests that dormancy arose as a response to drier climatic conditions, where it became advantageous to wait for a moist period before germinating. This evolutionary breakthrough appears to have opened a floodgate: previously inhospitable areas, such as dry mountain slopes, could now be tolerated, as were soon covered by trees. Seeds offered further advantages to their bearers: they increased the success rate of fertilised gametophytes, and because a nutrient store could be "packaged" in with the embryo, the seeds could germinate rapidly in inhospitable environments, reaching a size where it could fend for itself more quickly. For example, without an endosperm, seedlings growing in arid environments would not have the reserves to grow roots deep enough to reach the water table before they expired. Likewise, seeds germinating in a gloomy understory require an additional reserve of energy to quickly grow high enough to capture sufficient light for self-sustenance. A combination of these advantages gave seed plants the ecological edge over the previously dominant genus Archaeopteris, this increasing the biodiversity of early forests. EVOLUTION OF FLOWERS

Flowers are organs possessed only by the group known as the angiosperms, a relatively late appearance on the evolutionary scene. Colourful and/or pungent structures surround the cones of plants such as cycads and gnetales, making a strict definition of the term "flower" elusive.

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The flowering plants have long been assumed to have evolved from within the "gymnosperms"; according to the traditional morphological view, they are closely allied to the gnetales. However, as noted above, recent molecular evidence is at odds to this hypothesis, and further suggests that gnetales are more closely related to some gymnosperm groups than angiosperms, and that extant gymnosperms form a distinct clade to the angiosperms, The relationship of stem groups to the angiosperms is of utmost importance in determining the evolution of flowers; stem groups provide an insight into the state of earlier "forks" on the path to the current state, If we identify an unrelated group as a stem group, then we will gain an incorrect image of the lineages' history, The traditional view that flowers arose by modification of a structure similar to that of the gnetales, for example, no longer bears weight in the light of the molecular data. Convergence increases our chances of misidentifying stem groups. Since the protection of the megagametophyte is evolutionarily desirable, it would be unsurprising if many separate groups stumbled upon protective encasements independently. Distinguishing ancestry in such a situation, especially where we usually only have fossils to go on, is tricky-to say the least, In flowers, this protection is offered by the carpel, an organ believed to represent an adapted leaf, recruited into a protective role, shielding the ovules. These ovules are further protected by a double-walled integument. Penetration of these protective layers needs something more that a free-floating microgametophyte. Angiosperms have pollen grains comprising just three cells, One cell is responsible for drilling down through the integuments, and creating a conduit for the two sperm cells to flow down. The megagametophyte has just seven cells; of these, one fuses with a sperm cell, forming the nucleus of the egg itself, and another other joins with the other sperm, and dedicates

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itself to forming a nutrient-rich endosperm. The other cells take auxilIary roles. This process of "double fertilisation" is unique and common to all angiosperms. In the fossil record, there are three intriguing groups which bore flower-like structures. The first is the Permian pteridosperm Glossopteris, which already bore recurved leaves resembling carpels. The Triassic Caytonia is more flowerlike still, with enclosed ovules-but only a single integument. Further, details of their pollen and stamens set them apart from true flowering plants. The Bennettitales bore remarkably flower-like organs, protected by whorls of bracts which may have played a similar role to the petals and sepals of true flowers. However, no true flowers are found in any groups save those extant today. Most morphological and molecular analyses place Amborella, the nymphaeales and Austrobaileyaceae in a basal clade dubbed "ANA". This clade appear to have diverged in the early Cretaceous, around 130 million years ago--around the same time as the earliest fossil angiosperm, and just after the first angiosperm-like pollen, 136 million years ago. The magnoliids diverged soon after, and a rapid radiation had produced eudicots and monocots by 125 million years ago. ADVANCES IN METABOLISM

The. most recent major innovation by the plants is the development of the C4 metabolic pathway. Photosynthesis is not quite as simple as adding water to CO2 to produce sugars and oxygen. A complex chemical pathway is involved in this miraculous reaction, facilitated along the way by a range of enzymes and co-enzymes. The enzyme RuBisCO is responsible for "fixing" CO2-that is, it attaches it to a carbon-based molecule to form a sugar, which can be used by the plant, releasing an oxygen molecule along the way. However, the enzyme is notoriously inefficient, and works just as effectively in the other direction through

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a process known as photorespiration. As well as a hot-offthe press sugar molecule, this also costs the plant energy required to "re-set" the RuBisCO molecule, Concentrating Carbon

To work around this inefficiency, C4 plants developed "carbon concentrating" mechanisms, These work by bombarding RuBisCO molecules with CO2, thereby increasing the amount of time they are performing the useful task of making sugars. The process of bombarding the RuBisCO requires more energy than allowing gasses to come and go where they please, but under the right conditions-Leo warm temperatures, low CO 2 concentrations, or high oxygen concentrations-pays off in terms of the decreased loss of sugar through photorespiration. One, C 4 metabolism, employs a so-called Kranz anatomy. This transports CO2 through an outer mesophyll layer, via a range of organic molecules, to a central bundle sheath cell, where, the CO2 is released. In this way, CO2 is concentrated near the site of RuBisCO operation. Because RuBisCO is operating in an environment with much more CO2 than it otherwise would be, it performs more efficiently. A second method, CAM photosynthesis, temporally separates photosynthesis from the action of RuBisCO. RuBisCO only operates during the day, when stomata are sealed and CO2 is provided by the breakdown of the chemical malate. More CO2 is then harvested from the atmosphere when stomata open, during the cool, moist nights, reducing water loss. These two pathways, with the same effect on RuBisCO, evolved a number of times independently-indeed, C 4 alone arose 18 times! The C4 construction is most famously used by a subset of grasses, while CAM is employed by many succulents and cacti. The trait appears to have emerged during the Oligocene, around 25 to 32 million

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years ago; however, they did not become ecologically significant until the Miocene, 6-7 million years ago. Remarkably, some charcoalified fossils preserve tissue organised into the Kranz anatomy, with intact bundle sheath cells, allowing the presence C4 metabolism to be identified without doubt at this time. In deducing their distribution and significance, we resort to the use of isotopic markers. C3 plants preferentially use the lighter of two isotopes of carbon in the atmosphere, 12C, which is more readily involved in the chemical pathways involved in its fixation. Because C4 metabolism involves a further chemical step, this effect is accentuated. Plant material can be analysed to deduce the ratio of the heavier 13C to 12(:. C3 plants are on average around 120/00 lighter than the atmospheric ratio, while C4 plants are about 270/00 lighter. It's troublesome procuring original fossil material in sufficient quantity to analyse the grass itself, but fortunately we have a good proxy: horses. Horses were globally widespread in the period of interest, and browsed almost exclusively on grasses. There's an old phrase in isotope palreontology, "you are what you eat (plus a little bit),,-this refers to the fact that organisms reflect the isotopic composition of whatever they eat, plus a small adjustment factor. There is a good record of horse teeth throughout the globe, and their d13C has been measured. The record shows a sharp negative inflection around 6-7 million years ago, during the Messinian, and this is interpreted as the rise of C4 plants on a global scale. While C 4 enhances the efficiency of RuBisCO, the concentration of carbon is highly energy intensive. This means that C4 plants only have an advantage over C 3 organisms in certain conditions: namely, high temperatures, low CO2 and high oxygen concentrations. C4 plants also need high levels of sunlight in order to thrive. Models suggest that without wildfires removing shadecasting trees and shrubs, there would be no space for C4

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plants. But wildfires have occurred for 400 million years.why did C 4 take so long to arise, and then appear independently so many times? The Carboniferous period had notoriously high oxygen levels-almost enough to allow spontaneous combustion-and very low CO2, but there is no C4 isotopic signature to be found. And there doesn't seem to be a sudden trigger for the Miocene rise. During the Micoene, the atmosphere and climate was relatively stable. If anything, it increased gradually from 149 million years ago before settling down to concentrations similar to the Holocene. This suggests that it did not have a key role in invoking C4 evolution. Grasses themselves had probably been around for 60 million years or more, so had had plenty of time to evolve C4, which in any case is present in a diverse range of groups and thus evolved independentl y. There is a strong signal of climate change in South Asia; increasing aridity-hence increasing fire frequency and intensity-may have led to an increase in the importance of grasslands. However, this is difficult to reconcile with the North American record. It is possible that the signal is entirely biological, forced by the fire- driven acceleration of grass evolution-which, both by increasing weathering and incorporating more carbon into sediments, reduced atmospheric CO2 levels. REFERENCES

Raven, J.A.; Edwards, D."Roots: evolutionary ongms and biogeochemical significance" . Journal of Experimental Botany 52 (90001): 381-401. 2001.

Kenrick, P., Crane P.R., The origin and early diversification of land plants. A cladistic study. Smithsonian Institution Press, Washington & London. 1997.

Gray, J. "The Microfossil Record of Early Land Plants: Advances in Understanding of Early Terrestrialization, 1970-1984". Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences (1934-1990) 309 (1138): 167-195. 1985,

5 Ecology of Fungi The fungi are heterotrophic organisms possessing a chitinous cell wall. The majority of species grow as multicellular filaments called hyphae forming a mycelium; some fungal species also grow as single cells. Sexual and asexual reproduction of the fungi is commonly via spores, often produced on specialized structures or in fruiting bodies. Some species have lost the ability to form specialized reproductive structures, and propagate solely by vegetative growth. Yeasts, molds, and mushrooms are examples of fungi. The fungi are a monophyletic group that is phylogenetically clearly distinct from the morphologically similar slime molds (myxomycetes) and water molds (oomycetes). The fungi are more closely related to animals than plants, yet the discipline of biology devoted to the study of fungi, known as mycology, often falls under a branch of botany. Occurring worldwide, most fungi are largely invisible to the naked eye, living for the most part in soil, dead matter, and as symbionts of plants, animals, or other fungi. They perform an essential role in all ecosystems in decomposing organic matter and are indispensable in nutrient cycling and exchange. Some fungi become noticeable when fruiting, either as mushrooms or molds. Many fungal species have long been used as a direct source of food, such as mushrooms and truffles and in

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fermentation of various food products, such as wine, beer, and soy sauce. More recently, fungi are being used as sources for antibiotics used in medicine and various enzymes, such as cellulases, pectinases, and proteases, important for industrial use or as active ingredients of detergents. Many fungi produce bioactive compounds called mycotoxins, such as alkaloids and polyketides that are toxic to animals including humans. Some fungi are used recreationally or in traditional ceremonies as a source of ' psychotropic compounds. Several species of the fungi are significant pathogens of humans and other animals, ' and losses due to diseases of crops (e.g., rice blast disease) or food spoilage caused by fungi can have a large impact on human food supply and local economies. Fruiting Body

'\

Figure 1. Basic structure of Q fungal body

The evolution of multicellular eukaryotes increased the size and complexity of organisms, allowing them to exploit the terrestrial habitat. Fungi first evolved in water but made the transition to land through the development of specialized structures that prevented their drying out. First classified as plants, fungi are now considered different enough from plants to be placed in a separate kingdom, and in fact are more like animals than plants. . .

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Fungi have a worldwide distribution, and grow in a wide range of habitats, including deserts. Most fungi grow in terrestrial environments, but several species occur only in aquatic habitats. Fungi along with bacteria are the primary decomposers of organic matter in most if not all terrestrial ecosystems worldwide. Based on observations of the ratio of the number of fungal species to the number of plant species in some environments, the fungal kingdom has been estimated to contain about 1.5 million species. Around 70,000 fungal species have been formally described by taxonomists, but the true dimension of fungal diversity is still unknown.ost fungi grow as thread-like filaments called hyphae, which form a mycelium, while others grow as single cells. Until recently many fungal species were described based mainly on morphological characteristics, such as the size and shape of spores or fruiting structures, and biological species concepts; the application of molecular tools, such as DNA sequencing, to study fungal diversity has greatly enhanced the resolution and added robustness to estimates of diversity within various taxonomic groups. Human use of fungi for food preparation or preservation and other purposes is extensive and has a long history: yeasts are required for fermentation of beer, wine and bread, some other fungal species are used in the production of soy sauce and tempeh. Mushroom farming and mushroom gathering are large industries in many countries. Many fungi are producers of antibiotics, including B-Iactam antibiotics such as penicillin and cephalosporin. Widespread use of these antibiotics for the treatment of bacterial diseases, such as tuberculosis, syphilis, leprosy, and many others began in the early 20th century and continues to playa major part in anti-bacterial chemotherapy. The study of the historical uses and sociological impact of fungi is known as ethnomycology.

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ECOLOGY

Although often inconspicuous, fungi occur in every environment on Earth and play very important roles in most ecosystems. Along with bacteria, fungi are the major decomposers in most terrestrial (and some aquatic) ecosystems, and therefore play a critical role in biogeochemical cycles and in many food webs. As decomposers, they play an indispensable role in nutrient cycling, especially as saprotrophs and symbionts, degrading organic matter to inorganic molecules, which can then re-enter anabolic metabolic pathways in plants or other organisms. Symbiosis

Many fungi have important symbiotic relationships with organisms from most if not all Kingdoms. These interactions can be mutualistic or antagonistic in nature, or in case of commensal fungi are of no apparent benefit or detriment to the host. Mycorrhizal symbiosis between plants and fungi is one of the most well-known plant-fungus associations and is of significant importance for plant growth and persistence in many ecosystems; over 90% of all plant species engage in some kind of mycorrhizal relationship with fungi and are dependent upon this relationship for survival. The mycorrhizal symbiosis is ancient, dating to at least 400 million years ago. It often increases the plant's uptake of inorganic compounds, such as nitrate and phosphate from soils having low concentrations of these key plant nutrients. In some mycorrhizal associations, the fungal partners may mediate plant-to-plant transfer of carbohydrates and other nutrients. Such mycorrhizal communities are called "common mycorrhizal networks". Lichens are formed by a symbiotic relationship between algae or cyanobacteria and fungi, in which individual

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photobiont cells are embedded in a tissue formed by the fungus. As in mycorrhizas, the photobiont provides sugars and other carbohydrates, while the fungus provides minerals and water. The functions of both symbiotic organisms are so closely intertwined that they function almost as a single organism. Many insects also engage in mutualistic relationships with various types of fungi. Several groups of ants cultivate fungi in the order Agaricales as their primary food source, while ambrosia beetles cultivate various species of fungi in the bark of trees that they infest. Termites on the African Savannah are also known to cultivate fungi. Fungi as Pathogens and Parasites

However, many fungi are parasites on plants, animals (including humans), and other fungi. Serious fungal pathogens of many cultivated plants causing extensive damage and losses to agriculture and forestry include the rice blast fungus Magnaporthe oryzae, tree pathogens such as Ophiostoma ulmi and Ophiostoma novo-ulmi causing Dutch elm disease, and Cryphonectria parasitica responsible for chestnut blight, and plant-pathogenic fungi in the genera Fusarium, Ustilago, Alternaria, and Cochliobolus; fungi with the potential to cause serious human diseases, especially in persons with immunodeficiencies, are in the genera Aspergillus, Candida, Cryptoccocus, Histoplasma, and Pneumocystis. Several pathogenic fungi are also responsible for relatively minor human diseases, such as athlete's foot and ringworm. Some fungi are predators of nematodes, which they capture using an array of specialized structures, such as constricting rings or adhesive nets. Nutrition and Autotrophy

Growth of fungi as hyphae on or in solid substrates or single cells in aquatic enviTonments is adapted to efficient

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extraction of nutrients from these environments, because these growth forms have high surface area to volume ratios. These adaptations in morphology are complemented by hydrolytic enzymes secreted into the environment for digestion of large organic molecules, such as polysaccharides, proteins, lipids, and other organic substrates into smaller molecules. These molecules are then absorbed as nutrients into the fungal cells. Traditionally, the fungi are considered heterotrophs, organisms that rely solely on carbon fixed by other organisms for metabolism. Fungi have evolved a remarkable metabolic versatility that allows many of them to use a large variety of organic substrates for growth, including simple compounds as nitrate, ammonia, acetate, or ethanol. Recent research raises the possibility that some fungi utilize the pigment melanin to extract energy from ionizing radiation, such as gamma radiation for "radiotrophic" growth. It has been proposed that this process might bear some similarity to photosynthesis in plants, but detailed biochemical data supporting the existence of this hypothetical pa4-hway are presently lacking. MORPHOLOGY

Microscopic Structures

Though fungi are part of the opisthokont clade, all phyla except for the chytrids have lost their posterior flagella. Fungi are unusual among the eukaryotes in having a cell wall that, besides glucans and other typical components, contains the biopolymer chitin. Many fungi grow as threadlike filamentous microscopic structures called hyphae, and an assemblage of intertwined and interconnected hyphae is called a mycelium. Hyphae can be septate, i.e., divided into hyphal compartments separated by a septum, each compartment containing one or more nuclei or can be coenocytic, i.e., lacking hyphal compartmentalization.

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However, septa have pores, such as the doliporus in the basidiomycetes that allow cytoplasm, organelles, and sometimes nuclei to pass through. Coenocytic hyphae are essentially multinucleate supercells. In some cases, fungi have developed specialized structures for nutrient uptake from living hosts; examples include haustoria in plantparasitic fungi of nearly all divisions, and arbuscules of several mycorrhizal fungi, which penetrate into the host cells for nutrient uptake by the fungus .. Macroscopic Structures

Fungal mycelia can become visible macroscopically, for example, as concentric rings on various surfaces, such as damp walls, and on other substrates, such as spoilt food, and are commonly and generically called mould; fungal mycelia grown on solid agar media in laboratory petri dishes are usually referred to as colonies, with many species exhibiting characteristic macroscopic growth morphologies and colours, due to spores or pigmentation.

PUeulI-_~"

YOUNG STAGE

Figure 2. Structure of a Toadstool Fungus

Specialized fungal structures important in sexual reproduction are the apothecia, perithecia, and c1eistotheda

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in the ascomycetes, and the fruiting bodies of the basidiomycetes, and a few ascomycetes. These reproductive structures can sometimes grow very large, and are well known as mushrooms. Structures for Substrate Penetration

Fungal hyphae are specifically adapted to growth on solid surfaces and within substrates, and can exert astoundingly large penetrative mechanical forces. The plant pathogen, Magnaporthe grise a, forms a structure called an appressorium specifically designed for penetration of plant tissues, and the pressure generated by the appressorium, which is directed against the plant epidermis can exceed 8 MPa. The generation of these mechanical pressures is the result of an interplay between physiological processes to increase intracellular turgor by production of osmolytes such as glycerol, and the morphology of the appressorium. REPRODUCI10N

Reproduction of fungi is complex, reflecting the heterogeneity in lifestyles and genetic make up within this group of organisms. Many fungi reproduce either sexually or asexually, depending on conditions in the environment. These conditions trigger genetically determined developmental programs leading to the expression of specialized structures for sexual or asexual reproduction. These structures aid both reproduction and efficient dissemination of spores or spore-containing propagules. Asexual Reproduction

Asexual reproduction via vegetative spores or through mycelial fragmentation is common in many fungal species and allows more rapid dispersal than sexual reproduction. In the case of the "Fungi imperfecti" or Deuteromycota, which lack a sexual cycle, it is the only means of propagation. Asexual spores, upon germination, may

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found a population that is clonal to the population from which the spore originated, and thus colonize new environments. Sexual Reproduction

Sexual reproduction with meiosis exists in all fungal phyla, except the Deuteromycota. It differs in many aspects from sexual reproduction in animals or plants. Many differences also exist between fungal groups and have been used to discrimina te fungal clades and species based on morphological differences in sexual structures and reproductive strategies. Experimental crosses between fungal isolates can also be used to identify species based on biological species concepts. The major fungal clades have initially been delineated based on the morphology of their sexual structures and spores; for example, the spore-containing structures, asci and basidia, can be used in the identification of ascomycetes and basidiomycetes, respectively. Many fungal species have elaborate vegetative incompatibility systems that allow mating only between individuals of opposite mating type, while others can mate and sexually reproduce with any other individual or itself. Species of the former mating system are called heterothallic, and of the latter homothallic. Most fungi have both a haploid and diploid stage in their life cycles. In all sexually reproducing fungi, compatible individuals combine by cell fusion of vegetative hyphae by anastomosis, required for the initiation of the sexual cycle. Ascomycetes and basidiomycetes go through a dikaryotic stage, in which the nuclei inherited from the two parents do not fuse immediately after cell fusion, but remain separate in the hyphal cells. In ascomycetes, dikaryotic hyphae of the hymenium form a characteristic hook at the hyphal septum. During cell

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division formation of the hook ensures proper distribution of the newly divided nuclei into the apical and basal hyphal compartments. An ascus (plural asci) is then formed, in which karyogamy (nuclear fusion) occurs. These asci are embedded in an ascocarp, or fruiting body, of the fungus. Karyogamy in the asci is followed immediately by meiosis and the production of ascospores. The ascospores are disseminated and germinate and may form a new haploid mycelium. Sexual reproduction in basidiomycetes is similar to that of the ascomycetes. Compatible haploid hyphae fuse to produce a dikaryotic mycelium. However, the dikaryotic phase is more extensive in the basidiomycetes, in many cases also present in the vegetatively growing mycelium. A specialized anatomical structure, called a clamp connection, is formed at each hyphal septum. As with the structurally similar hook in the ascomycetes, formation of the clamp connection in the baSidiomycetes is required for controlled transfer of nuclei during cell division, to maintain the dikaryotic stage with two genetically different nuclei in each hypha I compartment. A basidiocarp is formed in which club-like structures known as basidia generate haploid basidiospores after karyogamy and meiosis. The most commonly known basidiocarps are mushrooms, but they may also take many other forms. In zygomycetes, haploid hyphae of two individuals fuse, fOrmlng a zygote, which develops into a zygospore. When the zygospore germinates, it quickly undergoes meiosis, generating new haploid hyphae, which in tum may form asexual sporangiospores. These sporangi6spores are means of rapid dispersal of the fungus and germinate into new genetically identical haploid fungal colonies, able to mate and undergo another sexual cycle followed by the generation of new zygospores, thus completing the lifecycle.

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Dispersal of Spores

Both asexual and sexual spores or sporangiospores of many fungal species are actively dispersed by forcible ejection from their reproductive structures. This ejection ensures exit of the spores from the reproductive structures as well as travelling through the air over long distances. Many fungi thereby possess specialized mechanical and physiological mechanisms as well as spore-surface structures, such as hydrophobins, for spore ejection. These mechanisms include, for example, forcible discharge of ascospores enabled by the structure of the ascus and accumulation of osmolytes in the fluids of the ascus that lead to explosive discharge of the ascospores into the air. The forcible discharge of single spores term~d ballistospores involves formation of a small drop of water (Buller's drop), which upon contact with the spore leads to its projectile release with an initial acceleration of more than 10,000 g. Other fungi rely on alternative mechanisms for spore release, such as external mechanical forces, exemplified by puffballs. Attracting insects, such as flies, to fruiting structures, by virtue of their having lively colours and a putrid odour, for dispersal of fungal spores is yet another strategy, most prominently used by the stinkhorns. Besides regular sexual reproduction with meiosis, some fungal species may exchange genetic material via parasexual processes, initiated by anastomosis between hyphae and plasmogamy of fungal cells. The frequency and relative importance of parasexual events is unclear and may be lower than other sexual processes. However, it is known to playa role in intraspecific hybridization and is also likely required for hybridization between fungal species, which has been associated with major events in fungal evolution. PHYLOGENY AND CLASSIFICATION

For a long time taxonomists considered fungi to be

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members of the Plant Kingdom. This early classification was based mainly on similarities in lifestyle: both fungi and plant are mainly sessile, have similarities in general morphology and growth habitat. Moreover, both groups possess a cell wall, which is absent in the Animal Kingdom. However, the fungi are now considered a separate kingdom, distinct from both plants and animals, from which they appear to have diverged approximately one billion years ago. Many studies have identified several distinct morphological, biochemical, and genetic features in the Fungi, clearly delineating this group from the other kingdoms. For these reasons, the fungi are placed in their own kingdom. Similar to animals and unlike most plants, fungi lack the capacity to synthesize organic carbon by chlorophyll-based photosynthesis; whereas plants store the reduced carbon as starch, fungi, like animals and some bacteria, use glycogen for storage of carbohydrates. A major component of the cell wall in many fungal species is the nitrogen-containing carbohydrate, chitin, also present in some animals, such as the insects and crustaceans, while the plant cell wall consists chiefly of the carbohydrate cellulose. The defining and unique characteristics of fungal cells include growth as hyphae, which are microscopic filaments of between 2-10 microns in diameter and up to several centimetres In length, and which combined form the fungal mycelium. Some fungi, such as yeasts, grow as single ovoid cells, similar to unicellular algae and the protists. Unlike many plants, most fungi lack an efficient vascular system, such as xylem or phloem for long-distance transport of water and nutrients; as an example for convergent evolution, some fungi, such as Armillaria, form rhizomorphs or mycelial cords, resembling and functionally related to, but morphologically distinct from, plant roots.

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Some characteristics shared between plants and. fungi include the presence of vacuoles in the cell, and a similar pathway in the biosynthesis of terpenes using mevalonic acid and pyrophosphate as biochemical precursors; plants however use an additional terpene biosynthesis pathway in the chloroplasts that is apparently absent in fungi. Ancestral traits shared among members of' the fungi include chitinous cell walls and heterotrophy by absorption. A further characteristic of the fungi that is absent from other eukaryotes, and shared only with some bacteria, is the biosynthesis of the amino acid, L~lysine, via the a-aminoadipate pathway. Similar to plants, fungi produce a plethora of secondary metabolites functioning as defensive compounds or for niche adaptation; however, biochemical pathways for the synthesis of similar. or even identical compounds often differ markedly between fungi and plants. EVOLUTIONARY HISTORY

The first organisms having features typical of fungi date to 1,200 million years ago, the Proterozoic. However, fungal fossils do not become common and uncontroversial until the early Devonian, when they are abundant in the Rhynie chert. Even though traditionally included in many botany . curricula and textbooks, fungi are now thought to be more closely related to animals than to plants and are placed with the animals in the monophyletic group of opisthokonts. For much of the Paleozoic Era, the fungi appear to have been aquatic, and consisted of organisms similar to the extant Chytrids in having flagellum-bearing spores. The early fossil record of the fungi is fragmentary, to say the least. The fungi probably colonized the land during the Cambrian, long before land plants. For some time after the Permian-Triassic extinction event, a fungal spike, originally thought to be an extraordinary abundance of fungal spores in sediments

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formed shortly after this event, sugf', ~sted that they were the dominant life form during this period-nearly 100% of the fossil record available from this period. However, the relative proportion of fungal spores relative to spores formed by algal species is difficult to assess, the spike did not appear world-wide, and in many places it did not fall on the Permian-Triassic boundary. Analyses using molecular phylogenetics support a monophyletic origin of the Fungi. The taxonomy of the Fungi is in a state of constant flux, especially due to recent research based on DNA comparisons. These current phylogenetic analyses often overturn classifications based on older and sometimes less discriminative methods based on morphological features and biological species concepts obtained from experimental matings. There is no unique generally accepted system at the higher taxonomic levels and there are constant name changes at every level, from species upwards. However, efforts among fungal researchers are now underway to establish and encourage usage of a unified and more consistent nomenclature. Fungal species can also have multiple scientific names depending on its life cycle and mode (sexual or asexual) of reproduction. Web sites such as Index Fungorum and ITIS define preferred up-to-date names, but do not always agree with each other. Taxonomic Groups

The major divisions (phyla) of fungi have been classified based mainly on their sexual reproductive structures. Currently, seven fungal divisions are proposed: Chytridiomycota

The Chytridiomycota are commonly known as chytrids. These fungi are ubiquitous with a worldwide distribution; chytrids produce zoospores that are capable of active

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movement through aqueous phases with a single flagellum. Consequently, some taxonomists had earlier classified them as protists on the basis of the flagellum. Molecular phylogenies, inferred from the rRNA-operon sequences representing the 185, 285, and 5.85 ribosomal subunits, suggest that the Chytrids are a basal fungal group divergent from the other fungal divisions, consisting of four major clades with some evidence for paraphyly or possibly polyphyly. Blastocladiomycota

The Blastocladiomycota were previously considered a taxonomic clade within the Chytridiomycota. Recent molecular data and ultrastructural characteristics, however, place the Blastocladiomycota as a sister clade to the Zygomycota, Glomeromycota, and Dikarya (Ascomycota and Basiomycota). The blastodadiomycetes are fungi that are saprotrophs and parasites of all eukaryotic groups and undergo sporic meiosis unlike their close relatives, the chytrids, which mostly exhibit zygotic meiosis. Neocallimastigomycota

The Neocallimastigomycota were earlier placed in the phylum Chytridomycota. Members of this small phylum are anaerobic organisms, living in the digestive system of larger herbivorous mammals and possibly in other terrestrial and aquatic environments. They lack mitochondria but contain hydrogenosomes of mitochondrial origin. Zygomycota

The Zygomycota contain the taxa, Zygomycetes and Trichomycetes, and reproduce sexually with meiospores called zygospores and asexually with sporangiospores. Black bread mold is a common species that belongs to this

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group; another is Pilobolus, which is capable of ejecting spores several meters through the air, Medically relevant genera include Mucor, Rhizomucor, and Rhizopus. Molecular phylogenetic investigation has shown the Zygomycota to be a polyphyletic phylum with evidence of paraphyly within this taxonomic group. Glomeromycota

Members of the Glomeromycota are fungi forming arbuscular mycorrhizae with higher plants, Only one species has been observed forming zygospores; all other species solely reproduce asexually. The symbiotic association between the Glomeromycota and plants is ancient, with evidence dating to 400 million years ago, Ascomycota

The Ascomycota, commonly known as sac fungi or ascomycetes, constitute the largest taxonomic group within the Eumycota. These fungi form meiotic spores called ascospores, which are enclosed in a special sac-like structure called an ascus. This division includes morels, a few mushrooms and truffles, single-celled yeasts (e.g., of the genera Saccharomyces, Kluyveromyces, Pichia, and Candida), and many filamentous fungi living as saprotrophs, parasites, and mutualistic symbionts. Prominent and important genera of filamentous ascomycetes include Aspergillus, Penicillium, Fusarium, and Claviceps. Many ascomycetes species have only been observed undergoing asexual reproduction (called anamorphic species), but molecular data has often been able to identify their closest teleomorphs in the Ascomycota. Because the products of meiosis are retained within the sac-like ascus, several ascomyctes have been used for elucidating prine ·-,les of genetics and heredity (e.g. :'\Jeurospora crassa).

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Basidiomycota

Members of the Basidiomycota, commonly known as the club fungi or basidiomycetes, produce meiospores called basidiospores on club-like stalks called basidia. Most common mushrooms belong to this group, as well as rust (fungus) and smut fungi, which are major pathogens of grains. Other important Basidiomyces include the maize pathogen,Ustilago maydis, human commensal species of the genus Malassezia, and the opportunistic human pathogen, Cryptococcus neoformans. PHYLOGENETIC RELATIONSHIPS

Because of some similarities in morphology and lifestyle, the slime molds (myxomycetes) and water molds (oomycetes) were formerly classified in the kingdom Fungi. Unlike true fungi, however, the cell walls of these organisms contain cellulose and lack chitin. Slime molds are unikonts like fungi, but are grouped in the Amoebozoa. Water molds are diploid bikonts, grouped in the Chromalveolate kingdom. Neither water molds nor slime molds are closely related to the true fungi, and, therefore, taxonomists no longer group them in the kingdom Fungi. Nonetheless, studies of the oomycetes and myxomycetes are still often included in mycology textbooks and primary research literature. REFERENCES

Alexopoulos, c.J., Charles W. Mims, M. Blackwell et al., Introductory Mycology, 4th ed. John Wiley and Sons, Hoboken NJ, 2004. Arora, David. (1986). "Mushrooms Demystified: A Comprehensive Guide to the Fleshy Fungi". 2nd ed. Ten Speed Press. Barea JM, Pozo MJ, Azc6n R, Azc6n-Aguilar C. "Microbial co-operation in the rhizosphere". J. Exp. Bot. 56: 1761-1778. 2005. Perotto S, Bonfante P. "Bacterial associations with mycorrhizal fungi: close and distant friends in the rhizosphere.". Trends Microbial. 5: 496-501. 1997

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Deacon JW. "Fungal Biology" Malden, MA: Blackwell Publishers. 2005. Deshpande MV. "Mycopesticide production by fermentation: potential and challenges.". Crit Rev Microbial. 25: 229-243. 1999. Perotto S, Bonfante P. "Bacterial associations with mycorrhizal fungi: close and distant friends in the rhizosphere.". Trends Microbial. 5: 496-501. 1997 Thomas MB, Read AF. "Can fungal biopesticides control malaria?". Nat Rev Microbial. 5: 377-383. 2007.

6 Ecology of Nonvascular Plants Plants are divide into two groups: plants lacking ligninimpregnated conducting cells (the nonvascular plants) and those containing lignin-impregnated conducting cells (the vascular plants). Living groups of nonvascular plants include the bryophytes: liverworts, hornworts, and mosses. Vascular plants are the more common plants like pines, ferns, com, and oaks. Fossil and biochemical evidence indicates plants are descended from multicellular green algae. Various green algal groups have been proposed for this ancestral type, with the Charophytes often being prominently mentioned. Cladistic studies support the inclusion of the Charophytes as sister taxa to the land plants. Algae dominated the oceans of the precambrian time over 700 million years ago. Between 500 and 400 million years ago, some algae made the transition to land, becoming plants by developing a series of adaptations to help them survive out of the water. Vascular plants appeared by 350 million years ago, with forests soon following by 300 million years ago. Seed plants next evolved, with flowering plants appearing around 140 million years ago. LIFE CYCLE OF PLANT

Plants have an alternation of generations: the diploid sporeproducing plant (sporophyte) alternates with the haploid

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gamete-producing plant (gametophyte). Animal life cycles have meiosis followed immediately by gametogenesis. Gametes are produced directly by meiosis. Male gametes are sperm. Female gametes are eggs or ova. The plant life cycle has mitosis occurring in spores, produced by meiosis, that germinate into the gametophyte phase. Gametophyte size ranges from three cells (in pollen) to several million. Alternation of generations occurs in plants, where the sporophyte phase is succeeded by the gametophyte phase. The sporophyte phase produces spores by meiosis within a sporangium. The gametophyte phase produces gametes by mitosis within an antheridium (producing sperm) and/or archegonium (producing eggs). These different stages of the flowering plant life cycle are shown in Figure 1.

Figure 1. Plant life cycle

Within the plant kingdom the dominance of phases varies. Nonvascular plants, the mosses and liverworts, have the gametophyte phase dominant. Vascular plants show a progression of increasing sporophyte dominance from the ferns and "fern allies" to angiosperms. HOMOSPORY AND HETEROSPORY

Plants have two further variations on their life cycles. Plants that produce bisexual gametophytes have those gametophyte&..germinate from isospores that are about all

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the same size. This state is reterred to as homospory. A generalised homosporous plant life cycle is shown in Figure 2. Homosporous plants produce bisexual gametophytes. Ferns are a classic example of a h01;nosporous plant.

Figure 2. A typical homosporous life cycle

. Plants that produce separate male and female gametophytes have those gametophytes germinate from (or within in the case of the more advanced plants) spores of .different sizes. The male gametophyte produces sperm, and is associated with smaller or microspores. The female gametophyte is associated with the larger or megaspores. Heterospory is considered by botanists as a significan~ step toward the development of the seed. A generaHsed heterosporous life cycle is shown in Figure 3.

Figure 3. Typical heterosporous life cycle

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ADAYfATIONS TO LIFE ON LAND

Organisms in water do not face many of the challenges that terrestrial creatures do. Water supports the organisr::., the moist surface of the creature is a superb surface for gas exchange, etc. For organisms to exist on land, a variety of challenges must be met. Drying out. Once removed from water and exposed to air, organisms must deal with the need to conserve water. A number of approaches have developed, such as the development of waterproof skin (in animals), living in very moist environments (amphibians, bryophytes), and production of a waterproof surface (the cuticle in plants, cork layers and bark in woody trees)." Gas exchange. Organisms that live in water are often able to exchange carbon dioxide and oxygen gases through their surfaces. These exchange surfaces are moist, thin layers across which diffusion can occur. Organismal response to the challenge of drying out tends to make these surfaces thicker, waterproof, and to retard gas exchange. Consequently, another method of gas exchange must be modified or developed. Many fish already had gills and swim bladders, so when some of them began moving between ponds, the swim bladder began to act as a gas exchange surface, ultimately evolving into the terrestrial lung. Many arthropods had gills or other internal respiratory surfaces that were modified to facilitate gas exchange on land. Plants are thought to share common ancestry with algae. The plant solution to gas exchange is a new structure, the guard cells that flank openings (stomata) in the above ground parts of the plant. By opening these guard cells the plant is abl~ to allow gas exchange by diffusion through the open stomata. Support. Organisms living in water are supported by the dense liquid they live in. Once on land, the

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organisms had to deal with the less dense air, which could not support their weight. Adaptations to this include animal skeletons and specialised plant cells/ tissues that support the plant. Conduction. Single celled organisms only have tyo move materials in, out, and within their cells. A multicellular creature must do this at each cell in the body, plus move material in, out, and within the organism. Adaptations to this include the circulatory systems of animals, and the specialised conducting tissues xylem and phloem in plants. Some multicellular algae and bryophytes also have specialised conducting cells. Reproduction. Organisms in water can release their gametes into the water, where the gametes will swim by flagella until they ecounter each other and fertilisation happens. On land, such a scenario is not possible. Land animals have had to develop specialised reproductive systems involving fertilisation when they return to water (amphibians), or internal fertilisation and an amniotic egg (reptiles, birds, and mammals). Insects developed similar mechanisms. Plants have also had to deal with this, either by living in moist environments like the ferns and bryophytes do, or by developing specialised delivery systems like pollen tubes to get the sperm cells to the egg. Bryophytes Plant scientists recognize two kinds of land plants, namely, bryophytes, or nonvascular land plants and tracheophytes,or vascular land plants. Bryophytes are small, herbaceous plants that grow closely packed together in mats or cushions on rocks, soil, or as epiphytes on the trunks and leaves of forest trees. Bryophytes are distinguished from tracheophytes by two important characters.

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First, in all bryophytes the ecologically persistent, photosynthetic phase of the life cycle is the haploid, gametophyte generation rather than the diploid sporophyte; bryophyte sporophytes are very short-lived, are attached to and nutritionally dependent on their gametophytes and consist of only an unbranched stalk, or seta, and a single, terminal sporangium. Second, bryophytes never form xylem tissue, the special lignin- containing, water-conducting tissue that is found in the sporophytes of all vascular plants. At one time, bryophytes were placed in a single phylum, intermediate in position between algae and vascular plants. Modern studies of ~ell ultrastructure and molecular biology, however,confirm that bryophytes comprise three separate evolutionary lineages, which are today recognized as mosses (phylum Bryophyta), liverworts (phylum Marchantiophyta) and hornworts (phylum Anthocerotophyta). Following a detailed analysis of land plant relationships, Kenrick and Crane proposed that the three groups of bryophytes represent a grade or structural level in plant evolution, identified by their "monosporangiate,r life cycle. Within this the geologically oldest group, sharing a fossil record with the oldest vascular plants in the Devonian era. Of the three phyla of bryophytes, greatest species diversity is found in the mosses, with up to 15,000 species recognized. A moss begins its life cycle when haploid spores, which are produced in the sporophyte capsule,land on a moist substrate and begin to germinate. From the onecelled spore, a highly branched system of filaments, called . the protonema, develops. Cell specialization occurs within the protonema to form a horizontal system of reddish-brown, anchoring filaments, called caulonemal filaments and upright, green filaments, called chloronemal filaments. Each protonema, which superficially resembles a filamentous alga, can spread over

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several centimeters to form a fuzzy green film over its substrate. As the protonema grows, some cells of the caulonemal filaments specialize to form leafy buds that will ultimately form the adult gametophyte shoots. Numerous shoots typically develop from each protonema so that, in fact, a single spore can give rise to a whole clump of moss plants. Each leafy shoot continues to grow apically, producing leaves in spiral arrangement on an elongating stem. In many mosses the stem is differentiated into a central strand of thin-walled water-conducting cells, called hydroids, surrounded by a parenchymatous cortex and a thick-walled epidermis. The leaves taper from a broad base to a pointed apex and have lamina that are only one-cell layer thick. A hydroid-containing midvein often extends from the stem into the leaf. Near the base of the shoot, reddish-brown, multicellular rhizoids emerge from the stem to anchor the moss to its substrate. Water and mineral nutrients required for the moss to grow are absorbed, not by the rhizoids,but rather by the thin leaves of the plant as rain water washes through the moss cushion. As is typical of bryophytes, mosses produce large, multicellular sex organs for reproduction. Many bryophytes are unisexual, or sexually dioicous. In mosses male sex organs, called antheridia, are produced in dusters at the tips of shoots or branches on the male plants and female sex organs, the archegonia, are produced in similar fashion on female plants. Numerous motile sperm are produced by mitosis inside the brightly colored, c1ubshaped antheridia while a single egg develops in the base of each vase-shaped archegonium. As the sperm mature, the antheridium swells and bursts open. Drops of rain water falling into the cluster of open antheridia splash the sperm to near-by females. Beating their two whiplash £lagellae, the sperm are able to move short distances in the water film that covers the plants to

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Sporangium -

Sp""'phylc (2,,)

;

Figure 4. The moss life cycle. The haploid gametophyte phase is free-living and photosynthetic. The diploid sporophyte grows from and is nourished by the gametophyte .

the open necks of the archegonia. Slimey mucilage secretions in the archegonial neck help pull the sp~ downward to the egg. The closely packed arrangement of the individual moss plants greatly facilitates fertilization. Rain forest bryophytes that hang in long festoons from the trees rely on torrential winds with the rain to transport their sperm from tree to tree, while the small pygmy mosses of exposed, ephemeral habitats depend on the drops of morning dew to move their sperm. Regardless of where

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they grow, all bryophytes require water for sperm dispersal and subsequent fertilization. Embryonic growth of the sporophyte begins within the archegonium soon after fertilization. At its base, or foot, the growing embryo forms a nutrient transfer zone, or placenta, with the gametophyte. Both organic nutrients and water move from the gametophyte into the sporophyte as it continues to grow. In mosses the sporophyte stalk, or seta, tears the archegonial enclosure early in development, leaving only the foot and the very base of the seta embedded in the gametophyte. The upper part of the archegonium remains over the tip of the sporophyte as a cap-like calyptra. Sporophyte growth ends with the formation of a sporangium or capsule at the tip of the seta. Within the capsule, water-resistant spores are formed by meiosis. As _ the mature capsule swells, the calyptra falls away. This allows the capsule to dry and break open at its tip. Special membranous structures, called peristome teeth, that are folded down into the spore mass,now bend outward, flinging the spores into the drying winds. Moss spores can travel great distances on the winds, even moving between continents on tne-le! streams. Their walls are highly protective, allowing some spores to remain viable for up to 40 years. Of course, if the spore lands in a suitable, moist habitat, germination will begin the cycle all over, again. Liverworts and hornworts are like mosses in the fundamental features of their life cycle, but differ greatly in organization of their mature gametophytes and sporophytes. Liverwort gametophytes can be either leafy shoots or flattened thalli. In the leafy forms, the leaves are arranged on the stem in one ventral and two lateral rows or ranks, rather than in spirals like the mosses. The leaves are one cell layer thick throughout, never have a midvein and are usually divided into two or more parts called lobes. The ventral leaves, which actually lie against the substrate,

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are usually much smaller than the lateral leaves and are ; hidden by the stem. Anchoring rhizoids, which arise near the ventral leaves, are colorless and unicellular. The flattened ribbon-like to leaf-like thallus of the thallose liverworts can be either simple or structurally differentiated into a system of dorsal air chambers and ventral storage tissues. In the latter type, the dorsal epidermis of the thallus is punctuated with scattered pores that open into the air chambers. Liverworts synthesize a vast array of volatile oils, which they store in unique organelles called oil bodies. These compounds impart an often spicy aroma to the plants and seem to discourage animals from feeding on them. Many of these compounds have potential as antimicrobial or anticancer pharmecuhcals. Liverwort sporophytes develop completely enclosed within gametophyte tissues until their capsules are ready to open. The seta, which is initially very short,consists of small, thin-walled, hyaline cells. Just prior to capsule opening, the seta cells lengthen, thereby increasing the length of the seta upto 20 times its original dimensions. This rapid elongation pushes the darkly pigmented capsule and upper part of the whitish seta out of the gametophytic tissues. With drying, the capsule opens by splitting into four segments, or valves. The spores are dispersed into the winds by the twisting motions of numerous intermixed sterile cells, called elaters. In contrast to mosses, which disperse their spores over several days, liverworts disperse the entire spore mass of a single capsule in just a few minutes. Hornworts resemble some liverworts in having simple, unspecialized thalloid gametophytes, but they differ in many other characters. For example, colonies of the symbiotic _yanobacterium Nostoc fill small cavities that are scattered throughout the ventral part of the hornwort thallus. When the thallus is viewed from above, these

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colonies appear as scattered blue-green dots. The cyanobacterium converts nitrogen gas from the air into ammonium, which the hornwort requires in its metabolism and the hornwort secretes carbohydrate- containing mucilage which supports the growth of the cyanobacterium. Hornworts also differ from all other land plants in having only one large, algal-like chloroplast in each thallus cell. Hornworts get their name from their long, hornshaped sporophytes. As in other bryophytes, the sporophyte is anchored in the gametophyte by a foot through which nutrient transfer from gametophyte to sporophyte occurs. The rest of the sporophyte, however, is actually an elongate sporangium in which meiosis and spore development take place. At the base of the sporangium, just above the foot, is a mitotically active meristeII1,which adds new cells to the spore-producing zone throughout the life span of the sporophyte. In fact, the sporangium can be releasing spores at its apex, at the same time that new spores are being produced by meiosis at its base. Spore release in hornworts takes place gradually over a long period of time, and the spores are mostly dispersed by water movements rather than by wind Mosses, liverworts and hornworts are found throughout the world in a variety of habitats. They flourish particularly well in moist, humid forests like the fog forests of the Pacific northwest or the montane rain forests of the southern hemisphere. Their ecological roles are many.They provide seed beds for the larger plants of the community, they capture and recycle nutrients that are washed with rainwater from the canopy and they bind the soil to keep it from eroding. In the northern hemisphere peatlands, wetlands often dominated by the moss Sphagnum, are particularly

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important bryophyte communities. This moss has exceptional water-holding capacity, and when dried and compressed, forms a coal-like fuel. Throughout northern Europe, Asia and North America, peat has been harvested for centuries for both fuel consumption and horticultural uses and today peatlands are managed as a sustainable resource. VASCULAR PLANTS GROUPS

The vascular plants have specialised transporting cells xylem and phloem. When we think of plants we invariably picture vascular plants. Vascular plants tend to be larger and more complex than bryophytes, and have a life cycle where the sporophyte is more prominent than the gametophyte. Vascular plants also demonstrate increased levels of organisation by having organs and organ systems. Vascular plants first developed during the Silurian Period, about 400 million years ago. The earliest vascular plants had no roots, leaves, fruits, or flowers, and reproduced by producing spores. Cooksonia, shown in Figure 5, is a typical early vascular ,plant.

Figure 5. Cooksonia fossil specimen (L) and reconstruction (R)

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It was less than 15 em tall, with stems that

dichotomously branched. Dichotomous branching appears a primitive or ancestral trait in vascular plants. Some branches terminated in sporangia that produced a single size of spore. Many scientists now consider "Cooksonia" an evolutionary grade rather than a true monophyletic taxon. Their main argument is that not all stems of Cooksonia-type plants have vascular tissue. The evolutionary situation of a grade would have some members of the group having the trait, others not. The shapes of sporangia on various specimens of Cooksonia also vary considerably. Rhynia, shown in Figure 6, is another early vascular plant. Like Cooksonia, it lacked leaves and roots. One of the species formerly assigned to this genus, R. major, has since been reclassified as Aglaophyton major.

Figure 6. Rhynia gwynne-vaughanii (L) stem cross section from the Rhynie Chert in Scotland

Aglaophyton major (Figure 7) a bryophyte, however, it does have a separate free-living sporophyte that is more prominent than the sporophyte, but appears to lack lignified conducting cells. The remaining species, R. gwynne-vaughanii is an undoubted vascular plant.

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Figure 7. Reconstruction of Aglaophyton major (A-C) and Lyonaphyton rhyniensis

Devonian plant lines included the trimerophytes and zosterophyllophytes, which have been interpreted as related to fems and lycophytes. Psilophytes

The Psilotales aretl:te least complex of all terrestrial vascular plants, and were once believed to be remnants of an otherwise extinct Devonian flora. This is primarily because psilophytes are the only living vascular plants to lack both roots and leaves. Though they have been considered "primitive," recent developmental and molecular evidence suggests that the group may actually be reduced from fernlike ancestors. There is not universal agreement on this, but we here treat them with the ferns for that reason. Despite the uncertainty of their relationships, psilophytes do structurally resemble certain early vascular plants, and are used as a model for understanding the ecology of these plants. This is ·a small group with only two genera, Psilotum, shown above left, and Tmesipteris,. above right, neither with many species. Both genera grow in tropical or

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subtropical regions, where they occur on rich soil or as epiphytes. Psilotum occurs in North America in the Caribbean, and along the Gulf and Atlantic Coasts to as far north as North Carolina, and has been reported from one locality in Arizona. It may also be found in tropical Asia and on Pacific islands. Tmesipteris grows in New Caledonia and nearby areas of the South Pacific, including Australia and New Zealand.

Figure 8. Psilotum nudum

In addition to its natural distribution, Psiloturn is also found as a common weed in greenhouses, and sometimes escapes cultivation in regions with mild climate. It occasionally becomes a nuisance, but is still very popular for its unusual growth form. In Japan, more than 100 unusual breeds have been produced, some of them highly prized by cultivators. Morphology

The psilophyte stern lacks roots; it is anchored instead by a horizontally creeping stern called a rhizome. The erect

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portion of the stem bears paired enations, outgrowths which look like miniature leaves, but unlike true leaves, the enations have no vascular tissue. These paired outgrowths lie immediately below the spore-producing synangia, which produce the spores. The synangia appear to be the product of three sporangia which became fused ~ver the course of evolution, and are borne on the tip of a short lateral branch. This is another feature in which the psilophytes differ from other living vascular plants; all other such plants produce their sporangia on their leaves. You can click on the picture of the synangia of Psilotum at at these structures. right, for a better look , . When the synangia mature, they open to release yellow to whitish spores, from which the gametophyte plants will later emerge, like the one shown at left. The gametophytes are very small, usually less than two millimeters long. They are subterranean and saprophytic, getting their nutrition by absorbing substances dissolVed in the environment. This is often aided by the presence of fungi which grow into the tissues of the gametophyte and through the surrounding . soil. Eventually, the gametophyte reaches sexual maturity, producing both egg and sperm cells. The multiflagellate sperm swim to the egg cells, where they unite to begin the sporophyte generation. Psilophyte gametophytes may even self-fertilize to produce a sporophyte plant. The resulting sporophyte begins its life as a dependent on its parent gametophyte, as in other seedless plants. But unlike the "bryophytes," the sporophyte eventually gains independence from its parent, and establishes itself in the environment. The mature sporophyte of Psilotum will often grow to 30 cm tall, and may grow even taller. It has no true leaves, and instead the stem is green and photosynthetic, being covered with stomates to,allow g"s exchange. As the crosssection at right shows, the stem has a central core of

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vascular tissue (protostele) which is usually lobed. The thick-walled cells in the center oflhis core are sometimes considered to be pith, in which case the v~scular arrangement would actually be a siphonostele. Surrounding the vascular tissue is a layer called the endodermis, which has specially packed cells to regulate flow of water and nutrients. Tmesipteris has similar reproductive structures and life history to that of Psilotum, but by contrast it has broad leaflike extensions of its stem, each with a single vascular bundle. These extensions may lie to either side of the stem, forming a flat growth, or they may be radially arranged. In any case, they are not considered leaves by most botanists, though this interpretation has been challenged by some workers. Lycophytes

The next group, the Division Lycophyta, have their sporangia organised into strobili (singular: strobilus). A strobilus is a series of sporangia and modified leaves closely grouped on a stem tip. The leaves in strobili are soft and fleshy as opposed to the hard, modified leaves in cones.

Figure 9. Steps in the evolution

of the microphyll leaf

Leaves that contained vascular tissue are another major advance for this group. The leaves in lycophytes, both

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Plant Ecology

living and fossil forms, are known ~ microphylls. This term does not imply any size constrain't, but rather refers to the absence of a leaf gap in the vascular supply of the stem at the point where the leaf vascular trace departs. Ferns and other plants have megaphylls, leaves that produce this leaf gap. Today there are fewer genera of lycophytes than during the group's heyday, the Paleozoic Era. Major living lycophytes include Lycopodium, Isoetes, and Selaginella (the so-called resurrection plant). LycopodiUm produces isospores that germinate in the soil and produce a bisexual gametophyte. These spores are all approximately the same size. Selaginella and Isoetes are heterosporous, and thus produce two sizes of spores: small spores (termed microspores) that germinate to produce the male gametophyte; and larger spores (megaspores) that germinate to produce the female gametophyte. The production of two sizes of spores, and also making separate unisexual gametophytes, is thought an important step toward the seed. Modern lycophytes are small, herbaceous plants. Many of the prominent fossil members of this group produced large amounts of wood and were significant trees in the Carboniferous-aged coal swamps. Selaginella is a heterosporous member of the lycophytes. Some species' of this genus are able to withstand drying out by going dormant until they are rehydrated. For this reason these forms of the genus are commonly called resurrection plants. Fossil Lycophytes: Baragwanathia and Drepanophycus

Baragwanathia is an undoubted lycophyte from the middle Silurian deposits of Australia. It has microphyllous leaves spirally attached to the stem, and sporangia clustered in some areas of the plant, although not in terminal strobili as in modern lycophytes.

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Figure 10. (a) Baragwanathia; (b) Drepanophycus

Drepanophycus ·is a middle Devonian lycophyte from the Northern Hemisphere. Its features are very similar to modern lycophytes. Lepidodendron and Sigillaria

The Lycophytes became significant elements of the world's flora during the Carboniferous time (the Mississippian and Pennsylvanian are terms used for this time span in the United States). These non-seed plants evolved into trees placed in the fossil genera Lepidodendron and Sigillaria, with heights reaching up to 40 meters and 20-30 meters respectively. Lepidodendron stems are composed of less wood (secondary xylem) that usually is found in gymnosperm and angiosperm trees. We know much about the anatomy of these coal-age lycopods because of an odd type of preservation known as a coal ball. Coal balls can be peeled and the plants that are anatomically preserved within theIr laboriously studied to learn the details of cell structure of these coal age plants. Additionally, we have some exceptional petrifactions and

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compressions that reveal different layers of the plants' structure. Estimates place the bulk, up tc 70%, of coal material as being derived from lycophytes. Lepidodendron, was a heterosporous lycophyte tree common in coal swamps of the Carboniferous time. As with many large plant fossils, one rarely if ever finds the entire tree preserved intact. Consequently there are a number of fossil plant genera that are "organ taxa" and represent only the leaves (such as Lepidophylloides), reproductive structures (Lepidostrobus), stem (Lepidodendron), spores (Lycospora), and roots (Stigmaria). Lepidodendron had leaves borne spirally on branches that dichotomously forked, with roots also arising spirally from the stigmarian axes, and both small (microspores) and large (megaspores) formed in strobili (a loose type of soft cone).

Lep1dophy1101des (leeves) . ~ Lep1dostrobus (cones)

Ulodendron (brench seers) 1) Aculeatum 2) Obavatum (outer berl<- mId trunk)

Figure 11. Lepidodendron

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Lepidodendron may have attained heigths of nearly 40 meters, with trunks nearly 2 meters in diameter. The trees branched extensively and produced a large number of leaves. When these leaves fell from the branches, they left behind them the leaf scars characteristic of the genus. Sigillaria was another arborescent lycopod, and is also common in coal-age deposits. In contrast to the spirally borne leaves of Lepidodendron, Sigillaria had leaved arranged in vertical rows along the stem. Sphenophyta

The division Sphenophyta contains once dominant plants (both arborescent as well as herbaceous) in Paleozoic forests, equisetophytes are today relegated to minor roles as herbaceous plants. Today only a single genus, Equisetum, survives. The group is defined by their jointed stems, with many leaves being produced at a node, production of isospores in cones borne at the tips of stems, and spores bearing elaters (devices to aid in spore dispersal). The gametophyte is small, bisexual, photosynthetic, and free-living. Silica concentrated in the stems give this group one of their common names: scouring rushes. These plants were reportedly used by American pioneers to scour the pots and pans. The fossil members of this group are often encountered in coal deposits of Carboniferous age in North America and Europe. The Ferns

A fern is anyone of a group of about 20,000 species of plants classified in the phylum or division Pteridophyta, also known as Filicophyta. The group is also referred to as Polypodiophyta, or Polypodiopsida when treated as a subdivision of tracheophyta (vascular plants). The study of ferns and other pteridophytes is called pteridology, and one who studies ferns and other pteridophytes is called a pteridologist. The term "pteridophyte" has traditionally

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been used to describe all seedless vascular plants, making it synonymous with "ferns and fern allies". This can be confusing since members of the fern phylum Pteridophyta are also sometimes referred to as pteridophytes. Life cycle

Ferns are vascular plants differing from the more primitive lycophytes by having true leaves (megaphylls), and they differ from seed plants in their mode of reproduction lacking flowers and seeds. Like all other vascular plants, they have a life cycle referred to as alternation of generations, characterized by a diploid sporophytic and a haploid gametophytic phase. Unlike the gymnosperms and angiosperms, the ferns' gametophyte is a free-living organism.

gametophyte (prothaJlt.m)

Figure 12. Fern life cycle

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The life cycle of a typical fern is as follows: A sporophyte (diploid) phase produces haploid spores by meiosis; A spore grows by cell division into a gametophyte, which typically consists of a photosynthetic prothallus The gametophyte produces gametes (often both sperm and eggs on the same prothallus) by mitosis A mobile, flagellate sperm fertilizes an egg that remains attached to the prothallus The fertilized egg is now a diploid zygote and grows by mitosis into a sporophyte (the typical "fern" plant). Ecology

The stereotypic image of ferns growing in moist shady woodland nooks is far from being a complete picture of the habitats where ferns can be found growing. Fern species live in a wide variety of habitats, from remote mountain elevations, to dry desert rock faces, to bodies of water or in open fields. Ferns in general may. be thought of as largely being specialists in marginal habitats, often succeeding in places where various environmental factors limit the success of flowering plants. Some ferns are among the world's most serious weed species, including the bracken fern growing in the British highlands, or the mosquito fern (Azolla) growing in tropical lakes, both species form large aggressively spreading colonies. There are four particular types of habitats that ferns are found in: moist, shady forests; crevices in rock faces, especially when sheltered from the full sun; acid wetlands including bogs and swamps; and tropical trees, where many species are epiphytes. Many ferns depend on associations with mycorrhizal fungi. Many ferns only grow within specific pH ranges; for instance, the climbing fern (Lygodium) of eastern North America will only grow in

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moist, intensely acid soils, while the bulblet bladder fern with an overlapping range, is only ever found on limestone. Structure

Like the sporophytes of seed plants, those of ferns consist of: Stems: Most often an underground creeping rhizome, but sometimes an above-ground creeping stolon, or an above-ground erect semi-woody trunk reaching up to 20 m in a few species. Leaf. The green, photosynthetic part of the plant. In ferns, it is often referred to as a frond, but this is because of the historical division between people who study ferns and people who study seed plants, rather than because of differences in structure. New leaves typically expand by the unrolling of a tight spiral called a crozier or fiddlehead. This uncurling of the leaf is termed circinate vemation. Leaves are divided into three types: Trophophyll: A leaf that does not produce spores, instead only producing sugars by photosynthesis. Analogous to the typical green leaves of seed plants. Sporophyll: A leaf that produces spores. These leaves are analogous to the scales of pine cones or to stamens and pistil in gymnosperms and angiosperms, respectively. Unlike the seed plants, however, the sporophylls of ferns are typically not very specialized, looking similar to trophophylls and producing sugars by photosynthesis as the trophophylls do. Brophophyll: A leaf that produces abnormally large amounts of spores. There leaves are also larger than: . the other leaves but bare a resemblance to trophopylls.

"',"V&"I§Y 01

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Roots:The underground non-photosynthetic structures that take up water and nutrients from soil. They are always fibrous and are structurally very similar to the roots of seed plants. The gametophytes of ferns, however, are very different from those of seed plants. They typically consist of: Prothallus: A green, photosynthetic structure that is one cell thick, usually heart or kidney shaped, 3-10 mm long and 2-8 mm broad. The prothallus produces gametes by means of: Antheridia: Small spherical structures that produce flagellate sperm. Archegonia: A flask-shaped structure that produces a single egg at the bottom, reached by the sperm by swimming down the neck. - Rhizoids: Root-like structures that consist of single greatly-elongated cells, water and mineral salts are absorbed over the whole structure. Rhizoids anchor the prothallus to the soil. One interesting difference between sporophytes and gametophytes might be summed up by the saying that "Nothing eats ferns, but everything eats gametophytes." This is an over-simplification, but it is true that gametophytes are often difficult to find in the field because they are far more likely to be food than are the sporophytes. Evolution and Oassification

Ferns first appear in the fossil record in the earlyCarboniferous period. By the Triassic, the first evidence of ferns related to several modem families appeared. The "great fern radiation" occurred in the late-Cretaceous, when many modem families of ferns first appeared. Ferns have traditionally been grouped in the Class Filices, but modem classifications ~ssign them their own division in the plant kingdom, called Pteridophyta.

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Traditionally, three discrete groups of plants have been considered ferns: two groups of eusporangiate femsfamilies Ophioglossaceae and Marattiaceae-and the leptosporangiate ferns. The Marattiaceae are a primitive group of tropical ferns with a large, fleshy rhizome, and are now thought to be a sibling taxon to the main group of ferns, the leptosporangiate ferns. Several other groups of plants were considered "fern allies": the clubmosses, spikemosses, and quillworts in the Lycopodiophyta, whisk ferns in Psilotaceae, and horsetails in the Equisetaceae. More recent genetic studies have shown that the Lycopodiophyta are only distantly related to any other vascular plants, having radiated evolutionarily at the base of the vascular plant clade, while both the whisk ferns and horsetails are as much "true" ferns as are the Ophioglossoids and Marattiaceae. In fact, the whisk ferns and Ophioglossoids are demonstrably a clade, and the horsetails and Marattiaceae are arguably another clade. Molecular data - which remain poorly constrained for many parts of the plants' phylogeny - have been supplemented by recent morphological observations supporting the inclusion of Equisetaceae within the ferns, notably relating to the construction of their sperm, and peculiarities of their roots. One possible means of treating this situation is to consider only the leptosporangiate ferns as "true" ferns, while considering the other three groups as "fern allies". In practice, numerous classification schemes have been proposed for ferns and fern allies, and there has been little consensus among them. A new classification by Smith et a1. is based on recent molecular systematic studies, in addition to morphological data. This classification divides ferns into four classes: Psilotopsida, Equisetopsida, Marattiopsida, Polypodiopsida.

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The last group includes most plants familiarly known as ferns. Modern research supports older ideas based on morphology that the Osmundaceae diverged early in the evolutionary history of the leptosporangiate ferns; in certain ways this family is intermediate between the eusporangiate ferns and the leptosporangiate ferns. Ferns are not as important economically as seed plants but have considerable importance. Some ferns are used for food, including the fiddleheads of bracken, Pteridium aquilinum, ostrich fern, Matteuccia struthiopteris, and cinnamon fern, Osmunda cinnamomea. Diplazium esculentum is also used by some tropical peoples as food. Ferns of the genus Azolla are very small, floating plants that do not look like ferns. Called mosquito fern, they are used as a biological fertilizer in the rice paddies of southeast Asia, taking advantage of their ability to fix nitrogen from the air into compounds that can then be used by other plants. A great many ferns are grown in horticulture as landscape plants, for cut foliage and as houseplants, especially the Boston fern. The Bird's Nest Fern, Asplenium nidus, is also popular, and the staghorn ferns, genus Platycerium, have a considerable following. Several ferns are noxious weeds or invasive species, including Japanese climbing fern (Lygodium japonicum), mosquito fern and sensitive fern (Onoclea sensibilis). Giant water fern (Salvinia molesta) is one of the world's worst aquatic weeds. The important fossil fuel coal consists of the remains of primitive plants, including ferns. REFERENCES

Glime, Janice M., Bryophyte Ecology, Volume 1. PhysiolOgical Ecology. Ebook sponsored by Michigan Technological University and the International Association of Bryologists. 2007. Moran, Robbin C. A Natural History of Ferns. Portland, OR: Timber Press. 2004.

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Lord, Thomas R. Ferns and Fern Allies of Pennsylvania. Indiana, P A; Pinelands Press. 2006. Raven, Peter H., Evert, Ray F., & Eichhorn, Susan E. Biology of Plants. New York: W. H. Freeman and Company. 2005. Schofield, W. B. Introduction to Bryology New York: Macmillan. 1985. Watson, E.V. The Structure and Life of Bryophytes. London: Hutchinson University library. 1971.

7 Ecology of Seed Plants The spermatophyte!>, which means "seed plants", are some of the most important organisms on Earth. Life on land as we know it is shaped largely by the activities of seed plants. Soils, forests, and food are three of the most apparent products of this group. Seed-producing plants are probably the most familiar plants to most people, unlike mosses, liverworts, horsetails, and most other seedless plants which are overlooked because of their size or inconspicuous appearance. Many seedplants are large or showy. Conifers are seed plants; they include pines, firs, yew, redwood, and many other large trees. The other major group of seed-plants are the flowering plants, including plants whose flowers are showy, but also many plants with reduced flowers, such as the oaks, grasses, and palms. Today, the seed plants are some of the most important organisms on earth. Life on land as we know it is shaped largely by the activities of seed plants. This large and important group appeared early in the evolution of vascular plants, and throughout the Late Paleozoic shared dominance of the land flora with ferns, lycophytes, and sphenopsids. Since the beginning of the Mesozoic, however, most trees and forests have consisted of seed plants.

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HISTORY AND EVOLUTION

The seed plants are often divided arbitrarily into two groups: the gymnosperms and the angiosperms. The basis for this distinction is that angiosperms produce flowers, while the gymnosperms do not. This is poor form, since it defines the gymnosperms by the absence of a character, and not by any features that the organisms actually share. The gymnosperms do share a number of features, but, as should be obvious from the above cladogram, they are not more closely related to each other than to the angiosperms. The features shared by gymnosperms were likely present in the early ancestors of the flowering plants as well. It should also be noted that the "progymnosperms" are represented by a box of a different color, in order to make it clear that they are not actually seed plants, but rather are included here because they are believed to be the closest relatives of the seed plants. The earliest seeds appear in the Late Devonian. The oldest known seed plant is Elkinsia polymorpha, a "seed fern" from Late Devonian (Famennian) of West Virginia. Though the fossils consist only of small seed-bearing shoots, these fragments are quite well-preserved. This has allowed us to learn details about the evolutionary development of the seed. Another such fossil from about this time is Archaeosperma, also known only from fragments. The earliest seed plants produced their seeds along their branches without specialized structures, such as cones or flowers, unlike most living seed plants. The seeds were produced singly or in pairs, and were surrounded by a loose cupule. This small cup-like structure was lobed in the earliest seeds, producing a somewhat sheltered chamber at one end of the seed. Within this cupule, the seed was enclosed by a more tighly appressed tissue called the integument. The integument is a layer of tissue found in all seeds; it is produced by the parent plant, and develops into

Ecology of Seed Plants

lSS

the seed coat. As the integument evolved to enclose the seed more tightly, an opening was left at one end, called the micropyle, which permitted pollen to enter and provide sperm to fertilize the egg cell. Both the integuments and cupule are believed to be the result of reduced and fused branches or leaves. In later seed plants, a small pollen chamber appears just inside the micropyle. In modem cycads and conifers, this chamber exudes sticky fluids to aid in pollen capture, and as the fluid dries, it pulls the pollen inside the micropyle. This structure is preserved in detail in a number of recently discovered permineralized Devonian seeds. Besides preserving the pollen drop, minerals replaced the original tissues gradually, such that fine detail of the cell walls can be studied - a few Permian seeds even have preserved embryos. Seed plants diversified and spread in the Late Paleozoic. By the end of the Devonian, a variety of early seed plants collectively known as "lyginopterids" appeared. These include Sphenopteris, a plant with fern-like leaves, but which bore seeds and cupules. The Carboniferous saw an increase in the number and kinds of seed plants. In the coal swamps of North America grew pteridosperms like Medullosa, a seed plant that resembles modern tree-ferns, but which bore seeds. Cordaites also grew in these swamps, and in a number of other habitats including ocean-edge environments similar to that of the modem mangrove. However, the cordaites are believed to be closer relatives of modem conifers. Both the medullosans and cordaites were small trees when compared to the great scale-trees which dominated these Late Paleozoic coal swamps. Seed plants were thus overshadowed in their early evolution by plants which did not produce seeds. By the Westphalian, the Voltziales first show up. These are believed to be the closest relatives of modem conifers, and in fact some paleobotanists classify

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them as conife~s. By the Permian, the seed plants were beginning to produce large trees, and by the Triassic, all major groups of seedplants had appeared, except for the flowering plants. LIFE CYCLE OF SEED PLANTS

Seed plants are heterosporous- they have 2 different spore sizes: megaspores and microspores. The generalized life cycle of plants has been modified to illustrate plants which have separate male and female gametophytes (megagametophyte and microgametophyte) produced by different sized spores (megaspores and microspores). sporophyte ~

/

~ ~

seed

- fertiliz8\" -- -- - -- - - - - -- -\ sperm eggs \

"---=

\...

2N

riOSr t---~ .

megaspores

mlcrospores

megagametoPhyt!) microgametophyte

Figure 1. Life cycle of seed plants

The evolutionary trend from nonvascular plants to seedless vascular plants to seed plants has been a reduction in the size of the gametophyte. In seed plants, the gametophyte is usually microscopic and is retained within the tissues of the sporophyte. The megasporangium is surrounded by layers of sporophyte tissue called the integument. The integument and structures within (megasporangium, megaspore) are the ovule. Microspores germinate within the sporophyte tissue and become pollen grains. The microgametophyte is

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contained within the tough, protective coat of the pollen grain. The entire microgametophyte (pollen grain) is transferred to the vicinity of the megagametophyte by a process of pollination. Wind or animals usually accomplish this transfer. When pollen reaches the female gametophyte, it produces an elongate structure (pollen tube) that grows to the egg cell. Sperm are transferred directly through this tube to the egg. The advantage of this process is that sperm do not have to swim long distances as they do in seedless plants. MORPHOLOGY OF SEED PLANTS

The seed includes three primary regions: the embryo, nutritive tissue, and seed coat. The embryo is the young sporophyte plant. This is what will grow into the new tree, shrub, vine, etc. The embryo is usually surrounded by some sort of nutritive tissue which will feed it during its early growth, until it can establish its own root system and leaves to support itself. The origin of this nutritive tissue varies from group to group of seed plants. Nutrients in the tissue are absorbed into the developing embryo by specially modified leaves called cotyledons. In some plants, the cotyledons may absorb all the nutrients before the seed is even dispersed, storing the food inside themselves. Around the whole seed is a layer called the seed coat. This layer may be thick or thin, depending on the species, but it often contains light-sensitive chemicals. When conditions are right - there is appropriate light and water - the seed coat may trigger the germination of the seed. Many plants use this to break a period of dormancy, when the embryo remains inactive. This dormancy can be very important for plants in seasonal habitats, or any environment where the water or light vary greatly over time.

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(seed coat)

pUnu!e--"'1II! eplcotyl embryo

~---mlcropyle

radicle

dlcot seed (bean)

Figure 2. Structure of a typical seed

The seed does not develop from just any part of the plant, but from special structures called ovules. The ovule is an immature seed, which does not yet contain a viable embryo. It is only when the egg cell inside the ovule is fertilized by sperm that the ovule is called a seed. The ovule is surrounded by integument tissues which produce the seed coat, and in the earliest seed plants another layer called the cupule enclosed the entire ovule/seed. While all plants may grow larger by primary growth from their branch tips, not all plants are capable of secondary growth. Secondary growth is the increase in diameter of existing tissues and organs, and this process results in secondary tissues. In seed plants, two kinds of secondary tissues are produced: wood and periderm. Wood is produced by the vascular cambium, a layer of cells whose job it is to divide off cells for new conducting tissues. The vascular cambium is a cylindrical region running through the entire stem of the plant, and branching into every twig and limb. When the cells of this cHmbium divide, they may produce new cells toward the outside of the cylinder, or

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toward the inside. Tho~ which split off to the outside of the cylinder become new phloem tissue, which transports the sap of the tree and thus moves food manufactured in the leaves down to the roots. Those cells which split off toward the inside of the cylinder become new xylem tissue, which transports water and minerals up from the roots, and also provides for movement of materials between the exterior and interior of the plant. When a great deal of xylem accumulates, it is called wood; plants with wood may be trees, shrubs, or stout vines. Periderm is the other product of secondary growth; it is produced by the cork cambium, a cylindrical layer of cells which develops not far under the outer skin (epidermis) of the plant. Like the vascular cambium, the cork cambium divides new cells toward the inside and toward the outside. Those which are produced toward the outside become the particularly important tissue called cork. Cork is important because it replaces the original outer layers of tissue as the plant grows. A young plant begins with a smooth intact surface, but the growth of two cambial layers producing new tissues strains this outer layer, causing it to rupture. Cork is produced to replace these lost tissues, and thus protect the inner tissues. Usually, this new cork will have a different texture, and may even later be replaced by additional new cork. Together, the periderm and the phloem (which lies just to the inside of the periderm) are called the bark. Secondary growth occurs only in seed plants and a few of their extinct close relatives, such as the progymnosperms. Even so, not all seed plants actually produce such secondary growth; the monocots are one such group which lacks this kind of growth. There are a number of other groups which do develop secondary tissues, such as some extinct lycophytes and sphenophytes, and their secondary growth occurs by a similar process, though it differs in a number of important details. The extinct scale trees, for

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instance, produced woody growth enough to reach heights of several dozen meters, but their primary support was their periderm (bark, not wood), and both cambia were unifacial, that is they divided new cells only toward one side of the cambium, and not to both sides as in seed plants. GYMNOSPERMS

Gymnosperms have seeds but not fruits or flowers. Gymnos means naked, sperm means seed: thus the term gymnosperm = naked seeds. Gymnosperms developed during the Paleozoic Era and became the dominant seed plant group during the early Mesozoic Era. The ancestors of gymnosperms were some now-extinct type of heterosporous fern or related group. There are 700 living species of gymnosperms placed into four divisions: conifers, cycads, ginkgos, and gnetophytes. Gymnosperms are undoubtedly the group from which the angiosperms developed, although, as Charles Darwin noted in Origin of Species, which group "remains an abominable mystery". Numerous gymnosperm groups have been proposed as flowering plant ancestors over the past century. Cycads

Cycads are placed in the Division Cycadophyta. They retain several fern-like features, notably pinnate leaves and circinate vernation. However, they usually produce cones of nonphotosynthetic reproductive structures, a distinctively unfemlike feature. Cycads, like all seed plants, are also heterosporous, unlike the ferns which are all homosporous. Cycad cones are unisexual, in fact the plants producing them are dioecious, having separate male and female plants. Cycads also proouce free-swimming sperm. Cycads were much more prominent in the forests of the Mesozoic than they are today. Presently, they are restricted to the tropics. Zamia floridana is the only cycad occurring I natively in the continental United States.

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Figure 3. Cycas revoluta female corn

Figure 4. Cycas revoluta male corn

Several species of Cycas, notably C. revoluta, are commonly encountered cultivated plants in warm, moist areas. Cycas revoluta leaves are often used in Palm Sunday services in some churches, both for .their feathery appearance and ease of obtaining from local greenhouses.

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Ginkgos

The ginkgoes also were a much more prominent group in the past than they are today. The sole survivor of this once robust and diverse group is Ginkgo biloba, the maidenhair tree shown in Figure 5.

Fanshaped leaves

The Ginkgo

tree

Figure 5. Ginkgo biloba

Extensively used as an ornamental plant, Ginkgo was thought extinct in the wild until it was discovered growing natively in a remote area of China. Ginkos are dioecious, with separate male and female plants. The males are more commonly planted since the females produce seeds that have a nasty odor. Pollination is by wind. Recently, Ginkgo has become the current herbal rave, although scientific studies have debunked the claim that the herbal supplement made from ginkgoes improves memory. Precise systematic placement of the ginkgoes has yet to bet determined. Ginkgoes have motile (swimming) sperm, a rarity among living seed plants, although the vegetative I

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anatomy of ginkgoes is more conifer-like (long shoot and short shoot morphology discussed below; structure of their wood). Ginkgoes, like the cycads, are dioecious, and also have similar seed features to cycads. Plants possibly allied to the modern ginkgoes have been found in Permian-aged and later rocks. These plants have been classified in the leaf-genera Ginkgoites and Baiera, although recent studies suggest these genera are really morphological variants and that the modern genus Ginkgo should be used to include these fossils . During the Mesozoic ginkgoes were worldwide in their distribution and important elements in the gymnosperm forests that dominated the land. Conifers The conifers remain a major group of gymnosperms that include the pines, spruce, fir, bald cypress and Norfolk Island Pine (Araucaria). The division Pinophyta contains approximately 550 species of conifers. The conifers are cone producing trees and shrubs that usually have evergreen needle-like leaves. Needles have a thick cuticle, sunken stomates, and a reduced surface area. The conifers, as a groupr are well adapted to withstand extremes in climate and occur in nearly all habitats from the equator to the subpolar regions. The taiga biome consists largely of various conifer species. Auracarias

The Araucariaceae are a very ancient family of conifers. They achieved maximum diversity in the Jurassic and Cretaceous periods, when they existed almost worldwide. At the end of the Cretaceous, when dinosaurs became extinct, so too did the Araucariaceae in the northern hemisphere. There are three genera with 41 species alive today, Agathis, Araucaria and Wollemia, all derived from

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the Antarctic flora and distributed largely in the southern hemisphere. By far the greatest diversity is in New Caledonia (18 species), with others in southern South America, New Zealand, Australia and Malesia, where Agathis ext~nds a short distance into the northern i hemisphere, reaching 18°N in the Philippines. All are evergreen trees, typically with a single stout trunk and ve~ regular whorls of branches, giving them a formal appearance. Several are very popular ornamental trees--in gardens, in subtropical regions, and some are also very important timber trees, producing wood of high quality. Several have edible seeds similar to pine nuts, and others produce valuable resin and amber. In the forests where they occur, they are usually dominant trees, often the largest species in the forest; the largest is Araucaria hunsteinii, reported to 89 m tall in New Guinea, with several other species reaching 50-65 m tall. The petrified wood of the famous Petrified Forest east of Holbrook, Arizona are fossil Araucariaceae. During the Upper (Late) Triassic the region was moist and mild. The trees washed from where they grew in seasonal flooding and accumulated on sandy delta mudflats, where they were buried by silt and periodically by layers of volcanic ash which mineralized the wood. The fossil trees belong generally to three species of Araucariaceae, the most common of them being Araucarioxylon arizonicum. Some of the segments of trunk represent giant trees that are estimated to have been over 50 meters tall when they were alive. Members of this group of conifers have numerous small, scale-like leaves spiraling around their stems. Araucaria, a major genus that gives its name to the group, is a common ornamental because of the symmetry and beauty of its growth form. The monkey puzzle tree, shown in Figure 6, is a species of Araucaria.

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....,.

Figure 6. Araucaria imbricata

The fossil record of Auracarias and similar plants is quite good. The fossil genus Auracarioxylon that grew in Arizona during the Triassic Period comprises the largest group of petrified wood in the Petrified Forest National Park of Arizona. Taxodiaceae

Taxodiaceae was at one time regarded as a distinct plant family comprising the following ten genera of coniferous trees: Athrotaxis Cryptomeria Cunninghamia

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Glyptostrobus Metasequoia Sciadopitys Sequoia Sequoiadendron Taiwania Taxodium However, recent research has shown that the Taxodiaceae, with the single exception of Sciadopitys, should be merged into the family Cupressaceae. There are no consistent characters by whi~h they can be separated, and genetic evidence demonstrates close relationships; this merging is now becoming widely accepted. The one exception, the genus Sciadopitys, is genetically very distinct from all other conifers, and now treated in a family of its own, Sciadopityaceae. Members of this group include some of the largest trees, and have been significant members of the forests of the world since the Mesozoic. Sequoia, shown in Figure 7, and Seq~oiadendron are major genera in this group. S~quoia is a genus in the cypress family Cupressaceae (formerly treated in Taxodiaceae), containing the single living ~pecies Sequoia sempervirens. Common names include Coast Redwood and California Redwood (it is one of three ~pecies of trees known as redwoods). It is an evergreen, long-lived, monoecious tree living for up to 2,200 years, and is the tallest tree in the world, reaching up to 115.5 m (379.1 ft) in height and 8 m (26 ft) diameter at breast height. The crown is c€>nical, with horizontal to slightly drooping branches. The bark is very thick, up t~ 30 cm (12 in), and quite soft, fibrous with a bright red-brown when freshly exposed (hence the name 'redwood'), weathering darker. The root system is composed of shallow, wide-spreading lateral roots.

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Figure 7. Sequoia plant

The leaves are variable, being 15-25 mm long and flat on young trees and shaded shoots in the lower crown of old trees, and scale-like, 5-10 mm long on shoots in full sun in the upper crown of older trees; there is a full range of transition between the two extremes. They are dark green above, and with two blue-white stomatal bands below. Leaf arrangement is spiral, but the larger shade leaves are twisted at the base to lie in a flat plane for maximum light capture. The seed cones are ovoid, 15-32 mm long, with 1525 spirally arranged scales; pollination is in late winter with maturation about 8-9 months after. Each cone scale bears 3-7 seeds, each seed 3-4 mm long and 0.5 mm broad, with two wings 1 mm wide. The seeds are released when the

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168

cone scales dry out and open at maturity. The pollen cones are oval, 4-6 mm long. The species is monoecious, with pollen and seed cones on same plant. Its genetic makeup is unusual among conifers, being a hexaploid and likely autoallopolyploid. The mitochondrial genome is paternally inherited. Pine Life Cycle

Pines have an interesting life cycle, that takes two years to complete. Not all seed plants have such a long time span to complete their life history: some flowering plants manage to do it in as little as a few weeks.

Figure 8. Pine Life Cycle

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169

The sporophyte, as in all other vascular plant groups, is the dominant, photosynthetic part of the life cycle: when you are holding pine needles in your hand you are holding sporophyte parts. Pines have specialised reproductive structures in whicn~ meiosis occurs: pine cones. Pollen grains are produced in the male cones, and contain the male gametophyte (which consists of only a very few cells). Pollen released from the male cones is carried by wind to the female cones, where it lands. The cones close and the next year the pollen grain germinates to produce a pollen tube that grows into the female gametophyte. The sperm cell (from the pollen grain) and egg cell fuse, forming the next generation sporophyte. The sporophyte develops into an embryo encased within a seed. The seed is later released to be transported by the wind to where it lands and germinates. If you have seen a large pine tree you realise there are hundreds or more female cones on such a tree. Pine pollen has been noted to travel great distances from the plant that produced it, if the wind is strong enough. To aid this transport pine pollen has two air sacs, and thus is quite distinctive. Gnetales

The Gnetales, shown in Figure 9, are an odd group: they have some angiosperm-like features but are not themselves angiosperms. Cladistic analyses support placement of the gnetales (or some portion of them) as outgroups for the flowering plants. Three distinctive genera comprise this group: Welwitschia, Gnetum, and Ephedra. Ephedra occurs in the western United States where it has the common name "Mormon tea". It is a natural source for the chemical ephedrine, although there is no evidence the Mormons in Utah (where the plant is extremely common) ever used it for tea. Welwitschia is limited to coastal deserts in South Africa, although fossil leaf, cuticle

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and pollen evidence indicates plants of this type were widespread during the Mesozoic Era. Welwitschia is noted for-its two long, prominent leaves. Gnetum: has leaves that look remarkably like those in angiosperms, as well as vessels in the xylem, generally considered an angiospermcharacteristic.

Figure 9. Ephedra

Among the gnetalean plants, Ephedra is perhaps the best known~ One folkloric name for the plant is "Mormon tea". This is a misnomer as there appears little or no evidence that members of a religion that bans stimulants such as caffeine ever brewed a tea from the plant. However, the plant does produce the drug ephedrine, a stimulant lately linked to deaths of athletes. Welwistchia is a very bizarre plant natively growing only in the coastal deserts of South Africa. The plant produces two long leaves and a crown of reproductive cones rimming a brown, central body. Pollen resembling Welwitschia has been found in many parts of the world, indicating a formerly more widespread distribution of this enigmatic plant.

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ANGIOSPERMS

Flowering plants, the angiosperms, were the last of the seed plant groups to evolve, appearing during the later part of the of the Age of Dinosaurs. All flowering plants produce flowers. Within the female parts of the flower angiosperms produce a diploid zygote and triploid endosperm. Fertilisation is accomplished by a variety of pollinators, including wind, animals, and water. Two sperm are released into the female gametophyte: one fuses with the egg to produce the zygote, the other helps form the nutritive tissue known as endosperm. The angiosperms (angios = hidden) produce modified leaves grouped into flowers that in tum develop fruits and seeds. There are presently 235,000 known living species. Flowers Flowers are collections of reproductive and sterile tissue arranged in a tight whorled array having very short internodes. Sterile parts of flowers are the sepals and petals. When these are similar in size and shape, they are termed tepals. Reproductive parts of the flower are the stamen (male, collectively termed the androecium) and carpel (often the carpel is referred to as the pistil, the female parts collectively termed the gynoecium). Lily flowers demonstrates these concepts. Flowers may be complete, where all parts of the flower are present and functional, or incomplete, where one or more parts of the flower are absent. Many angiosperms produce a single flower on the tip of a shoot. Other plants produce a stalk bearing numerous flowers, termed an inflorescence, such as is seen in many orchids. Many flowers show adaptations for insect pollination, bearing numerous white or yellow petals. Others, like the grasses, oaks, and elms, are wind pollinated and have their petals reduced and often inconspicuous.

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Angiosperm Life Cycle

Flowering plants also exhibit the typical plant alternation of generations, shown in Figure 10.

Figure 10. Angiosperm life cycle

The dominant phase is the sporophyte, with the gametophyte being much reduced in size and wholly dependant on the sporophyte for nutrition. The is not a unique angiosperm condition, but occurs in all seed plants as well. What makes the angiosperms unique is their flowers and the "double fertilisation" that occurs. Technically this is not double fertilisation, but rather a single egg-sperm fusion (fertilisation proper) plus a fusion of the second of two sperm cells with two haploid cells in the female gametophyte to produce triploid (3n) endosperm, a nutritive tissue for the developing embryo.

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Angiosperm Systematics

The flowering plants, the division Magnoliophyta, contain more than 235,000 species, six times the number of species of all other plants combined. The flowering plants divide into two large groups, informally named the monocots and the dicots. The techjnical names for these groups are the class Magnoliopsida for dieots and the class Liliopsida for monocots. The-Elirotyledons are in the class Magnoliopsida and have these features: either woody or herbaceous, flower parts usually in fours and fives, leaves usually net-veined, vascular bundles arranged in a circle within the stem, and produce two cotyledons (seed leaves) at germination. Prominent dieot families include the mustards, maples, cacti, peas and roses. Several dieot families are noteworthy because of the illegal drugs derived from them: the Cannabinaceae (marijuana) and Papaveraceae (poppies from which opium and heroin are derived). Erythroxylum coca (in the dieot family Erythroxylaceae) is the plant from which the illegal drug cocaine is extracted. Not all dieot plants are misused to produce illegal drugs. Notable dieot families with legitimate uses include the pea family, whieh includes the crop plants beans, clover, and peas as well as many ornamental landscape plants such as Acacias. Beans are an excellent source of nonanimal protein as well as fiber. Chocolate and cola are products of the plant family Sterculiaceae. CoHee is produced from CoHea arabica, a plant in the family Rubiaceae, while tea comes from Camelia sinensis (Theaceae), a plant native to China. The class Liliopsida has plants that are herbaceous (a majority are, only palms and bamboo stand out as monocot trees), flower parts are in threes, leaves are usually parallelveined, vascular bundles are scattered within the stem, and produce one cotyledon (seed leaf) at germination. Monocot families include lilies, palms, orchids, irises, and grasses.

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The monocot family Poaceae (known previously as the' Gramineae) includes the grasses such as com, oats, wheat, rye, and rice that are staple food products as well as ornamental plants such as crabgrass and tiff grass. The importance of this plant family to modem civilisation cannot be overstated, as the first six plants mentioned in the previous sentence provide 75% of our food, either directly as food we eat or indirectly as 'food for animals we eat. Evolution of Flowering Plants

Several evolutionary trends within the/plant kingdom have been noted. The mODophyletic nateure of this kingdom is not in dispute, with the first major division being between ~scular and nonvascular plants. Wihin the vascular plants . we see increasing changes in the relationship between sporophyte and gametophyte, culminating in flowering plants. Developing from green algal ancestors, plants show a trend for reduction of the complexity, size, and dominance of the gametophyte generation. In nonvascular plants the gametophyte is the conspicuous, photosynthetic, freeliving phase of the life cycle. Conversely, the angiosperm gametophyte is reduced to between three and eight cells and is dependent on the free-living, photosynthetic sporophyte for its nutrition. Plants also developed and refined the root-shoot-Ieafaxis with its specialized conducting cells of the xylem and phloem. The earliest vascular plants, such as Cooksonia and Rhynia, were little, more than naked (unleafed) photosynthetic stems. Some plants later developed secondary growth that produced wood. Numerous leaf modifications are known, including "carnivorous" plants such as the Venus flytrap, as well as plants that have reduced or lost leaves, such as Psilotum and the cacti. A third trend is the development of the seed to promote the dormancy of the embryo. The seed allows the plant to

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wait out harsh environmental conditions. With the development of the seed during the Paleozoic era plants became less prone to mass extinctions. The fourth trend in plant evolution is the encasing of a seed within a fruit. The only plant group that prodl,lces true fruit is the flowering plants, the angiosperms. Fruits serve to protect the seed, as well as aid in seed dispersal. Land plants have existed for about 425 million years. Early land plants reproduced by spores like their aquatic counterparts. Marine organisms can easily scatter copies of themselves to float away and grow elsewhere. Land plants soon found it advantageous to protect their copies from drying out and other hazards by enclosing them in a case, the seed. Early seed bearing plants, like the ginkgo, and conifers (such as pines and firs), did not produce flowers. The earliest fossil of an angiosperm, or flowering plant, Archaefructus liaoningensis, is dated to about 125 million years BP. Pollen, considered directly linked to flower development, has been found in the fossil record perhaps as long ago as 130 million years. While there is only hard evidence of such flowers existing about 130 million years ago, there is some circumstantial evidence that they may have existed 250 million years ago. A chemical used by plants to defend their flowers, oleanane, hasl5een detected in fossil plants that old, including gigantopterids, which evolved at that time and bear many of the traits of modem, flowering plants, though they are not known to be flowering plants themselves, because only their stems and prickles have been found preserved in detail, one of the earliest examples of petrification. The apparently sudden appearance of relatively modem flowers in the fossil record posed such a problem for the theory of evolution that it was called an "abominable mystery" by Charles Darwin. However the fossil record has grown since the time of Darwin, and recently discovered

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angiosperm fossils such as Archaefructus, along with further discoveries of fossil gymnosperms, suggest how angiosperm characteristics may have been acquired in a series of steps. Several groups of extinct gymnosperms, particularly seed ferns, have been proposed as the ancestors of flowering plants but there is no continuous fossil evidence showing exactly how flowers evolved. Some older fossils, such as the upper Triassic Sanmiguelia, have been suggested. Based on current evidence, some propose that the ancestors of the angiosperms diverged from an unknown group of gymnosperms during the late Triassic (245-202 million years ago). The relationship of the earlier gigantopterids to flowering plants is still enigmatic. A close relationship between Angiosperms and Gnetophytes, suggested on the basis of morphological evidence, has been disputed on the basis of molecular evidence that suggest Gnetophytes are more closely related to other gymnosperms. Recent DNA analysis (molecular systematics) show that Amborella trichopoda, found on the Pacific island of New Caledonia, belongs to a sister group of the other flowering plants, and morphological studies suggest that it has features which may have been characteristic of the earliest flowering plants. The great angiosperm radiation, when a great diversity of angiosperms appear in the fossil record, occurred in the mid-Cretaceous (approximately 100 million years ago). However, a study in 2007 estimated that the division of the five most recent (the genus Ceratophyllum, the family Chloranthaceae, the eudicots, the magnoliids, and the monocots) of the eight main groups occurred around 140 million years ago. By the late Cretaceous, angiosperms appear to have become the predominant group of land plants, and many fossil plants recognizable as belonging to modern families (including beech, oak, maple, and magnolia) appeared.

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However, some authors have proposed an earlier origin for angiosperms, sometime in the Paleozoic (251 million years ago or more). It is generally assumed that the function of flowers, from the start, was to involve mobile animals in their reproduction processes. Pollen can be scattered without bright colors and obvious shapes. Expending energy on these structures would appear to be a liability, unless they provide sigriificant benefit. Island genetics provides one proposed explanation for the sudden, fully developed appearance of flowering plants. Island genetics is believed to be a common source of speciation in general, especially when it comes to radical adaptations which seem to have required inferior transitional forms. Flowering plants may have evolved in an isolated setting like an island or island chain, where the plants bearing them were able to develop a highly specialized relationship with some specific animal. Such a relationship, with a hypothetical wasp carrying pollen from one plant to another much the way fig wasps do today, could result in both the plant(s) and their partners developing a high degree of specialization. Note that the wasp example is not incidental; bees, which apparently evolved specifically due to mutualistic plant relationships, are descended from wasps. Animals are also involved in the distribution of seeds. Fruit, which is formed by the enlargement flower parts, is frequently a seed disbursal tool which depends upon animals, who eat or otherwise disturb it, incidentally scattering the seeds it contains. While many such mutualistic relationships remain too fragile to survive competition with mainland animals and spread, flowers proved to be an unusually effective means of production, spreading to become the dominant form of land plant life. Flowers are derived from leaf and stem components, arising from a combination of genes normally responsible

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for forming new shoots. The most primitive flowers are thought to have had a variable number of flower parts, often separate from (but in contact with) each other. The flowers would have tended to grow in a spiral pattern, to be bisexual (in plants, this means both male and female parts on the same flower), and to be dominated by the ovary (female part). As flowers grew more advanced, some variations developed parts fused together, with a much more specific number and design, and with either specific sexes per flower or plant, or at least ovary inferior". Flower evolution continues to the present day; modern flowers have been so profoundly influenced by humans that some of them cannot be pollinated in nature. Many modern, domesticated flowers used to be simple weeds, which only sprouted when the ground was disturbed. Some of them tended to grow with human crops, perhaps already having symbiotic companion plant relationships with them, and the prettiest did not get plucked because of their beauty, developing a dependence upon and special adaptation to human affection. II

REFERENCES

Gifford, Ernest M., Adriance S. Foster. Morphology and Evolution of Vascular Plants. Third edition. WH Freeman and Company, New York. 1989. Cronquist, Arthur. An Integrated System of Classification of Flowering Plants. Columbia Univ. Press, New York. 1981. Raven, P.H., R.F. Evert, S.E. Eichhorn. Biology of Plants, 7th Edition. W.H. Freeman. 2004. Simpson, M.G. Plant Systematics. Elsevier Academic Press. 2006. Stewart W. N. & Rothwell G. W. Paleobotany and the Evolution of Plants. Cambridge Univ. Press, NY, USA. 521pp. Taylor T. N. & Taylor E. L. The Biology and Evolution of Fossil Plants. Prentice Hall, NJ, USA. 982pp. 1993.

8 Plant Community and Ecosystem Dynamics A community is the set of all populations that inhabit a certain area. Communities can have different sizes and boundaries. These are often identified with some difficulty. An ecosystem is a higher level of organisation the community plus its physical environment. Ecosystems include both the biological and physical components affecting the community/ecosystem. Ecologists find that within a community many populations are not randomly distributed. This recognition that there was a pattern and process of spatial distribution of species was a major accomplishment of ecology. Two of the most important patterns are open community structure and the relative rarity of species within a community. If species within a community have similar geographic range and density peaks, the community is said to be a closed community, a discrete unit with sharp boundaries known as ecotones. An open community, however, has its populations without ecotones and distributed more or less randomly. In a forest, where we find an open community structure, there is a gradient of soil moisture. Plants have different tolerances to this gradient and occur at different places along the continuum. Where the physical environment has abrupt transitions, we find sharp boundaries developing between populations. For example,

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an ecotone develops at a beach separating water and land. Open structure provides some protection for the community. Lacking boundaries, it is harder for a community to be destroyed in an all or nothing fashion. Species can come and go within communities over time, yet the community as a whole persists. In general, communities are less fragile and more flexible than some earlier concepts would suggest. Most species in a community are far less abundant than the dominant species that provide a community its name: for example oak-hickory, pine, etc. Populations of just a few species are dominant within a community, no matter what community we examine. Resource partitioning is thought to be the main cause for this distribution. CLASSIFICATION OF COMMUNITIES

There are two basic categories of communities: terrestrial (land) and aquatic (water). These two basic types of community contain eight smaller units known as biomes. A biome is a large-scale category containing many communities of a similar nature, whose distribution is largely controlled by climate: Terrestrial biomes: tundra, grassland, desert, taiga, temperate forest, tropical forest. Aquatic biomes: marine, freshwater. Terrestrial Biomes

Tundra and Desert

The tundra and desert biomes occupy the most extreme environments, with little or no moisture and extremes of temperature acting as harsh selective agents on organisms that occupy these areas. These two biomes have the fewest numbers of species due to the stringent environmental conditions. In other words, not everyone can live there due to the specialised adaptations required by the environment.

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Tropical Rain Forests

Tropical rain forests occur in regions near the equator. The climate is always warm (between 20° and 25° C) with plenty of rainfall (at least 190 em/year). The rain forest is probably the richest biome, both in diversity and in total biomass. The tropical rain forest has a complex structure, with many levels of life. More than half of all terrestrial species live in this biome. While diversity is high, dominance by a particular species is low. While some animals live on the ground, most rain forest animals live in the trees. Many of these animals spend their entire life in the forest canopy. Insects are so abundant in tropical rain forests that the majority have not yet been identified. Charles Darwin noted the number of species found on a single tree, and suggested the richness of the rain forest would stagger the future systematist with the size of the catalogue of animal species found there. Termites are critical in the decomposition and nutrient cycling of wood. Birds tend to be brightly colored, often making them sought after as exotic pets. Amphibians and reptiles are well represented. Lemurs, sloths, and monkeys feed on fruits in tropical rain forest trees. The largest carnivores are the cats. Encroachment and destruction of habitat put all these animals and plants at risk. Epiphytes are plants that grow on other plants. These epiphytes have their own roots to absorb moisture and minerals, and use the other plant more as an aid to grow taller. Some tropical forests in India, Southeast Asia, West Africa, Central and South American are seasonal and have trees that shed leaves in dry season. The warm, moist climate supports high productivity as well as rapid decomposition of detritus. With its yearlong growing season, tropical forests have a rapid cycling of nutrients. Soils in tropical rain forests tend to have very little organic matter since most of the

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organif: carbon is tied up in the standing biomass of the plants. These tropical soils, termed laterites, make poor agricultural soils after the forest has been cleared. About 17 million hectares of rain forest are destroyed each year. Estimates indicate the forests will be destroyed within 100 years. Rainfall and climate patterns could change as a result. Temperate Forests

The temperate forest biome occurs south of the taiga in eastern North America, eastern Asia, and much of Europe. Rainfall is abundant and there is a well-defined growing season of between 140 and 300 days. The eabtern United States and Canada are covered by this biome's natural vegetation, the eastern deciduous forest. Dominant plants include beech, maple, oak; and other deciduous hardwood trees. Trees of a deciduous forest have broad leaves, which they lose in the fall and grow again in the spring. Sufficient sunlight penetrates the canopy to support a well-developed understory compQsed of shrubs, a layer of herbaceous plants, and then often a ground cover of mosses and ferns. This stratification beneath the canopy provides a numerous habitats for a variety of insects and birds. The deciduous forest also contains many members of the rodent family, which serve as a food source for bobcats, wolves, and foxes. This area also is a home for deer and black bears. Winters are not as cold as in the taiga, so many amphibian and reptiles are able to survive. Shrubland (Chaparral)

The shrubland biome is dominated by shrubs with small but thick evergreen leaves that are often coated with a thick, waxy cuticle, and with thick underground stems that survive the dry summers and frequent fires. Shrublands occur in parts of South America, western Australia, central Chile, and around the Mediterranean Sea. Dense shrubland

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in California, where the summers are hot and very dry, is known as chaparral. This Mediterranean-type shrubland lacks an understory and ground litter, and is also highly flammable. The seeds of many species require the heat and scarring action of fire to induce germination. Grasslands

Grasslands occur in temperate and tropical areas with reduced rainfall or prolonged dry seasons. Grasslands occur in the Americas, Africa, Asia, and Australia. Soils in this region are deep and rich and are excellent for agriculture. Grasslands are almost entirely devoid of trees, and can support large herds of grazing animals. Natural grasslands once covered over 40 percent of the earth's land surface. In temperate areas where rainfall is between 10 and 30 inches a year, grassland is the climax community because it is too wet for desert and too dry for forests. Most grasslands have now been utilised to grow crops, especially wheat and corn. Grasses are the dominant plants, vvhile grazing and burrowing species are the dominant animals. The extensive root systems of grasses allows them to recover quickly from grazing, flooding, drought, and sometimes fire. Temperate grasslands include the Russian steppes, the South American pampas, and North American prairies. A tall-grass prairie occurs where moisture is not quite sufficient to support trees. Animal life includes mice, prairie dogs, rabbits, and animals that feed on them (hawks and snakes). Prairies once contained large herds of buffalo and pronghorn antelope, but with human activity these once great herds ahve dwindled. The savanna is a tropical grassland that contains some trees. The savanna contains the greatest variety and numbers of herbivores (antelopes, zebras, and wildebeests,

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among others). This environment supports a large population of carnivores (lions, cheetahs, hyenas, and leopards). Any plant litter not consumed by grazers is attacked by termites and other decomposers. Once again, human activities are threatening this biome, reducing the range for herbivores and carnivores. Deserts

Deserts are characterised by dry conditions (usually less than 10 inches per year; 25 cm) and a wide temperature range. The dry air leads to wide daily temperature fluctuations from freezing at night to over 120 degrees during the day. Most deserts occur at latitudef. of 300 N or S where descending air masses are dry. Some deserts occur in the rainshadow of tall mountain ranges or in coastal areas near cold offshore currents. Plants in this biome have developed a series of adaptations to conserve water and deal with these temperature extremes. Photosynthetic modifications are another strategy to life in the drylands. The Sahara and a few other deserts have almost no vegetation. Most deserts, however, are home to a variety of plants, all adapted to heat and lack of abundant water (succulents and cacti). Animal life of the Sonoran desert includes arthropods (especially insects and spiders), reptiles (lizards and snakes), running birds (the roadrunner of the American southwest and Warner Brothers cartoon fame), rodents (kangaroo rat and pack rat), and a few larger birds and mammals (hawks, owls, and coyotes). Taiga (Boreal Forest)

The taiga is a coniferous forest extending across most of the northern area of northern Eurasia and North America. This forest belt also occurs in a few other areas, where it has different names: the montane coniferous forest when near mountain tops; and the temperate rain forest along the Pacific Coast as far south as California.

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The taiga receives between 10 and 40 inches of rain per year and has a short growing season. Winters are cold and short, while summers tend to be cool. The taiga is noted for its great stands of spruce, fir, hemlock, and pine. These trees have thick protective leaves and bark, as well as needlelike (evergreen) leaves can withstand the weight of accumulated snow. Taiga forests have a limited understory of plants, and a forest floor covered by low-lying mosses and lichens. Conifers, alders, birch and willow are common plants; wolves, grizzly bears, moose, and caribou are common animals. Dominance of a few species is pronounced, but diversity is low when compared to temperate and tropical biomes. Tundra

The tundra covers the northernmost regions of North America and Eurasia, about 20% of the Earth's land area. This biome receives about 20 cm (8-10 inches) of rainfall annually. Snow melt makes water plentiful during summer months. Winters are long and dark, followed by very short summers. Water is frozen most of the time, producing frozen soil, permafrost. Vegetation includes no trees, but rather patches of grass and shrubs; grazing musk ox, reindeer, and caribou exist along with wolves, lynx, and rodents. A few animals highly adapted to cold live in the tundra year-round (lemming, ptarmigan). During the summer the tundra hosts numerous insects and migratory animals. The ground is nearly completely covered with sedges and short grasses during the short summer. There are also plenty of patches of lichens and mosses. Dwarf woody shrubs flower and produce seeds quickly during the short growing season. The alpine tundra occurs above the timberline on mountain ranges, and may contain many of the same plants as the arctic tundra.

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Climate and Terrestrial Biomes

Climate controls biome distribution by an altitudinal gradient and a latitudinal gradient. With increases of either altitude or latitude, cooler and drier conditions occur. Cooler conditions can cause aridity since cooler air can hold less water vapor than can warmer air. Deserts can occur in warm areas due to a blockage of air circulation patterns that form a rain shadow, or from atmospheric circulation patters. Warm air rises, producing low pressure areas. Cooler air sinks, producing high pressure areas. The tropics tend to be atmospheric low pressure zones the arctic areas atmospheric highs. Relative humidity is a measure of how much water an air mass at a given temperature can hold. In short, warm air can hold more moisture than can cold air. This basic physical feature of air helps explain the distribution of some of the world's great deserts. The warm, moist air masses in the tropics rise upward in the atmosphere as they heat. The pressure of air rising forces air in the upper atmosphere to flow away north and south. This air at higher elevations is cooler and loses much of its moisture as rainfall. When the air masses begin to descend they heat up and begin to draw moisture from the lands they descend upon, at 30 degrees north and south of the equator. Many of the world's deserts are at approximately 30 degrees latitude. Rain shadow deserts also form when cool, dry air masses descend after passing over a tall mountain range, such as the Coast Range and Sierras in California. The Sonoran desert in Arizona is a doubly caused desert, being at 30 degrees latitude as well as in the rain shadow of California mountains. Aquatic Biomes

Conditions in water are generally less harsh than those on land. Aquatic organisms are buoyed by water support, and

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do not usually have to deal with desiccation. Despite covering 71 % of the Earth's surface, areas of the open ocean are a vast aquatic desert containing few nutrients and very little life. Clearcut biome distinctions in water, like those on land, are difficult to make. Dissolved nutrients controls many local aquatic distributions. Aquatic communities are classified into: freshwater communities and marine (saltwater or oceanic) communities. Marine Biame

The marine biome contains more dissolved minerals than the freshwater biome. Over 70% of the Earth's surface is covered in water, by far the vast majority of that being saltwater. There are two basic cat~gories to this biome: benthic and pelagic. Benthic communities (bottom dwellers) are subdiv.ided by depth: the shore/shelf and deep sea. Pelagic communities include planktonic (floating) and nektonic (swimming) organisms. The upper 200 meters of the water column is the euphotic zone to which light can penetrate. Coastal Communities

Estuaries are bays where rivers empty into the sea. Erosion brings down nutrients and tides wash in salt water; forms nutrient trap. Estuaries have high production for organisms that can tolerate changing salinity. Estuaries are called "nurseries of the sea" because many young marine fish develop in this protected environment before moving as adults into the wide open seas. Seashores

Rocky shorelines offer anchorage for sessile organisms. Seaweeds are main photosynthesizers and use holdfasts to anchor. Barnacles glue themselves to stone. Oysters and mussels attach themselves by threads. Limpets and

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periwinkles either hide in crevices or fasten flat to rocks. Sandy beaches and shores are shifting strata. Permanent residents therefore burrow underground. Worms live permanently in tubes. Amphipods and ghost crabs burrow above high tide and feed at night. Coral Reefs

Areas of biological abundance in shallow, warm tropical waters. Stony corals have calcium carbonate exoskeleton and may include algae. Most form colonies; may associate with zooxanthellae dinoflagellates. Reef is densely populated with animal life. The Great Barrier Reef of Australia suffers from heavy predation by crown-of-thorns sea star, perhaps because humans have harvested its predator, the giant triton. Oceans

Oceans cover about three-quarters of the Earth's surface. Oceanic organisms are placed in either pelagic (open water) or benthic (ocean floor) categories. Pelagic division is divided into neritic and three levels of pelagic provinces. Neritic province has greater concentration of organisms because sunlight penetrates; nutrients are found here. Epipelagic zone is brightly lit, has much photosynthetic phytoplankton, that support zooplankton that are food for fish, squid, dolphins, and whales. Mesopelagic zone is semi-dark and contains carnivores; adapted organisms tend to be translucent, red colored, or luminescent; for example: shrimps, squids, lantern and hatchet fishes. The bathypelagic zone is completely dark and largest in size; it has strange-looking fish. Benthic division includes organisms on continental shelf (sublittoral), continental slope (bathyal), and the abyssal plain. Sublittoral zone harbors seaweed that becomes sparse where deeper; most dependent on slow rain of plankton

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and detritus from sunlit water above. Bathyal zone continues with thinning of sublittoral organisms. Abyssal zone is mainly animals at soil-water interface of dark abyssal plain; in spite of high pressure, darkness and coldness, many invertebrates thrive here among sea urchins and tubeworms. Thermal vents along oceanic ridges form a very unique community. Molten magma heats seawater to 350°C, reacting with sulfate to form hydrogen sulfide (H2S). Chemosynthetic bacteria obtain enEJrgy by oxidizing hydrogen sulfide. The resulting food chain supports a community of tubeworms and clams.

Freshvvater

Bio~e

The fresh\water biome is subdivided into two zones: running waters and standing waters. Larger bodies of freshwater are less prone to stratification (where oxygen decreases with depth). The upper layers have abundant oxygen, the lowermost layers are oxygen-poor. Mixing between upper and lower layers in a pond or lake occurs during seasonal changes known as spring and fall overturn. Lakes are larger than ponds, and are stratified in summer and winter. The epilimnion is the upper surface layer. It is warm in summer. The hypolimnion is the cold lower layer. A sudden drop in temperature occurs at the middle of the thermocline. Layering prevents mixing between the lower hypolimnion (rich in nutrients) and the upper epilimnion (which has oxygen absorbed from its, surface). The epilimnion warms in spring and cools in fall, causing a temporary mixing. As a consequence, phytoplankton become more abundant due to the increased amounts of nutrients. Life zones also exist in lakes and ponds. The littoral zone is closest to shore. The limnetic zone is the sunlit body of the lake. Below the level of sunlight penetration is the

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dark profundal zone. At the soil-water interface we find the benthic zone. The term benthos is applied to animals and other organisms that live on or in the benthic zone. Rapidly flowing, bubbling streams have insects and fish adapted to oxygen-rich water. Slow moving streams have aquatic life more similar to lake and pond life. COMMUNITY DENSITY

Communities are made up of species adapted to the conditions of that community. Diversity and stability help define a community and are important in environmental studies. Species diversity decreases as we move away from the tropics. Species diversity is a measure of the different types of organisms in a community (also referred to as species richness). Latitudinal diversity gradient refers to species richness decreasing steadily going away from the equator. A hectare of tropical rain forest contains 40-100 tree species, while a hectare of temperate zone forest contains 10:-30 tree species. In marked contrast, a hectare of taiga contains only a paltry 1-5 species! Habitat destruction in tropical countries will cause many more extinctions per hectare than it would in higher latitudes. Environmental stability is greater in tropical areas, where a relatively stable/constant environment allows more different kinds of species to thrive. Equatorial communities are older because they have been less disturbed by glaciers and other climate changes, allowing time for new species to evolve. Equatorial areas also have a longer growing season. The depth diversity gradient is found in aquatic communities. Increasing species richness with increasing water depth. This gradient is established by environmental stability and the increasing availability of nutrients. Community stability refers to the ability of communities to remain unchanged over time. During the

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1950s and 1960s, stability was equated to diversity: diverse communities were also stable communities. Mathematical modeling during the 1970s showed that increased diversity can actually increase interdependence among species and lead to a cascade effect when a keystone species is removed. Thus, the relation is more complex than previously thought. CHANGE IN COMMUNITIES

Biological communities, like the organisms that comprise them, can and do change over time. Ecological time focuses on community events that occur over decades or centuries. Geological time focuses on events lasting thousands of years or more. Community succession is the sequential replacement of species by immigration of new species and local extinction of older ones following. a disturbance that creates unoccupied habitats for colonization. The initial rapid colonizer species are the pioneer community. Eventually a climax community of more or less stable but slower growing species eventually develops. During succession productivity declines and diversity increases. These trends tend to increase the biomass (total weight of living tissue) in a community. Succession occurs because each community stage prepares the environment for the stage following it. Primary succession begins with bare rock and takes a very long time to occur. Weathering by wind and rain plus the actions of pioneer species such as lichens and mosses begin the buildup of soil. Herbaceous plants, including the grasses, grow on deeper soil and shade out shorter pioneer species. Pine trees or deciduous trees eventually take root and in most biomes will form a climax community of plants that are stabile in the environment. The young produced by climax species can live in that environment, unlike the young produced by successional species.

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Secondary succession occurs when an environment has been disturbed, such as by fire, geological activity, or human intervention. This form of succession often begins in an abandoned field with soil layers already in place. Compared to primary succession, which must take long periods of time to build or accumulate soil, secondary succession occurs rapidly. The herbaceous pioneering plants give way to pines, which in turn may give way to a hardwood deciduous forest. Early researchers assumed climax communities were determined for each environment. Today we recognise the outcome of competition among whatever species are present as establishing the climax community. Climax communities tend to be more stable than successional communities. Early stages of succession show the most growth and are most productive. Pioneer communities lack diversity, make poor use of inputs, and lose heat and nutrients. As succession proceeds, species variety increases and nutrients are recycled more. Climax communities make fuller use of inputs and maintain themselves, thus, they are more stable. Human activity replaces climax communities with simpler communities. Communities are composed of species that evolve, so the community must also evolve. Comparing marine communities of 500 million years ago with modern communities shows modem communities composed of quite different organisms. Modem communities also tend to be more complex, although this may be a reflection of the nature of the fossil record as well as differences between biological and fossil species. The basic effect of human activity on communities is community simplification, an overall reduction of species diversity. Agriculture is a purposeful human intervention in which we create a monoculture of a single favored (crop) species such as com. Most of the agricultural species are derived from pioneering communities. Inadvertent human

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intervention can simplify communities and produce stressed communities that have fewer species as well as a superabundance of some species. Disturbances favor early successional species that can grow and reproduce rapidly. ECOSYSTEMS AND COMMUNITIES

Ecosystems include both living and nonliving components. These living, or biotic, components include habitats and niches occupied by organisms. Nonliving, or abiotic, components include soil, water, light, inorganic nutrients, and weather. An organism's place of residence, where it can be found, is its habitat. A niche is is often viewed as the role of that organism in the community, factors limiting its life, and how it acquires food. Producers, a major niche in all ecosystems, are autotrophic, usually photosynthetic, organisms. In terrestrial ecosystems, producers are usually green plants. Freshwater and marine ecosystems frequently have algae as the dominant producers. Consumers are heterotrophic organisms that eat food produced by another organism. Herbivores are a type of consumer that feeds directly on green plants. Since herbivores take their food directly from the producer level, we refer to them as primary consumers. Carnivores feed on other animals and are secondary or tertiary consumers. Omnivores, the feeding method used by humans, feed on both plants and animals. Decomposers are organisms, mostly bacteria and fungi that recycle nutrients from decaying organic material. Decomposers break down detritus, nonliving organic matter, into inorganic matter. Small soil organisms are critical in helping bacteria and fungi shred leaf litter and form rich soil. . Even if communities do differ in structure, they have some common uniting processes such as energy flow and matter cycling. Energy flows move through feeding relationships. The term ecological niche refers to how an organism functions in an ecosystem. Food webs, food

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chains, and food pyramids are three ways of representing energy flow. Producers absorb solar energy and convert it to chemical bonds from inorganic nutrients taken from environment. Energy content of organic food passes up food chain; eventually all energy is lost as heat, therefore requiring continual input. Original inorganic elements are mostly returned to soil and producers; can be used again by producers and no new input is required. Energy flow in ecosystems, as with all other energy, must follow the two laws of thermodynamics. Recall that the first law states that energy is neither created nor destroyed, but instead changes from one form to another (potential to kinetic). The second law mandates that when energy is transformed from one form to another, some usable energy is lost as heat. Thus, in any food chain, some energy must be lost as we move up the chain. The ultimate source of energy for nearly all life is the Sun. Recently, scientists discovered an exception to this once unchallenged truism: communities of organisms around ocean vents where food chain begins with chemosynthetic bacteria that oxidise hydrogen sulfide generated by inorganic chemical reactions inside the Earth's crust. In this special case, the source of energy is the internal heat engine of the Earth. Food chains indicate who eats whom in an ecosystem. Represent one path of energy flow through an ecosystem. Natural ecosystems have numerous interconnected food chains. Each level of producer and consumers is a trophic level. Some primary consumers feed on plants and make grazing food chains; others feed on detritus. The population size in an undisturbed ecosystem is limited by the food supply, competition, predation, and parasitism. Food webs help determine consequences of perturbations: if titmice and vireos fed on beetles and earthworms, insecticides that killed beetles would increase competition between birds and probably increase predation of earthworms, etc.

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The trophic structure of an ecosystem forms an ecological pyramid. The base of this pyramid represents the producer trophic level. At the apex is the highest level consumer, the top predator. Other pyramids can be recognised in an ecosystem. A pyramid of numbers is based on how many organisms occupy each trophic level. The pyramid of biomass is calculated by multiplying the average weight for organisms times the number of organisms at each trophic level. An energy pyramid illustrates the amounts of energy available at each successive trophic level. The energy pyramid always shows a decrease moving up trophic levels because: Only a certain amount of food is captured and eaten by organisms on the next trophic level. Some of food that is eaten cannot be digested and exits digestive tract as undigested waste. Only a portion of digested food becomes part of the organism's body; rest is used as source of energy. Substantial portion of food energy goes to build up temporary ATP in mitochondria that is then used to synthesize proteins, lipids, carbohydrates, fuel contraction of muscles, nerve conduction, and other functions. Only about 10% of the energy available at a particular trophic level is incorporated into tissues at the next level. Thus, a larger population can be sustained by eating grain than by eating grain-fed animals since 100 kg of grain would result in 10 human kg but if fed to cattle, the result, by the time that reaches the human is a paltry 1 human fg! A food chain is a series of organisms each feeding on the one preceding it. There are two types of food chain: decomposer and grazer. Grazer food chains begin with algae and plants and end in a carnivore. Decomposer chains are composed of waste and decomposing organisms such as fungi and bacteria.

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Food chains are simplifications of complex relationships. A food web is a more realistic and accurate 'depiction of energy flow. Food webs are networks of feeding interactions among species. The food pyramid provides a detailed view of energy flow in an ecosystem: The first level consists of the producers. All higher levels are consumers. The shorter the food chain the more energy is available to organisms. Most humans occupy a top carnivore role, about 2% of all calories available from producers ever reach the tissues of top carnivores. Leakage of energy occurs between each feeding level. Most natural ecosystems therefore do not have more than five levels to 'their food pyramids. Large carnivores are rare because there is so little energy available to them atop the pyramid. Food generation by producers varies greatly between ecosystems. Net primary productivity (NPP) is the rate at which producer biomass is formed. Tropical forests and swamps are the most productive terrestrial ecofystems. Reefs and estuaries are the most productive aquatic ecosystems. All of these productive areas are in danger from human activity. Humans redirect nearly 40% of the net primary productivity and directly or indirectly use nearly 40% of all the land food pyramid. REFERENCES

Grime, J.P. "Biodiversity and Ecosystem Function: The Debate Deepens." Science Vol. 277. no. 533029 Aug 1997 pp. 1260 -1261. 25 May 2007 Lindeman, R.L. 194;2. 'The trophic-dynamic aspect of ecology". Ecology 23: 399-418. Patten, B.C. 1959. "An Introduction to the Cybernetics of the Ecosystem: The Trophic-Dynamic Aspect". Ecology 40, no. 2.: 221-231. Robert Ulanowicz (1997). Ecology, the Ascendant Perspective. Columbia . Univ. Press. Tansley, A.G. 1935. "The use and abuse of vegetational concepts and terms". Ecology 16: 284-307.

9 Ecology of Weeds and Invasive Plants A "plant growing out .of place," that is, plants growing where they are not wanted, at least by some people, is a common, accepted explanation for what weeds are. This notion of undesirability imparts so much human value to the idea of weediness that it is usually necessary to recognise who is making the determination as well as the characteristics of the plants themselves. Weeds are recognized worldwide as an important type of undesirable, economiC pest, especially in agriculture. However, the value of any plant is unquestionably determined by the perceptions of its viewers. These perceptions also influence the human activities directed at this category of vegetation. Weeds are little more than plants that have aroused a level of human dislike at some particular place or time. \ Unfortunately, the anthropomorphic view of weeds provides little insight into why and where they exist, their ' interactions and associations with crops, native plants, and other organisms, or even how to manage them effectively. Weeds are found worldwide and have proven to be successful organisms in the environments that they inhabit. Therefore, it is important to explore whether weeds posses common traits that distinguish them from other plants or whether they are only set apart by local notions of usefulness. "

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Characteristics of Weeds

A list of biological characteristics that describe weeds was proposed in the 1970s and continues to be used today, but it seems unlikely that any plant species could possess all of those "ideal" weedy traits. However, Herbert Baker, botanist and originator of the list, suggests that a species might possess various combinations of the characteristics, resulting in a range of weediness from minor to major weeds. These are: Germination requirements fulfilled in many environments Discontinuous germination (internally controlled) and great longevity of seed Rapid growth through vegetative phase to flowering Continuous seed production for as long as growing conditions permit Self-compatibility but not complete autogamy or apomixis Cross-pollination, when it occurs, by unspecialized visitors or wind Very high seed output in favorable environmental circumstances Production of some seed in a wide range of environmental conditions; tolerance and plasticity Adaptations for short-distance dispersal and longdistance dispersal If perennial, vigorous vegetative reproduction or regeneration from fragments If perennial, brittleness, so as not to be drawn from the ground easily Ability to complete interspecifically by special means. Ecological success in the form of weediness cannot be measured solely from the perspective of noxiousness. The

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number of individuals, the range of habitats occupied, and the ability to continue the species through time must be considered foremost when evaluating success of a species. Weeds can be described in either anthropomorphic or biological terms. Weeds emerge from such descriptions as organisms that may possess a particular suite of biological characteristics but also have the distinction of negative human selection. Thus, a definition of a weed as any plant that is objectionable or interferes with the activities or welfare of man seems to describe sufficiently this category of vegetation. A sample of definitions of weeds published over the past century was presented by Randall, who also argued that the most important criterion was problemcausing plants that interfere with land use. Zimmerman believes that the term "weed" should be used to describe plants that (1) colonize disturbed habitats, (2) are not members of the original plant community, (3) are locally abundant, and (4) are economically of little value (or are costly to control). Aldrich defines weeds as plants that originated under a natural environment and, in response to (human) imposed or natural conditions, are interfering associates of crops and human activities. Each of these definitions implies that weeds have some common biological traits but also a level of relative undesirability as determined by particular people. Whether or not a plant is a weed depends on the context in which someone finds it and on the perspectives and objectives of those involved in dealing with it. Rejma'nek, on the other hand, believes that weeds, colonisers, and naturalized species (including invasive plants) reflect three overlapping concepts. In his view, weeds are plants growing where they are not desired (anthropomorphic defi-nition), colonisers occur early in succession (ecological definition), and invasive plants are plants that become locally established and spread to areas where they are not native (biogeographical definition).

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The most important criterion for weediness is interference at some place or time with the values and activities of people-farmers, foresters, land managers, and many other segments of human society. However, the abundance of weeds is often of more concern than the mere presence of them. For instance, farmers and land managers are usually less concerned about the occurrence of a few isolated plants in a field, even noxious ones, than the occupation of land by vast numbers of weeds. Therefore, the relative abundance of plants, their location, and the potential use of the land they occupy should also be considered in weed definitions. When abundance is applied as a criterion for weediness, it implies a condition of the land as well as a class of vegetation and a form of human discrimination. Weed abundance also may be an indicator or symptom of land mismanagement or neglect. Agrestals

Agrestals are weeds of tilled, arable land. They require the nearly continual disturbance of agriculture to occupy the land. Holzner et al. indicate that every cropping system, for example, cereals, root crops, and orcl:tards, also has its special complement of weeds, which may be·eifuer natfve·::: plants or exotics that have been naturalised into the local flora. A list of the 76 worst agricultural weeds in the world was developed by Holm and his associates and has become the standard by which agrestals are compared. The top 18 weeds on this list are given in Table 1. An additional 104 of the weeds that cause the greatest impacts on agriculture was reviewed by Holm et al. in 1997. As a group these 180 agricultural weeds are estimated to cause over 90% of the loss of crop productivity worldwide. Agrestals have evolved as either specialists or colonizers during the course of agricultural history. Specialised weeds (specialists) have evolved a narrow adaptation to a single crop or sometimes crop cu1tivar and

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its particular growing conditions. Perhaps the most extreme • example of how human activities influence weed species distribution and composition are crop mimics. These are weeds that have evolved life cycles or morphological features so similar to a crop that the two species cannot be distinguished or separated easily. Table 1. Scientific and Common names of certain annual weed species considered the world's 18 worst Species

Common name

Amaranthus hybridus Amaranthus spinosus Avenafatua Chenopodium album Convolvulus arvensis Cynodon dactylon Cyperus esculentus Cyperus rotundus Digitaria sanguinalis Echinochloa colonum Echinochloa crus-galli Eichhornia crassipes Eleusine indica Imperata cylindrica Paspalum conjugatum Portulaca oleracea Rottboellia exaltata Sorghum halepense

Smooth pigweed Spiny amaranthus Wild oat Common lambsquarters Field bindweed Bermudagrass Yellow nutsedge Purple nutsedge Large crabgrass Junglerice Bamyardgrass Waterhyacinth Goosegrass Cogongrass Sour paspalum Common purslane Itchgrass Johnsongrass

Since agrestals that are specialists have evolved along with the cultural practices of a particular crop, any change in practices usually disfavors the weed. Colonisers, on the other hand, are plants with characteristics that allow them to rapidly occupy and dominate disturbed areas. Weeds are major constraints to crop production, yet as primary producers, they also can be important components in an

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. agroecosystem. It is in this context that weeds are sometimes perceived as an ecological "good". Awareness of the importance of weeds on arable land for their role in other trophic levels is growing as natural landscapes become rare or disappear due to the expansion of human-occupied landscapes. The weed flora in many parts of the world has changed over the past century, with some species declining in abundance while others have increased. These changes in the weed flora reflect improved agricultural efficiency, the use of different crops in arable rotations, and the use of more broad-spectrum herbicide combinations.. Many weed species of arable land support a high diversity of insects, so the reduction in abundance of weed host plants can affect associated insects and, therefore, the abundance of other taxa. For example, in the United Kingdom a number of insect groups and farmlandassociated birds (notably the grey partridge, Perdix perdix) have undergone marked population decline, which is associated with changes in agricultural practices over the past 30 years. Thus, it seems that weeds may have a general role in supporting biodiversity within agroecosystems. CHARACfERISTICS OF INVASIVE PLANTS

Invasive plants, unlike agricultural weeds, are generally defined as those that can successfully establish, become naturalised, and spread to new natural habitats apparently without further assistance from humans. They are also generally nonnative or exotic in the new habitat and are often relatively new introductions to an ecoregion. Invasive plants respond readily to human-induced changes in the environment such as disturbance but also may initiate environmental change through their dominance on the landscape. In addition, the spatial and temporal extent of their impact may be expressed at scales ranging from local to global.

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Some ecological impacts believed to be caused by invasive plants are:.;:; follows: Reduction of biodiversity Loss or encroachment upon endangered and threatened species and their habitats Loss of habitat for native insects, birds, and other wildlife Loss of food sources for wildlife Changes to natural ecological processes such as plant community succession Alteratio~s to the frequency and intensity of natural fires Disruptions of native plant-animal associations such as pollination, seed dispersal, and hvst-plant relationships It is widely believed that the most effective way to limit plant invasions is to prevent the introduction of exotic species, which may be difficult because of the ongoing expansion in global travel and trade, changes in environments at all scales (local to global), and increasing development of land for human use. Although the traits of an "ideal weed" have also been ascribed to invasive plants, few empirical studies have tested this concept. The biological characteristics of invasive plants appear in many cases to be dependent upon the habitat in which they occur. Thus, general descriptions of invasive plants remain inconclusive. Some useful generalisations have been made, however, from reviews of empirical evidence or broadscale analyses of floras or databases. For example, Reichard and Hamilton, using a regression tree analysis of biological and environmental traits of invasive plants, suggest that species known to be invasive elsewhere should be limited in introduction to a new area with a similar environment, where they might .Iso be invasive.

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-CLASSIFICATION SYS~ OF WEEDS AND INVASIVE PLANTS

Massive amounts o~ money, time, and energy are expended on weeds and inv~.sive plants because of their economic and ecological costs and impacts on agricultural and natural systems. Because of the magnitude of these effects, it is important that scientists and land managers consider carefully the metaphors they use to describe these two categories of vegetation. Larson points out that metaphors allow people to understand abstract or perplexing subjects in term of something they already know about, a common referent. Thus, weeds and especially invasive plants are often described in militaristic terms, which probably date to Elton's classic The Ecology of Invasions by Animals and Plants. Davis points out that such terms as alien, exotic, invader, and invasion commonly used by invasion ecologists contrast markedly to ~e less evocative terms such as coloniser, founding population, introduced plant, nonnative, spread, or migration, which could be used to describe weeds and "invasive" plants. It should be noted that a similarly militaristic terminology has been used for decades in the pest management field. From a management point of view, there is little doubt that the "invasion" terminology and metaphors have been useful in pointing out the significance of weeds to land managers and policymakers. From a strictly scientific point of view, however, it is difficult to argue against returning to the more value-neutral terminology used by Baker and Stebbins in their early classic, The Genetics of Colonising Species. Since this text is designed to fulfill a dual role for both scientists and land mangers and because the notion of "weed" is itself value laden, we have chosen to use the language of both scientists and managers that is in conventional use to discuss this important class of vegetation.

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Botanical classification is the systematic grouping of plants using criteria that distinguish among types of vegetation. These criteria may be biologically meaningful, based on phylogenetic or evolutionary evidence, or artificial and based on structural or other visible or functional attributes. Some common methods used to classify weeds are by taxonomic relationships, life history, habitat, physiology, and degree of undesirability. Weeds and invasive plants can also be classified by ecological behavior related to invasion and evolutionary strategies related to carbon allocation. Taxonomic Classification

Systematics is the scientific study of biological organisms and their evolutionary relationships. Ideally, organisms are classified systematically according to their presumed genetic relationships, although often this information is unknown. The basis of modem classification is taxonomy, the identification, naming, and grouping of plants according to their traits in common. The accepted taxonomic system used today classifies organisms into a hierarchy of categories: kingdom, phylum (also called division in some botany texts), class, order, family, genus, and species. Recent evidence has shown that an additional category, the domain, occurs above the level of the kingdom; the three recognised domains are Bacteria, Archaea, and Eukarya. All land plants are placed in the domain Eukarya and the kingdom Plantae. Most weeds occur in the phylum Anthophyta (angiosperms, flowering plants), although notable exceptions occur (e.g., some ferns, which are seedless, and conifers, seed plants that have no flowers, are considered weeds). Angiosperms are further divided into the classes Dicotyledones (dicots) and Monocotyledones (monocots).

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The next level of classification is the order. Although systematists do not agree on the exact number of orders, the commonly accepted Cronquist system recognises 64 orders of dicots and 19 orders of monocots. The orders are divided further into families, which, like classes and orders, are comprised of plants whose morphological similarities are greater than their differEnces. Approximately 383 angiosperm families are currently recognised (318 dicot and 65 monocot). The level of genus includes plants that have common characteristics and that are presumed to be genetically related. The narrowest category of classification is the species, which consists of plants that can interbreed freely (the biological species concept). For practical purposes, however, most species are grouped largely on the basis of anatomical and morphological characteristics (the morphological species concept). At this point in taxonomic classification, the plant group is given a name, called a scientific name or Latin binomial, which consists of both the genus and species names of the plant. There are approximately 250,000 species of flowering plants in the world (depending upon which authority is used). However, less than 250 of these, about 0.1%, are troublesome enough to be called major agricultural weeds throughout the world. It is far more difficult to estimate the number of invasive plant species in nonagricultural habitats worldwide. In the United States, by one estimate, introduced invasive plants comprise from 8 to 47% of the total flora of most states. Of the 250 recognised major agricultural weeds, nearly 70% occur in only 12 plant families and over 40% are found in only two families, Poaceae (grass family) and Asteraceae (aster or composite family). Although these observations are fruitful areas of speculation for plant evolutionary biologists, it should be noted that about 75% of world food production is provided

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by only a dozen crops: barley, maize, millet, oats, rice, sorghum, sugarcane, wheat, cassava, soybean, sweet potato, and white potato. Eight of these crops (the first eight in the list above) are also members of the grass family. The distribution of both the world's worst agricultural weeds and its major crops is quite taxonomically restricted, again pointing to the extreme discrimination and selection that humans apply to vegetation. It is sometimes necessary to distinguish only broadly among weed species, for example when broad-scale methods of weed control are used. In such situations, distinction among grasses and sedges (monocot) and broadleaf (dicot) plants may be sufficient, and a much abbreviated system of classification is satisfactory. Such a system was once in common use by weed control specialists; a typical description of weeds by this method is shown below: ' Dicots. Plants whose seedlings produce two cotyledons or seed leaves. Usually typified by netted leaf venation and flowering parts in fours, fives, or multiples thereof. Examples include mustards (Brassica spp.), nightshades (Solanum spp.), and morningglory (Convolvulus spp.). Commonly called broadleaved plants. Monocots. Plants whose seedlings bear only one cotyledon. Typified by parallel leaf venation and flower parts in threes or multiples of three. Most weeds are found in only two groups, grasses and sedges, although other groups exist. Grasses. Leaves usually have a ligule or at times an auricle. The leaf sheaths are split around the stem with the stem being round or flattened in cross section with hollow internodes. Sedges. Leaves lack ligules and auricles and the leaf sheaths are continuous around the stem. In many species the stem is triangular in cross section with solid internodes.

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Classification by Life History

Another method used to classify weeds is by the life cycle of the plant. The length of life, seas~ of growth, and\time and method of reproduction are used to classify weeds in this way. Annuals. An annual plant completes its life cycle from seed to seed in one year or less. Annuals are often divided into two groups, winter and summe!", according to the plant's time of germination, maturation, and death: Winter annuals. These plants uS!lally germinate in the fall or winter, grow throughout the spring, and set seed. and die by early summer.. Summer annuals. These plants germinate in the sP9Jtg, grow throughout the summer, set seed by autumn, and die before winter. In mild climates, however, it is usual for some winter annuals to germinate in late summer or autumn and for some summer annuals to live throughout the winter. Annual plants are the largest single category of weeds. Biennials. These plants live longer than one but less than two years. During the first growth phase, biennials develop vegetatively from a seedling into a rosette. Because of this growth habit, biennials sometimes can be confused with winter annuals. After a cold period, vegetative growth resumes, and floral initiation, seed production, and death occur. Biennials are often large plants when mature and have thick fleshy roots. Relatively few weed species are biennials, but some annual plants may behave as biennials under certain conditions and some biennials may behave as short-lived perennials in mild climates. Perennials. Perennial plants live for longer than two years and may reproduce several times before dying. These plants are characterised by renewed vegetative growth year after year from the same root system:

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Simple herbaceous perennials. Simple herbaceous perennials reproduce almost exclusively from seed and normally do not reproduce vegetatively. However, if the root system of these plants is injured or cut, each piece usually regenerates into another plant. Dandelion, plantain, and sulfur cinquefoil are examples of simple herbaceous perennials. Creeping herbaceous perennials. Creeping herbaceous perennials survive over the winter and produce new vegetative structures from asexual reproductive organs such as rhizo(Iles, tubers, stolons, bulbs, corms, and roots. These plants also reproduce sexually from seed. Most aquatic. weeds, except algae, are creeping perennial plants. Woody plants. This is a special category of perennial weed. Plants in this group are characterised by stems that have secondary growth, producing wood and bark, which results in an incremental increase in diameter each year. Some tree, some shrub, and many vine species are considered to be woody weeds.

Figure 1. Habitats of aquatic weeds

Classification by Habitat

Weeds can be classified according to where they grow.

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Most weeds are terrestrial, that is, tound on land, but some are restricted to the aquatic environment. Some weeds only infest a particular crop or cropping system, complex of plant communities, or growing condition. Therefore, it is common to find lists and descriptions of weeds that are usually found in particular environments, such as arable land, pastures and rangeland, forests, ' rights-of-way, or wildlands. Weeds can be classified by their habitat as follows: Aquatic weeds. Aquatic weeds are plants that are modified structurally to live in water. They have been categorised further based on their location in the aqueous environment. Floating weeds. These plants rest upon the water surface. Their roots hang freely into the water or sometimes attach to the bottom of shallow ponds or streams. Emergent weeds. These typical plants of natural marshlands are often found along the shorelines of ponds and canals. They stand erect and are always rooted into very moist soil. . Submerged weeds. Although a few floating stems or leaves may exist on the water surface, these plants grow completely under water. Some weeds and invasive plantS occur mainly in riparian habitats, along rivers, streams, or other watercourses. These terrestrial plants, such as Japanese knotweed (PolygQJ\um cuspidatum), Himalaya blackberry (Rubus armenicus), reed canarygrass (Phalaris arundinacea), and saltcedar (Tamarix spp.), require the frequent disturbance or high water table associated with rivers, streams, lakes, or ponds. These plants can alter the hydrology of an area and also reduce human access to areas where they occur. Physiological Classification

Plants differ in their responses to temperature, light, day

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length, and other factors of the environment. These differences in plant physiology and biochemistry have also been used as a basis for weed classification. Most plants, called C3 plants, use the Calvin-Benson cycle exclusively as a method of fixing carbon dioxide, water, and light energy into sugars. This terminology is used because the first stable product of photosynthesis in such plants (phosphoglyceric acid) has three carbon atoms. In some plants, called C 4 plants, the first stable photosynthetic products are four-carbon atom sugars, such as oxaloacetate, malate, and aspartate. This physiological distinction may not seem significant as a means of categorising weeds. However, these differences in photosynthetic pathway result in substantial biochemical, anatomical, and morphological variation among species. Because of these differences, C4 weeds are often more efficient at photosynthesis and can be more competitive than C3 weeds and crops, especially in hot, dry climates. Of the 18 worst weeds in the world noted by Holm et al., 14 have the C4 pathway of carbon fixation. Classification by Day Length

Classification by day length is based on a photoperiodic response of flower initiation in plants. Three distinct classes of day length response are known: sho~t day, long day, and day neutral. Although these responses are named for the length of the light period, it is now known that plants detect and respond to the length of the dark period (e.g., shortday plants are actually long-night plants). Weeds that have a short-day response to day length, such a-s lambsquarters (Chenopodium album) and cocklebur (Xanthium spp.), are stimulated to flower when days are short and maintain vegetative growth when days are long. Long-day weeds, like henbane (Hysocyamus' niger) and dogfennel (Eupatorium capillifolium), maintain

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vegetative growth when days are short but are induced to flower under long-day conditions. Other weeds (e.g., nightshades) remain vegetative or flower irrespective of the photoperiodic condition. Undesirability Classification

The term noxious weed is a legal term that refers to any plant species capable of becoming detrimental, destructive, or difficult to control. Legally, a noxious weed is any plant designated by a federal, state, or county government as injurious to public health, agriculture, recreation, wildlife, or property. Many states, provinces, and countries maintain at least one official list of such weeds so tbat their introduction can be prevented or restricted. Noxious weeds usually create a particularly undesirable condition in crops, forest plantations, grazed rangeland, or pastures. For example, the presence of noxious weed seed in seed crops can prevent the sale and distribution of that crop across national and international boundaries. Poisonous weeds, which can be landscape ornamentals or occur in pastures and rangeland, represent a special kind of undesirability, since they can be a direct threat to human or animal health. Ecological Classification

Weeds, and in particular invasive plants, are often classified using ecological categories related to population behavior. The flora includes many weeds, which may also be colonisers (taxa appearing early in vegetation succession) or naturalised species (exotic species that form sustainable populations without direct human assistance). By this classification scheme, invasive plants are a subset of naturalised species that are spreading. Not all naturalised taxa are invasive, however, nor are all colonisers considered to be weeds.

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Groves and Cousens and Mortimer divide the process of invasion by an exotic species into the phases' of introduction, colonisation, and naturalisation. These three phases of invasion are defined as follows: Introduction. As a result of dispersal, propagules arrive at a site beyond their previous geographical range and establish populations of adult plants. Colonisation. The plants in the founding population reproduce and increase in number to form a colony that is self-perpetuating. Naturalisation. The species establishes new selfperpetuating populations, undergoes widespread dispersal, and becomes incorporated into the resident flora. Richardson et al., however, argue that colonisation as used by Cousens and Mortimer is a component of naturalisation, and the term invasion should be distinguished from naturalisation and used to describe widespread dispersal and incorporation of an exotic species into the resident flora. Such differences of opinion on terminology pertaining to invasion will likely diminish as further knowledge is gained about the ecological processes involved. Weed species can be organised according to evolutionary strategies that are based on genetically determined patterns of carbon resource allocation. One prevalent theory holds that two fundamental external factors limit the amount of plant material (vegetation) that can accumulate within an area. These factors are stress and disturbance. When the extremes of these factors are considered, the following possible strategies of evolutionary development emerge: Stress tolerators. These are plants that survive in unproductive environments by reducing their biomass allocation for vegetative growth and reproduction and increasing their allocation to maintenance and defense.

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They exhibit characteristic!> -hat ensure the endurance of relatively mature individuals in Competitors. These are plants that have evolved characteristics that maximise the capt..lre of environmental resources in productive but relatively undisturbed conditions. These plants have extensive vegetative growth and are abundant during the early and intermediate stages of succession. Ruderals. Ruderals are plants that are found in highly disturbed but potentially productive environments. These plants are usually herbs, characteristically having a short life span, rapid growth, and high seed production. They occupy the earliest stages of succession. Most herbaceous weed species fall into one of two combined strategies, competitive ruderals or stress-tolerant competitors. Plants possessing the competitive ruderal strategy have rapid early growth rates and competition between individual plants occurs before flowering. Such plants occupy fertile sites and periodic disturb?l1ce (e.g., annual tillage) favors their abundance and distribution. Many annual, biennial, and herbaceous perennial weed species found on arable land fit the criteria for the competitive ruderal tactic. Stress-tolerant competitors are primarily trees or shrubs, although some perennial herbs also fall into this category. Common characteristics of these weeds are rapid dry-matter production, large stem extension, and high leaf area production. WEEDS AND INVASIVE PLANTS IN PRODUCTION SYSTEMS

Some weed species have even achieved worldwide prominence. Most weeds are important, however, from a more local perspective. The local distribution of weeds is influenced by biotic and abiotic environmental factors that determine habitat types and human activities. Abiotic factors that affect weed occurrence are soil type, soil pH,

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soil moisture, light quantity and quality, precipitation pattern, and variation in air, soil, and water temperatures. Disturbed areas also are higher in susceptibility to invasion than habitats that exist for long periods of time in late succession. Biological factors, such as the incidence of insects and diseases on either weeds or associated crops, grazing activities of animals, and plant competition, also can influence the distribution of weeds. It is for all of these reasons that human land uses, such as farming, forestry, range management, and recreation, are major causes of local and regional patterns of weed distribution. Plant species react in different ways when their habitats are disturbed by humans; some species flourish because of the disturbance, whereas others migrate or die and are replaced. Weeds and Agricultural Land

Weeds must have been known to early farmers because hoes and other grubbing" implements, artifacts of those ancient times, have been found at archeological sites. In addition, many references account for the detrimental effects of weeds on crop yields, from the early writings of Theophrastus and the Bible to more recent books. Even today, weeds are considered to be just an incidental part of food production in most parts of the world, where farmers are simply people with hoes. The use of modern mechanical and chemical tools to control weeds is actually little more than a century old, even though weeds have been associated with humans since agriculture began. Human action is the most important factor determining the occurrence and distribution of agricultural weed species. TMany agrestals that accompanied crops for centuries in Europe have now become locally extinct, retreating to their climatic optimum where most survive outside cultivated fields. Other we~ds have increased in both prominence and abundance as agricultural practices 1/

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change. Holzner and Immenon suggest several causes for such changes in weed species composition: Improved seed cleaning, which results in the local eradication of "specialists" that are unable to grow outside arable land and depend on being sown with the crop Abandonment of crops, which leads to loss of specialised weeds "Leveling" of environmental conditions, which results in a uniform weed flora . Increased reliance on crop monocultures, which tends to simplify the weed flora Combine harvesting, which allows some weed species to shed seed in the field and distributes the seed of others Reduced-tillage and "no-tillage" operations, which promote perennial species Reduced competitive ability of short-stature crops and crops treated with chemical growth regulators Extensive use of herbicides, which causes sensitive species to become locally extinct or to evolve resistance to the chemical Weed Control

A goal of agriculture for the last half century or more has been to develop efficient methods of weed control in crops, forest plantations, rangelands, and noncrdp situations. The search for cost-effective ways to control weeds has often focused on tillage and herbicides as a means to reduce labor requirements and production costs or increase yields. Below are some reasons to control weeds in cropland. The threat of weeds to crop productivity accounts for most of the human effort devoted to weed control. It is estimated that 10-15% of the total market value of farm products in the United States is lost because of weeds. This

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loss amounts to about $8 billion to $10 billion per year. Direct losses to forests and rangeland are more difficult to estimate than agricultural losses. Walstad and Kuch believe that nearly 30% reduction in wood productivity could result because of weed occupation during the early stages of forest plantation formation. The U.S. Forest Service estimates that about 3.5 million acres of National Forest System lands are infested with invasive plants. Weeds have a detrimental effect on crop quality as well as quantity, especially crops that must meet size, color, nutrient content, or contamination-free standards. For example, yields of alfalfa hay in California are often highest during the first cutting when annual weeds are present. However, hay quality is also low when weeds are present in the crop. For example, protein content can fall from over 20% to below 10% when the hay contains large amounts of weeds. Such decreases in grade or quality often mean lowered revenue for growers, since a premium price is usually paid for commodities of high quality. In some cropping systems, the crop seed and weed seed are so similar in weight and shape that separation at harvest is. difficult. Examples are alfalfa and dodder (Cuscuta spp.) seed, soybean seed and nightshade fruits, and pea seed that are mixed with the immature flowers of Canada thistle (Cirsium arvensis). If the weed material is not removed from these crops by screening, lower price for the commodity will result. For seed crops, the presence of a few noxious weed seed, even less than 1%, usually makes the commodity unmarketable. Weed control is a major reason for many cultural practices associated with crop production. For example, weeds are killed during plowing and cultivation (tillage) to prepare seedbeds for planting. A report by the U.S. National Research Council indicates that 92-97% of the acreage planted to com, cotton, soybean, and citrus are treated with herbicides each year. In addition 87% of all

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citrus acreage and 75% of potato and vegetable crops acreage in the United States are chemically treated for weed control. According to the US Environmental Protection Agency, 60% of the total pesticide sales in the United States in 1999 was for herbicides. There is no doubt that weed control is a costly endeavor in the production of most crops. Weeds also interfere with harvesting operations, often making harvest more expensive and less efficient. For example, weeds sometimes get wrapped around rollers or cylinders of mechanical harvesters, causing equipment breakdowns and longer harvest times. Up to 50% loss in efficiency and 20% loss of yield can result from weed presence at harvest time. Some weed species act as alternate hosts or harbor insects, pathogens, nematodes, or rodents thdt are crop pests. Numerous specific examples exist of various pest organisms that benefit from the presence of weeds. For example, aphids and cabbage root maggots live on wild mustard, later attacking cabbage and other cole crops. Nightshades are hosts of the Colorado potato beetle. Disease organisms, such as maize dwarf mosaic and maize chlorotic dwarf virus, use Johnsongrass (Sorghum halepense) rhizomes to overwinter. Black stem rust uses barberry (Berberis thunbergii), quackgrass (Agropyron repens), and wild oat (Avena fatua) as hosts prior to infesting cereal crops. Rodent damage to orchards can be prevented by weeding around trees before winter. It also is possible for weeds to aid in thE' prevalence or spread of certain bene-fidal organisms that are used to control other pests. In such cases, the weeds act as an alternate source of food or cover for the beneficial organisms, allowing them to survive when thE' preferred host is not available. Improve A nimal Health. Some weeds are poisonous to animals. However, plants toxic to one species of animal may be harmless to others. For example, larkspur (Delphinium spp.) will kill cattle if

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eaten in sufficient quantity, but sheep and horses are relatively unaffected by this rangeland weed. In contrast, fiddleneck (Amsinckia spp.) is highly toxic to horses, while other livestock are relatively tolerant of it. It is estimated that up to 10% of range-grazing livestock may become afflicted by poisonous plants at some time during each growing season: In addition to direct poisoning, animals may experience other discomforts from association with certain weed species. Some plants contain chemicals that make animals abnormally sensitive to the sun, a phenomenon called photosensitization. Other plants contain teratogenic materials that result in fetal malformations. For example, malformed lambs can result if false hellebore (Veratrum californicum) is ingested by sheep around the fourteenth day of gestation. Bracken fern (Pteridium aquilinum) causes a disease of cattle called "red water" because of the blood-colored urine that is its symptom. This weed causes cancer of the bladder if eaten in sufficient quantities. Weeds affect a number of human activities that are difficul t to assess in monetary terms. The presence of weeds can reduce real estate values because of the unkempt and unsightly appearance of the property. Dense moistureholding weed growth aids the deterioration of wooden and metal structures and machinery, further reducing property value. In fire-prone ecosystems, weeds can provide fuel to carry fire, further endangering structures and property. Access and enjoyment of recreation areas are also reduced by weed presence. Some rivers and lakes in the tropics and subtropics are clogged by aquatic weeds, making travel on them nearly impossible. Ross and Lembi provide an interesting example of how weeds influence transportation costs. They indicate that in 1969 and 1970,487,000 tons of wild oat seed were inadvertently transported from Canada to the United States . along with 16 million tons of grain. The transportation costs

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for the wild oat were estimated at $2 million, which did not include the $2 million cost for cleaning the grain to remove contamination. Weeds are kept free from highway intersections to prevent accidents. Airports and railways also keep signs and lights free of weeds so that maximum visibility can be maintained. Power line rights-of-way are kept free of tall growing vegetation to prevent power outages if trees contact power lines during storms and to increase access to downed power lines. Toxicants or irritants produced by weeds can cause serious health problems for some people. These discomforts or illnesses include hay fever, dermatitis, and direct poisoning. Hay fever afflicts millions of people each year. It is caused by an adVerse effect of proteins associated with the pollen of certain plants on the respiratory system of susceptible people. Ragweed is best known for causing hay fever. However, pollen from many other broadleaved plants, grasses, trees, and shrubs causes similar allergic reaction~. Each year, many people are troubled by poison ivy (Rhus radicans), poison oak (R. diversiloba), and poison sumac (R. vernix). These plants produce and store a toxic substance called urushiol that causes intense itching and rash upon contact with the skin. Many plants contain toxic substances that when ingested cause sickness or death to humans. Toxic substances in weeds include alkaloids, glycosides, oxalates, resins and resinoids, volatile oils, acrid juices, phytotoxins (toxalbumens), and minerals. There are few poisons, including synthetic substances and minerals, that approach the strength and violence of illnesses caused by some plant-produced toxins. Weeds in Forests

There are many natural conditions such as climate, soil type and fertility, topography, and events like hurricanes and wildfire that shape forested landscapes. Following

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"catastrophic" disturbances, it is common for forests to undergo a sequence of vegetation changes that result in a forest nearly identical to the one previously destroyed. This process of natural forest reestablishment through successive changes in vegetation composition is called secondary succession. Following a radical disturbance, like a fire or clearcut, a new patch in the physical environment is once again available for colonisation by plants. In such situations, "pioneer" tree (e.g., poplar, birch, alder, and some conifers) or shrub species are quick to colonise the disturbed areas and can dominate them for years to decades. This rapid recolonisation by usually native pioneer species, although a normal stage in succession, can delay the revegetation of disturbed sites with more economically desirable trees. The major disturbance to forests of any region is the harvesting of wood by humans. It was estimated in 1989 that each year the world loses 37 million acres of forest in this manner and current estimates remain unchanged. In temperate conifer forests logging, especially without any follow up reforestation activities, led to the gradual replacement of conifers by less desirable herbaceous, shrub or hardwood species. In Canada large-scale weed problems have occurred due to exploitation forestry, which strives to maximise profits and minimize costs. Weed problems were exacerbated by poor choice of forested stands to harvest, season and method of harvesting, intensity of utilisation, and lack of attention to regeneration. Walstad and his associates similarly indicated that hardwoods occupy 32% of the prime timberland in western Oregon that was once dominated by conifers. Forest Regeneration

Most forests regenerate naturally following disturbance given enough time. However, logging activities and land

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clearing are the principal disturbance factors that both set up and modify the natural patterns and time frames of succession so that native and exotic weed species are favored and even dominate many/forest types. The ability of a site to regenerate, as well as ihe composition of species following such disturbances, is most dependent on the type, frequency, and severity of the tree removal operation. In the coastal Douglas-fir forests of the U.S. Pacific Northwest, the impact of both native and exotic plants is currently restricted to the earliest stages of forest succession that follow logging and fires. Ruderal exotic forbs, such as Canada thistle or woodland groundsel (Senecio jacobaea), and some exotic shrubs, such as Scotch broom (Cytisus scoparius), displace native early seral vegetation in some locations and reduce tree regeneration in others. Though exotic plants are typically eliminated from the plant community after a few years to a decade of forest stand development", exotic shade-tolerant species are capable of persisting and/or invading forest understories if relatively open stand conditions are maintained through clearcutting or severe silvicultural thinning. In particular, false-brome (Brachypodium sylvaticum) poses a serious threat to forest understory communities in that region. Several techniques, collectively known as artificial regeneration, have been used successfully to replant many logged-over areas in many countries. This method usually involves collecting seed of preferred tree species, germinating and growing the seedling trees in nurseries, outplanting them to field sites, and following this by intensive chemical weed control. Wagner et al. (2006), surveying 60 studies, found that the most intensive vegetation management treatments always improved crop tree growth, although results varied by location, tree species, and length of time from experiment initiation. Despite these successes in projected crop tree biomass yield, important questions still remain about the ecological

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(Balandier et al. 2006), social, and economic desirability of converting vast acreages of naturally regenerated forests into tree farms. Weeds in Rangelands

The destruction and replacement of vegetation by humans are now common occurrences over most of the world, with a loss in primary productivity and floristic diversity often being the result. The invasion of exotic plants is both a cause and a consequence of such environmental manipulation. However, it is rare that invaders cause the replacement of most or all of the plant and animal species in a disturbed ecosystem. A possible exception to this generalisation is rangeland weeds. In this system of production, species replacement following disturbance has been so complete that only a sketchy picture of predisturbance conditions remain. We offer the sagebrush (Artemisia tridentata)-cheatgrass (Bromus tectorum) steppe as an example. The chance introduction of cheatgrass before the turn of the last century to the Great Basin of North America altered the entire native shrub ecosystem of that region. D' Antonio and Vitousek after Billings indicate that its introduction provides a classical case of biological impoverishment where the concomitant environmental change allows successful replacement of indigenous vegetation. In this case, native perennial bunchgrasses and shrubs, particularly sagebrush, were first grazed by large herbivores, then invaded by cheatgrass, and subsequently subjected to range fires. Original Vegetation and Early Land Use History of Great Basin. Billings and others indicate that the western Great Basin was not part of the bison range of the North American Great Plains because the rhizomatous Col grasses on which the bison thrived cannot grow on the summerdry steppes of this region.

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Rather, perennial C3 bunchgrasses of the genera Poa, Festuca, Agropyron, and Stipa dominated the grass stratum of this sagebrush ecological formation. Apparently, the native bunchgrasses of the region also did not carry fire well because range fires in the sagebrush-bunchgrass steppe, in contrast to the Great Plains, were rare. The native ungulate herbivores were antelope, deer, desert bighorn sheep, and elk which, because of their smaller size and numbers than bison, created a relatively light impact on the sagebrush-grass community. WEED AND INVASIVE PLANTS IN LESS MANAGED HABITATS

Certain forests, deserts, prairies, beaches, marshes, estuaries, and riparian areas have been protected from disturbance or designated as wilderness throughout the world. Wilderness and similarly managed natural areas, such as national parks and monuments, provide many benefits to society. These benefits include the preservation of biodiversity, unique natural features, and watersheds as well as opportunities for recreation and personal fulfillment. Although land management agencies place a high priority on protection of natural ecosystems and wilderness areas, some of these benefits are threatened by increasing levels of human activity within and outside areas designated for protection. The introduction of exotic species into such areas is of particular concern due to the potential for irreversible impacts on the natural ecosystems that such areas represent. Three research areas were identified by the Aldo Leopold Wilderness Research Institute to address the question of exotic plant invasion into wilderness: Understanding the introduction, spread, and distribution of exotic species within wilderness Understanding the effects of exotic species on wilderness values

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Identifying and evaluating management options and their consequences Parks and her associates) examined the patterns of invasive plant diversity in mountainous ecoregions of the northwestern United States. Their analysis found that altered riparian systems an~ disturbed forests were especially vulnerable to exotic plant invasion. Conversely, alpine areas, forests, and grasslands designated as wilderness were still relatively unaffected by invasive plants, with introductions often being restricted to campsites, roads, or trails. The predominance of wilderness throughout much of the western United States is believed to contribute to the lower incidence of invasive plants in mountainous ecoregions of that area compared to other regions. Human settlement and intense land use at low elevations were identified as factors that enhance invasive plant introductions. There are many examples of the widespread regional or even global distribution of weeds. One of the earliest examples is that of Hitchcock and Clothier which describes the distribution of native and introduced weeds in Kansas as that land was being developed for agriculture. A similar study was accomplished by Mason, who described the occurrence of wild oat throughout several provinces of central Canada. These studies are augmented by more recent descriptions of widespread infestations of weed species, for example, leafy spurge (Euphorbia esula), purple loosestrife (Lythrum salicaria), downy brome (also known as cheatgrass) (B. tectorum), Paterson's curse (Echium platagineum), and lantana (Lantana camara). The ability to disperse widely is a common characteristic of many weed and invasive plant species, which has been exacerbated in recent decades by increasingly global movement of humans and goods.

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Any harmful organism that is spreading or has the capacity to spread poses a threat to uninfested areas without regard for ownership boundaries. Thus, a spreading species represents a problem to more people than just those whose land it currently occupies. Such situations make a strong case for legislation (weed laws), quarantine districts, or other governmental interventions to reduce or slow the spread of weeds and invasive plants. Furthermore, governmental objectives for weed suppression may be less constrained by cash flow than those of individual farmers, rant.pers, or forest land owners. REFERENCES

Baskin, Yvonne. A Plague of Rats and Rubbervines: The Growing Threat Of Species Invasions. Island Press, 2003. Burdick, Alan. Out of Eden: An Odyssey of Ecological Invasion. Farrar Straus and Giroux, 2005. Coates, Peter. American Perceptions of Immigrant and Invasive Species: Strangers on the Land. University of California Press, 2007. Elton, Charles S. The Ecology ofInvasions by Animals and Plants. University of Chicago Press, -2000. Janick, Jules. Horticultural Science. San Francisco: W.H. Freeman, 1979. Lockwood, Juli~; Martha Hoopes, Michael Marchetti Invasion Ecology. Blackwell Publishing, 2006. McNeeley, Jeffrey A. The Great Reshuffling: Human Dimensions Of Invasive Alien Species. World Conservation Union (IUCN), 200l. Van Driesche, Jason; Roy Van Driesche Nature Out of Place: Biological Invasions In The Global Age. Island Press, 2004. Terrill, Ceiridwen. Unnatural Landscapes: Tracking Invasive Species. University of Arizona Press, 2007.

10 Phage Ecology and Plants Bacteriophages (phages), potentially the most numerous "organisms" on Earth, are the viruses of bacteria, more generally, of prokaryotes. Phage ecology is the study of the interaction of bacteriophages with their environments. Phage ecology is increasingly an important component of sessions and symposiums associated with phage meetings as well as general microbiological meetings. Phages are obligate intracellular parasites meaning that they are able to reproduce only while infecting bacteria. Phages therefore are found only within environments that contain bacteria. Most environments contain bacteria, including our own bodies. Often these bacteria are found in large numbers. As a rule of thumb, many phage biologists expect that phage population densities will exceed bacterial densities by a ratio of 10-to-1 or more. As there exist estimates of bacterial numbers on Earth of approximately 1030, there consequently is an expectation that 1031 or more individual virus particles exist, making phages the most numerous category of "organisms" on our planet. Bacteria appear to be highly diverse and there possibly are millions of species. Phage-~cological interactions therefore are quantitatively vast: huge numbers of interactions. Phage-ecological interactions are also qualitatively divers\.!: There are huge numbers of

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environment types, bacterial-host types, and also individual phage types. The phage consists of a nucleic acid core that is made up, depending on the phage, of DNA or, less often, of RNA. Surrounding this nucleic-acid genome is '.a proteinbased capsid. The capsid plays three important roles in the phage life cycle: (i) protecting the phage genome during the extracellular search; (ii) effecting phage adsorption, which is the attachment of the virion particle to a susceptible bacterium; and (iii) the subsequent delivery of the phage genome into the cytoplasm of the now-infected bacterium. The extracellular search occurs via phage diffusion through an aqueous milieu. During this period the phage must avoid physical damage while waiting to encounter a susceptible bacterium. The likelihood that an individual phage will find a bacterium to adsorb is a function of time, the phage diffusion rate, and the local density of phagesusceptible bacteria, with more bacteria resulting in faster phage adsorption. A slightly different set of parameters governs the likelihood of phage attack on bacteria, with adsorption a function of time and the phage diffusion rate, but also of phage density, with more phage resulting in more bacteria infected.

Figure 1. Structure of Bacteriophage

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Phage adsorption to bacteria furthermore is a functIon of phagebacteria chemical and physical interaction. Phage display proteins with high affinity to specific bacterial surface molecules-an association analogous to antigen recognition by immune systems. The host range of most phages, Le., the species that they are capable of productively infecting, consequently is relatively narrowtypically limited to only a single bacterial genus, species, or, often, even to onsly a limited number of strains within a given species. Thus, while total phage densities can be enormous-as many as 100 million or more per gram of soil or m1 of aquatic environment the actual density of phages capable of infecting a particular bacterial strain usually is much smaller. Following uptake, the phage genome can rapidly subvert host-cell functions, directing bacterial metabolism during the phage latent period towards phage production. 'Oepending on the phage, these virions either may ,accumulate within the bacterial cytoplasm or, for filamentous phage, virions instead are released-over the course of an extended latent period- across an otherwise intact cell envelope. PHAGE ECOLOGY

Phage ecology is the study of the interaction of phage with their biotic and abiotic environments. The scale of phage ecology is at once both exhilarating and intimidating. As a guiding principle toward understanding phage ecology we therefore seek generalizations, plus look to more established scientific discipiines for guidance, the most obvious being general ecology. Toward that end we can speak of phage "organismal" ecology, population ecology, community ecology, and ecosystem ecology. Phage ecology from these perspectives will be described in turn. Phage ecology also may be considered (though mostly less well formally explored) from perspectives of phage

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behavioral ecology, evolutionary ecology, functional ecology, landscape ecology, mathematical e~ology, molecular ecology, physiological ecology and spatial ecology. Phage ecology additionally draws from microbiology, particularly in terms of environmental microbiology, but also from an enormous catalog of study of phage and phage-bacterial interactions in terms of their physiology and, especially, their molecular biology. Following the traditions of ecology we can differentiate phage ecology into four general categories: Phage organismal ecology is the study of the adaptations that phage employ to increases their likelihood of transmission between hosts such as virion desiccation resistance, ability during infection to repair ultraviolet (UV) light-mediated nucleic-acid damage, and so on. Phage population ecology is the study of phage lifehistory characteristics, particularly as they apply to phage growth and intraspecific (between-phage) competition. U~derstanding phage population growth within spatially structured environments, such as within the phyllosphere or rhizosphere, is particularly challenging. Phage community ecology focuses on the stability of phage-containing environments such as the propensity of phage to drive phage-sensitive bacteria to extinction. Phage community ecology is complicated by the continuous co-evolution of bacteria with their phage predators. In addition, phage play important roles in the horizontal transfer of DNA between bacteria. Phage ecosystem ecology considers the phage impact on energy flow and nutrient cycling within ecosystems. Phage, for example, can disrupt the soil bacteria responsible for nitrogen cycling.

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Phage Organismal Ecology

Phage "organismal" ecology is primarily the study of the evolutionary ecological impact of phage growth parameters: latent period, plus eclipse period rise period burst size, plus rate of intracellular phage-progeny maturation adsorption constant, plus rates of virion diffusion virion decay (inactivation) rates host range, plus resistance to restriction resistance to abortive infection various temperate-phage properties, including rates of reduction to lysogeny rates of lysogen induction the tendency of at least some phage to enter into (and then subsequently leave) a not very well understood state known (inconsistently) as pseudolysogeny. Another way of envisioning phage "organismal" ecology is that it is the study of phage adaptations that contribute to phage survival and transmission to new hosts or environments. Phage organismal" ecology is the most closely aligned of phage ecology disciplines~ith the classical molecular and molecular genetic analyses of bacteriophage. From the perspective of ecological subdisciplines, we can also consider phage behavioral ecology, functional ecology, and physiological ecology under the heading of phage "organismal" ecology. However, as notEi!d, these II

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subdisciplines are not as well developed as more general considerations of phage "organismal" ecology. Phage growth parameters often evolve over the course of phage experimental adaptation studies. In the mid 191Os, when phage were first discovered, the concept of phage was very much a whole-culture phenomenon, where various types of bacterial cultures (on solid media, in broth) were visibly cleared by phage action. Though from the start there was some sense, especially by Felix d'HereIle, that phage consisted of individual "organisms", in fact it wasn't until the late 1930s through the 1940s that phage were studied, with rigor, as individuals, e.g., by ~lectron microscopy and single-step growth experiments: Note, though, that for practical reasons much of "organismal" phage study is of their properties in bulk culture (many phage) rather than the properties of individual phage virions or individual infections. This, somewhat whole-organismal view of phage biology saw its heyday during the 1940s and 1950s, before giving way to much more biochemical, molecular genetic, and molecular biological analyses of phage, as seen during the 1960s and onward. This shift, paralleled in much of the rest of microbiology, represented a retreat from a much more ecological view of phages. However, the organismal view of phage biology lives on as a foundation of phage ecological understanding. Indeed, it represents a key thread that ties together the ecological thinking on phage ecology with the more "modern" considerations of phage as molecular model systems. The basic experimental toolkit of phage "organismal" ecology consists of the single-step growth (or one-step growth; example) experiment and the phage adsorption curve (example). Single-step growth is a means of determining the phage latent period (example), which is approximately equivalent (depending on how it is defined)

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to the phage period of infection. Single-step growth experiments also are employed to determine a phage's burst size, which is the number of phage (on average) that are produced per phage-infected bacterium. The adsorption curve is obtained by measuring the rate at which phage virion particles attach to bacteria. This is usually done by separating free phage from phage-infected bacteria in some manner so that either the loss of not currently infecting (free) phage or the gain of infected bacteria may be measured over time. Phage Population Ecology

A population is a group of individuals which either do or can interbreed or, if incapable of interbreeding, then are recently derived from a single individual. Population ecology considers characteristics that are apparent in populations of individuals but either are not apparent or are much less apparent among individuals. These characteristics include so-called intraspecific interactions, that "is betwee~ individuals making up the same population, and cah include competition as well as cooperation. Competition can be eith~r in terms of rates of population growth or in terms of retention of population sizes. Respectively, these are population-density independent and· dependent effects. Phage population ecology considers issues of rates of phage population growth, but also phage-phage interactions as can occur . when two or more phage adsorb an individual bacterium. Phage Community Ecology

A conuriimity consists of all of the biol9gical individuals found within a given enviroIiment, particularly when more than one species is present. Community ecology studies those characteristics of "Communities that either are not

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apparent or which are much less apparent if a community consists of only a single population. Community ecology thus deals with interspecific interactions. Interspedfic interactions, like intraspecific interactions, can range from cooperative to competitive but also to quite antagonistic. An important consequence of these interactions is coevolution. The interaction of phage with bacteria is the primary concern of phage community ecologists. Phage, however, are capable of interacting with species other than bacteria, e.g., such as phage-encoded exotoxin interaction with animals. Phage therapy is an example of applied phage community ecology. Phage Ecosystem Ecology

An ecosystem consists of both the biotic and abiotic components of an environment. Abiotic entities are not alive and so an ecosystem essentially is a community combined with the non-living environment within which that ecosystem exists. Ecosystem ecology naturally differs from community ecology in terms of the impact of the community on these abiotic entities, and vice versa. In practice, the portion of the abiotic environment of most concern to ecosystem ecolOgists is inorganic nutrients and energy. Phage impact the movement of nutrients and energy within ecosystems primarily by lysing bacteria. Phage can also impact abiotic factors via the encoding of exotoxins. Phage ecosystem ecologists are primarily concerned with the phage impact on the global carbon cycle, especially within the context of a phenomenon know as the microbial loop. PHAGE TEMPERANCE AND VIRULENCE

Population and organismal ecologies are concerned with· the adaptations that organisms employ to enhance their·

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Darwinian fitness over the course of their life cycles. For phage, the life-cycle steps most under their control are the durability of the yirion particle, the breadth of the host range, and the details of the infection strategy. In this section we take a population-ecology approach to contrasting two infection strategies: temperance versus virulence. Subsequently we consider the impact of plants on lysogeny. For lytic phage, progeny release can occur only following the total destruction (lysis) of the host bacterial cell. One can differentiate lytic phage into two types, temperate phage and obligately lytic (virulent) phage. Only temperate phage can display lysogeny, an infection that stalls shortly after the introduction of the phage genome into the host cell, which then (in most cases) integrates as a prophage into the bacterial chromosome. During lysogeny phages neither produce virions nor lyse bacteria. A temperate phage does not obligately enter into a lysogenic relationship with its host bacterium; in fact many temperate phage infections result in the immediate production of phage progeny, i.e., a lytic cycle rather than lysogeny. This decision is determined by characteristics of the infecting phage and the metabolic state of the host. When not induced, a phage in the lysogenic state replicates as a giant gene complex along with the host cell's genome. Lysogeny typically results in bacterial resistance to infection by similar phage. Mutant phage that are able to bypass this resistance are described as vir mutants. One advantage that can be associated with such virulence is an ability to actively infect homologous lysogens (bacteria lysogenised by the same phage), though establishment of this Vir phenotype can require multiple phage mutations. Note, though, that a diversity of virulent phage exist that are not vir mutants but instead are unrelated to temperate phage. In addition to providing a safe home to the temperate-phage genome, and blocking

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the replication of non-virulent homologous phage, lysogeny has the potential to alter the phenotype of the host cell, a process known as phage (or lysogenic) conversion. PHAGE LYSOGENY

What advantages are bestowed upon a phage that displays a temperate lifestyle? We can describe obligately lytic phage as essentially semelparous, with the acquisition of a bacterial cell resulting in only a single reproductive episode. As such these phage are perhaps best understood as adapted to a so-called r strategy of population growth, with a life-history emphasis on rapid population increase when resources are plentiful. Aiding in this strategy is their (i) avoidance of physiological tradeoffs required for the display of lysogeny, (ii) a commitment of all progeny to lytic growth (rather than some fraction to lysogen formation), or (iii) an ability to infect lysogens. These advantages, however, come with requirements for long-term survival as free phage when bacteria are less numerous, and/or dissemination to new environments to find new bacterial hosts. Temperate phage can also display the semelparous infection strategy of virulent phage. A fraction of infections, however, will instead result in lysogeny. Lysogeny represents an iteroparity of sorts, i.e., more than one reproductive episode per lifetime, at least so long as one is willing to accept a clonally related population of lysogtt?S as a single individual and a sporadic induction of lytic cycles within this population as consecutive repro.ducti'l:~ events. Such a life-history approach is the more K-like strategy whereby temperate prophage, by less-rapidly killing off their bacterial resource, may more readily sustain their population size at an environment's carrying capacity. Filamentous phage similarly display a more semelparous life-history strategy than virulent phage, with (i) a longer-

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term maintenance of the host infection, (ii) only a few phage progeny released over a given interval, and (iii) with multiple intervals over which these phage progeny are released. Though lysogeny often is framed as an adaptation to survival within relatively unstable environments, i.e., during "hard times", one could similarly argue that lysogeny bestqws-a competitive advantage on phage that is useful when bacterial populations are relatively stable, i.e., not fluctuating in size. Lysogeny thusly can be an effective K strategy so long as lysogens are not being actively killed by lytic phage. Exposure to . lytic phage may be less likely in wellstructured environments that limit phage diffusion and in which lysogen--nrlcrocolonies exist as relatively few cells, with low lysogen number minimising the potential for temperate-phage mutation to anti-lysogen virulence. Of course, arguing that wellstructured environments can favor lysogeny over virulence is nearly a restatement of the "hard times" hypotheSIs .whereby the "long periods of dearth" required for this dominance is posited to be a consequence of environmental structure rather than solely of temporal variation in resource availability. Lysogeny and Plants

Plants can induce pacteriallysogens (that is, cause them to initiate their lytic cycle), a strategy that plants could employ towards the elimination of bacterial pathogens. An extract of inulberry leaves could induce lysogens of Pseudomonas syringae. It may be· that these induced prophage are "abandoning ship" in response to the plant's release of antibacterial compounds, with the phage simply following an evolutionary algorithm based on the logic that it is better to take one's chances as a free phage than to continue to infect a dying bacterium.

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Also consistent with a shift away from lysogeny in the lysogeny-lytic cycle balance, Menzel et al have reported a negative impact of certain plant-growth regulators and herbicides on the initial establishment of lysogeny. For many bacteria, plant association marks not a high likelihood of plant-induced bacterium death but, instead~ a period of effective bacterial growth. Given heterogeneous bacterial populations it could be advantageous for temperate phage to lyse host bacteria when times are good since not only are healthy, uninfected bacteria potentially present, but those bacteria also could contain nonhomologous prophage capable of infecting and then lysing their own uninduced lysogens. That is, more effective bacterial growth could tip an environment from conditions that disfavor phage lytic growth and favor lysogen survival (Le., environments in which it is better, from the phage perspective, to be a bacterium) to conditions that favor lytic growth and thereby disfavor a continued display of lysogeny. PHAGE RHIZOSPHERE

From the phage perspective much of what is of interest in understanding the phage-plant interaction has to do with the extracellular search, a province of phage organismal ecology. How exactly do phage manage to find new bacteria to infect before succumbing to the ravages of environmentally induced virion decay? Answering this question within the rhizosphere-the region consisting of plant roots and surrounding soil-involves examining the impact of soil structure and chemistry on the mobility and survival of free phage and their hosts. In most crop environments, with a few notable exceptions, most of the time the soil is only partially hydrated. The lack of a continuous aqueous phase greatly complicates predictions regarding the diffusion of free phage, as for any soil colloid. This situation is complicated

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ftu$er by the propensity of free phage to become trapped within biofilms or reversibly adsorbed to particles, such as clays, that are commonly found in the soil. This non-specific and often reversible phage adsorbtion within soils is a function of sorbent and virion surface chemistry, virion size, and pH. Moreover, acidic soils can permanently inactivate free phage. We can only speculate whether these phage-soil interactions help or hinder the phage in their search for a suitable host. Since sorption to solid substrates lowers the diffusion rates of free -phage, it localises them to specific spatial regions, and thereby limits the maximum rate at which they can encounter a suitable host cell. On the other hand, adsorption to clays has been suggested to ,have a protective effec~- by holding phage within 'a hydrated environment. Both states are essentially in opposition only within undisturbed soil. ' If the soil itself is disseminated, becomes fully hydrated, or is otherwise well mixed, then associated phage, whether free or bound, may be disseminated. Soil-adsorbed phage thus could serve as a viable infectivity pool that is tapped only as bacteria grow, diffuse, or swim into the phage vicinity, or if the soil particle i:self is transferred into or onto a bacterium-containing environment. Phage can attack bacteria directly associated with plant roots. Given the close proximity of roots to soil it seems obvious that phage attack must occur via phage diffusion from either surrounding soil or from neighboring roots . . Rhizosphere bacteria, however, may gain an upper hand in what likely is a constant battle between the phage predator and bacteria prey by some combination of: (i) relatively' low viable counts of phage capable of infecting specific bacteria; (ii) relatively low rates of phage diffusion within soil, particularly under drier conditions or following phage adsorption to ~oil;

Plant Ecology

(iii) relatively high rates of free-phage inactivation within

soil; and (iv) physical (or spatial) refuges that provide bacteria with a degree of physical protection from phage. Indeed, one can envisage phage replication as equivalent to a nuclear chain reaction with anything damping the mobility or production of the phage "neutron" serving to limit depletion of the bacterial "fuel": Significant bacterial depletion occurs only once bacteria first have achieved a critical "mass". Phage researchers have significant experience handling phage within environments containing relatively low phage densities-and in which diffusion (and mixing) is limited-since these are the conditions under which phage growth typically occurs in solid media. Solid-media growth involves mixing a small number of phage with a large excess of bacteria, which is then poured into a thin layer over a regular plate of agar growth medium. The soft-agar -ih the top layer impedes phage an~ bacterial diffusion while the bottom agar layer maintains a relatively constant chemical and physical state for bacterial and phage growth. Within the soft agar layer, phage populations grow as plaques, which are expanding regions of phageinduced bacterial lysis, each originating from a single phage infection. Plaques appear transparent or translucent against the background of the typically more-opaque bacterial lawn growing within the agar-based substrate. Of particular relevance to understanding host-phage ecology within the rhizosphere therefore is determination of the degree to which phage growth in soil systems approximates the better understood solid-phase phage growth in laboratory media. Though within agar plaquedevelopment theory as well as techniques for plaquegrowth quantification have been fairly well developed, to our knowledge similarly finescale investigation has not been attempted within a soil-based medium.

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Individual soils likely vary' spatially and temporally with regardto plaque-like growth-particularly as a function of soil composition, degree of hydration, density and physiological state of host bacteria, and rates of free-phage inactivation. Nevertheless, we predict that the basic principle of phage solid-phase population growth, i.e., a phagediffusion mediated expanding sphere of bacterial infection, could still apply. Thus, without active mixing of soil, e.g., via the action of invertebrates or other localized soil disruptions, we speculate that bacterial microcolonies within the rhizosphere may display periods of boom or bust with regard to phage attack, with increasing microcolony size, or mere time, increasing the likelihood of phagemicrocolony encounter. Infection of one bacterium within a localised bacterial clone could result in the destruction of part or all of a genetically homogeneous bacterial, microcolony. Means by which such coordinated attack may be thwarted could include: (i) variation in the physiological or anatomical state ot the bacteria making up a clone, so that not all bacteria are equally susceptible to phage attack (including the formation of spores for those species that are able to, as well as hyphael aging for streptomycetes; (ii) display of motility such that bacteria progeny minimise co-location and thereby avoid serial infection (though with the caveat that active movement through soil might increase the likelihood that individual bacteria encounter phage); and (iii) sequestration away from the soil such as within root nodules colonised by rhizobia or perhaps following bacterial penetration into plants upon infection. Rarely encountering microcolonies could be antithetical to the prosperity or even survival of obljgately lytic phage, but

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could provide numerous stable, otherwise phage-free niches for temperate-phage survival as bacteriallysogens. PHAGE PHYLLOSPHERE

How phage interact with their bacterial hosts in the phyllosphere, the aerial plant structures, is even less understood than the phage ecology of the rhizosphere. The phyllosphere presents a less hospitable environmentrelative to the rhizosphere-given the exposure to UV, intense visible light, and desiccation that is likely on many above-soil plant surfaces. In the case of phages of the plant-pathogen Erwinia amylovora, some studies have noted that phage are less readily isolated from the aerial portions of trees, even during times of active infection by the host. By contrast, phage could almost always be isolated from the soil around infected trees. That would suggest that the phage reservoir is located in the soil, possibly with phage multiplying on stray bacteria that fall from the tree to the ground below. Other phage studies on the same host bacterium, however, have found abundant E. amylovora phages in the phyllosphere of infected trees. If the virion reservoir is the soil for at least some phage that attack plant epiphytes, then how do phage reach the phyllosphere of a tree, which may have foliage 3 or more feet off the ground? One possible explanation is that phage invade the phyllosphere upon plant germination and then remain a part of plant normal flora. Alternatively, phages could move from plant to plant within the phyllosphere with soil remaining a phyllosphere phage sink rather than source habitat. The habitats of otherwise plant-associated phage can range beyond plants or plant-associated soil. Irrigation waters and agricultural drainage are known to contain phage capable of inf~cting plant-associated bacteria. Erwinia phage have been isolated from lakes as well as

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from sewage. The latter phage perhaps display broad host ranges, attacking enteric bacteria associated with humans as well as the bacteria associated with plants. Perhaps similarly, phage infecting phytopathogenic Pseudomonas have been isolated from sewage while Agrobacteriuminfecting phage have been isolated from feces. Erwinia phage have also been isolated from a com flea beetle. This association between phage and insect is of interest since arthropods are known vectors for viruses that directly infect plants. However, the degree to which arthropods are responsible for phage transmission remains an open question. Indeed, phage movement within and between plants as well as between soil and plants presumably follows paths similar to those employed by bacteria, i.e., carriage by animals, dust, soil, seeds, and water, including splashing caused by hard rain, plus various human activities such as pruning. A case can be made that rain, by promoting epiphytic bacterial growth, can simultaneously supply phage with (i) healthy hosts, (ii) water in which to diffuse between bacteria, and (iii) a means of disseminating about individual plants .as well as among populations of pla:nts. PHAGE AND AGRICULTURE

Though plants are surrounded by phage, the vast bulk of the phage impact on plants is mediated through plantassociated bacteria. Plant-associated symbiotic bacteria can range from helpful (mutuals) to harmful (pathogens), and the phage impact on bacteria also can range from mutualistic to parasitic. Despite these complications, the phage impact, either negative or positive on plants, tends to be limited (i) to phage-induced bacterial lysis, (ii) to selection for phageresistance within bacterial communities, or (iii) to phageassociated modification of bacterial phenotypes (phage conversion). These we consider in order:

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There have been a number of reports of phage presencE!, in relatively small-scale experiments, that result in reduced plant growth or reduction in plant nitrogen content. 'Experimentally induced decline in the plant-protective bacterium, Pseudomonas fluorescens, has also been noted. However, it is uncertain how often wild phages negatively impact on plant growth under naturally occurring conditions. We speculate that extrapolation of observations from small to large scales is challenging due to ignorance of naturally occurring phage-bacteria dynamics and spatial heterogeneities that exist across large plots. Indeed, we are aware of only one report suggesting that phage may have obstructed plant growth on a large scale in a non-experimental agricultural setting, as mediated by reductions in soil rhizobia. Further exploration of this "extrapolation" issue could be difficult assuming reluctance to conduct large-scale field tests of phage that are antagonistic to beneficial bacteria. It is typically assumed that reduced plant growth correlated with phage presence is a consequence of . phage antagonism against beneficial bacteria, e.g., phage-induced lysis. Lytic phage can also indirectly reduce the fitness of susceptible bacteria. This fitness reduction can be manifest either as a density decline of bacteria inhabiting specific niches, or by a decline only of susceptible bacteria, the latter suggesting a replacement of susceptible bacteria by similar but phage-resistant bacteria. The above-cited Demolon and Dunez study, for example, has been much discussed in the plantphage literature, ,with some authors concluding that the observed negative phage impact resulted from selection for a phage-resistant bacterial phenotype that was less effective at nitrogen fixation. This interpretation is consistent with more modern

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phage community-ecology theory, which posits that bacterial resistance to phage attack often comes at some metabolic cost to bacteria. Consequently, a bacterium. that does not display phage resistance may bEt ablEtio invade and even drive to extinction a phage-resistant population of otherwise-identical bacteria, at least so long as phage are not present. From a plant's perspective, this change in bacterial prevalence upon phage attack is irrelevant unless phagesensitive and phageresistant bacteria display differences in their ability to interact with plants. Such differences are often observed, though it is important to note that for many studies only a fraction of bacterial- mutations to phage resistance result in significant change in plant-interaction phenotypes. Phage T4- and phage fEC2-resistant, mostly lypopolysaccharide (LPS)-defective mutants of the soft-rot Erwinias E. carotovora and E. chrysanthemi, for example, generally do not display a loss of virulence. Given a relative rarity of avirulence in phage-resistant mutants, an invasion of phage into a pathogen population and subsequent selection for phage-resistant variants may not impact negatively on the virulence of the surviving bacteria. Rather, the community impact may be seen as a decHpe of the pathogen population followed by recovery by similarly virulent but phageresistant bacteria. Phage-resistant mutants of Ralstonia solanacearum, which display various defects in LPS synthesis, were predominantly avirulent in tobacco seedlings, as were approximately 50% of selected phage-resistant Xanthomonas campestris mutants. Mutational phage resistance resulting from a loss of pili also has been shown to reduce Rhizobium nodulation in clover. Mutation to phage-resistance by P. fluorescens similarly can result in reduced protection of radishes from Fusarium. wilt. The mechanism by which this loss occurs, however, appears to

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be not so much from a decline in P. fluorescens colonising ability as due to insufficient induction of cross-reactive systemic resistance by plants. Like phage resistance, modification of bacterial phenotype can result from phage conversion, Le., the expression of prophage genes over the course of lyso-genic infection. Little is known of phage conversion positively affecting plant growth. There exists some evidence for the converse, however: lysogeny negatively impacting soybeanBradyrhizobium japonicum interaction. An example of phage conversion that could possibly be interpreted as positively affecting plant fitness, by poisoning a plant predator, occurs in the case of toxic annual ryegrass, Lolium rigidum. The developing seeds of this grass are susceptible to infection and gall formation by a nematode, Anguina funesta. If these nematodes are carrying certain strains of the bacterium Clavibacter toxicus, then the galls will contain corynetoxin. Related to tunicamycin-like antibiotics, corynetoxin inhibits protein glycosylation and can be fatal to graz~ng animals (Le., sheep) that consume the infected grass. Some phages are associated with the ability of C. toxicus to produce corynetoxin. It is not clear how the phage causes the production of the toxin, as the complicated glycolipid structure of corynetoxins would require an elaborate metabolic pathway to be expressed from the phage genome. However, associations between phage and bacterial virulence factors are quite common, being responsible for much of the virulence associated with such important human pathogens as Corynebacterium diphtheria, Escherichia coli, and Vibrio cholerae. Note that the likelihood of phage conversion of non-toxogenic C. toxicus strains into toxigenic strains seems to be enhanced via the . application various herbicides at the same time as phage exposure. Also of interest, phage Xf and Xf-2 infection of

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X. campestris results in an increase in this bacterium's virulence towards rice. PHAGE THERAPY

Phage have been proposed as plant-pathogen control agents in a process known as phage therapy: the application of specific phages to specific ecosystems in order to reduce the population size of specific bacteria. That is, phage therapy is a form of biological control-the use of one organism to suppress another. Like other methods of biological control, one advantage of phage therapy is a reduction in the usage of chemical agents against pest species, which, in the case of phage, means a reduction in the usage of chemical antibiotics. Phage therapy was ~xPlored extensively by early phage workers as a means of controlling plant pathogens. Circumstances in which phage therapy of plants or plant products has been attempted include against Salmonella associated with freshcut fruit, to disinfest Streptomyces scabies-infected potato seedtuber, against bacterial leaf spot of mungbeans, against Xanthomonas pruniassociated bacterial spot of peaches, to control of X. campestris infections of peach trees as well as cabbage and pepper diseases, to control Ralstonia solanacearum, and to control soft rot and fire blight associated with Erwinia. Phage therapy has been used successfully against bacterial blotch of mushrooms caused by Pseudomonas tolaasii. In studies notable for the employment of phage hostrange mutants, phage therapy has also been employed against bacterial blight of geraniums and bacteria spot of tomatoes, both caused by pathovars of X. campestris. Phage can also be used to bias the survival of more-effective mutualistic bacteria. Basit et al., for example, have isolated phage that are ineffective against a preferred inoculum of B. japonicum but effective against naturally occurring competitors. By coating seeds with phage effective only

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against these potential competitors they can enhance nitrogen fixation. Though seemingly effective in certain situations, it is likely that phage therapy againSt bacterial plant pathogens will not prove to be a "magic bullet" in all cases. Johnson proposed a general biological control model which suggests that the success of a particular treatment will be influenced by agent and target densities. An important component of this model is the possibility of the target residing in spatial refuges into which the biological control agent cannot penetrate. We would propose several additional factors that could contribute to the success or failure of a potential phage therapy system, such as the location or niche in which the target pathogen population resides (including the potential for refuges), the presence of adequate water as a medium for virion diffusion, rates of virion decay, the timing of phage application, phage in situ infection fecundity, and the relative fitness of phageresistant bacterial mutants. Furthermore, due to the diversity of bacteria and their phages, extrapolation of phage therapy practices from one pathogen system to different systems will not always be practicable. REFERENCES

Abedon, S. T. 'Phage Ecology", In R Calendar and S. T. Abedon (eds.), The Bacteriophages. Oxford University Press, Oxford. 2006. Ackermann, H.-W., and M. S. DuBow. Viruses of Prokaryotes, CRC Press, Boca Raton, Florida. 1987. Chibani-Chennoufi, 5., A. Bruttin, M. L. Dillmann, and H. Briissow. "Phage-host interaction: an ecological perspective" J. Bacteriol. 186:3677-3686. 2004. Goyal, S. M., C. P. Gerba, and G. Bitton. Phage Ecology. CRC Press, Boca Raton, Florida. 1987. Paul, J. H., and C. A. Kellogg. "Ecology of bacteriophages in nature", p. 211-246. In C. J. Hurst (ed.), Viral Ecology. Academic Press, San Diego. 2000.

11 Ecology of Plant Diseases Plant pathology is the scientific study of plant diseases caused by pathogens (infectious diseases) and environmental conditions (physiological factors). Organisms that cause infectious disease include fungi, oomycetes, bacteria, viruses, viroids, virus-like organisms, phytoplasmas, protozoa, nematodes and parasitic plants. Not included are insects, mites, vertebrate or other pests that affect plant health by consumption of plant tissues. Plant pathology also involves the study of the identification, etiology, disease cycle, economic impact, epidemiology, how plant diseases affect humans and animals, pathosystem genetics and management of plant diseases. Plant pathogens cause mortality and reduce fecundity of individual plants, drive host population dynamics, and affect the structure and composition of natural plant communities. Pathogens are responsible for both numerical changes in host populations and evolutionary changes through selection for resistant genotypes. Unking such ecological and evolutionary dynamics has been the focus of a growing body of literature on the effects of plant diseases in natural ecosystems. A guiding principle is the importance of understanding the spatial and temporal scales at which plants and pathogens interact. This chapter describes the ecology of plant diseases in natural

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ecosystems, focusing in particular on the effects of diseases on populations of plants, the maintenance of plant species diversity as well as the process of rapid evolutionary changes in host-pathogen symbioses. ORGANISMS CAUSING PLANT DISEASES

Phytopathogenic Fungi

The majority of phytopathogenic fungi belong to the Ascomycetes and the Basidiomycetes. The fungi reproduce both sexually and asexually via the production of spores. These spores may be spread long distances by air or water, or they may be soil bourne. Many soil bourne spores, normally zoospores and capable of living saprotrophically, caring out the first part of their lifecycle in the soil. Fungal diseases can be controlled through the use of fungicides in agriculture, however new races of fungi often evolve that are resistant to various fungicides. Oomycetes

The oomycetes are fungal-like organisms that until recently used to be mistaken for fungi. They include some of the most destructive plant pathogens including the genus Phytophthora which includes the casual agents of potato lat~ blight and sudden oak death. Despite not being closely related to the fungi, the oomycetes have developed very similar infection strategies and so many plant pathologists group them with fungal pathogens. Bacteria

Most bacteria that are associated with plants are actually saprotrophic, and do no harm to the plant itself. However, a small number, around 100 species, are able to cause disease. Bacterial diseases are much more prevalent in subtropical and tropical regions of the world.

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Most plant pathogenic bacteria are rod shaped (bacilli). In order to be able to colonise the plant they have specific pathogenicity factors. There are 4 main bacterial pathogenicity factors: 1. Cell wall degrading enzymes. Used to break down the plant cell wall in order to release the nutrients inside. Used by pathogens such as Erwinia to cause soft rot. 2. Toxins. These can be non-host specific, and damage all plants, or host specific and only cause damage on a host plant. 3. Phytohormones. For example agrobacterium changes the level of Auxin to cause tumours. 4. Exopolysaccharides. These are produced by bacteria and block xylem vessels, often leading to the death of the plant. Phytoplasmas and Spiroplasmas

Phytoplasma, formerly known as 'Mycoplasma-like organisms' or MLOs, are specialised bacteria that are obligate parasites of plant phloem tissue, and some insects. They were first discovered by scientists in 1967 when they were named mycoplasma like organisms or MLOs. They can't be cultured in vitro in cell-free media. They are characterised by their lack of a cell wall, a pleiomorphic or filamentous shape, normally with a diameter less than 1 micrometer, and their very small genomes. Phytoplasmas are pathogens of important crops, including coconuts and sugarcane, causing a wide variety of symptoms that ranges from mild yellowing to death of infected plants. They are most prevalent in tropical and sub-tropical regions of the world. Phytoplasmas require a vector to be transmitted from plant to plant and this normally takes the form of sap sucking insects such as leaf hoppers in which they are also able to replicate.

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Phytoplasmas were first identified in 1967 in plants that were thought to be infected with viruses, but ultrathin sections of the plants phloem revealed the presence of mycoplasma like organisms. Being mollicutes, phytplasmas lack cell walls and instead are bound by a triple layered unit membrane. The cell membranes of all phytoplasmas studied so far usually contain a single immunodominant protein that makes up the majority of the protein content of the cell membrane. Their shape is normally pleiomorphic or filamentous and normally have a diameter of less than 1 micrometer. Like other prokaryotes, DNA is free in the cytoplasm. They are believed to reproduce through Binary fission. Symptoms

A common symptom caused by phytoplasma infection is phyllody, the production of leaf like structures in place of flowers. Evidence suggests that the phytoplasma downregulates a gene involved in petal formation and genes involved in the maintenance of the apical meristem. This causes sepals to form where petals should. Other symptoms, such as the yellowing of leaves, are thought to be caused by the phytoplasma's presence in the phloem affecting its function, and changing the transport of carbohydrates. Phytoplasma infected plants may also suffer from virescence - the development of green flowers due to the loss of pigment in the petal cells. Many phytoplasma infected plants gain a bushy or witch's broom appearance due to changes in normal growth patterns caused by the infection. Most plants show apical dominance but phytoplasma infection can cause the proliferation of auxiliary (side) shoots and an increase in size of the internodes. Such symptoms are actually useful in the commercial productiun of poinsettia. The infection is necessary to produce more axillary shoots that enable to

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production of pionsettia plants that have more than one flower. Phytoplasmas may cause many other symptoms that are induced because of the stress placed on the plant by infection rather than specific pathogencity of the phytoplasma. Photosynthesis, especially photosystem II, is inhibited in many phytoplasma infected plants. Phytoplasma infected plants often show yellowing which is caused by the breakdown of chlorophyll, whose biosynthesis is also inhibited. Transmission

Movement between plants The phytoplasmas are mainly spread by insects of the families Cicadellidea (leafhoppers) and Fulgoridea (planthoppers) which feed on the phloem tissues of infected plants picking up the phytoplasmas and transmitting them to the next plant they feed on. For this reason the host range of phytoplasmas is strongly dependent upon its insect vector. Phytoplasmas contain a major antigenic protein that makes up the majority of their cell surface proteins and this has been shown to interact with insect microfilament complexes and,is believed to the determining factor is insect-phytoplasma interation. Phytoplasmas may overwinter in insect vectors or perrinial plants. Phytoplasmas can have varying affects on their insect hosts, examples of both reduced and increased fitness have been seen. Phytoplasmas will be found in most of the major organs of an infected insect host once they are established. They will enter the insects body through the stylet and then move through the intestine and bein absorbed into the haemolymph. From here they proceeded to colonise the salivary glands, a process that can take up to three weeks. The time between phytoplasmas being taken up by the

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insect and the phytoplasmas reaching an infectious titre in the salivary gland is called the latency period. Phytoplasmas can also be spread via vegetative propergation such as the grafting of a piece of infected plant onto a healthy plant. Movement within plants

Phytoplasmas are able to move within the pholem from source to sink and they are able to pass through sieve tube elements, but spread more slowly than solutes, for this and other reasons I.110Vement by passive translocation is not supported. Detection and Diagnosis

Before molecular techniques were developed the diagnosis of phytoplasma diseases was difficult due to the fact that they could not be cultured. Thus classical diagnostic techniques such as observation of symptoms were used. Ultrathin sections of the phloem tissue from suspected phytoplasma infected plants would also be examined for their presence.Another diagnostic technique used was to treat infected plants with antibiotics such as tetracycline to see if this cured the plant. Molecular diagnostic techniques for the detection of phytoplasma began to emerge in the 1980s and included ELISA based methods. In the early 1990's peR-based methods were developed that were far more sensitive than those that used ELISA, and RFLP analysiS allowed the accurate identification of different strains and species of phytoplasma. There are also techniques that allow the assessement of the level of infection. Control

Phytoplasmas are normally controlled by the breeding and pla!lting of disease resistance varieties of crops (believed to

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the most economically viable option) and by the control of the insect vector. Tissue culture can be used to produce clones of phytoplasma infected plants that are healthy. The chances of gaining healthy plants in this manner can be enhanced by the use of cryotherapy, freezing the plant samples in liquid nitrogen before using them for tissue culture. Work has also been carried out investigating tHe effectiveness of plantibodies targeted against phytoplasmas. Tetracyclines are bacteriostatic to phytoplasmas, that is they inhibit their growth. However without continuous use of the antibiotic disease symptoms will reappear. Thus tetracycline is not a viable control agent in agriculture, but is used to protect ornamental coconut trees. Spiroplasma

Spiroplasma is a genus of Mollicutes, a group of small bacteria without cell walls. Spiroplasma shares the simple metabolism, parasitic lifestyle, fried-egg colony morphology and small genome of other Mollicutes, but has a distinctive helical morphology, unlike Mycoplasma. It has a spiral shape and moves in a corkscrew motion. Most spiroplasmas are found either in the gut or hemolymph of insects, or in the phloem of plants. Spiroplasmas are fastidious organisms, which require a rich culture medium. Typically they grow well at 30°C, but not at 37°C. A few species, notably Spiroplasma mirum, grow well at 37°C (human body temperature), and cause cataracts and neurological damage in suckling mice. The best studied species of spiroplasmas are Spiroplasma citri, the causative agent of Citrus Stuborn Disease, and Spiroplasma kunkelii, the causative agent of Com Stunt Disease. There is some disputed evidence for the role of spiroplasmas in the etiology of Transmissible Spongiform Encephalopathies (TSEs), due primarily to the work of Dr. Bastian, summarized be~ow. Other researchers, such as

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Leach et al. have failed to replicate this work, while the prion model for TSEs has gained very wide acceptance. The most recent work of Alexeeva et aL appears to refute the role of spiroplasmas in the best small animal scrapie model (hamsters). Bastian et aL have responded to this challenge with the isolation of a spiroplasma species from scrapieinfected tissue, grown it in cell-free culture, and demonstrated its infectivity in ruminants. According to Frank 0. Bastian, MD: "spiroplasmas contain internal fibrillar proteins, that have morphological and immunological similarities to scrapieand CJD-related fibrillar proteins. This comparison is noteworthy since mycoplasmologists consider these fibril proteins unique to this prokaryoteJn vivo and in vitro experimental Spiroplasma infections produce cytopathic effects similar to those of the scrapie agent Experimental Spiroplasma brain infection in the suckling rat is characterized by vacuolar encephalopathy with localization of the microbe to gray matter.[... ] Spiralins are chemically bound to Spiroplasma-associated fibrils (SpFs) and are separated with difficulty.' SpFs are unique internal fibrils of spiroplasmas with a molecular weight of 55 kDa. Recently, SpFs have been shown to bear close morphological resemblance to scrapie-associated fibrils (SAPS), ' and show cross-reactivity using SAP antibody."

In addition, a Spiroplasma species had been shown to kill

males of the Plain Tiger butterfly on infection, leading to interesting consequences for population genetics and consequently speciation similar to the effects caused by some strains of Wolbachia Plant Viruses

There are many types of plant virus, and some are even ~symptomatic. Normally plant viruses only cause a loss of yield. Therefore it is not economically viable to try to :ontrol them, the exception being when they infect :>erennial species, such as fruit trees.

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Most plant viruses have small, single stranded RNA genomes. These genomes may only encode 3 or 4 proteins: a replicase, a coat protein, a movement protein to allow cell to cell movement and sometimes a protein that allows transmission by a vector. Plant viruses must be transmitted from plant to plant by a vector. This is normally an insect, but some fungi, nematodes and protozoa have been shown to be viral vectors. Plant viruses need to be transmitted by a vector, most often insects such as leafhoppers. One class of viruses, the Rhabdoviridae, have been proposed to actually be insect viruses that have evolved to replicate in plants. The chosen insect vector of a plant virus will often be the determining factor in that virus' host range: it can only infect plants that the insect vector feeds upon. This was shown in part when the old world white fly made it to the USA, where it transferred many plant viruses onto new hosts. Depending on the way they are transmitted, plant viruses are classified as non-persistent, semi-persistent and persistent. In non-persistent transmission, viruses become attached to the distal tip of the stylet of the insect and on the next plant it feeds on, it inoculates it with the virus. Semi-persistent viral transmission involves the virus entering the foregut of the insect. Those viruses that manage to pass through the gut into the haemolymph and then to the salivary glands are known as persistent. There are two sub-classes of persistent viruses: propergative and circulative. Propergative viruses are able to replicate in both the plant and the insect (and may have originally been insect viruses), whereas circulative can not. Many plant viruses encode within their. genome polypeptides with domains essential for transmission by insects. In non-persistent and semi-persistent viruses, these domains are in the coat protein and another protein known as the helper component. A bridging hypothesis has been

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proposed to explain how these proteins aid in insectmediated viral transmission. The helper component will .bind to the specific domain of the coat protein, and then the insect mouthparts--creating a bridge. In persistent propergative viruses, such as tomato spotted wilt virus (TSWV), there is often a lipid coat surrounding the proteins that is not seen in the other classes of plant viruses. In the case of TSWV, 2 viral proteins are expressed in this lipid envelope. It has been proposed that the viruses bind via these proteins and are then taken into the insect cell by receptor-mediated endocytosis. The discovery of plant viruses causing disease is often accredited to Martinus Beijerinck who discoved, in 1898, that even after passing infective tree sap through a porcelain filter remained infectious but was sterile of microorganisms. After the initial discovery of the 'viral concept' there was need to classify any other known viral diseases based on the mode of transmission even though microscopic observation proved fruitless. In 1939 Holmes published a classification list of 129 plant viruses. This was expanded and in 1999 there were 977 officially recognised, and some provisional, plant virus species. The pUrification of the TMV (the first purification) was first performed by Wendell Stanley, who published his findings in 1936. He later was accredited with the Nobel Prize in Chemistry in 1946. In the 1950s a discovery by two labs simultaneously proved that the purified RNA of the TMV was infectious which reinforced the argument, that had a lot of opposition at the time, that RNA was carrying genetic information to ('ncie for the production of new infectious particles. More recently the research has been focused on the manipulation and modification of plant virus genomes do discover function and for commercial gain in the agriculture business by using viral-derived sequences to

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provide understanding of novel forms of resistance. The recent boom in technology allowing humans to manipulate plant viruses has really helped bring the subject out of an Aristolean science age and into the 21st century. Structure

Viruses are very small and can only be seen under an electron microscope. The ~tructure of a virus is given by its coat of proteins, which surround the viral genome. Assembly of viral particles takes place spontaneously. Over' 50% of known plant viruses are rod shaped. Exact length is normally dependent on the genome but it is usually between 300-500 nm with a diameter of 15-20 nm. Protein subunits can be placed around the circumference of a circle to form a disc. In the presence of the viral genome, the discs are stacked, then a tube is created with room for the nucleic acid genome in the m1ddle The second most common structure amongst plant viruses a~e isometric particles. They are 40-50 nm in diameter. In cases when there is only a single coat protein, the basic structure consists of 60 T subunits, where T is an integer. Some viruses may have 2 coat proteins are the formation of the particle is analogous to a football. There are three genera of Geminiviridae that possess geminate particles which are like two isometric particles stuck together. A very small number of plant viruses have, in addition to their coat proteins, a lipid envelope. This is derived from the plant cell membrane as the virus particle buds off from the cell. Transmission of plant viruses

Through sap It implies direct transfer of sap by contact of and wounded

plant with a healthy one. Such process occurs during

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Eco~ogy

agricultural practices by tools, hands, or by animal feeding on the plant. Generally TMV, potato viruses and cucumber mosaic viruses are transmitted via sap.

Insects Plant viruses need to be transmitted by a vector, most often insects such as leafhopperso One class of viruses, the Rhabdoviridae, have been proposed to actually be insect viruses that have evolved to replicate in plantso The chosen insect vector of a plant virus will often be the determining factor in that virus' host range: it can only infect plants that the insect vector feeds upon. This was shown in part when the old world white fly made it to the USA, where it transferred many plant viruses onto new hosts. Depending on the way they are transmitted, plant viruses are classified as non-persistent, semi-persistent and persistent. In non-persistent transmission, viruses become attached to the distal tip of the stylet of the insect and on the next plant it feeds on, it inoculates it with the virus.[2] Semi-persistent viral transmission involves the virus entering the foregut of the insect. Those viruses that manage to pass through the gut into the haemolymph and then to the salivary glands are known as persistent. There are two sub-classes of persistent viruses: propergative and circulative. Propergative viruses are able to replicate in both the plant and the insect, whereas circulative can not. Many plant viruses encode within their genome polypeptides with domains essential for transmission by insects. In non-persistent and semi-persistent viruses, these domains are in the coat protein and another protein known as the helper component. A bridging hypothesis has been proposed to explain how these proteins aid in insectmediated viral transmission. The helper component will bind to the specific domain of the coat protein, and then the insect mouthparts - creating a bridge.

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In persistent prop ergative viruses, such as tomato spotted wilt virus (TSWV), there is often a lipid coat surrounding the proteins that is not seen in the other classes of plant viruses. In the case of TSWV, 2 viral proteins are expressed in this lipid envelope. It has been proposed that the viruses bind via these proteins and are then taken into the insect cell by receptor-mediated endocytosis. Plasmodiophorids

A number of viral genera are transmitted, both persistently and non-persistently, by soil bourne zoosporic protozoa. These protozoa are not phytopathogenic themselves, but parasitic. Transmission of the virus takes place when they become associated with the plant roots. Seed and pollen borne viruses

Plant virus transmission from generation to generation occurs in about 20% of plant viruses. When viruses are transmitted by seeds, the seed is infected in the generative cells and the virus is maintained in the germ cells and sometimes, but less often, in the seed coat. When the growth and development of plants is delayed because of situations like unfavourable weather, there is an increase in the amount of virus infections in seeds. There does not seem to be a correlation between the location of the seed on the plant and its chances of being infected. Little is known about the mechanisms involved in the transmission of plant viruses via seeds, although it is known that it is environmentally influenced and that seed transmission occurs because of a direct invasion of the embryo via the ovule or by an indirect route with an attack on the embryo mediated by infected gametes. These processes can occur concurrently or separately depending on the host plant. It is unknown how the virus is able to directly invade and cross the embryo and boundary between the parental and progeny generations in the ovule.

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Many plants species can be infected through seeds including but not limited to the families Leguminoseae, Solanacease, Compositae, Rosaceae, Curcurbitaceae, Gramineae. Nematodes

Nematodes are small, multicelluar wormlike creatures. Many live freely in the soil, but there are some species which parasitize plant roots. They are mostly a problem in tropical and subtropical regions of the world, where they may infect crops. Root knot nematodes have quite a large host range, whereas cyst nematodes tend to only be able to infect a few species. Nematodes are able to cause radical changes in root cells in order to facilitate their lifestyle. Protozoa

There are a few examples of plant diseases caused by protozoa. They are transmitted as zoospores which are very durable, and may be able to survive in a resting state in the soil for many years. They have also been shown to transmit plant viruses. When the motile zoospores come into contact with a root hair they produce a plasmodium and invade the roots. PHYSIOLOGICAL PLANT DISORDERS

Physiological plant disorders are caused by nonpathological disorders such as poor light, weather damage, water-logging or a lack of nutrients, and affect the functioning of the plant system. Physiological disorder are distinguished from plant diseases caused by pathogens, such as a virus or fungus. Whilst the symptoms of physiological disorders may appear disease-like, they can usually be prevented by altering environmental conditions. However, once a plant shows symptoms of nutrient deficiency it is likely that that season's yields will be reduced.

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Causes of physiological disorders can be identified by examining: Where symptoms first appear on a plant- on new leaves, old leaves or all over? The pattern of any discolouration or yellowing- is it all over, between the veins or around the edges? If only the veins are yellow deficiency is probably not . involved. Note general patterns rather than looking at individual plants- are the symptoms distributed throughout a group of plants of tPte same type growing together. In the case of a deficiency all of the plants should be similarly effected, although distribution will depend on past treatments applied to the soil. Soil analysis, such as determining pH, can help to confirm the presence of physiological disorders. Recent conditions, such as heavy rains, dry spells, frosts, etc, may also help to determine the cause of plant disorders. WEATHER DAMAGE

Frost and cold are major causes of crop damage to tender plants, although hardy plants can also suffer if new growth is exposed to a hard frost following a period of warm weather. Symptoms will often appear overnight, affecting many types of plants. Leaves and stems may tum black, and buds and flowers mar be discoloUfed, and frosted blooms may not produce fruit. Many annual plants, or plants grown in frost free areas, can suffer from damage when the air temperature drops below 400 Fahrenheit. Tropical plants may begin to experience cold damage when the temperature is 42-480 Fahrenheit, symptoms include wilting of the top of the stems and/ or leaves, and blackening or softening of the plant tissue. Frost or cold damage can be avoided by ensuring that tender plants are properly hardened before planting, ~nd

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that they are not planted too early in the season, before the risk of frost has passed. Avoid planting susceptible plants in frost pockets, or where they will receive early morning sun. Protect young buds and bloom with horticultural fleece if frost is forecast. Cold, drying easterly winds can also severely inhibit spring growth even without an actual frost, thus adequate shelter or the use of windbreaks is important. Drought can cause plants to suffer from water stress and wilt. Adequate irrigation is required during prolonged hot, dry periods. Rather than shallow daily watering, during a drought water should be directed towards the roots, ensuring that the soil is thoroughly soaked two or three times a week. Mulches also help preserve soil moisture and keep roots cool. Heavy rains, particularly after prolonged dry periods, can also cause roots to split, onion saddleback, tomatoes split and potatoes to become deformed or hollow. Using mulches or adding organic matter such as leaf mold, compost or well rotted manure to the soil will help to act as a 'buffer' between sudden changes in conditions. Waterlogging can occur on poorly drained soils, particularly following heavy rains. Plants can become yellow and stunted, and will tend to be more prone to drought and diseases. Improving drainage will help to alleviate this problem. Hail can cause damage to soft skinned fruits, and may also allow brown rot or other fungi to penetrate the plant. Brown spot markings or lines on one side of a mature apple are indicative of a spring hailstorm. Plants affected by salt stress are unable to take water from soil, due to an osmotic imbalance between soil and plant. NUTRIENT DEFICIENOES

Poor growth and a variety of complaints such as leaf discolouration can be caused by a lack of plant foods. This may be due to shortages of necessary nutrients, or because

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the nutrients are present but not available to the plant. Th~ latter can be caused by incorrect pH, shortages of water or an excess of another nutrient. Generally, the key to avoiding nutrient deficiencies is to ensure that the soil is healthy and contains plenty of well rotted organic matter rather than by feeding or treating individual plants. Boron Deficiency

Boron (B) deficiency is a rare disorder affecting plants growing above a granite bedrock, which is low in boron. Boron may be present but locked up in soils with a high pH, and the deficiency may be worse in wet seasons. Symptoms include dying growing tips and bushy stunted growt~. Crop-specific symptoms include: Beetroot: rough, cankered patches on roots, internal brown rot. Cabbage: distorted leaves, hollow areas in stems. Cauliflower: poor development of curds, and brown patches. Stems, leafstalks and midribs roughened. Celery: leaf stalks develop cracks on the upper surface, inner tissue is reddish brown. Pears: new shoots die back in spring, fruits develop hard brown flecks in the skin. Strawberries: Stunted growth, foliage small, yellow and puckered at tips. Fruits are small and pale. Swede (rutabaga> and turnip: brown or grey concentric rings develop inside the roots. Arecaceae: brown spots on fronds & lower productivity. Boron deficiency can be avoided by improving the moisture retaining capacity of light soils, and ensuring pH is kept below 7. Borax can be raked into the soil at 35 g/m2. Calcium Deficiency

Calcium (Ca) deficiency is a plant disorder that can be

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caused iJy insufficient calcium in the growing medium, but is more frequently a product of a compromised nutrient mobility system in the plant. This may be due to water shortages, which slow the transportation of calcium to the plant, or can be caused by excessive usage of potassium or nitrogen fertilizers. Calcium deficiency symptoms appear initially as generally stunted plant growth, necrotic leaf margins on young leaves or curling of the leaves, and eventual death of terminal buds and root tips. Generally the new growth of the plant is affected first. The mature leaves may be affected if the problem persists. Crop-specific symptoms include: Apple. 'Bitter pit'-fruit skins develop pits, brown patches appear in flesh and taste becomes bitter. Can occur when fruit is in storage. Bramley apples are particularly susceptible. Cabbage and Brussels sprouts. Internal browning. Carrot. Cavity spot'-oval spots develop into craters which may be invaded by other disease causing organisms. , Celery. Stunted growth, central leaves stunted. Tomatoes and peppers. 'Blossom end rot'-Symptoms start as sunken, dry decaying areas at the blossom end of the fruit, furthest away from the stem, not all fruit on a truss is necessarily affected. Sometimes rapid growth from high-nitrogen fertilizers may cause blossom end rot. Treatment

Calcium deficiency can be rectified by adding Agricultural lime to acid soils, aimittg at a pH of 6.5, unless the plant in question specifically prefers acidic soil. Organic matter should be added to the soil in order to improve its moistureretaining capacity.

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Plant damage is difficult to reverse, so take corrective action immediately. Make supplemental applications bf calcium nitrate at 200 ppm nitrogen. Test and correct the pH if needed because calcium deficiency is often associated with low pH. Iron Deficiency

Iron (Fe) deficiency is a plant disorder also known as 'limeinduced chlorosis'. A deficiency in the soil is rare. Iron can be unavailable if pH is too high or if the soil is waterlogged, or has been overfertilised with phosphorus. Can be confused with manganese deficiency. Any plants may be affected, but raspberries and pears are particularly susceptible, as well as most acid-loving plants such as azaleas and camellias. Symptoms include leaves turning yellow or brown in the margins between the veins which may remain green, while young leaves may appear to be bleached. Fruit is of poor quality and quantity. Iron deficiency can be avoided by choosing appropriate soil for the growing conditions (e.g., avoid growing acid loving plants on lime soils), or by adding well-rotted manure or compost. Magnesium Deficiency

Magnesium (Mg) deficiency is a plant disorder. Magnesium can be easily washed out of light soils in wet seasons. Excessive potassium fertiliser usage can cause also Mg to become unavailable to the growing plant. This disorder partic~arly affects potatoes, tomatoes, apples, currants <;md gooseberries, and chrysanthemums. Symptoms include, yellowing between leaf veins, which stay green, giving a marbled appearance. This begins with older leaves from late June, but spreads to younger growth. Can be confused with virus, or natural aging in the

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case of tomato plants. Fruits are small and woody. Mg deficiency can be rectified in the short term by applying a foliar feed fortnightly, with epsom salts diluted at a rate of 200g per 10 litres of water (807 per 21;2 gal) after flowering. In the longer term add dolomitic limestone if soil pH allows, or other Mg containing rocks such as Kieserite. Reduce usage of potash fertilsers if this may be contributing to the problem. Manganese Deficiency

Manganese (Mn) deficiency is a plant disorder that is often confused with, and occurs with, iron deficiency. Most common in poorly drained soils, also where organic matter levels are high. Manganese may be unavailable to plants where pH is high. Affected plants include onion, apple, peas, French beans, cherry and raspberry, and symptoms include yellowing of leaves with smallest leaf veins remaining green to produce a 'chequered' effect. The plant may seem to grow away from the problem so that younger leaves may appear to be unaffected. Brown spots may appear on leaf surfaces, and severely affected leaves turn brown and wither. Nitrogen Deficiency

Nitrogen (N) deficiency in plants can occur when woody material such as sawdust is added to the soil. Soil organisms will utilise any nitrogen in order to break this down, thus making it temporarily unavailable to growing plants. 'Nitrogen robbery' is more likely on light soils and those low in organic matter content, although all soils are susceptible. Cold weather, especially early in the season, can also cause a temporary shortage. All vegetables apart from nitrogen fixing legumes are prone to this disorder. Symptoms include poor plant

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growth, leaves are pale green or yellow in the case of brassicas. Lower leaves show symptoms first. Leaves in this state are said to be etiolated with reduceq chlorophyll. Flowering and fruiting may be delayed. Prevention and control of nitrogen deficiency can be achieved in the short term by using grass mowings as a mulch, or foliar feeding with manure, and in the longer term by building up levels of organic matter in the soil. Sowing green manure crops such as grazing rye to cover soil over the winter will help to prevent nitrogen leaching, while leguminous green manures such as winter tares will fix additional nitrogen from the atmosphere. Phosphorus Deficiency

Phosphorus (P) deficiency is a plant disorder that is most common in areas of high rainfall, especially on acid, clay or poor chalk soils. Cold weather can cause a temporary deficiency. All plants may be affected, although this is an uncommon disorder. Particularly susceptible are carrots, lettuce, spinach, apples, currants and gooseberries. Symptoms include poor growth, and leaves that turn blue/ green but not yellow-oldest leaves are 'affected first. Fruits are small and acid tasting. Phosphorus deficiency may be confused with nitrogen deficiency. It can be controlled by applying organic sources of phosphorus such as rock phosphate. Plants that are naturally adapted to low levels of available soil phosphorus, however, are more likely to suffer from phosphate poisoning: the key is to provide the right level for any particular plant type, neither too high nor too low. Potassium Deficiency

Potassium deficiency, also known as potash deficiency, is a plant disorder that is most common on light, sandy soils,

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as well as chalky or peaty soils with a low clay content. It is also founden--heavy clays with a poor structure. Plants require potassium ions (K+) for protein synthesis and for the opening and closing of stomata, which is regulated by proton pumps to make surrounding guard cells either turgid or flaccid. A deficiency of potassium ions can impair a plants ability to maintain these processes. The deficiency most commonly affects fruits and vegetables, notably potatoes, tomatoes, apples, currants, and gooseberries, and typical symptoms are brown scorching and curling of leaf tips, and yellowing of leaf veins. Purple spots may also appear on the leaf undersides. Deficient plants may be more prone to frost damage and disease, and their symptoms can often be confused with wind scorch or drought. Prevention and cure can be achieved in the shorter term by feeding with home-made comfrey liquid, adding seaweed meal, composted bracken or other organic potassium-rich fertilisers. In the longer term the soil structure should be improved by adding plenty of well rotted compost or manure. Wood ash has high potassium content, but should be composted first as it is in a highly soluble form. EPIDEMIC DISEASES OF PLANTS

Plant Disease epidemiology is the study of disease in plant populations. Much like diseases of humans and animals, plant diseases occur due to pathogens such as bacteria, viruses, fungi, oomycetes, nematodes, phytoplasmas, protozoa, and parasitic plants. Plant disease epidemiologists strive for an understanding of the cause and effects of disease and develop strategies to intervene in situations where crop losses may occur. Typically successful intervention will lead to a low enough level of disease to be acceptable, depending upon the value of the crop.

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Plant disease epidemiology is often looked at from a multi-disciplinary approach, requiring biol9gical, statistical, agronomic and ecological perspectives. Biology is necessary for understanding the pathogen and its life cycle. It is also necessary for understanding the physiology of the crop and how the pathogen is adversely affecting it. Agronomic practices often influence disease incidence for better or for worse. Ecological influences are numerous. Native species of plants may serve as reservoirs for pathogens that cause disease in crops. Statistical models are often applied in order to summarize and describe the complexity of plant disease epidemiology, so that disease processes can be more readily understood. For example, comparisons between patterns of disease progress for different diseases, cultivars, management strategies, or environmental settings can help in determining how plant diseases may best be managed. Policy can be influential in the occurrence of diseases, through actions such as restrictions on imports from sources where a disease occurs. In 1963 J. E. van der Plank published "Plant Diseases: Epidemics and Control", a seminal work that created a theoretical framework for the study of the epidemiology of plant diseases. This book provides a theoretical framework based on experiments in many different host pathogen systems and moved the study of plant disease epidemiology forward rapidly, especially for fungal foliar pathogens. Using this framework we can now model and determine thresholds for epidemics that take place in a homogeneous environment such as a mono-cultural crop field. Disease epidemics in plants can cause huge losses in yield of crops as well threatening to wipe out an entire species such as was the case with Dutch Elm Disease and could occur with Sudden Oak Death. An epidemic of potato late blight, caused by Phytophthora infestans, led to the Great Irish Famine and the loss of many lives.

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Commonly the elements of an epidemic are referred to as the "disease triangle": a susceptible host, pathogen, and conducive environment. For disease to occur all three of these must be present. Below is an illustration of this point. Where all three items meet there is disease. The fourth element missing from this illustration for an epidemic to occur, is time. As long as all three of these elements are present disease can initiate, an epidemic will only ensue if all three continue to be present. Anyone of the three might be removed from the equation though. The host might outgrow susceptibility as with high-temperature adult-plant resistance, the environment changes and is not conducive for the pathogen to cause disease, or the pathogen is controlled through a fungicide application for instance. Sometimes a fourth factor of time is added as the time at which a particular infection occurs, and the length of time conditions remain viable for that infection, can also play an important role in epidemics. The age of the plant species can also playa role, as certain species change in their levels of disease resistance as they mature; a process known as ontogenic resistance. If all of the criteria are not met, such as a susceptible host and pathogen are present but the environment is not conducive to the pathogen infecting and causing disease, disease cannot occur. For example, com is planted into a field with com residue that has the fungus Cercospora zeamaydis, the causal agent of Grey leaf spot of com, but if the weather is too dry and there is no leaf wetness the spores of the fungus in the residue cannot germinate and initiate --infection. Likewise, it stands to reason if the host is susceptible and the environment favours the development of disease but the pathogen is not present there is no disease. Taking the example above, the com is planted into a ploughed field where there is no com residue with the fungus Cercospora zea-maydis, the causal agent of Grey leaf spot of com,

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present but the weather means long periods of leaf wetness, there is no infection initiated. When a pathogen requires a vector to be spread then for an epidemic to occur the vector must be plentiful and active. Monocyclic epidemics are caused by pathogens with a low birth rate and death rate meaning they only have one infection cycle per season. They are typical of soil born diseases such as Fusarium wilt of flax. Polycyclic epidemics are caused by pathogens capable of several infection cycles a season. These are most often caused by airborne diseases such as powdery mildew. Bimodal polycyclic epidemics can also occur. For example in brown rot of stone fruits the blossoms and the fruits are infected at different times. For some diseases it is important to consider the disease occurrence over several growing seasons, especially if growing' the crops in monoculture year after year or growing perennial plants. Such conditions can mean that the inoculum produced in one season can be carried over to the next leading to a build of an inoculum over the years. In the tropics there are no dear cut breaks between growing seasons as there are in temperate regions and this can lead to accumulation of innoculum. Epidemics that occur under these conditions are referred to as polyetic epidemics and can be caused by both monocylcic and polycyclic pathogens. Apple powdery mildew is an example of a polyetic epidemic caused by a polycyclic pathogen and Dutch Elm disease a polyetic epidemic caused by a monocyclic pathogen. REFERENCES

Agrios, George. Plant Pathology. Academic Press. 2005. Arneson, PA "Plant disease epidemiology: temporal aspects". Plant Health Instructor. American Phytopathological Society. 2001. Dickinson, M . Molecular Plant Pathology. BIOS Scientific Publishers. 2003. George N. Agrios. Plant Pathology, Academic Press. New York. 1997.

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Madden, Laurence; Gareth Hughes, Frank Van Den Bosch. Study of Plant Disease Epidemics. American Phytopathological Society. 2007. Milton Zaitlin. Discoveries in Plant Biology, New York 14853, USA. Pp.: 105-110. 1998. Drenth, A "Fungal epidemics - does spatial structure matter?". New Phytologist 163: 4-7. 2004. Shultz, T.R; Line, R.F "High-Temperature, Adult-Plant Resistance to Wheat Stripe Rust and Effects on Yield Components". Agronomy Journal 84: 170-175. American Society of Agronomy. 1992.

12 Plant Ecology and Climate Change In contemporary ecology, there are at least four prominent research speciality areas that study vegetation change: succession ecology, invasion biology, gap/patch dynamics, and global change effects on plant communities. The underlying processes studied in each of these areas are basically the same. First, colonization, establishment, turnover, persistence, and spread' are fundamental events and processes that interact to produce vegetation change in all four subdisciplines; second, whatever the nature of vegetation change, it is often initiated or greatly in.uenced by, disturbance and/ or changes in interactions with other trophic levels; third, local and long-distance dispersal allow new species to enter existing plant communities; fourth, facilitation and inhibition, as well as interactions with species from other trophic levels, strongly in.uence vegetation change; and .fth, in all cases, changes in community composition affect, and are affected by, ecosystem processes (Figure 1). These four research areas focus on different causes of vegetation change, e.g., species introduced from other regions of the world, disturbances that create gaps and initiate succession, and global change. Given that these four research areas seekto illuminate the mechanisms that cause vegetation change, and that the phenomena under

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study often interact (e.g., gaps and climate change may facilitate invasions), one would. expect there to be considerable information exchange among these research areas. Time I ntrod uction of New Dis p&f&al Within Native Species The Community

OngOing or Inlt'l11lltll!nt C1ISp&rsal

Introduction of Hew Non-Natiw Species.

111~llr tll!ltl!tl!l!tl!!l!l Mutualisms Herbivory Disease Predation

Re produ ction DisperSoal

Climate Change

Resou rce Fluctuations

Microbes Pollinators HerolVofll.

Changes in establishment. spread. pgrsisfllnce,and ecosystem procssSGs

).

Vegetation change

Figure 1. The same factors change plant communities regardless of the speciality area in which the research is conducted.

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PLANTS AND CLIMATE

Climatic extremes appear more important than climatic averages in predicting patterns of vegetation distribution. The poleward limits of many tree species are determined by frost sensitivity and freezing tolerance. For example, -40°C is the lower limit of supercooling for most ringporous hardwood trees. Below this temperature intracellular freezing occurs, killing the cells and therefore the organism. The northern limit of these tree species correlates closely with the record low temperature of 40°C. Conifers, which have a different mechanism of freezing tolerance, can withstand the temperature of liquid nitrogen and have a correspondingly more extreme latitudinal limit to their distribution. Similarly, chilling-sensitive broad-leaved tropical evergreen trees are killed at 15°C, and their poleward limit coincides with record low temperatures of 15°C. Woodward recognised several major physiognomic types of vegetation: conifer forest, deciduous forests, broadleaved evergreen forests, shrubland, and herbdominated vegetation, based on the assumptions that climate governs the low-temperature or low-moisture limit of distribution, and that the mesic limits of distribution are determined by the environmental tolerance of a competitively superior but less tolerant physiognomic type, Woodward successfully predicted the global distribution of most major physiognomic types of vegetation. These observations suggest that the lowtemperature or low-moisture limit of a physiognomic type is governed by the fundamental niche (Le., phYSiological tolerance) of the plants, whereas additional limits of plant distribution may be determined by other, more complex factors, probably mediated by competition. These patterns also indicate that extreme events, which are a function of environmental fluctuation, are more useful than average conditions in

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predicting northern limits of physiognomic types. As climate changes, the location and frequency of these extreme events will certainly change, leading to changes in distribution of biomes. Understanding of patterns of environmental variability are critical to predictions of future vegetation distribution. Although extreme events predict distributions of general physiognomic types, they are inadequate to predict finer distribution patterns of species or functional groups of species, i.e., groups of species which show similar responses to change in environment. The greater diversity of response by species than by physiognomic types occurs because each species has a unique range of environmental conditions under which it occurs, which seldom coincides precisely with their limits of physiological tolerance. Correlation of the current distribution of plants with their current average climatic conditions has been the main basis of explaining past patterns of vegetation change and predictions of future vegetation distribution. These studies have led to several important generalisations: (i) Each species shows a unique pattern of distribution with climate and responds most strongly to different patterns of climatic factors, so that complex climatic changes cause species to migrate with different patterns and to form new associations. Thus, current plant communities are temporary associations among species that last only hundreds or a few thousand years. These communities did not exist as entities in the past, and there is no reason to expect their continued coexistence in the future. Because of the highly individualistic response of species to climate and because this response depends on interactions with other species in the community, it is difficult to determine which aspects of climate are particularly

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critical to the distribution of a species, making predictions of future distribution difficult. (ii) Changes in the distribution of a species may lag significantly behind climatic changes where dispersal limits the rate of species migration. Thus, predictions of the future response to climate requires some understanding of factors governing the regeneration phase. till) Projections of the future response of vegetation to climate are quite sensitive to availability of soil resources, so that knowledge of climate alone is inadequate to predict future species distribution. (iv) Finally, in complex and more diverse communities such as dry or wet tropical forest there are so many important species that we can never hope to understand their climatic controls well enough to predict future vegetation changes. What is needed is a simplified classification of species into functional groups whose environmental controls can be predicted from general ecological principles. The question is whether there are any predictable responses of groups of species that make a functionalgroup classification practical. Plants' Interaction with Environment

As plants interact with the environment directly by exchanging water and energy, they are very sensitive to storms, droughts and floods . These weather events can severely damage crop yield. In this unit we look at how plants are affected by changes in temperature, humidity and rainfall. Probably the most important role of plants in the environment is the production of oxygen (02) and the absorption of carbon dioxide (C02) from the atmosphere during the process of photosynthesis. Photosynthesis is the

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basic process for plant life. Plants also affect and change their surroundings to make them more suitable for growth. Figure 2 shows some of the more minor ways in which plants interact with their environment. Environmental conditions, such as light intensity, temperature, water availability and wind strength, affect plant growth.

Leavtllt ~ bnlnd\_ absorb sound ilnd block erosion-causing rainfall

Branches, leaves provide shade and reduoe windspMd

EvapotraMpiration from Il!!iIves cools

surrounding ilir

Rooa ~Iizesoil. prevent erosion

Figure 2. Plant's interact with environment

Plants also modify the environment around them, they release water which cools the air, breakdown the soil to make it suitable for their roots and for other plants and animals and decrease the speed of the wind. As you know, plants need water to live and grow. High temperatures reduce the availability of water and decrease crop yields. Young plants are especially vulnerable to extreme weather conditions.

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Let's now look at some of the mechanisms plants use to cope with extreme weather events such as high temperatures, floods or droughts. High Temperatures

High temperatures affect crops directly by increasing the rate at which they loose water (their evaporation rate), in the same way that high temperatures make us sweat. Plants have very small pores, called stomata, spread over their leaves. These stomata help the plants control how much water they contain. In figure below you can see that the stomata are made up of two guard cells which open or close the pore depending on how much water the plant needs. During dry periods, the guard cells are closed so that the plants do not loose too much water. Under normal . weather conditions, the stomata are open.

Gucrd cells Stana

Stanatal q:>enlng

Figure 3. Working of stomata

Each plant type has different structural characteristics and so all plants don't grow well at the same temperature. When the optimal temperature value for a particular plant is exceeded, the plant tends not to grow as well leading to a drop in yield. Most plants are very sensitive to high temperatures, although the extent varies depending on the age of the plant and its ability to withstand poor situations.

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High temperatures also increase the amount of water lost from the soil by evaporation. This also affects plant growth as the soil is their main source of water. Precipitation

Precipitation (rainfall) is the primary source of soil moisture, and rainfall amount is probably the most important factor determining the crop yield. A change in climate can cause an increase or a decrease in the amount of precipitation which falls.

primary root

-.....,MIi'-~ ._ _~

root hairs

__ root tip rootcap---

Figure 4. Plant roots

Roots are one of the main ways plants get water from the environment. In many parts of the world, plants have much longer roots than trunks or branches. Sometimes a bush that is only around 30 cm high can have roots that go as deep as two meters into the soil. This happens in places where there is not much rain during the year, like the deserts or the very arid dry regions of the world. Dry periods can badly affect plant growth but the amount of damage depends on the ability of the plant to expand its root system and how much water the soil can hold onto. High humidity, frost and hail may also damage certain crops.

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High temperatures are normally accompanied by dry periods and both the warmth and the lack of water can have negative effects on plant growth. Roots are unable to find water in the soil and stomata have to close to prevent water loss from the plant. This causes the temperature of the plant to rise and this can damage the plant. When plants struggle to grow because of high temperatures or lack of water, they are said to be under stress. Excessively wet years, on the other hand, may cause crop yields to fall. The soil becomes waterlogged and the plant roots rot in the excess water. Intense bursts of rainfall may also damage young plants, both because of the hard impact of the rain drops on the tender plants and because heavy rain can cause soil erosion. VEGETATION AND NATURAL RESOURCES

Given the difficulties of predicting direct species reponses to climate and the importance of soil resources in mediating plant responses to climate, the correlation of vegetation with soil resources may provide an effective means of predicting the response of functional groups of plants to climate change (figure 5). Soil resource availability will respond in predictable ways to altered climate. In cold climates, increased temperature will stimulate microbial activity and nutrient cycling, thus increasing the availability of commonly limiting nutrients. such as nitrogen and phosphorus. In temperate mesic-to-dry climates, increased temperature and potential evapotranspiration combined with moderate changes in precipitation will decrease soil moisture in mid-continental regions. In tropical coastal dry climates, moderate increases in temperature with high variations in precipitation are expected.

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Figure 5. Interrelationship between climate, soil resources, and functional . groups of organisms.

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lV1S

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Figure 6. Total annual precipitation of the last fifteen years at the tropical deciduous forest ill the Biological Reserve of Chamela, Jalisco, Mexico.

Rates of nutrient cycling correlate closely with availability of most soil resources. Forest clearing for agriculture creates earlv successional habitat with associated increases in light availability. Local and regional variations in climatic projections and soil fertility provide logical bases for refining these predictions< The major point is that J

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patterns of change in soil resources can be predicted from defined scenarios of climatic change. Soil moisture responds readily and predictably to changes in climate. However, considerable research is necessary to determine to the magnitude and timing of the response of soil fertility to changes in climate. At present it is also difficult to predict how changes in the seasonality of climate affect soil resources and the plant phenological patterns necessary to exploit these soil resources effectively. Changes in availability of soil resources lead to predictable changes in the types of plants that can be expected. In brief, high-resource environments support a high relative growth rate (RGR) through high capacities for photosynthesis and nutrient uptake, which in turn require high tissue-nitrogen concentrations. Continued high rates of resource capture require high rates of root and leaf turnover, a process that can be supported at relatively low cost in a high-resource environment. Conversely, in low-resource environments there are inadequate resources to support rapid growth, so plants are constrained to grow slowly and are likely to achieve a small size. Because of low tissue-nutrient concentrations, plants in low-resource environments have low potentials to photosynthesize, transpire, and absorb nutrients. Plants in these environments have high root-to-shoot ratios to maximise capture of scarce soil resources. Tissue-turnover rates are low, causing tissue nutrients to be retained for --a long time but also requiring effective chemical defense against herbivores and pathogens. These chemical defenses reduce litter quality and reinforce the low nutrient availability in these sites. Experimental evidence supporting these predictions has emerged mainly from studies in temperate ecosystems, although recent studies in tropical rain forests suggest similar patterns with respect to relative growth rate, root-

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shoot ratios and mineral nutrient requirements. Less is known about the highly diverse tropical deciduous forest in which the availability of soil resources is restricted by seasonal drought. In these forests tree-seedlings occupying resource-rich disturbed areas (e.g., Heliocarpus pallidus) show high relative growth rates, high demands for mineral nutrients and a low root/shoot ratio. These species also tend to be more sensitive to water and mineral nutrient stress. ENVIRONMENTAL RESPONSES OF PLANTS

Differences in resource requirements determine plant response to environmental fluctuations. Plants adapted to high availability of soil resources are sensitive to changes in nutrient supply and other resources such as light and water. In these habitats plants respond plastically to localised depletion zones around root systems through morphological changes in roots, res:ulting in reallocation of absorptive surfaces from depleted zones into the resource-rich areas. This system of patch exploitation requires shoots and roots with a short life span and high rate of tissue turnover. A high degree of morphological plasticity associated with active foraging will be of selective advantage only where it promotes access to large reserves of light, water, and mineral nutrients. Conversely, in habitats where productivity is chronically resource-limited, there is less morphological plasticity. On infertile soils we expect that resource capture and survival will depend on successful exploitation of resource pulses. In unproductive habitats, in which growth is frequently uncoupled from r~source capture, plasticity would involve reversible physiological changes rather than reallocation of biomass to facilitate exploitation of resource pulses. These generalisations come from screening programs which document the responses of plants to fluctuations in resource supply.

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The general predictions of the laboratory experiments described above are consistent with responses of communities to field manipulations. In artic tundra, when light temperature, and nutrients were manipulated to simulate patterns expected with climatic change, each species initially showed a species-specific, unique response to our manipulations, which was difficult to predict from general principles. However, after nine years of treatment, there continued to be species-specific, unpredictable responses to temperature, but relatively predictabl responses to nutrients: For example Betula nana and other rapidly growing deciduous shrubs increased growth in response to nutrient addition, whereas Ledum palustre and other slowly growing evergreen species responded negatively to nutrient addition. Similarly, all species except the most shade-tolerant understory species responded negatively to reduction in light intensity. These results suggest several important conclusions. First, the resource responses observed in the field were consistent with predictions of laboratory experiments in that rapidly growing species responded positively to nutrient addition but slowly growing species responded negatively to this improvement in resource supply, presumably through changes in competitive balance. Secondly, it was easier to predict responses to altered resource supply than to altered climate. The relatively minor increase in nutrient availability that occurred nine years after initiation of our temperature treatment suggests that our experiment was too short for climate to strongly alter soil resource supply. A third result of these experiments was that manipulations which benefited some species reduced the biomass of other species. Therefore, the overall production of the ecosystem was affected much less than the productivity of individual species or functional groups. Thus, the contrasting responses of individual species and functional groups

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buffer ecosystem processes such as production and nutrient cycling, rendering them relatively resistant to environmental change. The buffered response of ecosystem processes is also seen in an unmanipulated tundra ecosystem sampled over a period- of years. Over a series of years in which the productivity of individual species varied 2-8-fold, there was no significant variation in productivity of the total community. Addition of water and nutrients to grassland also gives predictable responses by functional groups of species. Rapidly growing forbs and grasses responded positively to nutrient and water addition, whereas the more slowly growing cactus responded negatively. Moreover, when grassland production was examined over a series of years, the productivity of individual species varied much less than did the productivity of the community as a whole, again indicating the extent to which ecosystem processes are buffered by the compensatory responses of individual species. ADAPTIVE STRATEGIES OF VEGETATION

Plants evolve a variety of adaptations to the light and moisture availability within a particular environment in order to flourish. Plants adaptations include those of leaf form and canopy structure (the roof of foliage formed by the crowns of trees). For instance, a hard, needle leaf structure is an adaptation to extreme temperatures and low moisture status in winter. The leaves of some rain forest trees have a special joint .at the bottom of their stalk that enables them to twist and turn to follow the light as the sun passes from east to west over head. Deciduous trees drop their leaves to cut transpiration loss during dry periods and when temperatures are very cold. Fleshy "leaves", like those of desert succulents or thick photosynthetic skin like that of the giant Saguaro cactus

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helps retain moisture. The Baobab tree, found in the wet/ dry tropical (savanna) climate stores water in its trunk to combat the long drought period experienced in that climate. Plants have adapted particular root structures to live in arid regions. Deep tap roots draw moisture hidden deep below the surface while extensive near - surface root systems catch moisture as it infiltrates into soil. Some desert grasses have rolled surfaces to reduce water loss from the inner surface and hairs which reduce air movement. Canopy structures reflect the environmental conditions vegetation grows in. The conical canopies of conifers help shed snow and catch low angle sun rays during the long winters where they grow. The rain forest displays a multi-layered canopy. Each layer possesses organisms adapted to the environmental conditions found in it. A canopy can be so thick and dense, like that found in the rain forest, that little light penetrates to the surface. The lack of light for understory growth creates an open forest that you can see into for some distance. Where canopy density is low, more light filters to the surface creating a thick ground cover and a closed forest. Rarely is any location dominated by a single specie of plant. A plant community refers to the associated plant species that form the natural vegetation of any place. For instance, a midlatitude forest is comprised of a community of trees, shrubs, ferns, grasses, and flowering herbs. Plant communities provide a habitat for animals and significantly modify the local environment. Plant communities affect soil type when organic material decomposes into the soil altering soil moisture retention, infiltration capacity, soil structure and soil chemiStry. Trees shade the forest floor, reducing incident' solar radiation and lowering temperatures of both the soil, and the air. Reduced incident light decreases evaporation

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keeping soils moister beneath the forest canopy. These impacts affect animal habitats and the diversity of animal species which are associated with these plant communities. An ecotone is a plant community in a distinct zone of transition between other more extensive communities. Ecotones vary in scale, from local (between forest and field) to global (savannas). Within an ecotone plants of different environmental tolerances often intermingle. For instance, grasses adapted to low moisture conditions intermingle with deciduous trees within a prairie - forest ecotone. Principle of Limiting Factors

The plants and animals that succeed in occupying a particular niche are those that can easily adapt to the unique environmental conditions of a site. Each plant and animal in the community has a specific range of tolerance for particular environmental conditions. Climate factors are the most important influence over the successful establishment of plant and animal communities. Two climatic factors are important, sunlight and moisture . Not •only is the amount of sunlight available important but the duration and quality of light are important too. For instance, at high altitudes the intense ultra violet light may inhibit the growth of particular plants. The intensity of light affects photosynthesis and rate of primary productivity. The duration of sunlight affects the flowering of plants and the activity patterns of animals. The availability of water is important for the survival of most life forms. But plants require water for a number of life processes like germination, growth and reproduction too. Principle" of limiting factors says that the maximum obtainable rate of photosynthesis is limited by whichever

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basic resource of plant growth is in least supply. The availability of energy and moisture varies geographically. At high latitudes the limiting factor is generally energy availability while in low latitudes moisture is the limiting factor to growth. The figure below shows the relationship between potential evapotranspiration (PE), a moisture index (MI), climate and vegetation. p

p

Cool Dry

Cool Wet

Moisture Index

Figure 7. Relationship between c1iTrUlte, vegetation potential evapotranspiration and the moisture index.

Potential evapotranspiration is the optimal amount of water entering the atmosphere as a result of evaporation and plant transpiration when there is an unlimited amount of moisture. Because evaporation and transpiration depend on energy availability, potential evapotranspiration is a measure of energy input. Highvalues of potential evapotranspiration relate to warm climates while low values to cool climates. The moisture index is a measure of moisture availability. High values of the moisture index means that plenty of water is available.

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Combining the two variables, potential evapotranspiration and moisture index we have a notion of what the climate is like in any part of the diagrams. For instance, high PE and large values of MI are indicative of warm and moist climates. Note that tundra and taiga (mostly conifers) are successfully established over a wide range of moisture conditions, from dry to moist, but always in cool environments. Other vegetation systems have more narrowly defined moisture and (emperature requirements. Plants of a particular region have adapted to the temperature and moisture conditions in which they live. Most gatd~.ners are familiar with plant hardiness (growing) zone maps. The zones are based on the minimum temperature experienced and thus tolerated by different species of plants. There have been recent signs that these zones are starting to shift due to global warming. PLANT SUCCESSION

Natural vegetation of a particular location evolves in a sequenCe of steps involving different plant communities. The evolutionary process is known as plant succession. Plant succession usually begins with a fairly simple community known as a pioneer community. The pioneer community, and each successive community alters the environment in such a way to permit new communities to occupy a site. These alterations of the environment include changes in site microclimate and soil conditions. A climax community is the result of a long period of plant succession. Climax communities usually exhibit a good deal of species diversity and thus are relatively stable systems. Disturbance renews a successional sequence. Plant succession was renewed after the explosion of Mt. St. Helens with the subsequent disruption of biotic communities that inhabited the region. Human

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disturbance related to tropical deforestation has renewed the successional sequence of plant communities in the tropical rain. forest. RESEARCH ON VEGETATION CHANGE

Most researchers who study vegetation change do so within a narrow conceptual framework. They do not regularly make use of the findings and insights of very similar studies being conducted in other research subdisciplines, nor do they try to make their findings and insights easily accessible to researchers in other areas. It seems obvious that plant ecology would benefit from better communication among the different research speciality areas. Indeed, communication across broad disciplinary horizons within ecology may be of value more generally. We propose three steps that individual researchers can take to increase the useful exchange of ideas and informa tion among these research areas. The fourth step should be undertaken by the scientific community as a whole. Invasion ecology, succession ecology, gap/patch dynamics, and studies of the effects of global change on plant communities all study vegetation in flux, that is, vegetation experiencing changes in species composition. Thus, each speciality area could be considered a part of a larger research initiative: the ecology of vegetation change. There is precedence for this perspective. Luken recognised vegetation change as the fundamental subject area relevant to all kinds of vegetation management. Thus, the first step is to be aware that related speciality areas exist. This step may be especially important for young researchers, such as doctoral students, who may have limited awareness of the scope of their and related research areas. If researchers began to envision themselves as studying vegetation change, rather than an invasion biologist, or succession ecologist, they would be less

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inclined to take a parochial perspective with respect to their research. The simplest step individual researchers can take to increase communication among speciality areas is to consciously seekout relevant ideas and data from related research areas. For example, an invasion ecologist investigating the community-wide consequences of an introduced species that is altering the soil-microbial community could recognise the value of seeking out findings obtained from similar studies conducted within a succession framework. Or, a researcher studying the effects of increases in nitrogen deposition on plant communities could recognise the relevance of the many invasion studies that have examined the impact of changing resource levels on plant community structure. A simple conceptual approach that could integrate succession and invasion ecology is to consider the various ways that species can facilitate and/or inhibit one another. Considering native and introduced species, and the possibility that each may inhibit or facilitate species in either group, eight types of potential interactions among the two groups of species can be identified. Studies of invasibility have shown that some native species are able to prevent, or at least slow, the establishment of introduced species. For example, Wedin and Tilman showed that the native grass Schizachyrium scoparium can inhibit the establishment and spread of the introduced grass Agropyron repens by its ability to depress levels of soil nitrogen. Studies in disturbed New Zealand forests have shown that Hakea sericea is inhibiting the reestablishment of the native shrub Leptospermum scoparium and native tree Kunzea ericoides. The latter two examples exemplify the fourth type of interaction. In a Hawaiian study, Carino and Daehler showed that an introduced legume, Chamaecrista nicitans, facilitated the subsequent invasion

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of another introduced species, the grass Pennisetum setaceum. The final three types of interactions are not typical subjects of either succession or invasion ecology, yet they do occur. Juniperus virginiana, a native tree in the US, has been found to facilitate the establishment of Rhamnus cathartica, an introduced and invasive tree, on some Mississippi River bluffs through a nurse plant effect on Rhamnus seedlings. In the western United States, crested wheatgrass (Agropyrum cristatum),- an introduced perennial, is known to impede the spread and establishment of the annual cheatgrass (Bromus tectorum), another introduced species. And, De Pietri showed that Rosa rubiginosa, a shrub introduced to Argentina, facilitated the reestablishment of several native woody species in disturbed subantarctic forests by reducing grazing herbivory on native seedlings growing beneath the thorny shrubs. This brief accounting of eight types of interaction between introduced and native species emph~sizes that it is the nature of the impact of the species that is important, not the place of origin. Regardless of the interaction type under investigation, all these studies are trying to answer the same two basic questions: what are the mechanisms that facilitate or inhibit the establishment and spread of particular plant species over time, and what are the consequences of these changes on community structure and ecosystem processes? Rather than the subdisciplines of succession and invasion ecology continuing down two parallel tracks, we think it makes more sense to -take a common and integrated approach. REFERENCES

Bartholomew, G.A., "The role of natural history in contemporary biology". Bioscience 36, 324-329. 1986.

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Christian, J.M., Wilson, S.D., "Long-term ecosystem impacts of an introduced grass in the northern Great Plains". Ecology 80,23972407.1999. Connell, J.H., Slatyer, R.O., "Mechanisms of succession in natural communities and their role in community stability and organization". Am. Nat. 111, 1119-1144. 1977. Davis, M.A., Thompson, K., Grime, J.P., Charles 5, "Elton and the dissociation of invasion ecology from the rest of ecology". Diversity Distrib. 7, 97-102. 2001. Glenn-Lewin, D.C., Peet, R.K., Veblen, T.T. (eds.), Plant Succession: Theory and Prediction. Chapman & Hall, New York. 1992.

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Index Alluvial soils 44 Anchoring rhizoids 134 Anti-bacterial chemotherapy 109 Aquatic environments 111 Aquatic organisms 186 Bacterial microcolonies 241 ~cterial phenotypes 243 Biogeochemical cycles 9 Bisexual gametophytes 127 Bryophytes 130 Chemosynthetic organisms 58 Coarse-textured soils 39 Continental drift theory 7 Crassulacean Acid Metabolism (CAM) 63 Devonian plant 138 Dikaryotic mycelium 116 Ecosystem ecology 229 Environmental microbiology 230 Epiphytes 181 Ethnomycology 109 Evapotranspiration 47

Field Capacity (FC) 53

Fine-textured soils 40 Fossillartd plants 5 Free-living organism 146 Gametophyte 126 Glyceraldehyde 3-Phosphate (G3P) 62 Gravitational water 53 Heterotrophs 58 Invasion biology 275 Land plants. 1 Latitudinal diversity gradient 190 Late Embryogenesis Abundant (LEA) 9 Lepidodendron 144 Life zones 189 Linnaeus' system 1 Liverwort sporophytes 134 Low-resource environments 285

Lypopolysaccharide (LPS) 245 Medium-textured soils 39 Moisture Index (Ml) 291 Molecular model systems 232 Motile sperm 131

Plant Ecology

302

Multicellular eukaryotes 108 Mycorrhizal networks 110 Net Primary Productivity (NPP) 196 Non-photosynthetic ancestors 2 Northern hemisphere peatlands 135 Nurse logs 74 Nutrient storage capacity 38

Phototropism 55 Physiological ecology 231 PhysiolOgical mechanisms 117 Plant life cycle 126 Plant nutrient ions 39 Plantae sensu lato 2 Plant·pathogenic fungi 111 Polypodiopsida 145 Potential Evapotranspiration (PE) 291 Psychotropic compounds 108 Purple nonsulfur bacteria 60

Oxygenic photosynthesis 61 Permanent Wilting Point (PWP) 53 Permian-triassic boundary 120 Phage community ecology 233 Phage community-ecology theory 245 Phage ecology 229 Phage ecosystem ecology 234 Phage organismal ecology 238 Phage population ecology 233 Phage-ecological interactions 227 Phage-resistant bacterial phenotype 244 Phosphoenolpyruvate (PEP)

Relative Growth Rate (RGR) 285 Relative Humidity (RH) 68 Secondaly minerals 39 Soil-plant-atmbSphere continuum 49 Sporophyte growth 133 Successional communities 192 Taiga forests 185 Temperate grasslands 183 Temperate-phage mutation 237 Terrestrial ecosystems 109 Triangular cross-section 73

63 Phosphoglyceraldehyde (PGAL) 62 Photosynthetic plants 4 Photosynthetic systems 60

Vascular plant 137 Vrridiplantae 1 Woody plants 74