Ecosystems of disturbed ground

ECOSYSTEMS OF THE WORLD 16

ECOSYSTEMS OF DISTURBED GROUND

ECOSYSTEMS OF THE WORLD Editor in Chief: David W. Goodall Centre for Ecosystem Studies, Edith Cowan University, Joondalup, W.A. (Australia)

I. TERRESTRIAL ECOSYSTEMS

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

A. Natural Terrestrial Ecosystems Wet Coastal Ecosystems Dry Coastal Ecosystems Polar and Alpine Tundra Mires: Swamp, Bog, Fen and Moor Temperate Deserts and Semi-Deserts Coniferous Forests Temperate Deciduous Forests Natural Grasslands Heathlands and Related Shrublands Temperate Broad-Leaved Evergreen Forests Mediterranean-Type Shrublands Hot Deserts and Arid Shrublands Tropical Savannas Tropical Rain Forest Ecosystems Wetland Forests Ecosystems of Disturbed Ground

17. 18. 19. 20. 21.

B. Managed Terrestrial Ecosystems Managed Grasslands Field Crop Ecosystems Tree Crop Ecosystems Greenhouse Ecosystems Bioindustrial Ecosystems

II. AQUATIC ECOSYSTEMS

22. 23.

A. Inland Aquatic Ecosystems River and Stream Ecosystems Lakes and Reservoirs

24. 25. 26. 27. 28.

B. Marine Ecosystems Intertidal and Littoral Ecosystems Coral Reefs Estuaries and Enclosed Seas Ecosystems of the Continental Shelves Ecosystems of the Deep Ocean

29.

C. Managed Aquatic Ecosystems Managed Aquatic Ecosystems

III. UNDERGROUND ECOSYSTEMS 30.

Subterranean Ecosystems

ECOSYSTEMS OF THE WORLD 16

ECOSYSTEMS OF DISTURBED GROUND Edited by Lawrence R. Walker Department of Biological Sciences, University of Nevada, Las Vegas, 4505 Maryland Parkway, Box 454004, Las Vegas, NV 89154-4004, USA

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cm. − − (Ecosystems of the world ; 16)

Includes bibliographical references. ISBN 0−444−82420−0 R.

1. Ecology. 2. Nature− −Effect of human beings on. II. Series.

QH545.A1E2824

I. Walker, L.

1999

577.27− −dc21

99-41984 CIP

ISBN 0 444 82420 0 (Volume) ISBN 0 444 41702 8 (Series) ∞ The paper used in this publication meets the requirements of ANSI/NISO Z39.48-1992 (Permanence of Paper). 

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PREFACE As the human population inexorably grows, its cumulative impacts on the earth’s resources are hard to ignore. The ability of the earth to support more humans is dependent on the ability of humans to manage natural resources wisely. Because disturbance alters resource levels, effective management requires understanding of the ecology of disturbance. Editorship of this book was undertaken with several goals in mind. First, I wanted to present an organized summary of the many types of disturbances that impact the earth, with as global a focus as the existing literature allowed. The book is organized into chapters that deal primarily with natural disturbances (Chapters 2–13), anthropogenic disturbances (Chapters 14–20), overviews of natural processes that occur across disturbance types (Chapters 21–27), and human interactions with and responses to disturbance (Chapters 28–30). Chapter 31 explores a hierarchical view of disturbance; Chapter 32 examines the concept that the consequences of growth of the human population themselves represent the ultimate disturbance, and suggests ways to ameliorate human impacts. “Natural” and “anthropogenic” disturbances generally are interrelated. The focus of this book is on disturbances that have a direct physical impact on terrestrial systems, excluding primarily atmospheric phenomena such as acid rain or increases in carbon dioxide and decreases in ozone (but not wind), and primarily aquatic phenomena such as cultural eutrophication or chemical, thermal, and bacterial pollution of waterways. A second purpose was to enhance understanding of the concept of disturbance in order to manage it better. The development of theory related to disturbance is in its infancy. One of the few book-length overviews of the topic was that provided by Pickett and White (1985), in which they focused on alterations of relatively pristine habitats. With the wealth of examples from around the world presented in this volume, perhaps we can make further progress toward a general theory of disturbance. Alternatively, one may recognize that site-specific characteristics do not allow such generalizations. To those ends, authors were given some freedom to interpret disturbance as they deemed appropriate. Authors addressing a particular

disturbance type or disturbance in a particular habitat (Chapters 2–20) were asked to address, as far as possible, the disturbance regime, the damage caused by the disturbance, the responses of the biota to the disturbance, and interactions between disturbance types and between disturbance and humans. Authors of process chapters (Chapters 21–27) were to seek generalizations from a global perspective of short- and longer-term responses across various types of disturbed ecosystems. In the final Chapter 33, Willig and I evaluate common threads in the previous chapters and make contributions toward the development of a theory about disturbance. A third goal was to provide insights for land managers on how to incorporate lessons about disturbance into their efforts. Some of the chapters (e.g., Chapters 11, 14–16, 18–20) explicitly address managed systems such as pasture-land, urban habitats, and agriculture; other chapters address management issues more generally. Any level of generalization that can be made about disturbance responses will aid managers of disturbed ecosystems. Human well-being on this increasingly crowded planet depends on the success to which land management policies (see Chapter 30) apply such lessons. I wish to thank the many chapter authors for their patience, hard work, excellent insights, and chapter reviews; the 36 non-author peer reviewers for their constructive criticisms that made my job easier; the series editor David Goodall for overall guidance and careful editing; Rachel Lawrence and Dorothy Dean for assistance with correspondence and proofing; the University of Nevada, Las Vegas (UNLV) for providing substantial logistical support; colleagues and graduate students of ecology at UNLV for engaging discussions; Michael Willig for helping me to synthesize the whole volume; and my wonderful wife, Elizabeth Powell, for her continual support. During the development of the ideas presented here, I was supported by NSF grants BSR-8811902 and DEB-9411973 to the Institute for Tropical Ecosystem Studies, University of Puerto Rico, and the International Institute of Tropical Forestry, as part of the Long-Term Ecological Research Program in v

vi

PREFACE

the Luquillo Experimental Forest. Additional support was provided by the U.S. Fish and Wildlife Service, the U.S. National Park Service, and the U.S. Forest Service. Finally, the completion of this book was facilitated by a sabbatical leave from UNLV. Lawrence R. Walker Editor

REFERENCES Pickett, S.T.A. and White, P.S., 1985. The Ecology of Natural Disturbance and Patch Dynamics. Academic Press, Orlando, Florida, 472 pp.

LIST OF CONTRIBUTORS E.B. ALLEN Department of Botany and Plant Sciences University of California Riverside, CA 92521-0124, USA

M.L. CADENASSO Institute of Ecosystem Studies Box AB Millbrook, NY 12545-0129, USA

M.F. ALLEN Department of Biology Soil Ecology and Restoration Group San Diego State University San Diego, CA 92182-0057, USA

C.R. CARROLL Institute of Ecology University of Georgia Athens, GA 30602, USA J.A. COOKE School of Life and Environmental Sciences University of Natal Durban 4041, South Africa

D. BAINBRIDGE Department of Biology Soil Ecology and Restoration Group San Diego State University San Diego, CA 92182-0057, USA

C.M. D’ANTONIO Department of Integrative Biology University of California Berkeley Berkeley, CA 94720, USA

A.H. BALDWIN Natural Resources Management Program Department of Biological Resources Engineering University of Maryland College Park, MD 29742, USA

R. DEL MORAL Department of Botany University of Washington Box 355325 Seattle, WA 98195-5325, USA

C.J. BARROW Centre for Developmental Studies University College of Swansea University of Wales Swansea SA2 8PP, United Kingdom

S. DEMARAIS Department of Wildlife and Fisheries Mississippi State University Mississippi State, MS 39762, USA

D. BINKLEY Department of Forest Sciences Colorado State University Fort Collins, CO 80523, USA

M.B. DICKINSON Department of Biological Sciences Florida State University Tallahassee, FL 32306-2043, USA

I.K. BRADBURY Dept. of Geography University of Liverpool P.O. Box 147 Liverpool L69 3BX, United Kingdom

C. DOLJANIN Department of Biology Soil Ecology and Restoration Group San Diego State University San Diego, CA 92182-0057, USA

N.V.L. BROKAW Manomet Center for Conservation Sciences P.O. Box 1770 Manomet, MA 02345, USA vii

viii

LIST OF CONTRIBUTORS

T.L. DUDLEY Department of Integrative Biology University of California Berkeley Berkeley, CA 94720, USA

G.S. HARTSHORN Organization for Tropical Studies Box 90630 Durham, NC 27708-0630, USA

G.E. ECKERT 1434 Pine Street Norristown, PA, USA

C. HARVEY Department of Entomology Cornell University 5126 Comstock Hall Ithaca, NY 14853-0901, USA

F. EDWARDS Department of Biology Soil Ecology and Restoration Group San Diego State University San Diego, CA 92182-0057, USA O. ENGELMARK Swedish Centre for Ecological Sustainability (Swecol) S-901 87 Ume¨a, Sweden C.M. GHERSA Departamento de Ecolog´ıa Facultad de Agronom´ıa, Universidad de Buenos Aires Av. San Martin 4453 1417 Buenos Aires, Argentina M. GIAMPIETRO Istituto Nazionale della Nutrizione Unit of Special Food Technology Via Ardeatina 546 00178 Rome, Italy S.Yu. GRISHIN Institute of Biology and Pedology Russian Academy of Sciences Vladivostok 690022, Russia P.J. GUERTIN Environmental Division U.S. Army Construction and Engineering Research Lab Box 4005 Champaign, IL 61820, USA S. HARNEY Department of Biology Soil Ecology and Restoration Group San Diego State University San Diego, CA 92182-0057, USA

C. HINKSON Department of Biology Soil Ecology and Restoration Group San Diego State University San Diego, CA 92182-0057, USA R.J. HOBBS CSIRO Wildlife and Ecology Private Bag, PO Wembley WA 6014, Australia D.W. JOHNSON Biological Sciences Center Desert Research Institute P.O. Box 60220 Reno, NV 89506, USA E.E. JORGENSEN Department of Range, Wildlife and Fisheries Management Texas Tech University Lubbock, TX 79409, USA ´ ´ V. KOMARKOV A Villa Elisabeth 5 CH-1854 Leysin, Switzerland ´ R.J.C. LEON Departamento de Ecolog´ıa Facultad de Agronom´ıa, Universidad de Buenos Aires Av. San Martin 4453 1417 Buenos Aires, Argentina J. LORETI Facultad de Agronom´ıa, Universidad de Buenos Aires Av. San Martin 4453 1417 Buenos Aires, Argentina

LIST OF CONTRIBUTORS

ix

M.D. LOWMAN The Mary Selby Botanical Gardens 811 S. Palm Ave. Sarasota, FL 34236, USA

S.T.A. PICKETT Institute of Ecosystem Studies Box AB Millbrook, NY 12545-0129, USA

R. MACALLER Department of Biology Soil Ecology and Restoration Group San Diego State University San Diego, CA 92182-0057, USA

D. PIMENTEL Department of Entomology Cornell University 5126 Comstock Hall Ithaca, NY 14853-0901, USA

M. MACK Department of Integrative Biology University of California Berkeley Berkeley, CA 94720, USA

M. RILLIG Department of Biology Soil Ecology and Restoration Group San Diego State University San Diego, CA 92182-0057, USA

J.A. MACMAHON College of Science Utah State University UMC 4400 Logan, UT 84322-4400, USA

P.W. RUNDEL Department of Biology University of California Los Angeles Los Angeles, CA 90024, USA

J.A. MATTHEWS Department of Geography University of Wales Swansea Singleton Park Swansea SA2 8PP, United Kingdom

T.D. SCHOWALTER Entomology Department Oregon State University Corvallis, OR 97331-2907, USA

M.A. MCGINLEY Ecology Program Department of Biological Sciences Texas Tech University Lubbock, TX 79409-313, USA

B. SCHULTZ Biological Sciences Center Desert Research Institute P.O. Box 60220 Reno, NV 89506, USA

K.L. MCKEE National Wetlands Research Center 700 Cajundome Blvd. Lafayette, LA 70506, USA

M. SEMMARTIN Facultad de Agronom´ıa, Universidad de Buenos Aires Av. San Martin 4453 1417 Buenos Aires, Argentina

M. OESTERHELD Departamento de Ecolog´ıa Facultad de Agronom´ıa, Universidad de Buenos Aires Av. San Martin 4453 1417 Buenos Aires, Argentina

¨ C. SIGUENZA Department of Botany and Plant Sciences University of California Riverside, CA 92521-0124, USA

J.M. PARUELO Facultad de Agronom´ıa, Universidad de Buenos Aires Av. San Martin 4453 1417 Buenos Aires, Argentina

R.E. SOJKA USDA Agricultural Research Service Northwest Irrigation and Soils Research Lab 3793N-3600E Kimberley, ID 83341, USA

x

LIST OF CONTRIBUTORS

U. STARFINGER ¨ Institut f¨ur Okologie Technische Universit¨at Berlin Schmidt-Ott Strasse 1 D-12165 Berlin, Germany

F. WIELGOLASKI Department of Biology, University of Oslo, Box 1045, Blindern N-0316 Oslo, Norway

H. SUKOPP ¨ Institut f¨ur Okologie Technische Universit¨at Berlin Schmidt-Ott Strasse 1 D-12165 Berlin, Germany

M.R. WILLIG Ecology Program Department of Biological Sciences and the Museum Texas Tech University Lubbock, TX 79409-313, USA

D.J. TAZIK Environmental Division U.S. Army Construction and Engineering Research Lab Box 4005 Champaign, IL 61820, USA

S.D. WILSON Department of Biology University of Regina Regina, Saskatchewan S45 0A2, Canada

L.R. WALKER Department of Biological Sciences University of Nevada, Las Vegas 4505 Maryland Parkway Box 454004 Las Vegas, NV 89154-4004, USA S.L. WEBB Biology Department Drew University Madison, NJ 07940-4000, USA D.F. WHIGHAM Smithsonian Environmental Research Center Box 28 Edgewater, MD 21037, USA J.L. WHITMORE Vegetation Management and Protection Research USDA Forest Service, Box 96090 Washington, DC 220090-6090, USA

J. WU Department of Life Sciences Arizona State University West Box 37100 Phoenix, AZ 85069, USA L.C. YOSHIDA Department of Botany and Plant Sciences University of California Riverside, CA 92521-0124, USA T.A. ZINK Department of Biology Soil Ecology and Restoration Group San Diego State University San Diego, CA 92182-0057, USA

CONTENTS PREFACE

. . . . . . . . . . . . . . . . . . . . . . .

LIST OF CONTRIBUTORS

. . . . . . . . . . . . . .

Chapter 1. AN INTRODUCTION TO TERRESTRIAL DISTURBANCES by L.R. Walker and M.R. Willig . . . . . . Chapter 2. DISTURBANCE REGIMES AND ECOSYSTEM RESPONSE ON RECENTLY-DEGLACIATED SUBSTRATES by J.A. Matthews . . . . . . . . . . . . .

v vii

1

17

Chapter 3. STRESS AND DISTURBANCE IN COLD REGION ECOSYSTEMS by V. Kom´arkov´a and F.E. Wielgolaski . .

39

Chapter 4. ECOLOGICAL EFFECTS OF EROSION by D. Pimentel and C. Harvey . . . . . . .

123

Chapter 5. VOLCANIC DISTURBANCES AND ECOSYSTEM RECOVERY by R. del Moral and S.Yu. Grishin . . . . .

137

Chapter 6. BOREAL FOREST DISTURBANCES by O. Engelmark . . . . . . . . . . . . . .

161

Chapter 7. DISTURBANCE BY WIND IN TEMPERATEZONE FORESTS by S.L. Webb . . . . . . . . . . . . . . . Chapter 8. BACKGROUND CANOPY GAP AND CATASTROPHIC WIND DISTURBANCES IN TROPICAL FORESTS by D.F. Whigham, M.B. Dickinson and N.V.L. Brokaw . . . . . . . . . . . . . . . Chapter 9. FOREST HERBIVORY: INSECTS by T.D. Schowalter and M.D. Lowman

. . .

Chapter 10. DISTURBANCE IN MEDITERRANEANCLIMATE SHRUBLANDS AND WOODLANDS by P.W. Rundel . . . . . . . . . . . . . . Chapter 11. GRAZING, FIRE, AND CLIMATE EFFECTS ON PRIMARY PRODUCTIVITY OF GRASSLANDS AND SAVANNAS by M. Oesterheld, J. Loreti, M. Semmartin and J.M. Paruelo . . . . . . . . . . . . . . . .

187

Chapter 12. DISTURBANCE IN DESERTS by J.A. MacMahon . . . . . . . . . . . .

307

Chapter 13. DISTURBANCE REGIMES IN NORTH AMERICAN WETLANDS by K.L. McKee and A.H. Baldwin . . . . .

331

Chapter 14. MINING by J.A. Cooke

. . . . . . . . . . . . . . .

365

Chapter 15. DISTURBANCE ASSOCIATED WITH MILITARY EXERCISES by S. Demarais, D.J. Tazik, P.J. Guertin and E.E. Jorgensen . . . . . . . . . . . . . . .

385

Chapter 16. DISTURBANCE IN URBAN ECOSYSTEMS by H. Sukopp and U. Starfinger . . . . . .

397

Chapter 17. DISTURBANCE AND BIOLOGICAL INVASIONS: DIRECT EFFECTS AND FEEDBACKS by C.M. D’Antonio, T.L. Dudley and M. Mack . . . . . . . . . . . . . . . . . .

413

Chapter 18. DISTURBANCE IN TEMPERATE FORESTS OF THE NORTHERN HEMISPHERE by D. Binkley . . . . . . . . . . . . . . .

453

Chapter 19. ANTHROPOGENIC DISTURBANCE AND TROPICAL FORESTRY: IMPLICATIONS FOR SUSTAINABLE MANAGEMENT by G.S. Hartshorn and J.L. Whitmore . . .

467

Chapter 20. SUCCESSIONAL CHANGES IN AGROECOSYSTEMS OF THE ROLLING PAMPA by C.M. Ghersa and R.J.C. Le´on

. . . . .

487

Chapter 21. PHYSICAL ASPECTS OF SOILS OF DISTURBED GROUND by R.E. Sojka . . . . . . . . . . . . . . .

503

Chapter 22. SOIL MICROORGANISMS by M.F. Allen, E.B. Allen, T.A. Zink, S. Harney, L.C. Yoshida, C. Sig¨uenza, F. Edwards, C. Hinkson, M. Rillig, D. Bainbridge, C. Doljanin and R. MacAller . . . . . . .

521

Chapter 23. RESPONSES OF CARBON AND NITROGEN CYCLES TO DISTURBANCE IN FORESTS AND RANGELANDS by D.W. Johnson and B. Schultz . . . . . .

545

223

253

271

287

xi

xii Chapter 24. DISTURBANCE AND PRIMARY PRODUCTION IN TERRESTRIAL ECOSYSTEMS by I.K. Bradbury . . . . . . . . . . . . . . Chapter 25. PATTERNS AND PROCESSES IN PRIMARY SUCCESSION by L.R. Walker . . . . . . . . . . . . . . Chapter 26. PLANT INTERACTIONS DURING SECONDARY SUCCESSION by S.D. Wilson . . . . . . . . . . . . . . Chapter 27. THE RESPONSE OF ANIMALS TO DISTURBANCE AND THEIR ROLES IN PATCH GENERATION by M.R. Willig and M.A. McGinley . . . . Chapter 28. HOW HUMANS RESPOND TO NATURAL OR ANTHROPOGENIC DISTURBANCE by C.J. Barrow . . . . . . . . . . . . . . .

CONTENTS

571

585

611

Chapter 31. PATCH DYNAMICS AND THE ECOLOGY OF DISTURBED GROUND: A FRAMEWORK FOR SYNTHESIS by S.T.A. Pickett, J. Wu and M.L. Cadenasso

707

Chapter 32. ECONOMIC GROWTH, HUMAN DISTURBANCE TO ECOLOGICAL SYSTEMS, AND SUSTAINABILITY by M. Giampietro . . . . . . . . . . . . .

723

Chapter 33. DISTURBANCE IN TERRESTRIAL ECOSYSTEMS: SALIENT THEMES, SYNTHESIS, AND FUTURE DIRECTIONS by M.R. Willig and L.R. Walker . . . . . .

747

GLOSSARY

769

. . . . . . . . . . . . . . . . . . . . . .

633 SYSTEMATIC LIST OF GENERA 659

AUTHOR INDEX

. . . . . . . . . .

773

. . . . . . . . . . . . . . . . . . .

777

SYSTEMATIC INDEX

Chapter 29. RESTORATION OF DISTURBED ECOSYSTEMS by R.J. Hobbs . . . . . . . . . . . . . . .

673

Chapter 30. ENVIRONMENTAL POLICIES AS INCENTIVES AND DISINCENTIVES TO LAND DISTURBANCE by G.E. Eckert and C.R. Carroll . . . . . .

689

GENERAL INDEX

. . . . . . . . . . . . . . . .

821

. . . . . . . . . . . . . . . . . .

833

Chapter 1

AN INTRODUCTION TO TERRESTRIAL DISTURBANCES Lawrence R. WALKER and Michael R. WILLIG

is now 5.8×109 and is projected to reach 10–12×109 by the year 2040. What are the consequences of such growth? What is the carrying capacity of the earth (Cohen, 1995)? Can human intelligence and technology prevent or even postpone a global collapse? Estimates of the ecological footprint (a concept that calculates how much arable land is needed to sustain a given level of energy consumption per member of a human population; Wackernagel and Rees, 1996) of those countries with the highest standards of living already are 15 times greater in area than the geographical space they occupy. Clearly, the world does not have the resources to sustain the entire human population at a standard of living similar to that in the more affluent nations of the world. Giampietro (Chapter 32, this volume) explores ways in which wise resource management and curtailment of resource abuse can improve the future prospects of humans and the biosphere.

WHY STUDY DISTURBANCE?

Dramatic, large-scale natural disturbances (e.g., volcanic eruptions, fires, hurricanes, floods) are important to understand because they destroy property, cause human injury, and disrupt emotional lives. Human interference with natural disturbances (e.g., fire suppression) may actually make them more destructive (e.g., larger, hotter fires: Bond and van Wilgen, 1996). Disturbances are also important to all living organisms because they have beneficial effects such as nutrient recycling, resetting of successional pathways, and maintenance of species diversity (Luken, 1990). The exponential increase in human population density guarantees that more people are affected by natural disturbances every year. It is clear that one needs to continue efforts to predict and avoid disturbances, minimize damage, and maximize the ability of human society to restore degraded systems. Some anthropogenic disturbances are well publicized (e.g., spills of oil or toxic waste, bomb explosions). Yet the more gradual disturbances that do not receive as much attention, such as urbanization, excavation of minerals, soil erosion as a result of agriculture, or logging of forests, may have far greater consequences. In fact, anthropogenic disturbances are ubiquitous and all ecosystems of the world are disturbed at least partially by human activities. Both natural and anthropogenic disturbances clearly impact the entire earth. Understanding how to live with or mitigate natural disturbances, and moderate the consequences of human actions, is imperative (Thomas, 1956; Botkin et al., 1989). The consequences of increased human population represent the ultimate disturbance. Humans currently consume or utilize 40% of the earth’s primary production (Vitousek et al., 1986). The human population

PERSPECTIVES ON DISTURBANCE

Disturbances have been the subject of many myths and legends. Gods have been associated with disturbances such as volcanoes (Ixtocewatl and Pococatepetl in Mexico; Vulcan in ancient Rome; Pele in Hawaii), windstorms (Luquillo in Puerto Rico; Hurakan in Mayan culture), floods (Janaina in Brazil; Poseidon in ancient Greece), and fire (Loki in Norse mythology; Prometheus in ancient Greece). The biblical Noah dealt with a flood, and Moses’ enemies were subjected to a herbivore (locust) outbreak. Disturbances have directly altered human history. Volcanoes have destroyed cities (e.g., Pompeii in Italy; St. Pierre in Martinique) and altered world climates (Krakatau in Indonesia) (Sheets and Grayson, 1979; 1

2

Simkin and Fiske, 1983). Hurricanes have repeatedly damaged buildings and biota (e.g., Hurricane Hugo in the Caribbean and the eastern United States: B´enitoEspinal and B´enito-Espinal, 1991; Finkl and Pilkey, 1991; Walker et al., 1991, 1996). Fertile soils along river floodplains (e.g., the Nile, Tigris, or Euphrates) have nurtured civilizations, but often at the cost of extensive losses of lives and property (Officer and Page, 1993). Famous fires have altered the histories of cities such as Chicago, Rome and San Francisco, and the vegetation of entire continents (Komarek, 1983). Biotic disturbances are perhaps most damaging. The Black Death killed one-third of all people in medieval Europe, and many Native Americans died from diseases such as smallpox and malaria introduced by Europeans (cf. Crosby, 1986; Officer and Page, 1993). Cultural and environmental concerns traditionally have been shaped by the interplay between resource availability and the local disturbance regime. Degradation of land caused by erosion and deforestation was noted by Greek and Roman writers, and Confucianism in China addressed environmental concerns (Barrow, 1991). Humans typically have responded to natural disturbances by management (use of fire by many native cultures), exploitation (use of early-successional plants for food), or avoidance (minimal use of deserts, lava fields, and glacial valleys). Attitudes toward natural resources can evolve from exploitation to conservation when human population densities reach local carrying capacities. However, the demise of some societies [e.g., the Maya in Central America, the Hohokam in Arizona (U.S.A.), and the Assyrians in Mesopotamia] has been attributed in part to the collapse of the local resource base from over-exploitation (Thomas, 1956). The remarkable ability of humans to accommodate to naturally or anthropogenically caused environmental change (or to migrate out of disturbed areas – as with the Dust Bowl in Oklahoma, U.S.A.: Worster, 1979) suggests that most disturbances modify but do not destroy cultures. Most landscapes are now the product of a long history of human land use (e.g., the Mediterranean basin: Rundel, Chapter 10, this volume). For the last 100–200 years, Western cultures have been systematically recording observations about various natural disturbances (e.g., volcanoes: Whittaker et al., 1989; glaciers: Chapin et al., 1994) and anthropogenic disturbances (e.g., changes in levels of atmospheric carbon dioxide: Vitousek, 1994) and ecosystem responses to disturbance (e.g., succession:

Lawrence R. WALKER and Michael R. WILLIG

Clements, 1928). Such long-term observations allow an examination of disturbance on various time scales with the partitioning of short-term fluctuations from longerterm cycles (Magnuson, 1990). They also facilitate the distinction of human impacts from natural fluctuations. Recognition of the role of humans in global warming or acid rain, and the growing impacts of mining, agriculture, and urbanization have increased environmental awareness in recent decades. This awareness has fostered the growth of environmental politics (e.g., the Green Parties in Europe), entrepreneurism (e.g., the purchase of natural areas by private agencies such as the Nature Conservancy operating from the United States), and cooperation at the local level (restoration activities), the regional level (credits to companies that reduce pollution), and the global level (relief of national debt in exchange for establishment of nature reserves). Interactions of culture and disturbance are further discussed in this volume by Ghersa and Leon (Chapter 20), Barrow (Chapter 28), Hobbs (Chapter 29), Eckert and Carroll (Chapter 30) and Giampietro (Chapter 32).

DEFINITIONS OF DISTURBANCE

As the literature on disturbance ecology has proliferated in the last two decades, so too has the lexicon. Nonetheless, maturation of the science requires a precise use of terminology along with straightforward clarification when terms are used in different ways. At the same time, terms should be sufficiently general so that they are useful to an appreciable segment of the practitioners in the discipline. On occasion, growth of a discipline can be stymied significantly by vague or illdefined terminology, in part because synthesis requires incisive understanding and in part because confusion over terminology can lead to division among practitioners who disagree about definitions. Such semantic differences can give the impression of disagreement over substantive or conceptual issues, lead to heated or senseless debate, and delay the maturation of a scientific discipline. We do not attempt to resolve such semantic and conceptual differences here. Indeed, authors contributing to this volume were given broad latitude in the use of terms so as to engender individual creativity. Nonetheless, we follow White and Pickett (1985) and provide an introduction to widely accepted meanings of selected terms in the lexicon of disturbance ecology,

AN INTRODUCTION TO TERRESTRIAL DISTURBANCES

so that the general reader will have an appreciation of the scope of the discipline, and specialists will be motivated to provide more detailed definitions or alternate terminology as appropriate (see Pickett et al., Chapter 31, this volume). A disturbance is a relatively discrete event in time and space that alters the structure of populations, communities, and ecosystems. It can do so by altering the density, the biomass, or the spatial distribution of the biota, by affecting the availability and distribution of resources and substrate, or by otherwise altering the physical environment. It often results in the creation of patches and the modification of spatial heterogeneity. Disturbance is a relative term that requires explicit delineation of the system of concern, including the spatial and temporal scale of the components of interest. The cause of a disturbance may be thought of as the agent or entity initiating the changes in the structure of the ecological system of interest. For example, highspeed winds are agents of disturbance for hurricanes. If the cause originates outside the system of interest, as is the situation for hurricanes, the disturbance is considered to be exogenous, whereas if the cause of the disturbance originates inside the system of interest, as when a tree-fall results from natural senescence, the disturbance is considered to be endogenous. Clearly, definition of the system of interest is integral to such considerations, and a clear distinction is not always possible. The likelihood of an exogenous disturbance may be affected by the state of the system of interest and characteristics of endogenous disturbances may be affected by characteristics of previous exogenous disturbances. Indeed, the dichotomy between purely endogenous and exogenous disturbances might more appropriately be considered as a continuum of intermediate possibilities. Disturbances are most often characterized by the central tendency, variability, and distribution of three attributes: frequency, extent, and magnitude. Frequency measures the number of events per unit of time or the probability that an event will occur. Extent is the actual physical area affected by a disturbance. It can be estimated from the area of a single event (e.g., a tree-fall), or from the sum of the areas affected by equivalent events over a particular time period (e.g., gap area created by all tree-falls in a year). Extent is often reported as the proportion of an entire landscape in which a particular disturbance occurred

3

in a given time period. Magnitude includes two interrelated attributes: intensity and severity. Intensity is the physical force of an event (e.g., wind-speed for hurricanes), whereas the impact on or consequences to the system of interest is the severity (e.g., the biomass of trees that were killed by passage of a hurricane). Intensity and severity are usually correlated, and the terms often are used interchangeably, at least in part, because the physical forces of many disturbances, especially those generated by the biota (e.g., treefalls, rodent mounds, insect outbreaks) are difficult to quantify. Clearly, severity reflects the response of the biota to the disturbance and may not be fully documented until a considerable time has elapsed since the disturbance event impinged on the system of interest. Most systems are simultaneously subjected to a number of disturbances (e.g., hurricanes, landslides, tree-falls, herbivory, droughts, and human activities all affect the structure and function of Caribbean forests). The sum of all disturbances at a particular place and time is termed the disturbance regime. The different disturbance events enhance or diminish the frequency, extent, or magnitude of other disturbances. Such interactions are considered synergisms, and are important considerations to address in understanding disturbance and recovery in ecological systems.

TYPES OF DISTURBANCE

Because virtually every habitat experiences some level of disturbance, no book can easily cover the entire topic. This book focuses on disturbances that physically impact the ground. It does not address atmospheric or aquatic disturbances. Primarily natural disturbances (Chapters 2–13) can be categorized by the four classical elements: earth, air, water, and fire (Table 1.1). Disturbances linked to the earth are independent of all causal factors other than tectonic forces (del Moral and Grishin, Chapter 5, this volume). Disturbances involving air, water, and fire are primarily driven by an interplay of climatic, topographic, and soil factors. In addition, biotic variables influence fire and are represented by both non-human disturbances (e.g., herbivory) and human disturbances (Table 1.1). Disturbances often trigger other disturbances, so that there is an interlacing web of disturbance interactions (for a detailed example, see Fig. 33.2 below). For instance, volcanoes can trigger earthquakes, earthquakes

4

Lawrence R. WALKER and Michael R. WILLIG

Table 1.1 Examples of some of the major types of disturbance of the earth 1 Element

Primary disturbance 2

Earth (tectonic)

earthquake (1) erosion (>50) volcano (1)

Air

hurricane (15) tornado (<1) tree-fall (nd)

Water

drought (30) flood (15) glacier (10)

Fire

fire (>50)

Biota – non-human

herbivory (nd) invasion (nd) other animal activity 3 (nd)

Biota – human

agriculture (45) forestry (10) mineral extraction (1) military activity 4 (1–40) transportation 5 (5) urban (3)

1

Data from many sources; nd = no data available. Approximate percent of earth’s terrestrial surface regularly affected by each disturbance is in parentheses. 3 Includes building, excavating, waste products, movement, death, diseases, parasites. 4 U.S.A., 1%; Vietnam, 40%. 5 Includes motorized and non-motorized transportation. 2

or hurricanes can trigger landslides, hurricanes or landslides can induce flooding, and flooding can cause landslides. These interactions may augment, diminish, or neutralize the interacting disturbances. Anthropogenic disturbances are, of course, always interacting with natural disturbances (e.g., road-building can trigger a landslide). A hierarchical view of disturbance types (cf. O’Neill et al., 1986; Pickett et al., 1987) may be most useful in examining disturbance interactions, and in making spatial and temporal scales explicit for each disturbance under consideration. When the common types of disturbance of the world (from Table 1.1) are compared by frequency, extent, and severity using a subjective ranking procedure (1, least; 5, most), several patterns emerge (Fig. 1.1). Primarily anthropogenic disturbances are usually greater in extent (mean score = 3.2) than

Fig. 1.1. The frequency, spatial extent, and severity of 19 types of disturbance throughout the world based on their subjectively ranked scores from 1 (least) to 5 (most). Intensity and severity scores were highly correlated and thus are represented on a single axis. Disturbances are: AG, agriculture; AN, animal activities; DR, drought; EA, earthquakes; ER, erosion; FI, fire; FL, flooding; FO, forestry; GL, glaciers; HE, herbivory; HU, hurricanes; IN, invasions; MI, mining; ML, military; TF, tree falls; TO, tornadoes; TR, transportation; UR, urban; VO, volcanoes. Anthropogenic disturbances are shaded. Uncircled letters occupy the same location as adjacent circles (VO, GL; EA, TO; IN, AG).

natural disturbances (mean score = 2.1), presumably because of the cosmopolitan distribution of humans. Anthropogenic disturbances are also slightly more severe (mean score = 3.8) than natural disturbances (mean score = 3.0), but similar in frequency (mean scores 3.1 and 2.8, respectively). Of the five most severe disturbance types (score = 5), natural disturbances (glaciers and volcanoes) were less extensive and frequent than anthropogenic disturbances (mining, transportation, urban development). Transportation was rated uniquely high in both extent and severity. Other outliers were herbivory, tree-falls, and animal activities, all of which received very low scores for severity, but high scores for frequency. Most important, perhaps, is the broad range of extent, severity, and frequency among the disturbance types, particularly those representing natural disturbances. At relatively large spatial scales (~104 –1010 m2 ) and long temporal scales (~102 –104 yr), many areas of the earth are dominated by only one or a few major disturbance types. Inside the front cover of this book, we have mapped areas where disturbances related to

AN INTRODUCTION TO TERRESTRIAL DISTURBANCES

earth, air, water, and fire predominate on terrestrial surfaces of the earth. Volcanoes and earthquakes result from plate tectonics (earth element) and predominate around the rim of the Pacific Ocean and in central Asia. Hurricanes (air element) develop in the tropics, but occasionally reach latitudes >45º N or S. Tornadoes reach further inland than hurricanes. Less severe windstorms are nearly ubiquitous at smaller spatial scales and were not included in the map. Floods or ice (excess of the water element) are important disturbances along river corridors and in boreal and polar regions. Drought (deficiency of the water element) is primarily a factor in mid-latitude, hot deserts, but also in northeastern Brazil (Mares et al., 1985). Droughts and floods are dictated largely by ocean currents, global wind patterns, and regional topography, although human activities often influence both droughts (e.g., desertification) and flooding (river channelization). Fire is the most ubiquitous type of terrestrial disturbance after human urban and agricultural activities (Bond and van Wilgen, 1996). It is important in tundra, coniferous forests, temperate grasslands and shrublands, and tropical grasslands and savannas, although only the most flammable biomes (coniferous forests and Mediterranean-climate shrublands) are shown. Biotic disturbances can be considered a fifth category of disturbance. Non-human biotic disturbances include plant and animal invasions, herbivory, and other animal activities (e.g., excavating, building, movement, waste products, disease, and parasitism). These activities are too ubiquitous and small in scale to map globally. In contrast, anthropogenic disturbances, equally ubiquitous but occurring at larger spatial scales,

5

can more readily be mapped globally (Fig. 1.2). There is a strong similarity between the distributions of human population (Fig. 1.2A) and common human disturbances (Figs. 1.2B, 1.2C, 1.2D). Current anthropogenic disturbances reflect human land-use patterns that are a consequence of historical settlements based primarily on the presence of soils suitable for agriculture (Fig. 1.2B), and appropriate waterways or land routes for transportation. More recent urbanization reflects primarily transportation centers (Fig. 1.2C) that have excellent access to power sources or to agricultural products (cf. Cronon, 1991). Many humans (45%) now live in or near cities, and this trend is accelerating. Nevertheless, some human activities such as mineral extraction (Fig. 1.2D) and military installations may actually promote low human population densities, but still represent severe disturbance (e.g., northern Alaska, northern Venezuela, eastern Saudi Arabia). Inside the back cover of this book, we have mapped all human influences together, using four hemeroby classes (see Sukopp and Starfinger, Chapter 16, this volume) representing degrees of human influence: (1) minimal: mountains, tundra, undeveloped forest; (2) moderate: low human population densities, some agriculture; (3) major: moderate human population densities, intense agriculture (e.g., deep plowing, clearcutting, biocides); and (4) maximal: high urban population densities, sealed or poisoned land surfaces. This measure of combined influences of humans emphasizes that most damage occurs where population densities are high. Agriculture and resource extraction, although often locally severe, do not alter the environment as much as pavement and urban buildings.

6

Lawrence R. WALKER and Michael R. WILLIG

Fig. 1.2A. Global distribution of four aspects of anthropogenic disturbance. A. Human population distribution. Grey levels indicate different population densities: <2 km−2 ; 2–20 km−2 ; 20–100 km−2 ; >100 km−2 . Various sources were used, including: Oxford World Atlas (1973) and The Times Atlas of the World (1990, 1995).

AN INTRODUCTION TO TERRESTRIAL DISTURBANCES

Fig. 1.2A (continued).

7

8

Lawrence R. WALKER and Michael R. WILLIG

Fig. 1.2B. Agriculture excluding forestry. Grey levels indicate relative intensity:

sparse;

low;

moderate;

high.

AN INTRODUCTION TO TERRESTRIAL DISTURBANCES

Fig. 1.2B (continued).

9

10

Lawrence R. WALKER and Michael R. WILLIG

Fig. 1.2C. Surface transportation (roads and railroads). Grey levels indicate relative intensity:

sparse;

low;

moderate;

high.

AN INTRODUCTION TO TERRESTRIAL DISTURBANCES

Fig. 1.2C (continued).

11

12

Lawrence R. WALKER and Michael R. WILLIG

Fig. 1.2D. Mineral resource extraction (including oil, gas and coal). Grey levels indicate relative intensity: high.

sparse;

low;

moderate;

AN INTRODUCTION TO TERRESTRIAL DISTURBANCES

Fig. 1.2D (continued).

13

14

Lawrence R. WALKER and Michael R. WILLIG

Table 1.2 Distribution of topics discussed in each chapter. Parentheses indicate minor topics Chapter

Element 1

Geographic region 2

Ecoregion 3

Trophic level 4

Theme 5

2

1,2,3

1,2,(3),(4),5,6

1,5

1,(2),3

1,3,(4),7,8

3

1,2,3,4,5

2,3,4,5,6,7,8

1,2,4,6

1,2,(3)

2,(4),5,6,7,9,12

4

1,2,3,5

(1),3,(5),6,7

(2),5,8

1,(2),3

1,5,6,10

5

1,(2),3

3,4,(5),6,8

4,5

1,2,3

1,2,3,8,10,12

6

2,4,5

3,5,6

5

1,2

2,3,4,7,(10)

7

2,5

3,4,5,6,7,8

5,6

1,2,3

1,2,3,(4),5,8,10,13

8

2,(4)

1,3,4,6,7,8

5,6

1

1,2,3,5,6,8,14

9

5

4,6

5

1,2

1,2,3,5,6,9,11

10

4,5

1,4,5,6,7

1,2,4,5,8

1

1,10,12

11

4,5

1,6,7

(1),2,3,(5)

1,2,(3)

1,5,6,7,9,14

12

2,3,4,5

1,3,4,5,6,7

1

1,2,(3)

1,2,3,(4),14

13

(2),3,4,5

3,6,8

6

1,2

1,2,3,4,5,6,(7),9,10,(14)

14

1,5

1,(3),4,5,6,(7)

1,2,3,5

1

1,3,5,6,13

15

5

4,5,6

1,2,4,5,6

1,2

1,10,13

16

5

(3),5,6

5,7

1,2

(2),3,(5),10,12

17

(1),(2),(3),4,5

1,4,5,6,8

1,2,3,4,5,6,8

1,2

1,3,4,5,(10),12

18

1,2,4,5

4,5,6

5,8

1,2

2,3,5,6,(10)

19

1,2,3,4,5

1,3,4,6,7,8

5

1,2

1,2,3,9,10,12,13

20

5

7

2,8

1,(3)

2,3,4,5,7,(8),10,12,14

21

1,3,5

6

(6),8

1,2

1,5

22

1,4,5

5,6

1,4,5

1,2,3

3,5,13,14

23

5

1,5,6

2,4,5

1,2,3

1,3,4,5,(6)

24

1,4,5

1,4,5,6

2,4,5

1,2

1,5,6

25

1,2,3,5

1,2,3,4,5,6,8

5

1,2,3

1,3,4,5,6

26

5

(4),6

2,5,(6),8

1

2,3,4,5,6

27

2,4,5

4,6,8

2,4,5,8

1,2,3

1,2,3,4,5,6,10,14

28

4,5

4,5,6,7

(2),(4),(5)

2

1,(2),5,7,9,13,14

29

5

4,6,7

2,4,5,(6),8

1,2,3

1,5,7,9,11,12,13,14

30

5

1,3,6,7,8

(1),2,5,6,8

1,2,3

3,7,9,10,13,14

31

1,2,3,4,5

6,7

5

2

2,(3),7,(13),14

32

5

1,3,4,5,6

8

1,2

5,6,8,14

1

Element: 1, earth; 2, air; 3, water; 4, fire; 5, biota. Geographic region: 1, Africa; 2, Antarctica; 3, Asia; 4, Australasia (Australia, New Zealand, Micronesia); 5, Europe; 6, North America; 7, South America; 8, Islands. 3 Ecoregion: 1, desert; 2, grassland; 3, savanna; 4, shrubland; 5, forest; 6, wetland; 7, urban; 8, agroecosystem. 4 Trophic level: 1, producer; 2, consumer; 3, decomposer. 5 Theme: 1, interactions; 2, spatial heterogeneity; 3, succession; 4, competition; 5, nutrient cycling; 6, productivity; 7, stability and resilience; 8, predictability; 9, thresholds; 10, biodiversity; 11, functional redundancy; 12, invasive species; 13, restoration and management; 14, modeling. 2

AN INTRODUCTION TO TERRESTRIAL DISTURBANCES DISTURBANCE THEMES

The chapters in this volume approach the topic of disturbance from many perspectives (Table 1.2). Each chapter addresses how at least one of the four basic elements or ethers (earth, air, water, fire) and the biota may be an agent of disturbance, and a few chapters address all of them. The most frequently covered type of disturbance is biotic (particularly human). The most frequently described geographical region is North America, but all regions of the world are discussed (more than simply a reference or brief mention) in the following rank order: North America  Australasia = Europe > Africa = Asia = South America > islands  Antarctica. This representation probably reflects both author bias and available literature, although all regions except Antarctica are discussed in at least ten chapters. Ecological regions were discussed in the order: forests  grasslands > shrublands = agroecosystems > deserts = wetlands  savannas > urban areas, again suggesting the distribution of available literature (and humans), and despite the global importance of urbanization. Most chapters address effects of disturbance on primary producers and consumers, but a substantial fraction also consider decomposers. Fourteen themes emerge in the following order: nutrient cycling > interactions = succession  spatial heterogeneity > productivity = biodiversity > competition = modeling > stability and resilience > thresholds = invasive species = restoration and management > predictability  functional redundancy. This order suggests that disturbances often interact and that there are intimate links between disturbance and nutrient cycling, succession, spatial heterogeneity, productivity, and biodiversity. In the last chapter of the volume, we examine the lessons learned from earlier chapters in this volume about the relationships between disturbance and these important themes. Ecologists have made great strides in understanding the role of disturbance in shaping natural systems. Successional responses and competitive interactions among species have generated particularly large numbers of papers. Other responses to disturbance (notably belowground processes) have received very little attention. Land managers have developed a broad base of knowledge about practical issues relating to intentional human disturbances such as agriculture. However, neither ecologists nor land managers have developed a

15

robust set of predictions about the consequences of disturbances. Much more integration of management and theory is needed in order to address the environmental challenges which humans face. Especially important to understand are the consequences of irregular natural disturbances such as hurricanes or volcanoes, the recent and overwhelming human impacts such as erosion or clear-cutting, and the interactions among them. This global compendium of examples of disturbed ground offers a sampling of the types of data that are available and some preliminary generalizations and conceptual models. We hope this book will stimulate more longterm monitoring of disturbed ground, experiments that address the mechanisms behind biotic responses to disturbance, and studies that compare responses within and among various types of disturbances (and ideally across gradients of disturbance severity). Such types of data are needed to provide the basis for predictions about disturbance. REFERENCES Barrow, C.J., 1991. Land Degradation. Cambridge University Press, Cambridge, 295 pp. B´enito-Espinal, D.B. and B´enito-Espinal, E. (Editors), 1991. L’Ouragan Hugo: genese, incidences g´eographiques et e´ cologiques sur la Guadeloupe. Co-editors: Parc National de la Guadeloupe, D´el´egation R´egionale a l’Action Culturelle, and Agence Guadeloup´eene de l’Environnement du Tourisme etdes Loisirs. Imprimerie D´esormeaux, Fort-de-France, Martinique. Bond, W.J. and van Wilgen, B.W., 1996. Fire and Plants. Chapman and Hall, London, 263 pp. Botkin, D.B., Caswell, M.F., Estes, J.E. and Orio, A.A. (Editors), 1989. Changing the Global Environment: Perspectives on Human Involvement. Academic Press, New York, 459 pp. Chapin III, F.S., Walker, L.R., Fastie, C.L. and Sharman, L.C., 1994. Mechanisms of primary succession following deglaciation at Glacier Bay, Alaska. Ecol. Monogr., 64: 149–175. Clements, F.E., 1928. Plant Succession and Indicators. H.W. Wilson, New York, 453 pp. Cohen, J.E., 1995. How Many People Can the Earth Support? Norton, New York, 532 pp. Cronon, W., 1991. Nature’s Metropolis: Chicago and the Great West. Norton, New York, 530 pp. Crosby, A.W., 1986. Ecological Imperialism – The Biological Expansion of Europe, 900–1900. Cambridge University Press, Cambridge, 368 pp. Finkl, C.W. and Pilkey, O.H. (Editors), 1991. Impacts of Hurricane Hugo: September 10–22, 1989. J. Coastal Res., 8: 356 pp. Special issue. Komarek, E.V., 1983. Fire as an anthropogenic factor in vegetation ecology. In: W. Holzner, M.J.A. Werger and I. Ikusima (Editors), Man’s Impact on Vegetation. Dr W. Junk, The Hague, pp. 77–82. Luken, J.O., 1990. Directing Ecological Succession. Chapman and Hall, London, 251 pp.

16 Magnuson, J.J., 1990. Long-term ecological research and the invisible present. BioScience, 40: 495–501. Mares, M.A., Willig, M.R. and Lacher Jr., T.E., 1985. The Brazilian caatinga in South American zoogeography: tropical mammals in a dry region. J. Biogeogr., 12: 57–69. Officer, C. and Page, J., 1993. Tales of the Earth. Oxford University Press, Oxford, 226 pp. O’Neill, R.V., DeAngelis, D.L., Waide, J.B. and Allen, T.F.H., 1986. A Hierarchical Concept of Ecosystems. Princeton University Press, Princeton, NJ, 253 pp. Oxford World Atlas, 1973. Saul B. Cohen (Geographical Editor), Oxford University Press, Oxford, 190 pp. Pickett, S.T.A., Collins, S.L. and Armesto, J.J., 1987. Models, mechanisms, and pathways of succession. Bot. Rev., 53: 335–371. Sheets, P.D. and Grayson, D.K., 1979. Volcanic Activity and Human Ecology. Academic Press, New York, 644 pp. Simkin, T. and Fiske, R.S., 1983. Krakatau 1883: The Volcanic Eruption and its Effects. Smithsonian Institution Press, Washington, DC, 464 pp. Thomas, W.L., 1956. Man’s Role in Changing the Face of the Earth. University of Chicago Press, Chicago, IL, 1193 pp. Times Atlas of the World, 1990. Times Books, London, 395 pp. Times Atlas of the World, 1995. Times Books, London, 374 pp.

Lawrence R. WALKER and Michael R. WILLIG Vitousek, P.M., 1994. Beyond global warming: ecology and global change. Ecology, 75: 1861–1876. Vitousek, P.M., Ehrlich, P.R., Ehrlich, A.H. and Matson, P.A., 1986. Human appropriation of the products of photosynthesis. BioScience, 36: 368–373. Wackernagel, M. and Rees, W., 1996. Our Ecological Footprint. New Society Publishers, Philadelphia, PA, 160 pp. Walker, L.R., Brokaw, N.V.L., Lodge, D.J. and Waide, R.B. (Editors), 1991. Ecosystem, plant and animal responses to hurricanes in the Caribbean. Biotropica, 23: 313–521. Walker, L.R., Silver, W.L., Willig, M.R. and Zimmerman, J.K. (Editors), 1996. Long term responses of Caribbean ecosystems to disturbance. Biotropica, 28: 414–614. White, P.S. and Pickett, S.T.A., 1985. Natural disturbance and patch dynamics: an introduction. In: S.T.A. Pickett and P.S. White (Editors), The Ecology of Natural Disturbance and Patch Dynamics. Academic Press, Orlando, Florida, pp. 3–13. Whittaker, R.J., Bush, M.B. and Richards, K., 1989. Plant recolonization and vegetation succession on the Krakatau Islands, Indonesia. Ecol. Monogr., 59: 59–123. Worster, D.E., 1979. Dustbowl: the Southern Plains in the 1930s. Oxford University Press, Oxford, 277 pp.

Chapter 2

DISTURBANCE REGIMES AND ECOSYSTEM RESPONSE ON RECENTLY-DEGLACIATED SUBSTRATES John A. MATTHEWS

Elven, 1978a, 1980; Matthews, 1978, 1979a; Paternoster, 1984). More recent developments include the ordination of communities (e.g., Matthews, 1979b–d; Matthews and Whittaker, 1987; Whittaker, 1989, 1991, 1993; Crouch, 1993; Vetaas, 1994) and autecological studies (St¨ocklin, 1990; St¨ocklin and B¨aumler, 1996). Studies outside of North America and Europe have been relatively few in number and, with the notable exception of work associated with the ice-free Antarctic landscapes (e.g., Smith, 1993; Walton, 1993; WynnWilliams, 1993; Lyons et al., 1997) and in New Zealand (e.g., Wardle, 1980, 1991; Sommerville et al., 1982; Burrows, 1990), generally less detailed. However, some investigations have also taken place in South America (Lawrence and Lawrence, 1959; Heusser, 1960, 1964; Rabassa et al., 1981; Veblen et al., 1989; Jordan, 1991), Africa (Coe, 1967; Spence, 1989; Mahaney, 1990), Irian Jaya (Hope, 1976) and Asia (e.g., Turmanina and Volodina, 1978; Solomina, 1989). Although recently-deglaciated terrain originates as a result of major disturbance (nudation) and is subjected to a wide range of disturbances, both the disturbances and their ecological effects are, paradoxically, poorly understood. Rather, emphasis has been placed on primary succession as ecosystem development following nudation. With few exceptions, most notably Oliver et al. (1985) and Whittaker (1991), development of these ecosystems has been viewed as a largely biological, deterministic process in a static physical environment, which is relatively immune from subsequent disturbance. This traditional, oversimplified view was challenged by Matthews (1992), who enlarged upon the concept of recently-deglaciated terrain as a geoecological landscape in which biological processes are coupled with abiotic processes of the physical

INTRODUCTION

Recently-deglaciated terrain holds a special place in the history of ecology, largely as a result of the classical studies on primary succession and soil development that have been carried out in these landscapes, in North America (e.g., Cooper, 1923a–c, 1939), the Alps (L¨udi, 1921, 1945, 1958; Braun-Blanquet and Jenny, 1926; Friedel, 1934, 1937, 1938; Negri, 1934) and Scandinavia (Fægri, 1933). These were some of the first detailed investigations of primary succession and soil development, and they took advantage of the spatial chronosequence of ecosystems that exists in front of a retreating glacier. Over the last 40 years, recently-deglaciated landscapes have continued to inspire important empirical and theoretical contributions to understanding ecological succession. Cooper’s early work at Glacier Bay, Alaska, has been followed by detailed research on the vegetation itself (including the long-term monitoring of change), on the soil changes that accompany vegetation change, and on the mechanisms of change (e.g., Crocker and Major, 1955; Lawrence, 1958, 1979; Mirsky, 1966; Lawrence et al., 1967; Reiners et al., 1971; Bormann and Sidle, 1990; Chapin et al., 1994; Fastie, 1995). Similar investigations have been made at other glaciers in North America (e.g., Scott, 1974; Birks, 1980; Sondheim and Standish, 1983; Fitter and Parsons, 1987; Blundon and Dale, 1990; Blundon et al., 1993; Helm and Allen, 1995). In Europe, studies of plant communities of recently-deglaciated substrates have generally involved phytosociological approaches, which have traditionally emphasized the classification and mapping of communities as a basis for understanding environmental relationships (e.g., Jochimsen, 1963, 1970; Persson, 1964; Richard, 1973; 17

18

environment. The term ‘geoecology’ was coined by Troll (1971) with the aim of placing the study of ecosystems in the broader context of the geographical landscape (German: ‘Landschaft’). It emphasizes the spatial organization of ecosystems in the landscape, and the broader framework of environmental processes, both natural and cultural, with which biological and ecological processes interact. Thus, successional changes on deglaciated substrates can be viewed as a result of the interaction of dynamic physical processes, including disturbance, with biotic processes. In this chapter, I focus on the importance of disturbance in the recently-deglaciated landscape, with particular attention to ecological effects on ecosystems. Of necessity, the chapter relies heavily on the studies reviewed in Matthews (1992) with the addition of the rather limited amount of more recent research. However, there has been no previous comprehensive review, ecological or otherwise, of disturbance in these environments. After summarizing the global distribution and general characteristics of recently-deglaciated terrain, this chapter develops three main themes: (1) the classification of disturbance types and their characteristics; (2) observed effects of disturbances on ecosystems, particularly the direct effects on the substrate and on plants; and (3) the role of disturbance as a driving force in primary succession.

GLOBAL DISTRIBUTION AND GENERAL CHARACTERISTICS OF DEGLACIATED SUBSTRATES

Although about 96% of Earth’s glacier ice is in Antarctica and Greenland, ice bodies – glaciers, ice caps and ice sheets – and hence deglaciated substrates occur on all continents in a remarkably wide range of polar and alpine environments. Glaciers accumulate wherever there is a surplus of snowfall over snowmelt for sufficient years to allow the consolidation of snow into ice; ice wastes away, glacier margins retreat and deglaciated substrates are exposed when melting (ablation) exceeds accumulation. On a global scale, the latitudinal and altitudinal distribution of glaciers is primarily dependent on low air temperatures: thus, glaciers occur at or close to sea level in some parts of the Arctic and Antarctic, but only above about 5000 m on tropical mountains. At a continental scale, glaciers are found at increasing altitudes from oceanic coasts towards continental interiors primarily

John A. MATTHEWS

in response to decreasing snowfall, and they may be absent from many cold but dry continental interiors because of precipitation starvation. Locally, other factors are influential, particularly wind and aspect through their effects on snow-drifting and topographic shading. A zone of recently-deglaciated terrain – the glacier foreland – currently occurs in front of most glaciers world wide. This has been produced by glacier retreat from the ‘Little Ice Age’ glacial maximum attained at various times during the last few centuries. In southern Norway, for example, most glaciers attained their ‘Little Ice Age’ maxima around the middle of the eighteenth century (Matthews, 1991; Bickerton and Matthews, 1993) (Fig. 2.1); in the Alps, some reached their maxima around AD 1600, others in the middle of the nineteenth century (Grove, 1988). Successively older terrain occurs with increasing distance from the glacier front, and a range of techniques are of potential use in dating glacier forelands with varying levels of accuracy (see Matthews, 1992). In some cases, older substrates may be available as a result of pre‘Little Ice Age’ (Neoglacial) glacier advances and retreats. The extent of the glacier foreland varies with the magnitude of glacier retreat. On the one hand, large valley glaciers in maritime climates tend to be relatively dynamic; glacier retreat rates may reach >100 m yr−1 , and the foreland may extend for several kilometres from the glacier; some small cirque glaciers in continental climates, on the other hand, are characterized by a narrow deglacierized zone <100 m wide. Glaciers that calve into lakes or other water bodies, have the greatest retreat rates and their forelands tend to be the most extensive: for example, at Glacier Bay, Alaska, retreat has totalled more than 100 km since about AD 1760 (Chapin et al., 1994). Recently-deglaciated landscapes are relatively barren (Fig. 2.2) but, as Jochimsen (1962) has aptly described them: “Das Gletschervorfeld – keine W¨uste” [the glacier foreland – not a desert]. Except on the most recently created land surfaces, close inspection reveals the presence of plant life, and the vegetation cover usually increases rapidly with increasing terrain age. Distinctive landforms, the products in particular of glacial and glacio-fluvial erosional and depositional processes, introduce a variety of habitats (e.g. Sugden and John, 1976; Drewry, 1986). The substrate itself is typically a till, that is, a diamicton or unsorted mixture

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Fig. 2.1. The glacier foreland of Bøverbreen, Jotunheimen, Norway: the relatively barren, recently-deglaciated zone between the glacier and the ‘Little Ice Age’ moraine ridge dating from about AD 1750. Note the mature mid-alpine vegetation in the foreground on terrain deglaciated about 9000 years B.P., as estimated by 14 C measurements.

of textures ranging from boulders to clays that have been deposited directly by glacier ice (e.g., Boulton and Eyles, 1979; Dreimanis, 1988). This is often absent in areas of glacier erosion, where bedrock outcrops occur, or have been reworked and replaced, most notably by sorted glacio-fluvial sands and gravels deposited by meltwater streams (Maizels, 1979; Maizels and Petch, 1985; Gurnell and Clark, 1987). Superimposed on the regional polar or alpine climate, and in addition to any climatic effects resulting from the local topography, are meso-climatic influences due to the proximity of a glacier. These include the likelihood of glacier winds, which are particularly strong on open terrain and where confined by valleys (e.g. Hoinkes, 1954; Nickling and Brazel, 1985). In summary, the environment may be described as relatively inhospitable to plants even without any consideration of disturbance. However, as deglaciated terrain is varied and occurs in a wide range of macroclimates, conditions for plant growth and ecosystem recovery are not easy to generalize and should not be viewed as uniform.

TYPES OF DISTURBANCE

Four types of disturbance can be recognized as affecting recently-deglaciated substrates and their ecosystems: (1) direct glacial disturbance, defined as disturbance caused by glacier contact; (2) glacier-dependent disturbance, caused by the proximity of a glacier; (3) glacier-conditioned disturbance, substrate disturbance conditioned by previous glacier presence; and (4) glacier-independent disturbance, which happens to occur on deglaciated terrain. Direct glacial disturbance Direct glacial disturbance is an effect of direct contact between the glacier and the ecosystem. Erosional disturbance of this type commonly accompanies a glacier advance, when ecosystems may be overridden and substrates bulldozed (Fig. 2.3). Also included is the direct deposition of debris from glacier ice, which may slide, roll, or flow down the glacier

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Fig. 2.2. The glacier foreland of Stegholbreen, an outlet of the Jostedalsbreen ice cap, Norway. Note the sharp trim-line between the glacier foreland and the mature, low-alpine, Betula pubescens woodland.

surface onto the proglacial area or may melt out from ice. The most important disturbance in the latter category is probably the sediment flows that originate as supraglacial debris liquefies in response to the melting of underlying ice (Lawson, 1981, 1982). A third, more unusual category follows glacier retreat and includes rock slides (Sigurdsson and Williams, 1991) and the slumping of lateral moraines as a result of the removal of glacial support (Fig. 2.4), and the slow melting of buried glacier ice, either in ice-cored moraines or beneath other parts of the glacier foreland (Wright, 1980; Mattson and Gardner, 1991; Krainer and Poscher, 1992; Winkler and Hagedorn, 1994). Whole ecosystems were destroyed during ‘Little Ice Age’ glacier expansion, which can be regarded as major nudation events on a global scale. Relics of these former ecosystems in the form of sub-fossil tree trunks, exhumed plant communities, and/or soil have occasionally been exposed during subsequent glacier retreat (e.g., Elven, 1978b; Smith, 1982; Bergsma et al., 1984). Smaller-scale destruction followed the minor readvances and accompanying moraine-ridge deposition that have punctuated the general retreat from

‘Little Ice Age’ limits (e.g., Bickerton and Matthews, 1993) and are currently occurring at many alpine and Scandinavian glaciers (e.g., Matthews et al., 1995; see also Fig. 2.3). Glacier-dependent disturbance Glacier proximity may result in indirect disturbance even though there is no direct contact between the glacier and the ecosystem. Glacio-climatic, glaciohydrologic and glacio-fluvial disturbance may be recognized as sub-types. Glacio-climatic disturbance is induced by local climatic changes as the glacier advances and retreats. Glacier winds are the most obvious manifestation of the glacier climate but others include the lowering of air temperatures, especially during the day, and the elevation of minimum air temperatures at night (Hoinkes, 1954; Lindr¨oth, 1971). As such effects are limited to a distance of about 200 m from the snout of cirque and small valley glaciers, they affect the ecosystems when the glacier approaches to within this threshold distance. Similarly, there is a soilmoisture gradient in summer associated with glacier

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Fig. 2.3. An advancing glacier bulldozing the substrate at Bergsetbreen, an outlet of the Jostedalsbreen ice cap, Norway. Note the small ‘push-moraine’ ridge, the damaged vegetation, and additional disturbance from blocks of ice fallen from the near-vertical ice front.

proximity, because there is continuous groundwater recharge from the melting ice close to the glacier snout. This gradient is likely to be much steeper than the soil temperature gradient, with high soil-moisture deficits occurring on well-drained sites even within 40 m of the glacier snout (Ballantyne and Matthews, 1982). Glaciofluvial disturbance by meltwater streams extends much further downstream and is present to a greater or lesser extent on all glacier forelands (Maizels, 1979; Odland et al., 1991). The characteristic braided-channel pattern of an outwash plain (plain sandur: extensive, unconfined area of glacio-fluvial deposits) or a valley train (valley sandur: an area of glacio-fluvial deposits confined by valley sides) is a response to the high sediment load of the streams, their variable discharge and their non-cohesive banks (Miall, 1983), which ensure that the anastomosing (inter-twining) complex of shallow, shifting channels remains active almost indefinitely (Fig. 2.5). Glacier-conditioned disturbance Recently-deglaciated substrates are susceptible to some

kinds of disturbances because they have been covered by glacier ice in the recent past. Such glacierconditioned disturbances represent the adjustment of the newly-deglaciated landscape to exposed, subaerial environmental conditions. They may also be termed paraglacial disturbances because they result from nonglacial processes that are directly conditioned by glaciation (cf. Church and Ryder, 1972; Matthews et al., 1998). Many kinds of disturbance are most effective immediately after deglaciation, and become less effective with the passage of time. These disturbances include mass movements, such as solifluction (Rose, 1991) and debris flow (Ballantyne and Benn, 1994; Ballantyne, 1995), cryogenic (frost-heave and frostsorting) processes (Ballantyne and Matthews, 1982, 1983), aeolian deflation and loess-like redeposition by wind (Boulton and Dent, 1974; Rose, 1991), and pervection (the down-profile removal of fine soil particles by soil water percolation). The newly-deglaciated substrates of proglacial areas are particularly susceptible to these disturbances, because of such characteristics as unconsolidated and meltwater-saturated sediments that dry out periodically, steep slopes, and the lack

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¨ Fig. 2.4. Large-scale disturbance of the eastern lateral moraine of the Marzellferner, Otztal Alps, Austria. To the left, sections of the moraine over 50 m in length have slipped towards the valley floor as a result of the removal of support following glacier retreat. Note also the extensive gully erosion on the proximal slope of the moraine, and the debris-covered glacier snout in the valley bottom (right).

of a significant vegetation cover. These disturbances decline as slopes stabilize (Welch, 1970; Sharp, 1984), sediments consolidate (Boulton and Paul, 1976) or are exhausted, the glacial-meltwater source retreats, and a protective vegetation cover becomes better established. Selected examples are illustrated in Figs. 2.6 and 2.7.

by humans (e.g., agriculture, forestry, tourism and scientific activity) which is best classified as exogenous glacier-independent disturbance, although tourists and scientists are often attracted by the presence of the glacier and hence in one sense may be considered as glacier-dependent.

Glacier-independent disturbance The occurrence of some disturbances on recentlydeglaciated substrates is entirely coincidental; they occur elsewhere and there is no reason to expect a greater frequency or magnitude on glacier forelands. The wide range of such disturbances that might occur may be classified as exogenous (e.g., those avalanches and debris-flows that descend onto the glacier foreland from valley-side slopes, and nivation processes associated with late-lying snowbeds) or endogenous (e.g., wind-throw disturbance on wooded forelands; Noble et al., 1984; Bormann and Sidle, 1990). The latter also includes animal infestations (e.g., Pignatelli and Bleuler, 1988), and disturbance

EFFECTS OF DISTURBANCE REGIMES ON ECOSYSTEMS

Because most ecological studies on recently-deglaciated terrain focus on succession, disturbances of all types have generally been actively avoided rather than investigated. Nevertheless, there is accumulating evidence of the diverse effects of a very wide range of disturbances on glacier-foreland ecosystems. Following the availability of evidence, emphasis in this section is given to the observed direct effects of disturbance on the substrate and on plants.

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Fig. 2.5. Glacio-fluvial disturbance on the outwash plain (sandur) in front of Veobreen, Jotunheimen, Norway. Note the characteristic braided-channel of the sandur, which is being crossed by a herd of reindeer (Rangifer tarandus) (centre).

Substrate modification Disturbance can physically remove substrate material en masse or differentially, can produce soil mixing, and can add substrate material to the soil profile. All these types of substrate modification have been observed on glacier forelands, and merge with more gradual soil changes that occur in response to the operation of continuous abiotic soil-forming processes. Soil profiles are shallow on recently-deglaciated terrain (Mellor, 1985; Messer, 1988) and many geomorphic processes are active. Truncation or removal of soil profiles, whether this is by direct glacial disturbance or other types of disturbance, is not unusual. The most effective processes in this are glacio-fluvial and massmovement processes; the former are particularly active near the valley axis downstream of valley glaciers; the latter near the glacier snout because of ice-melting and paraglacial effects (see above). Differential removal of fine particles is less obvious, but is ubiquitous on unvegetated areas, both close to the glacier and in eroded areas elsewhere. Deflation of silt and fine sand by strong winds is thought to be the main

cause of the stony layer that develops rapidly on newlyexposed till surfaces in Iceland (Boulton and Dent, 1974). Surface wash by running water and frost-sorting processes may also contribute to the development of this stony lag. The amount of material removed may sometimes be estimated from exposed plant roots or from stones that remain isolated above the general level of the surrounding terrain (Rose, 1991). Subsurface removal of silt and clay down the soil profile by pervection affects not only the near-surface layer but deeper layers as well. At first, this leads to the rapid formation of a sub-surface, silt-rich layer but, after 10– 30 years, the sub-surface layer is degraded (Romans et al., 1980; Frenot et al., 1995). Substrate mixing commonly occurs due to cryoturbation on Norwegian glacier forelands. Heaving and subsequent settling as ice lenses grow and melt is one source of disturbance; additional disturbance results from the differential heave of particles of different size, which produces frost sorting. Frost heave may continue throughout the winter in frost-susceptible, especially silt-rich substrates as frost penetrates into the ground, provided there is an adequate water supply for the

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Fig. 2.6. Disturbance due to slow mass movement: gelifluction (solifluction) on the glacier foreland of Styggedalsbreen, Jotunheimen. The downslope movement (upper left to lower right) of the substrate is reflected in the surface microrelief, and has probably been retarded by a surficial crust of mosses.

growth of ice lenses. Larger particles heave at faster rates than smaller ones, which produces boulder-cored frost boils (Harris and Matthews, 1984) where large boulders break the ground surface, and sorted patterned ground when the larger particles have migrated to the margins (gutters) around domed centres of fines (Ballantyne and Matthews, 1982, 1983). On sloping terrain, the thawing of ice-rich sediments leads to downslope movement, partly as a result of settling (frost creep) but mostly as a result of flow (gelifluction) which occurs following the saturation of the soil and its loss of cohesion. Gelifluction and frost sorting are the main processes leading respectively to solifluction lobes and stone stripes (Figs. 2.6 and 2.7). Eroded material tends to be deposited down-slope, where substrate burial can occur. Aeolian deposition of organic material as well as fine mineral particles, removed by wind from close to the glacier or from other exposed substrates, occurs in sheltered localities further away from the glacier and in vegetated areas (Boulton and Dent, 1974; Rose, 1991). Baranowski and P˛ekala (1982) found that mean annual deposition of up to

174 g m−2 , including 4% organic material, occurred in the end-moraine zone of the glacier Werenskioldbreen in Svalbard. Gellatly (1987) described how successive intervals of deposition produce successive buried (fAh) horizons in distal parts of the glacier foreland of Classen Glacier, New Zealand. Plant damage Many processes that disturb the substrate also damage the plants; additional disturbances may damage plants without modifying the substrate. Despite its wide recognition, there have been relatively few descriptions of plant damage as a direct effect of disturbance on recently-deglaciated terrain. Strong winds have major effects on open glacier forelands where plants may be isolated or vegetation cover thin. In Iceland, where a Racomitrium canescens– Stereocaulon spp. moss–lichen heath is the first plant community to achieve a complete ground cover, this is easily stripped off once disturbed (Boulton and Dent, 1974). Eroded turf, injuries to shrubs and

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Fig. 2.7. Substrate disturbance indicated by small-scale stone stripes produced by a combination of frost heaving and sorting, and downslope movement, on the glacier foreland of Svellnosbreen, Jotunheimen, Norway. Note lens cap for scale.

crippling of shrubs were attributed by Friedel (1936) to the destructive effects of winds in front of the Pasterze Gletscher in the Austrian Alps, where the lower altitudinal limit of Elynetum (an alpine grass– heath community) is depressed by as much as 500 m. Friedel (1936) also described the erosion of moraine crests and the deposition of loess-like fine sediments up to 3 m thick in vegetated areas. Heusser (1956) recognized wind-trained and wind-sheared trees which stand above the winter snow cover near the Columbia Glacier (British Columbia, Canada) (see also, Lutz, 1930; Oke, 1987). The proximate cause of such damage may be physical abrasion by snow or mineral particles transported by wind (niveo–aeolian erosion), or physiological effects due, for example, to winter dessication and frost damage. In several studies, growth suppression has been detected in the width of tree-rings following the approach of an advancing glacier (Lawrence, 1950; Bray and Struik, 1963; Holzhauser, 1984; Villalba et al., 1990). Suppression is manifest in narrow rings, which can be attributed to glacier-dependent disturbance of the local climate of the tree. This effect should be distinguished

from possible growth stimulation if competition is reduced following the thinning-out of surrounding trees (Bray and Struik, 1963). Root breakage and exposure to desiccation, caused by mobility of the substrate, are the main destructive effects on plants of frost-heave, related cryogenic processes, and mass-movement processes. Large roots, such as tap roots, and those of woody shrubs, are likely to be more susceptible to damage by frost-heave than the fibrous roots of grasses. Tap roots can be heaved out of the ground, much like relatively large soil particles. Even roots that extend below the depth of winter frost penetration, and hence become anchored in the substrate, may still be damaged by being snapped below the crown as the soil nearer the ground surface heaves (cf. Perfect et al., 1987, 1988; Goulet, 1995). Glacio-fluvial disturbance is most noticeable after major flood events and along channel banks (Sidle and Milner, 1989). However, more subtle effects are likely to be widespread on floodplains following the annual spring snow-melt flood, and also after more frequent rainfall-induced summer flooding. For example, data on seedling survival for a three-year period from the

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foreland of the Exit Glacier in Alaska showed only 30% survival in flooded plots compared with 80% survival in non-flooded areas. Mortality was, moreover, greatest among young seedlings less than one month old (Helm and Allen, 1995). Indirect effects on plants The indirect effects of disturbance on plants, and the effects on animals and micro-organisms, are even more poorly understood. Some of the above-mentioned substrate modifications, for example, have potential impacts on nutrient availability to plants. Flood events may provide nutrient flushes from meltwater. Soil mixing by cryoturbation on recently-deglaciated terrain is likely to counteract not only the loss of fines through pervection but also the loss of nutrients in solution by leaching. This may be termed a ‘dry flush’ effect (sensu Pearsall, 1971), which is well known in temperate uplands and mountains. On forested terrain that has been deglaciated for longer, wind-throw may perform a similar function in releasing immobilized nutrients and improving decomposition (Bormann and Sidle, 1990; Bormann et al., 1995; see also below). Aeolian deposition of nutrients may also be beneficial. Disturbance not only leads to a high turnover, but may also be necessary for the maintenance of certain species’ populations. This has been argued strongly by Brandani (1983), who suggested that some trees, such as Alnus, Betula and Populus, are favoured by the regular disturbance regime of the glacial river valley, aspects of which include adverse temperature and moisture conditions in winter, and high water tables and flooding in spring. He saw large annual seed crops, small wind- and water-dispersed seeds, lack of seed dormancy, rapid germination, early flowering and seed dispersal, short life-spans, and the ability to sprout when damaged, as some of the disturbance-adapted traits shown in this environment (see also, Ryvarden, 1971, 1975; Chapin, 1993). Whittaker (1993) demonstrated variations in plant size-frequency distributions and behaviour patterns on the foreland of the Storbreen glacier in Norway, which were explicable in relation to patterns of succession, site moisture and disturbance. The most striking behavioural adaptation amongst the shrub species studied was the predominance of sexual reproduction on the younger terrain; establishment of ramets by vegetative means occurred increasingly on older terrain, and it was the characteristic mode of establishment in

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mature vegetation outside the glacier-foreland boundary. Sexual reproduction by seed seems to be the main reproductive strategy on glacier forelands as well as on fellfields in the High Arctic (Callaghan, 1987). However, vivipary is a prominent reproductive strategy among the pioneer grasses at Storbreen and other Jotunheimen glaciers (e.g., Deschampsia alpina, Festuca vivipara and Poa alpina ssp. vivipara), and, according to Callaghan (1987), 20% of the vascular plant species on the foreland of K˚arsa Glacier in northern Sweden were found to be viviparous. Furthermore, on recently-deglaciated nunataks at Omnsbreen in southern Norway, wind transport of viviparous seedlings across the ground away from the parent plants (pseudo-viviparous tumbling) was found to be important in the initial stage of colonization (Elven, 1980). Observations such as these led Callaghan (1987) to suggest that this combination of sexual and vegetative reproduction is an adaptation to high levels of both stress and disturbance – the missing strategy in Grime’s (1979) classification of species into three primary plant strategies (competitors, stress tolerators, and ruderals). Effects on animals and micro-organisms The effects of disturbance on the animals of recentlydeglaciated terrain have received little attention. Although the important early investigations of Janetschek (1949) in the Alps emphasized successional concepts, and hence the relationship between terrain age and the animal populations, recent work by Gereben (1995) on the occurrence of six species of ground beetles (Nebria spp.) suggests that habitat differences, including substrate instability, may be more important than terrain age in accounting for the ground-beetle assemblages. Little is known about the effects of disturbance on micro-organisms. Relevant research has been carried out on recently-deglaciated soils in Antarctica, where Wynn-Williams (1986, 1993) has shown that microbial assemblages of bacteria and algae form raft-like, mucilaginous coatings on frost-disturbed deglaciated soils. However, as with the black-crust phenomenon on glacier forelands in lower latitudes (Worley, 1973), emphasis has been placed on the role of these structures in stabilizing the substrate, rather than as an effect of disturbance. Davey and Rothery (1993) found differences in the species composition of micro-algal communities between frost-sorted polygons on Signy

DISTURBANCE REGIMES AND ECOSYSTEM RESPONSE ON RECENTLY-DEGLACIATED SUBSTRATES

Island that were not correlated with a wide range of environmental factors, and suggested control by the vagaries of the colonization process. However, it is possible that differences in the intensity of disturbance may account for the observed pattern.

PRIMARY SUCCESSION AND DISTURBANCE

It is clear from the above that deglaciated terrain is a highly disturbed, dynamic physical environment. How does this affect the nature of succession? The general question is approached here in terms of five more specific questions: (1) Does deglaciation yield a sterile landscape? (2) How does disturbance affect the rate of succession? (3) To what extent does disturbance force successional trajectories? (4) Do seres diverge due to disturbance? (5) What is the relationship between disturbance, stabilization and retrogression? Does deglaciation yield a sterile landscape? A useful distinction can be made between primary disturbance, whereby a new land surface is created, and secondary disturbance that subsequently affects the site (cf. Oliver et al., 1985). The classical distinction between primary and secondary succession insists that there is no vestige of a previous ecosystem in the case of primary succession, and it has usually been assumed that where deglaciation is the primary disturbance this initiates primary succession. The validity of this assumption may be questioned on several grounds. Most importantly, recent research in the Antarctic has suggested that colonization by micro-organisms, particularly Cyanobacteria and algae, may be a necessary prerequisite to a pioneer plant succession (Wynn-Williams, 1986, 1993). These micro-organisms (‘primary colonizers’) are believed to play a role in this weathering of the substrate, in stabilization of the surface, and possibly as an organic- and nutrientenriched medium for ‘secondary colonizers’ such as bryophytes and microlichens (Smith, 1990, 1993). Because bacteria have been discovered in subglacial sediments of glaciers in the French Alps (Moiroud, 1970; Moiroud and Gonnet, 1977), and viable forms that are apparently 12 000 years old have been reported from Antarctic ice cores at −55ºC (Cameron and

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Morelli, 1974; Abyzov et al., 1987; see also, Gilichinsky and Wagener, 1995), it would seem that newlydeglaciated terrain may not be as sterile as previously thought. However, the following arguments can be advanced in favour of continuing to regard the initial deglaciated terrain as an essentially sterile system: (1) it cannot be certain that the samples from both subglacial sites and ice cores had not been contaminated (WynnWilliams, 1993), (2) the global atmosphere may well be a much more important source of propagules than the subglacial sediments (cf. Broady et al., 1987); (3) as microbial populations in the proglacial area are much more abundant in the rhizosphere than in the adjacent free soil (Moiroud, 1975; Moiroud and Gonnet, 1977), the rhizosphere is an alternative source of propagules in the subglacial sediments; and (4) dependence of secondary colonizers on such primary colonization is not proven – the primary colonizers may co-exist with the secondary colonizers in much the same way as cryptogams and phanerogams may colonize at the same time on temperate forelands (Matthews, 1992). Thus, the possible existence of micro-organisms in the substrate prior to deglaciation does not appear to be of any special significance for succession. Several other aspects of initial colonization are of relevance to the question of whether deglaciation initiates a true primary succession; but, in the writer’s opinion, these do not negate use of the term “primary succession” in the context of recentlydeglaciated terrain. They nevertheless may alter the rate of succession and possibly its course. First, again from the Antarctic, has come the concept of a glacier propagule bank (Smith, 1993). Newlydeglaciated terrain is strategically located with respect to a pool of viable propagules that have collected on the glacier surface and are released onto the proglacial area as the snow and ice melt. This may best be classified as an aspect of plant immigration that affects primary succession in ways still to be investigated. Second, whole ecosystems sometimes develop on supraglacial material, may be transported on the surface of the glacier, may undergo succession, may function as a propagule source and may survive deposition in whole or in part on the glacier foreland (Stephens, 1969; Post and Streveler, 1976; Rabassa et al., 1981; Veblen et al., 1989). Such ‘ecosystems-in-transit’ undoubtedly accelerate succession on forelands where they occur. Third, landslides, avalanches and debris flows often operate in a similar fashion as loci of succession

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beneath the steep slopes associated with the forelands of valley glaciers. How does disturbance affect succession rate? Traditionally, the effect of disturbance on succession has been regarded as setting the process back to an earlier stage, or reducing the rate of succession that would occur in its absence. Examples of this, ranging from the mosaic of successional stages that can be observed on the various surfaces of slightly varying height comprising a sandur, to the bare centres of patterned ground disturbed by cryoturbation, are abundant on recently-deglaciated substrates. In addition, disturbance has been viewed as one of the allogenic factors that slows the attainment of a relatively stable (or ‘climax’) state from a few hundred years in the most favourable environments to many thousands of years in the most severe environments (e.g., Turmanina and Volodina, 1978; Solomina, 1989; Matthews, 1992). Much less attention has been given to the possibility that disturbance may accelerate succession [but see del Moral and Bliss (1993), and del Moral and Grishin (Chapter 5, this volume) in the context of volcanic substrates]. Plant damage, examples of which are provided above, is most likely to retard succession. Thus, although the initial colonization seemed as rapid at Exit Glacier as on non-flooded forelands, flooding of the deglaciated area led to a longer initial colonization phase of up to 30 years, an effect of the high seedling mortality (Helm and Allen, 1995). Other examples can be cited where initial colonization has been delayed by substrate disturbance, an outcome that is particularly likely where glacier-conditioned (paraglacial) disturbance is involved. According to Zollitsch (1969), the initiation of succession on the foreland of the Pasterze Glacier may be delayed by disturbance for 20 years. In the Tien Shan, terminal moraines tend to be colonized more rapidly than lateral moraines because the latter are characterized by steeper, more unstable slopes. Similarly, according to Spence and Shaw (1983), succession at the Schoolroom Glacier (Teton Range, Wyoming, U.S.A.) is retarded on moraine slopes compared to relatively stable moraine crests. The slow melting of buried ice appears to produce the longest delay in moraine stabilization and hence the initiation of succession. For example, on the ice-cored moraines of the Klutlan Glacier, Yukon Territory (Canada), it is estimated that 75 years

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is necessary for substrate stabilization (Birks, 1980; Wright, 1980). Kuc (1964) also has reported a major lag in colonization at Hyrnebreen in Svalbard. The examples relating to initial colonization clearly demonstrate that some disturbances can accelerate the early stages of succession by aiding the immigration of propagules with or without substrate attached. Other processes of succession – ecesis, competition, reaction and gradual allogenic change – may also be affected by disturbance, which hence may slow or accelerate succession. Under less severe, intermediate disturbance regimes, safe sites may be created by disturbance, or damage to one species may create opportunities for another. The ability of a wide variety of early colonizers to co-exist and thrive at the same site in apparently inhospitable conditions, thanks to a combination of a lack of competition with relatively high disturbance levels (Stork, 1963) seems to be a manifestation of this. Similarly, disturbance probably contributes to the early peak in species diversity that characterizes many glacier-foreland successions (e.g., Persson, 1964; Zollitsch, 1969; Elven and Ryvarden, 1975; Matthews, 1978; Sommerville et al., 1982). In summarizing vegetation–environment relationships on the Storbreen glacier foreland in Jotunheimen (Norway), Whittaker (1989) concluded that explanation of the vegetation pattern in the deglaciated landscape was improved by viewing environmental variables as multivariate ‘factor complexes’, and that the disturbance regime (especially as reflected in cryoturbation) formed an intimate component of a terrain-age factor complex. The negative correlation, high levels of disturbance being associated with early successional stages, suggests that the main effect of disturbance is to reduce the rate of succession (see also Whittaker, 1987). However, this may be an oversimplification, because subsequent analysis of directly measured change over a 12-year time interval (Whittaker, 1991) demonstrated that, although the greatest amount of vegetation change occurred on terrain ages of 10–20 years, this was largely nondirectional in the successional sense. Furthermore, progressive (successional) change was occurring on terrain ages of 20–50 years, where disturbance levels were still high, which leaves open the possibility of an enhanced succession rate due to disturbance. An interesting observation of the interactions between two types of disturbance with opposite effects on succession rate has been made at Austerdalsbreen, southern Norway, by Whittaker and Petch (personal

DISTURBANCE REGIMES AND ECOSYSTEM RESPONSE ON RECENTLY-DEGLACIATED SUBSTRATES

communication). In the terminal-moraine zone of this glacier foreland, vegetation is generally heavily grazed by domestic animals, especially sheep, and succession is arrested at an early stage dominated by grasses and shrubs, even where terrain age is about 250 years. The most advanced vegetation – Betula pubescens woodland – occurs in a part of the foreland deglaciated about 60 years ago. This woodland is growing on an area of abandoned channel bars isolated from the rest of the foreland by major, active meltwater channels. In effect, therefore, fluvial disturbance protects the area from extensive, persistent disturbance from the ‘artificial’ grazing regime. To what extent does disturbance force successional trajectories? That disturbance is a potential forcing factor in primary succession on recently-deglaciated substrates cannot be doubted. The extent to which allogenic factors in general and disturbance in particular controls the direction or trajectory of succession by deflecting it along modified pathways, or creating very different pathways, is much less clear. Disturbance may exert control by altering the initial conditions of a succession and/or by continual intervention as an influx variable (Jenny, 1961, 1980; see also Matthews, 1992). The former was emphasized by Oliver et al. (1985) in their study of Nooksack Cirque, Washington, where secondary disturbances affected 63% of the total deglaciated area. The vegetation mosaic was influenced by more than one type of disturbance which, in decreasing order of area affected, were: avalanches (29%), rockslides (22%), intermittent snowfields (late-lying snow in certain years; 19%), creeping snowfields – large snowfields where the deformation of snow under its own weight (snow creep) may disturb underlying vegetation (15%), and glacio-fluvial streams (5%). Although forest development followed a general four-stage succession – the stand initiation, stem exclusion, understorey reinitiation and old-growth stages – significant differences were detected in forest composition depending on the type of disturbance. Thus, stands developing after avalanche and rock-fall disturbance were characterized by a higher proportion of Abies amabilis (mean proportion of stand overstorey 69.8%) than stands developing after soil mass movements and glacier retreat (23.9%). The same stands, subject to avalanche and rock fall, were characterized by less Tsuga heterophylla and Alnus

29

sinuata. A second example of the lasting effects of initial disturbance is provided by differences between forests developing in abandoned meltwater channels and forests developing on adjacent moraines (Sigafoos and Hendricks, 1972; Birks, 1980; Dale and MacIsaac, 1989). Qualitative differences in species composition were detectable long after stabilization of the substrate. However, the precise mechanisms by which these differences were brought about are unknown. Some disturbances temporarily stabilize a community; in other cases disturbance precipitates a change. Frequent inundation by glacio-fluvial meltwater in both the Alps and Norway favours a distinct community dominated by the moss Pohlia gracilis (Fægri, 1933; Jochimsen, 1970; Elven, 1978b). This community enters and leaves the succession in response to shifting stream courses, and may be maintained by one or more of the following: (1) the moisture regime; (2) the high concentration of silt; and (3) nutrient supply (a wet-flush effect). Aeolian deposition of silt or sand can have a similar effect or, depending on the rate of deposition, may lead to replacement change. For example, on the foreland of Klutlan Glacier, Yukon Territory, Canada, wind-blown silt trapped by taller plants supports acrocarpous mosses (Birks, 1980); in front of Muldrow Glacier, Alaska, the accumulation of such material is unfavourable to the clump-forming pioneer species, Astragalus nutzotinensis (Viereck, 1966). Particularly on relatively young terrain, local disturbances clearly cause changes in the direction as well as in the rate of succession. Matthews (1992) suggested that allogenic factors, including disturbance, may be more important than autogenic factors in the replacement of pioneer species by later colonizers. The allogenic processes cited include: (1) cryoturbation and other geomorphological processes of erosion and deposition, which physically damage plants; (2) pervection, frost-sorting, and deflation, which produce textural changes in the substrate and hence alter the nature and availability of micro-sites for plant establishment; (3) acidification and other chemical changes in the substrate produced by leaching, which are likely to be effective in altering nutrient supply even in the absence of significant inputs from litter; (4) local climatic amelioration following glacier retreat and consequent distancing from the glacier climate; and (5) reduction of water supply and hence increasing drought (‘desertification’) in response to falling water tables following glacier retreat (cf. Whittaker, 1991).

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Although some allogenic processes (particularly those associated with the paraglacial phase) are less important in later successional stages, and autogenesis increases in importance as vegetation cover, biomass and the influence of plants on their environment increase, some types of allogenic change continue to influence successional trajectories. In addition, exogenous and endogenous glacier-independent disturbances (see above) are just as likely to affect the distal foreland as the pioneer zone. Specific cases of disturbances cited in the literature as either actual or potential causal factors in the later stages of succession include: (1) surface runoff and meltwater disturbances on glacier forelands in the Austrian Alps (Zollitsch, 1969; Jochimsen, 1970); (2) avalanches and grazing by chamois (Rupicapra rupicapra) at the Rhˆone Glacier in the Swiss Alps (Richard, 1973; Schubiger-Bossard, 1988); (3) bears (Ursus arctos) and tourists at Glacier Bay, Alaska (Lawrence, 1979); (4) wood cutting at Nigardsbreen, southern Norway (Fægri, 1933, 1986); (5) species introduction by scientists at Omnsbreen, southern Norway (Elven, 1980); (6) seals and penguins on Heard Island, Antarctica (Scott, 1990); (7) needle ice at Tyndall Glacier, Mount Kenya (Coe, 1967); (8) solifluction at the Tasman Glacier, New Zealand (Archer et al., 1973); (9) needle ice, solifluction, snow creep and interfacial ice (at the interface between snow cover and substrate) at Helm Glacier, British Columbia (Brink, 1964); (10) water-table fluctuations in response to minor climatic changes in the Ben Ohau Range, New Zealand (Archer, 1973); and (11) wind throw at Glacier Bay, Alaska, which may contribute to the formation of muskeg from forest (Noble et al., 1984; see below, p. 31). Do seres diverge due to disturbance? The question of whether succession is convergent or divergent was a major theme of my early research on the Storbreen glacier foreland, Jotunheimen, southern Norway (Matthews 1979b–d). A variety of multivariate statistical techniques of classification and ordination were used to analyse within- and between-community variability, and the concept of convergence towards a relatively uniform landscape in later successional stages was tested. These analyses revealed that succession in this alpine environment was characterized by strongly divergent seres: the herbaceous pioneer community exhibited relatively little within-type variability, whereas late-successional heath communities

John A. MATTHEWS

were characterized by greater within-type variability and were relatively discrete due to greater between-type differences. Other research that has suggested divergence in glacier-foreland succession includes studies by Kuc (1964) on Svalbard, Elven (1978c) at Svartisen, northern Norway, and Spence (1989) on Mount Kenya. In contrast, Fægri (1933) and Vetaas (1994) concluded that succession is convergent on the wooded forelands dominated by Betula pubescens in the sub-alpine zone around the Jostedalsbreen ice cap in southern Norway. Birks (1980) also envisaged rapid convergence on the moraines of the Klutlan Glacier, Yukon Territory, but only after Picea glauca became dominant. More complex schemes involving divergence followed by partial convergence have been depicted by Zollitsch (1969) and by Jochimsen (1970) based on alpine forelands in Austria, whereas Schubiger-Bossard (1988) saw habitat-dependent seres developing in parallel on the foreland of the Rhˆone Glacier, Swiss Alps, without any convergent or divergent trend. Matthews (1992) proposed that convergence is largely a function of relatively strong biotic (autogenic) control in the later successional stages, and that divergence is largely associated with allogenic control, which is widespread in the early stages of succession and in relatively severe physical environments. Thus, convergence tends to occur on temperate, boreal and sub-alpine forelands where a small number of tree species are dominant, while divergence is characteristic in mid-alpine and polar regions and also in sites with severe micro-environments where habitat conditions are otherwise favourable to convergence. However, the evidence is not conclusive, especially in the low-alpine zone where both convergent and divergent trends have been inferred. To the writer’s knowledge, the relationship of disturbance sensu stricto to convergence and divergence has never been examined in any detail and is not clear. Three possible hypotheses can be proposed. First, the effect of disturbance may be non-directional in the sense that it slows or disrupts the successional processes that would otherwise occur, without changing the trajectory; this interpretation emphasizes the stochastic nature of disturbance and suggests that trajectories would be less predictable but not necessarily different. Second, the association of early divergent successional change with a highly disturbed landscape, and later convergent succession with lower levels of disturbance and strong biotic control, suggests a causal relationship. Thus, disturbance may

DISTURBANCE REGIMES AND ECOSYSTEM RESPONSE ON RECENTLY-DEGLACIATED SUBSTRATES

provide opportunities for different species to become established, and hence alternative successional pathways to be initiated. This interpretation is consistent with the idea that strong biotic control, especially if associated with a tree cover, reduces disturbance and the opportunities for divergence by imposing its own environmental uniformity. Third, if disturbance is sufficiently ubiquitous yet not too disruptive, it may force successional trajectories along a common pathway, and hence it may lead to convergence; frequent low-level cryoturbation in high-alpine and polar environments provides a possible example. The last two interpretations both view disturbance in much the same way as any gradual allogenic environmental forcing factor that is relatively predictable. In stating the problem in terms of competing hypotheses, it is hoped that this will stimulate hypothesistesting. However, it is realized that the three alternatives may apply to different stages of the same succession, and that each may be more or less appropriate on particular forelands depending on regional or local circumstances: indeed, all three may well be found amongst the diverse environments available on glacier forelands in different parts of the globe. What is the relationship between disturbance, stabilization and retrogression? A tendency to progress towards relative stability in the later stages, in the sense that communities are more likely to be replaced by similar communities than different ones, has for long been recognized as a possible effect of the interacting processes responsible for successional change (Clements, 1928; MacMahon, 1980). It is now realized, however, that relatively mature, late-successional communities are not only subject to disturbance but also may require disturbance for the maintenance of species composition (Pickett et al., 1987; Veblen, 1992). Although there is the problem of obtaining sufficiently long chronosequences, several studies of glacier forelands have yielded relevant information. Retrogression, or at least retrogressive tendencies, have been described from the later stages of forest succession at Glacier Bay, Alaska (Lawrence, 1958), Klutlan Glacier, Yukon Territory, Canada (Birks, 1980) and Franz Josef Glacier, New Zealand (Burrows, 1990). In each case it would appear that prolonged succession without disturbance is related to such factors as low soil fertility and waterlogging, which lead to retrogression.

31

Signs of retrogression include a long-term decline in productivity (Bormann and Sidle, 1990), and the presence of widely spaced and/or stunted trees (Birks, 1980; Burrows, 1990). Allogenic as well as autogenic processes appear to cause retrogression, which may well occur even in polar ecosystems. For example, on terrain deglaciated for at least 3000–5000 years in front of the Sørsdal Glacier, Vestfold Hills, Antarctica, simple lichen assemblages consisting of only one or two species in low abundance have been attributed to ‘climax collapse’ in response to the accumulation to detrimental or toxic levels of wind-blown salts and loose sand (Seppelt et al., 1988). Perhaps the best-researched example of retrogression is the eventual replacement of Picea sitchensis forest by muskeg (paludification) in southeastern Alaska. Several processes are potentially involved. First, the spruce forest may deteriorate as nitrogen, phosphorus and other nutrients are immobilized by accumulation in soil horizons and bolewood after more than 100 years of succession (Bormann and Sidle, 1990). Second, invasion of the forest by Sphagnum spp. may lead to further acidification and deterioration of conditions for forest growth leading to its replacement after periods ranging from 800 to several thousand years (Noble et al., 1984). Third, the internal drainage system of the soil may deteriorate due to podzolization and iron-pan development, possibly over a period of about 500 years (Ugolini and Mann, 1979). Fourth, allogenic changes in climate or fire regime may intervene to accelerate or reverse retrogression (Noble et al., 1984). Disturbance, particularly as induced by windthrow, which releases immobilized nutrients and improves soil drainage and decomposition, is seen as a means of spruce-forest maintenance (Bormann and Sidle, 1990), even though wind-throw may itself contribute to muskeg formation by creating pits that aid Sphagnum colonization of the forest floor (Noble et al., 1984). According to Bormann et al. (1995), windthrow or disturbances that mimic windthrow may be required at intervals of 200– 400 years to maintain the productive capacity of the soil in this and related ecosystems. It would seem, therefore, on the basis of the limited evidence available from glacier forelands, that disturbance may be associated with both the maintenance of stability and retrogression in mature ecosystems. Once again, glacier-foreland ecosystems provide an appropriate venue for further research.

32 CONCLUSION

The ecosystems of recently-deglaciated substrates come into existence following a major type of primary disturbance. They are quite widespread at present due to the world-wide tendency of glaciers to retreat following the ‘Little Ice Age’ episode of glacier expansion, the effects of which are in some respects analogous to the larger-scale glaciations and deglaciations that have periodically affected the Earth’s ecosystems through geological time. Deglaciation presents a more-orless sterile landscape for the invasion of organisms and initiates a primary succession. Throughout this succession, the ecosystems are subjected to several types of secondary disturbance, of which some (direct glacial disturbance, glacier-dependent disturbance and glacier-conditioned disturbance) are unique to recentlydeglaciated terrain and others (glacier-independent disturbance) also occur elsewhere. The direct and indirect effects of these various aspects of the disturbance regime are manifest in the form of substrate modification, plant damage, plant behaviour, and the rate and direction of succession. Disturbance is integral to the nature of succession on recently-deglaciated terrain. In general, disturbance tends to delay the initiation of succession, to slow the rate of succession or produce rapid reversals to earlier stages, to cause divergence of successional trajectories (especially in the early stages), and to maintain the productivity and stability of mature communities in the late stages of succession. However, in some circumstances, disturbance may drive forward progressive change, cause successional convergence and bring about the slow retrogression of mature communities towards less productive communities of lesser stature. The ecosystems of recently-deglaciated substrates exist in a glacier-foreland landscape where a generally severe climatic environment (characterized in particular by low temperatures, especially in winter, and also commonly subjected to drought and strong winds) is combined with high levels of disturbance. In addition, many gradual allogenic processes, such as leaching, pervection, deflation, and desiccation, affect the newlyexposed substrate and influence species’ establishment and succession. Polar and high-alpine forelands are characterized by particularly severe climatic conditions, high levels of disturbance associated with frost, snow and wind, and a predominance of allogenic over autogenic change. Initial colonization and the early stages of succession are also characterized by relatively severe

John A. MATTHEWS

physical environmental conditions, and experience the full brunt of the glacier climate concurrently with intensive paraglacial disturbance of the substrate and minimal autogenic influences. Thus, whereas life in recently-deglaciated landscapes exemplifies disturbed ecosystems, there are problems in separating the effects of disturbance sensu stricto from the effects of gradual allogenic processes against a general background of severity of the physical environment. This emphasizes a theme touched on at several points earlier in this chapter – that is, the major deficiencies in knowledge concerning both the nature of the disturbance regime and its precise role in the development and maintenance of the ecosystems of recently-deglaciated terrain. First, details on the magnitude and frequency of the different types of disturbance are lacking. This could be remedied by studies of the disturbances per se. Second, the effects of the various disturbances on the ecosystems and the mechanisms by which disturbances bring about change in the ecosystems need to be investigated using a combination of descriptive, monitoring, experimental, and modelling approaches. Glacier-foreland ecosystems provide one of the few opportunities to elucidate causal relationships between disturbance regime and primary succession, and also the broader question of the relative importance of allogenic and autogenic processes in succession. To this end there is an urgent need for the conservation of selected forelands in National Parks or smaller reserves so that the scientific importance of these unique ecosystems can continue to be exploited.

ACKNOWLEDGEMENTS

I thank Drs Roger del Moral, Lawrence Walker and Robert J. Whittaker for their comments on the manuscript and Alan Cutliffe for preparing the photographs. This chapter constitutes Jotunheimen Research Expeditions, Contribution No. 130.

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Chapter 3

STRESS AND DISTURBANCE IN COLD REGION ECOSYSTEMS ´ ´ and F.E. WIELGOLASKI V. KOMARKOV A

the poles are the polar fronts, which separate polar and mid-latitude wind belts and impede the transfer of heat from warmer lower latitudes. The southern boundary of the Arctic is the Arctic tree-line, located at the average summer position of the Arctic polar front (Bryson, 1966; Barry and Hare, 1974). It may be approximated by the 10ºC mean isotherm of the warmest month (K¨oppen, 1936) and by net radiation and potential evapotranspiration characteristics (Hare, 1950; Budyko, 1956). The Antarctic Polar Front and the associated 10ºC February surface air isotherm are located over the Southern Ocean. The sharply-defined Antarctic Convergence, an expression of the front, is the boundary of the Antarctic zone. It occurs where the temperature minimum of the Antarctic surface water sinks below warmer, less dense, nutrient-poorer Subantarctic surface water (Deacon, 1934; Lutjeharms et al., 1985; Kanda and Kom´arkov´a, 1997). At comparable latitudes, the climate is considerably more severe in the Antarctic than in the Arctic (Schenck, 1928). The mean air temperature in the Arctic is at least 3ºC higher (Cameron, 1969) and the winter sea ice is much less extensive. Mountainous cold regions are delimited by the average position of the lower horizontal boundary of cold air, indicated by the disappearance of trees at the tree-line, by shrubs, or by freeze–thaw ground phenomena. The tree-line rises from sea level in the polar regions to 4500 m above sea level, and snow-line to 6500 m above sea level on the Tibetan Plateau. Treeline and snow-line are higher in warm, dry, continental, and protected conditions and on massive mountain ranges, and lower in cold, wet, oceanic, and severe climatic conditions on less massive mountain ranges (Hermes, 1955; L¨ove, 1970; Troll, 1973). Stress has been defined in terms of energy change (Ivanovici and Wiebe, 1981), and as an unfavorable deflection from what is usual or expected (Odum

INTRODUCTION

Cold dominates large areas surrounding the North and South Poles and high elevation lands at all latitudes. For six months of polar winter night, the pole that is turned away from the sun experiences a net heat loss. During the polar summer day, the angle of the sun’s radiation is low, and proportionally more solar radiation is absorbed, reflected, and scattered along its longer path through the atmosphere than in the tropics. Mountainous regions are cold because of the low density of heat-absorbing particles, especially water vapor, in the atmosphere of high altitudes. The temperature decrease is about 1 to 3ºC per 300 m rise in elevation. Cold regions present conditions marginal to life. As latitude or elevation increases, resources decrease, the growing season becomes shorter, the complexity of food webs and ecosystems (e.g., numbers of niches or organisms) declines, and productivity and rates of nutrient cycling decrease (Wielgolaski et al., 1981). With increasing cold, the dominant plant groups change from trees to shrubs at the tree-line (Tikhomirov, 1962) to dwarf-shrubs, followed by sedges, grasses and other herbs, then bryophytes and lichens, and finally to simple ecosystems with only microorganisms. Despite the overall simplicity of high-latitude and high-elevation areas (Fig. 3.1), steep environmental gradients, and varied rocks, surface characteristics, and disturbances can combine to produce high spatial and temporal diversity in ecosystems. This diversity also reflects numerous stresses and disturbances ranging from frequent, narrow-amplitude, brief fluctuations of environmental factors to infrequent catastrophic ecosystem destruction, all more common than in most other biomes. The boundaries of cold air masses centered at 39

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Fig. 3.1. The mountain ranges on the Tibetan Plateau, looking northwest from the main chain of the Himalaya on the border between Nepal and Tibet. The elevation of the snow-line is about 5000 m. May 1994.

et al., 1979; Barrett, 1981). “Stress” here includes environmental influences which impede the function of a biotic unit, “subsidy” includes favorable or function-supporting influences, and “disturbance” includes destructive, space-freeing, matter-removing, structure-changing events (Grime, 1979; Odum et al., 1979; Grime et al., 1988). Ecosystem resistance (the ability to remain unaffected by disturbance) modifies the type and degree of damage a disturbance will cause. Ecosystem resilience [the ability to recover to a more or less persistent state (Holling, 1973; Pimm, 1984; Neubert and Caswell, 1997)] determines the rate (elasticity: Westman, 1986; Westman and O’Leary, 1986), degree, and direction of recovery from this damage. Persistence refers to presence over time, irrespective of disturbance, and constancy to low amplitude of variation over time. Ecosystems are stable if they return to their original variable states after a disturbance (Boesch, 1974; Boesch and Rosenberg, 1981; Rejm´anek, 1996a). In this chapter, we attempt to characterize the diverse cold-region ecosystems through the stresses and disturbances that affect them, and differentiate them on this basis from the ecosystems of other biomes. We also try to evaluate the importance of the resistance and resilience of cold-region ecosystems to disturbance

for their continued existence under new natural and human-related stresses and disturbances.

NATURAL DISTURBANCES

Periodic partial destruction or constant disturbance pressure are an integral part of ecosystems (Sprugel, 1976; Auerback, 1981; D.M. Raup, 1981). Cold regions have higher levels (intensity, frequency, diversity, scope, duration) of natural stresses and disturbances than most other biomes. In cold regions, unusually large heat- and gravity-related energy exchanges not only produce many disturbances, but also modify and spread disturbances originating from other causes (nutrient availability, surface disturbance by animals or humans, pollution). Heat and gravity gradients contribute to the high energy of wind, to the changes of state of water, and to other factors causing disturbances that vary with slope orientation and insolation (Moser, 1970) Persistent cold stress keeps the number of organisms low, enabling stress-tolerators to dominate in the absence of better competitors that are cold-intolerant. Such superior competitors may be established in coldregion margins, but most of them eventually disappear

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Fig. 3.2. Summary of patch-creating surface disturbances and their intensities, and frequencies in some cold-region landscapes. Infrequent regional disturbances (glaciations), large patch disturbances that occur every few thousands or hundreds of years (lakes, sand dunes, landslides, other mass movement, snow-patches, volcanic ash deposits), and frequent small patch disturbances (animal burrowing and trampling, nutrient deficiency or excess, freeze–thaw effects, drought, waterlogging, and freezing) are integral to most cold-region landscapes. After Kom´arkov´a (1993).

during unfavorable climatic fluctuations (Holdgate, 1964; Edwards and Greene, 1973). Cold leads to slow chemical reactions, decomposition, and nutrient cycling. Despite low production, organic matter accumulates in sinks located in relatively warm and moist habitats, but it is exported and redistributed from most areas by disturbance agents (wind, water, gravity). Other disturbances include drought, high precipitation, and waterlogging; deficient light; excessive light and ultraviolet radiation; deficient or excess nutrients; wind; volcanism; and grazing and other animal activities. Material sinks include peats (Schell and Ziemann, 1983) and peaty soils under tussocks and turfs, which are deep only in wet, flat, and relatively warm areas; Antarctic mossbanks (Collins, 1976; Fenton, 1980); ornithogenic soils (Ugolini, 1972; Tatur, 1989); lake stromatolites (Parker et al., 1981; Wharton et al., 1983); lake and ocean sediments; and stable air in the polar regions (Kerr, 1981) and in mountain valleys without drainage. In cold regions at high altitudes or latitudes where cold stress is constant and deep, there are no resident terrestrial higher organisms. Fluctuating environments of cold regions at middle and low altitudes and latitudes, less constantly hostile, require multi-purpose

adaptations, and do not lend themselves to truly opportunistic strategies such as those appropriate for warm deserts, or highly specialized ones as are seen in tropical rainforests. Many resident organisms live at a low metabolic level and, in alpine plants, longterm slow growth produces fewer cells than in plants of warmer environments (Clarke, 1980, 1985; K¨orner, 1994). Low and dense plant growth forms (mats, cushions, tussocks), surface hair, small leaves, and other adaptations trap heat and moisture, and efficiently store water and nutrients (Dahl, 1951; Salisbury and Spomer, 1964; Biebl, 1968). Animal migration and periods of reduced activity usually both secure resources (food, space, heat) and avoid stress and potential damage during unfavorable periods (nights, winters). Small organisms live or temporarily hide in habitats under low light and darkness (within or under rocks, soil, water, snow, or ice), where fluctuations of moisture, radiation, and temperature are limited. Marine fish follow zooplankton into deep water when phytoplankton production at the surface ceases in winter, also avoiding the risk of freezing. However, even normal diurnal, seasonal, and longterm environmental fluctuations create numerous large disturbed patches in populations or on the ground

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(Fig. 3.2). Most disturbances, however, occur during exceptional periods when fluctuations are wider or longer than average: unusually low summer temperatures, ephemeral summer snow and freezing, reduced amounts of snow in winter, late spring and early summer droughts, or unusually heavy rains. Such infrequent disturbances to which organisms cannot adapt cause most damage (Nitecki, 1981), especially to organisms at the edges of their geographical distribution. Some disturbances caused by changes in natural environmental factors may be triggered by human disturbances. These may originate at lower latitudes, but have the greatest effect in polar regions (climatic warming, thinning of the stratospheric ozone layer). Solar radiation as a disturbing factor The pronounced seasonality of light and darkness at high latitudes is unique (Dunbar, 1968, 1985; HolmHansen et al., 1977). Light stress and photoinhibition, which limit productivity, especially in the presence of additional stress factors, are the result of low temperatures, bright light, high albedo, and long periods of daylight (Adamson et al., 1988a, Howard-Williams and Vincent, 1989; Adamson and Seppelt, 1990). Low light and darkness-specific adaptations are rare, but microorganisms living in habitats which are poorly illuminated even during polar summer may show them: maximum photosynthetic rates at relatively low light irradiance, heterotrophic nutrition under both darkness and low light intensity (Palmisano and Sullivan, 1985), increased absorbency of blue-green light (Soo Hoo et al., 1987), and sensitive photoreceptive pigments (Palmisano et al., 1985; Rivkin and Putt, 1987a,b). They may be exposed to bright light when their habitat is disturbed (Vincent and Quesada, 1994). Having evolved under solar ultraviolet (UV) radiation, especially under the more injurious UV-B (280–320 nm wavelength), most organisms have some ability to avoid or screen it (pigmentation, hair, vertical migration driven by light and nutrient availability) to detoxify the highly reactive oxidants it generates, especially in hyperoxic aquatic environments (Craig et al., 1992), or to repair the damage it causes [e.g., to molecules of ribonucleic acid (RNA) or deoxyribonucleic acid (DNA)]. Protective adaptations are common in alpine regions, where the exposure to UV and shorter-wave radiation is higher than in the lowlands (intensity increases by 14–18% with

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every 1000 m in altitude), especially on sunny slopes. Such adaptations may be less common at high than at low latitudes, perhaps because polar organisms evolved under conditions of lower UV-B intensity, or because avoidance of UV is more common (Calkins and Thordardottir, 1980; Caldwell, 1968, 1979). Antarctic intertidal and subtidal invertebrates and macrophytes, and other aquatic organisms, contain mycosporine-like amino acids (MAA) that strongly absorb ultraviolet radiation (Karentz et al., 1991, 1992; Marchant, 1992). Some marine polar algae, terrestrial lichens and mosses (Adamson and Adamson, 1992), and many Cyanobacteria have other protective compounds, walls, or shells, e.g., Nostoc commune (Scherer et al., 1988), or scytonemin and related pigments, e.g., Phaeocystis (Vincent and Quesada, 1994; see also Marchant et al., 1991; Davidson and Marchant, 1994). Secondary carotenoids potentially limit photoinhibition and photodamage by quenching excess photochemical energy and trapping toxic oxidants such as free oxygen radicals, e.g., the snow alga Chlamydomonas nivalis (Bidigare et al., 1993) and Cyanobacteria (Vincent and Quesada, 1994). Melanin production in microorganisms may be an adaptation specific to Antarctic conditions (Abyzov, 1993). The polyploidy of many lower organisms in cold regions may mask the effects of single DNA mutations (Melchers, 1946; Favarger, 1961; L¨ove and L¨ove, 1974). Plant surfaces and animal skin may be protected from both cold and UV exposure by hairs or plumage (Griffiths and Fairney, 1988; Karentz, 1994). Because of the relatively high surface albedo, the eyes of Antarctic birds have higher thresholds for corneal damage by ultraviolet radiation than the eyes of birds from temperate areas (Hemmingsen and Douglas, 1970; Karentz, 1994). Temperature fluctuations Most cold-region organisms show some behavioral, structural or functional adaptations to cold and to periodic temperature fluctuations (Clarke, 1985, 1990; Block et al., 1992). At middle and high latitudes on land, seasonal fluctuations often have a large amplitude, which narrows with increasing latitude and elevation and the oceanity of the climate. The temperature of free water on land may range from below −10ºC to above 20ºC (Vincent, 1988). In polar oceans, temperatures range within only a few degrees of 0ºC throughout the year. Although bare ground on land heats and cools rapidly, most organisms live in the surface layer, with

STRESS AND DISTURBANCE IN COLD REGION ECOSYSTEMS

slower wind speeds, greater warmth, and the protection by snow and ice during unfavorable periods (Sørensen, 1941); the ground beneath a snow layer 50 cm-thick is warmed to about 0ºC by geothermal heat. Most microorganisms, and some other organisms or their tissues, are active below 0ºC; others employ winter hardening (Kainm¨uller, 1975; Sakai and Larcher, 1987), antifreeze mechanisms (Duman and DeVries, 1975; Block and Duman, 1989), countercurrent exchange (seals, humans), are insulated, maintain a high metabolic rate, or tolerate declines in body temperature. Most lower organisms can freeze or become supercooled temporarily without harm (Tyurina, 1957; Larcher, 1980; Schulze et al., 1985). An inverse relation between frost survival and atmospheric pressure may exist in the mountains (Halloy and Gonz´alez, 1993). Small size is energetically advantageous to ectothermic but not to endothermic organisms; however, small endothermic organisms can easily find shelter. Small ectothermic amphibians and reptiles can live only on the margins of cold regions. During cold periods, birds and mammals maintain body temperature by raising their metabolic rate biochemically, by shivering, or by other muscular activity; the blood volume and heat loss are lowered through reduced protein and water intake, and the conservation of urea. Down to a certain temperature threshold, the problems of living at middle and high latitudes center on the seasonal availability of resources (Clarke, 1980, 1990), as water and nutrients are frozen during long, cold winters (Ellis-Evans, 1990; Staley and Herwig, 1993). Essential activities (reproduction) may be spread out over several growing seasons, or occur rapidly within one. In long-lived perennial plants (which are often polyploid), reserves and living green tissues facilitate the survival of winters and unfavorable summers, winter pre-formation of shoots and flowers, and rapid growth and development in the spring. Plants accumulate resources over several years until the surplus is sufficient for flower production (Resvoll, 1917). Asexual reproduction may be important when the production of reproductive cells, propagules or seed supply is limited, or if needle ice or drought restrict germination and seedling establishment; but many vascular plants in cold regions spread by seeds even at the edges of their distribution areas, where pollinators may be absent and the taxa autogamous (Walter, 1964, 1968, 1985; Longton, 1972, 1988). In many resident heterotrophic organisms, often longlived, growth in the spring appears to be limited by

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primary production or food availability, leading to slow growth rates and delayed maturity (Mani, 1968, 1974; Østbye et al., 1975). Antarctic marine invertebrates hatch as advanced larvae or miniature adults, avoiding free-living larval stages (Clarke, 1990), and terrestrial ectothermic vertebrates are viviparous. Reproduction coincides with the environmentally and nutritionally most favorable periods; other energy-costly activities (molting) take place after reproduction is over. Many small mammals stay active in winter, as they cannot store enough energy for hibernation on account of their high metabolic rates. Low survival rates of small birds and mammals are compensated for by frequent reproduction, and rapid growth and development when conditions are favorable (Batzli, 1981). During periods of low temperatures, in the polar summer or during nights on tropical mountains, ice crystals tear cells and tissues internally and abrade them externally, and food, water, and nutrients may not be available. Sudden changes may cause die-back or die-off in terrestrial plants, and thermal shock, frostbite, hypothermia, and death in terrestrial animals. Cold spells arrest snowmelt and freeze water again in the spring (Rouse, 1984; Kane et al., 1991a, 1992). Late snowmelt enhances grubbing by lesser snow geese (Chen caerulescens caerulescens) for graminoid roots and rhizomes, with consequent damage (Jefferies, 1988; Jefferies et al., 1992). During cold summers reproduction may not occur, and organisms that have to reproduce annually may be eliminated. During summer snowstorms and hailstorms, large terrestrial wild and domestic herbivores may lack shelter, and are unable to find food. Winter ice may persist on usually icefree lakes (McLaren, 1964; Adams et al., 1989), snowpatches or snow may not melt (Moser, 1973), and freezing may produce hypersaline lake water (Schmidt et al., 1991). Abnormally high heat supply leads to the formation of extrazonal ecosystems (Baker et al., 1964; Longton and Holdgate, 1979; Broady, 1993). Too much summer heat may lead to damage or death in insulated, overexerted endotherms, in plants with heataccumulating growth forms, or in seedlings on bare soil (Pisek et al., 1967; Larcher and Wagner, 1976; Larcher, 1980). ˇ Abnormal winters are usually damaging (Stursa et al., 1973). A sudden early onset of winter is destructive, especially to endotherms that migrate to avoid winter cold and lack of resources. Loons that cannot take off from frozen lakes in the Arctic and bird chicks that are unable to fly may freeze. Molting

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adult penguins, and entire broods of penguin chicks still in down and not yet insulated for swimming, also freeze. Large sea mammals may be trapped by early sea ice and starve. Exceptionally cold winters with little snow cause widespread mortality among organisms reliant on snow protection. Non-migrating populations of livestock, inhabiting areas that remain bare of snow during average winters, may also perish during prolonged winter snowstorms (in the lower alpine region on the Tibetan Plateau: Shi, 1985; Schaller, 1993), and then the human pastoralists depending on them starve. In January 1997, snowstorms and temperatures of −36ºC caused the deaths of nearly a million livestock animals in the winter grazing regions of western China (Anonymous, 1997). Caribou (Rangifer tarandus), wolves, and foxes starve during winters with deep and long snow cover or a thick ice crust, which prevent them from reaching their usual food source or migrating (Scheffer, 1951; Vibe, 1967; Klein, 1968). Exceptionally long winters may kill off hibernators, which run out of moisture and energy supplies. During unusually warm winter periods (Gilbert and McKenna Neuman, 1988), the energy of some hibernators may be depleted; when melted snow and ground refreeze afterwards, unprotected organisms may be damaged. Hypoxia in cold-region ecosystems The partial pressure of atmospheric gases decreases with increasing elevation, while their proportions remain the same as at sea level. Hypoxia has a noticeable effect on lowland endotherms (Grover, 1974). Cattle cannot live indefinitely at elevations above about 3000 m, and humans cannot live indefinitely above 5500 m (Cern´ık and Sekyra, 1969; Franz, 1979). This diminishes human disturbances at high elevations such as mining, religious observances, and expeditionary tourism. Unlike the adverse effects of cold, the effects of hypoxia cannot be mitigated by behavioral adjustments, apart from descending to lower elevations or using an artificial source of oxygen. At high elevations, the cardiac and work outputs of lowland visitors are low (Frisancho, 1975). Native highlanders develop a relatively low blood pressure and large chests, hearts, and lung volume (Penaloza et al., 1963), but their reproduction and development are slowed. The efficiency of fires and engines also decreases, by 3% per 300 m. Low atmospheric pressure and low partial pressure of oxygen contribute to an altitudinal

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ceiling of about 3600 m for most small single-engine aircraft; superchargers are effective on helicopters used for rescues, construction, and supply. Cooking times increase, as the temperature at which water boils drops with decreasing atmospheric pressure (by 1ºC per 300 m), leading to a shift to foods which do not require cooking (air-dried meat, and roasted barley flour or tsampa in Tibet, freeze-dried potatoes or chu˜no in the Andes: Price, 1981). Hypoxia can occur even in polar regions. During long winters, plants in cold regions may be encased in ice; they are then exposed to hypoxia or anoxia, of which some common arctic taxa are highly tolerant (Crawford et al., 1994). Water and precipitation as disturbance factors In cold regions, water is potentially available in liquid form when temperatures rise above 0ºC. Water mediates most heat transfers, turning heat fluctuations into disturbances; water volume increases when it freezes into ice, pushing away the surrounding materials, which collapse back when the ice thaws again. A heat subsidy may be counteracted by an accompanying drought. Drops in temperature during summer are amplified, and rises in temperature during winter are counteracted by freeze-desiccation, which may cause plant die-back and death (Larcher, 1957; Sakai and Larcher, 1987; Kullman, 1993). Summer waterlogging followed by freezing kills higher organisms in low-lying areas. Precipitation mostly decreases with increasing latitude and elevation. At 8000 m, the water-vapor content of the atmosphere decreases to <1% of that at sea level. Orographic uplift leads to heavy precipitation, erosion, and many streams on steep windward slopes. On lee slopes and adjacent lowlands there may be rain-shadow deserts. Seasonal storms in desert areas, or monsoons in the Himalaya, may be responsible for most of the annual precipitation, erosion, and mass movement. Unusually high precipitation events (McCann and Cogley, 1972; Ryan, 1993; Greenland, 1995) may lead to an above-average number of landslides (Selkirk et al., 1988, 1990), avalanches, floods and other forms of mass movement, which may reach the lowlands below, especially when the mountain slopes are deforested. Storm surges remove material from beaches (Harper et al., 1988). Many cold-region terrestrial organisms have adaptations to drought and wind-caused loss of heat and water vapor tension above their surfaces. In Antarctica,

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Fig. 3.3. Soil moisture in the root zones of Deschampsia antarctica (solid line) and Colobanthus quitensis (dashed line) and precipitation on Stepping Stone Island in Arthur Harbor, Antarctic Peninsula. Two periods of soil drought developed after extended periods without precipitation in January and February 1984 on the Deschampsia site. The soil drought was associated with a die-back of the monitored Deschampsia tussock and other vascular plants on the island (Kom´arkov´a, 1984).

large expanses of rocks and soil are bare of macroorganisms, even lichens, on account of drought. High soil salinity and winds limit the moisture availability in continental cold deserts, where some organisms live in environments where temperature and moisture fluctuations are diminished and bright light is absent – in lakes (Light and Heywood, 1973, 1975; Parker et al., 1981) and inside porous rocks (Friedmann, 1982; Nienow and Friedmann, 1993). Invertebrates are often able to resist desiccation (Pickup, 1988; Worland and Block, 1986), which is common in normally wet habitats and shallow pools (Hawes et al., 1993). Many lower organisms rapidly resume normal function after being severely desiccated (Scherer et al., 1984; Peat and Potts, 1987). Poorly weathered and shallow soils with low content of organic matter have only low water-holding capacity, which diminishes with increasing latitude and elevation; life increasingly depends on meltwater, and must be able to withstand wide fluctuations in its availability (Siple, 1938; Llano, 1962; Longton, 1988). During an exceptionally long, warm, dry Antarctic summer (Morrison, 1990), vascular plants died back after two periods of soil drought at 64ºS (Fig. 3.3); no vascular plants died back in comparable habitats at 62ºS (Kom´arkov´a, 1984). During average summers, water stress for established plants is minor even in polar deserts (K¨orner, 1994), where during unusually moist and warm seasons seedlings abound in places which are

thermally advantageous (Gold and Bliss, 1995; Gold, 1998). Water cannot move below rock or the surface of ground ice, resulting in waterlogged soils on flat ground, and creep and downward flow on slopes. Small mammals are excluded from waterlogged sites with a shallow depth of thaw. Polygon ponds, beaded streams, and string bogs are associated with ground ice. Streams (Fig. 3.4) and thaw lakes (Fig. 3.5), which move in a direction perpendicular to the prevailing wind, rework the flat Alaskan Arctic at intervals of several thousand years (Britton, 1967; Everett, 1979; Billings and Peterson, 1980). During dry summers, meander cutbanks, pointbars which may border on sand-dune fields, and dry river bottoms are exposed to erosion by wind. Ponds and the surface peat dry out, because there is no additional input of thaw water from the wellinsulated ground ice below. Snow and ice as disturbing factors Snow and ice provide habitats which fluctuate at various spatial and temporal scales. Locally, shortlived microorganisms often live within the snow or ice, sustained by nutrients from windblown materials (Kol, 1968; Palmisano and Sullivan, 1985; HowardWilliams et al., 1990). Needle-ice crystals, 1–3 cm long (Mackay and Mathews, 1974; Lawler, 1988) form nightly in moist, unconsolidated surface materials,

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Fig. 3.4. Meandering rivers redistribute loose sand deposits underlain by ground ice on the Alaskan Arctic Coastal Plain, creating point-bars. Melting of ground ice leads to natural thermokarst and collapse on a cut-bank near Atqasuk.

Fig. 3.5. Thaw lakes travel across the flat Alaskan Arctic Coastal Plain in the direction perpendicular to the prevailing wind, loosening sand and creating sand-dune fields near Atqasuk. Low-center polygons and caribou trails appear on older surfaces on the right.

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Fig. 3.6. Fields of frost-shattered blocks in the foreground and talus, solifluction, and rock streams on far slopes. Mountains of Nin Jin Kan Shan, at elevations between 5000 and 5600 m, near Karo La Pass east of Gyantse, Tibet. Middle to upper alpine belt.

particularly on recently disturbed surfaces such as newly deglaciated areas moistened by melt-water. This freezing and thawing promotes small-scale erosion, sorts particles, and inhibits colonization by plants (Bryan, 1946; H.M. Raup, 1951, 1965, 1981). It also shatters rocks, sometimes creating large land-forms (Fig. 3.6) (Thorn, 1979; Williams and Smith, 1989; Coutard and Francou, 1989). Lake, stream, and sea ice scour the shores in the spring, and the subsequent melting of snow and ice may cause increased erosion, flooding (Marsh and Hey, 1989), and input of materials into sinks. Seasonal melting of the permafrost (which may extend to depths of hundreds of meters; Ives, 1973) typically occurs to depths of 0.5–5 m, depending on soil moisture content and insulation of the surface by organic matter. In unconsolidated materials, seasonal freezing and thawing of ice wedges (Chambers, 1966a,b, 1967; Lachenbruch, 1966), usually arranged in polygons several meters in diameter, moves and sorts particles in land-form patterns that change on the scale of hundreds of years (Everett, 1979). Loss of insulation and melting of ground ice cause ground collapse, subsidence, and mass movement (“thermokarst”: Fig. 3.7; Haag and Bliss, 1974; Babb, 1977; Lewkowicz, 1987). Ground ice aids the preservation of human food, but

it prevents burial of the dead, waste disposal, and the maintenance of safe water supplies. With increasing elevation and latitude, temperature fluctuations above 0ºC decrease, then disappear. Through melting, refreezing, and/or compaction increasing toward the bottom of the snowpack, snow becomes ice and grows into glaciers that sculpt the surfaces of most cold regions. Snow erodes nivation depressions (Fig. 3.8; Thorn, 1976; Thorn and Hall, 1980; Caine, 1992a, 1995a), in which the growing season is considerably shortened (Billings and Bliss, 1959; Holway and Ward, 1963; Canaday and Fonda, 1974), and they gradually become cirques (Fig. 3.9). Glaciers quarry and grind the surrounding rocks as they move at rates from several centimeters to several meters per day. Loosened fine materials may be deposited as loess (Smalley, 1966). Along with glacial streams, glaciers carry rocks and other materials downslope, depositing moraines on their sides (Fig. 3.10), and terminal moraines when they retreat. Snow and glaciers dam valleys, sometimes creating ephemeral lakes. Glaciers may surge and lakes may be sources of outburst floods (Visser, 1932; Thorarinsson, 1957; Hewitt, 1965), especially during eruptions of volcanoes covered by ice (Schoonmaker, 1998). Glaciers flow into lowland forests, flattening them by their weight; they may destroy villages and agricultural

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Fig. 3.7. Frost-sorted stone circles on Ardley Island, South Shetland Islands, Antarctica (62º13 S, 58º56 W). Diameter about 1.8 m.

Fig. 3.8. The upper part of “Martinelli’s snowpatch” and nivation depression on the southeast lee slope of Niwot Ridge, Front Range, Colorado Rocky Mountains. The rocky centers are free of vegetation because the snow-patch does not melt completely in most summers – only when snow-poor winters are followed by long, warm summers. The snow-patch fluctuations suggest that only small, shallow snow-patches are likely to disappear soon under the current levels of climatic warming, unless the warming is accompanied by drought. In the foreground, coniferous trees and willows (Salix spp.) grow in krummholz ribbons in response to the dominant westerly winds.

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Fig. 3.9. A cirque in the Krkonose Mountains, located between the Czech Republic and Poland, in the lee of the dominant westerly winds. During forest advances towards the flat plateaus above, rare cold-region plants survived in cirques, kept forest-free since the last glaciation by avalanches (Jen´ık, 1961). In the foreground, shrubs and krummholz (Pinus mugo ssp. mughus) with flexible stems withstand snow creep and avalanches. Photograph by J. Ruzicka.

Fig. 3.10. A glacier and side moraines covered by fresh snow on the north side of Dhaulagiri, Nepal Himalaya. Ice from hanging glaciers scours the free-fall rock face on the right.

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Fig. 3.11. Tree krummholz (Picea engelmannii), stunted by wind desiccation and abraded by wind-driven ice crystals and sand, on the lee east slope of Niwot Ridge, Front Range of the Rocky Mountains in Colorado. Snow protects the near-ground plant parts. Small soil particles are being blown away from winter snow-free ridge-tops covered by fellfields (see also Fig. 3.13).

fields on valley floors during minor glacial advances, and ecosystem zones and belts during major ones. Most cold region surfaces are scoured by glaciers at intervals between 20 000 and 50 000 years. Some surfaces in the proximity of glaciers are disturbed more often, while others in cold-region margins have not been glaciated for more than 100 000 years. Glaciers absorb and release large amounts of heat before responding to temperature changes by advancing or retreating. Sea ice surrounds Antarctica in a seasonally variable ring that extends from about 55º to 70ºS; more than 90% of this ice melts in the summer (Horner et al., 1992). The cold air and water associated with the Antarctic ice cap span almost 50º of latitude, and their thermal inertia will delay the onset of warming or cooling in Antarctica (King, 1991). The present annual accumulation of snow and ice in the Antarctic ice cap is equivalent to a lowering of the global sea-level by 6 mm, whereas the annual average rate of rise in the global sea level over the last century has been about 1 mm (Barnett, 1982, 1984; Budd, 1991). Wind disturbance Cold regions significantly influence both global atmospheric circulation and local winds and weather.

Mountains block winds, storms, and tornadoes, cause upper-air disturbances, and influence jet streams. Latitudinally-oriented mountain ranges (European Alps) deflect invasions of the Arctic air. The Tibetan Plateau affects the seasonal variation of the atmospheric boundary layer and westerly circulation over Asia, causes seasonal displacement of the subtropical westerly jet stream, and also may control the inter-annual variability of the general circulation of the atmosphere and the monsoonal weather patterns of southern and eastern Asia (Flohn, 1974, 1981; Reiter and Ding, 1981). Steep temperature gradients in cold regions produce some of the strongest winds on earth. Wind speed, and the frequency of unusually strong wind-storms that redistribute surface materials, usually increase with elevation and latitude. Cool air surrounding glaciers and snow may descend with destructive force to the lowlands below ( f¨ohn, chinook). Wind exacerbates the loss of moisture and heat (“wind-chill”) from the surfaces of organisms (Siple and Passel, 1945; Warren Wilson, 1959; Rees, 1993), and most cold-region biota have some structural or behavioral adaptations which mitigate its effects. In many areas the tree-line is wind-determined. At its upper altitudinal limit, krummholz grows in the lee of boulders, protruding terrain, or in stripes oriented in the direction of the prevailing wind (Figs. 3.8 and

STRESS AND DISTURBANCE IN COLD REGION ECOSYSTEMS

3.11). In the White Mountains of New Hampshire, on lee slopes (east-facing slopes at mid-latitudes in the Northern Hemisphere), downslope winds produce a wide krummholz belt (Reiners and Lang, 1979), and the same occurs in the Colorado Front Range (Marr, 1961), where the tree-line is lower on the lee slope than on the windward west slope by an average of 65 m (Kom´arkov´a, 1979). The redistribution of snow by prevailing winds to lee slopes produces a marked asymmetry of mountain ranges, with more numerous snow-patches, glaciers and cirques, and longer glacial valleys in the lee, e.g., Central European mountains (Jen´ık, 1961), Presidential Range, New Hampshire (L¨ove, 1965), Colorado Front Range (Kom´arkov´a, 1979) (Figs. 3.8 and 3.9). Wind redistributes sand from rivers, and moves dunes (Black, 1951), thaws lakes (Rosenfeld and Hussey, 1958; Rex, 1960; Carson and Hussey, 1962) and modifies other land-forms (Figs. 3.4 and 3.5). Wind removes fine mineral particles and dead organic material from summits, ridges, polar deserts, and deglaciated and other bare areas, reducing them to fellfields, pavements, or sands with little available moisture, nutrients, or snow protection in winter (Fig. 3.11), and depositing the windblown particles in lee areas, sometimes distant (Warren Wilson, 1958; Teeri and Barrett, 1975; Thorn and Darmody, 1985a,b). Wind erosion is promoted by surface disturbances (Thorn, 1978). Wind also disperses plant propagules (Bonde, 1969; Rudolph, 1970; Miller and Ambrose, 1976) and birds throughout the cold regions, and deposits and redistributes nutrients and pollutants. Gases, droplets, dust, and other small particles, such as those dispersed by volcanic eruptions, can be carried around the globe (Franz, 1979; Rahn and McCaffrey, 1980; Bodhaine et al., 1981). Disturbance by mass movement The abrupt gradients of temperature and elevation of cold regions contribute to a level of mass movement much higher than that in temperate regions. The effects of gravity increase with elevation and slope angle. Vegetation, which impedes erosion, stabilizes slopes, and increases the water-holding capacity of soils, is often scanty in cold regions, the soils are shallow, and most precipitation and meltwater run off. Water, snow, and ice significantly increase erosional rates by decreasing the friction between the falling particles and the ground.

51

Runoff and streams remove particles and solutes to lowlands, and undercut slopes (Caine, 1992b, 1995b). Rates of erosion and solute removal (Litaor, 1993) increase with water volume during high precipitation events, spring melting of snow and ice, surface microdisturbances (Friedmann and Weed, 1987), and rainy seasons. Erosional rates may reach 1 mm per year in high-relief mountain ranges with a humid climate, but in continental areas with gentle relief they are considerably lower (Caine, 1974). Some high mountain ranges (e.g., the Appalachians) have been worn down by erosion, and their populations of coldadapted organisms are probably extinct. Chances of migration are better when erosion of mountain ranges coincides with cooling of lowlands. Saturation of slope materials by water from rainstorms or snow-melt increases their weight and may cause abrupt sliding, especially when they are underlain by sloping rocks with layers of shingle. Vegetated solifluction lobes, terraces and similar features creep down at speeds of several centimeters per year (Benedict, 1970), more on steep slopes, and under diurnal or seasonal frost-heave (Bunza, 1975). Scree and its accumulations continue to move at rates of up to 20 cm per year (Benedict, 1970; Price, 1991; Matsuoka and Moriwaki, 1992); see also Fig. 3.6. Rock glaciers, with steep front sides and ice cores, develop in areas with low snow-fall and a continual supply of rocks from above. Their rates of movement are up to 50 cm per year, and they are usually not vegetated. Rockfalls, mud-flows, debris flows, landslides, avalanches (Voellmy, 1955; Luckman, 1977, 1978), floods, lahars, and other forms of mass movement vary in velocity, extent, and severity, but often destroy forests, crops, transportation corridors, and human settlements. The incidence and intensity of disturbance increase particularly during earthquakes, volcanic eruptions, and unusually wet years, and after deforestation. In low-elevation mountains, avalanches prevented forest establishment in lee cirques during the warm postglacial period, enabling cold-region plants to survive there while the summit plateaus were forested, e.g., Central European mountains (Jen´ık, 1961), Presidential Range, New Hampshire, USA (L¨ove, 1965); see also Fig. 3.9. Volcanic eruptions create bare patches of lava and tephra reaching into the lowlands. They may cause melting of snow and ice and floods; by introduction of particles and droplets into the atmosphere they may cause climatic cooling, reducing crop production

52

worldwide; and they may affect precipitation chemistry, contributing to acid rain (Williams, 1969; Lewis and Grant, 1981; Wood and del Moral, 1987). Many such disturbances originate in cold regions, e.g., Iceland (Arnalds, 1987; Arnalds et al., 1987), Aleutian Islands, Scotia Arc (Collins, 1969); and from other volcanoes rising from plate boundaries and hot spots into coldregion altitudes. In the presence of moisture, new volcanic ash offers opportunities for habitat diversification, organismic colonization, and geographical range expansion (Young and Kl¨ay, 1971). Fire Unlike forests or grasslands in which fire is often important, most cold regions have little combustible material. Even in the margins of the cold regions with relatively high production, litter is often removed by wind, runoff, gravity, or snow creep. Most fires occur in the Arctic tundra only during exceptionally dry, warm summers when accumulated peat dries out. In western and northern Alaska, 3800 km2 burned during the dry summer in 1977, but the fire frequency per annum ranged from 0.4 to 12 fires per 5000 km2 over a period of 23 years (Racine et al., 1987). Tundra fires are usually an extension of forest fires (Cochrane and Rowe, 1969; Douglas and Ballard, 1971; Wein and Bliss, 1973a); they can also be initiated by lightning or by humans. Arctic fires burning only for a short time do not affect the ground. Long-burning fires over ground ice increase the depth of thaw and may cause surface subsidence on slopes, particularly when the thick insulating vegetation and the surface layer of organic matter are burned; there may be a surplus of released moisture. The increase in depth of the thawed layer above permafrost may return to the original within six years, or it may continue even for 20 years, and burned areas may maintain reduced heat loss, higher temperatures and moisture, and consequently higher nutrient availability, lower albedos, and higher net radiation (Mackay, 1970, 1995; Bliss and Wein, 1972). Disturbances through effects on soil and nutrients In most cold regions, the influence of parent material is stronger than in other biomes; soils are shallow and erosion rates high, and essential or toxic elements

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released from minerals are continually available (Fernald, 1907; Braun-Blanquet and Jenny, 1926; Franz, 1979). However, even on alpine slopes, leaching by rain containing weak carbonic acid acidifies limestone soils so that their original ecosystems are invaded by acidophilous taxa and are usually impoverished (BraunBlanquet and Jenny, 1926). Calciphilous dominants and soil buffers may counteract these changes. On Subantarctic Macquarie Island, serpentinite, harzburgite, and basalt outcrops support areas of small and sparse vegetation with a high proportion of bare ground and low Ca/Mg ratio, unlike the surrounding dense vegetation on gabbro (Adamson et al., 1993). In the coldest areas, low temperatures dominate and rock weathering is slow. A high toxic salt content is common in the dry valleys of the southern Antarctic, where salts released by in situ weathering accumulate (Miotke, 1988). Sea spray accumulates on marine beaches, and the plants there have a high salt tolerance (Huntley, 1971; Gremmen, 1982), such as Poa flabellata (R.I.L. Smith, 1985a; Dawson and Bliss, 1987; Bliss, 1988). Low-growing and sparse vegetation does not efficiently protect soil from erosion and scavenge nutrients from the air. In most areas, wind, snow, ice, and runoff export most of the fine materials. The availability of nutrients is low and their turnover slow (Warren Wilson, 1957; Haag, 1974; McKendrick et al., 1978), but organisms are adapted to these conditions; plants have leaf nitrogen content higher than plants of temperate climates (Chapin et al., 1975; Chapin and Oechel, 1983; K¨orner and Larcher, 1988). Slow decomposition leads to organic matter accumulation in some terrestrial ecosystems (marshes, lowlands, tussock and turf communities; mossbanks in the Antarctic), but only some of them produce enough organic matter to support herbivorous mammals and their predators. The low productivity of tundra plants can be increased by fertilization (Chapin et al., 1975; McKendrick and Mitchell, 1978a,b). However, artificial addition of nitrogen and phosphorus causes changes in species composition and eventually in ecosystems, as grasses respond more quickly than shrubs and sedges (Jeffrey, 1971; Hegg, 1984a,b). Once the disturbance by fertilization ceases, taxa enhanced by it are again out-competed by strong competitors (usually tussock or turf-forming Cyperaceae). In many areas, nutrients supplied by the atmosphere (Wilson and House, 1965; Desideri et al., 1994; Cress et al., 1995), sea spray (Jenkin, 1975; Benninghoff and Benninghoff, 1985; Karl, 1993), or imported and

STRESS AND DISTURBANCE IN COLD REGION ECOSYSTEMS

53

Fig. 3.12. Nests of gentoo penguins (Pygoscelis papua) at Point Thomas, King George Island, South Shetland Islands, northern Antarctic. The bare area surrounding the nest contains excrement rich in krill chitin and toxic levels of nutrients. The surrounding area with lower nutrient concentrations is colonized by algae, mainly Prasiola crispa. Farther away, nutrients are diluted enough to subsidize abnormally luxuriant growth of Deschampsia antarctica, tolerant of elevated nutrient levels. Normal Deschampsia grassland is about 20 m away from the nest. January 1984.

redistributed by strong winds, water, or animals play a greater role in the distribution and production of organisms than nutrients that are cycled or supplied locally (e.g., by nitrogen fixation: Fogg and Stewart, 1968; Croome, 1973; V.R. Smith, 1985). The dependence of organisms on nutrient subsidies increases with altitude and latitude. Most nutrients that support the productivity of cold, barren ecosystems are probably blown in from more vegetated areas or from the ocean by wind (Swan, 1963, 1967; Papp, 1978; Edwards, 1988), deposited on windward slopes, and transferred to lee snow-patches and glaciers, where they accumulate. Meltwater from glaciers and snow carries high loads of remineralized inorganic nutrients, leachates, and particulate detritus (Benninghoff and Benninghoff, 1985), which are channeled to ecosystems below, and then by streams to the lowlands or washed out to sea. From the Antarctic Plateau, they are carried to the coastal ice-free areas (Allen et al., 1967; Claridge and Campbell, 1968). Nutrients are also transferred to land by nesting and breeding marine birds and mammals through their feces and urine, molted skin, fur and feathers, regurgitated shells and other material, and carcasses (Siegfried et al., 1978; Batzli, 1983; Ryan and Watkins, 1989). For

example, South American condors (Sarcorampus papa) nest in the high Andes, but migrate daily to the sea where they feed on dead fish (Lettau, 1967; Price, 1981). Natural nutrient enrichment by nitrogen and phosphorus in urine and feces near small-mammal burrows, bird posts, and nests can significantly enhance the diversity and luxuriance of vegetation (Figs. 3.12 and 3.13). Enrichment also occurs at carcass sites, because the concentration of most nutrients (except potassium) is much greater in herbivores than in plants (McKendrick et al., 1980). Grazing by lesser snow geese increases net aboveground primary production of coastal salt marshes by 40–100% (Jefferies, 1988). In the Antarctic, penguins nest in colonies on ornithogenic soils created by long-term accumulation of droppings (Ugolini, 1972; Heine and Speir, 1989; Tatur, 1989), which they also deposit on snow-patches where they cool down after fishing. Fresh penguin droppings contain about 2.55% phosphorus and a high nitrogen content, reflecting the high protein diet of penguins (Ugolini, 1972; Pietr et al., 1983). High levels of nutrients in rookeries are lethal to most organisms (Rudolph, 1967). Penguin droppings contain the antibiotic acrylic acid (Sieburth, 1961, 1963),

54

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Fig. 3.13. An area disturbed by pocket gophers on a ridge-top on Niwot Ridge, Front Range of the Rocky Mountains in Colorado. The animals burrow and graze below ground, increasing soil nutrients through their feces and urine. The patch is colonized by nutrient-subsidized grasses and dicotyledons, contrasting with the surrounding wind-blown fellfield free from snow in winter (Fig. 3.11) with sparse cushion plants.

and their decomposition yields oxalic acid (Akiyama et al. 1986, 1988). At a distance from the rookeries, diluted nutrients provide a subsidy to vascular plants (Fig. 3.12). Burrowing petrels (Procellariidae) are the most important source of nutrients in vegetated areas on Marion Island (Burger et al., 1978; Smith, 1979; Lindeboom, 1984), where contributions from elephant seals (Mirounga leonina) affect plant and invertebrate distribution (Huntley, 1971; Smith, 1976; Gremmen, 1982). Giant petrels (Macronectes giganteus) and other scavenging and predatory seabirds redistribute the materials derived from carcasses of penguins, seals, etc., as guano, eggs, feathers, droppings, or their own carcasses (Conroy, 1972; Williams et al., 1978; Hunter, 1985). Growing populations of Antarctic fur seals (Arctocephalus gazella) on Signy Island in the northern Antarctic increase the output of nutrients from land into the ocean (Ellis-Evans 1985, 1990; Smith, 1988). Animals as disturbing factors In cold oceans, which have been harvested by humans for centuries, little is known about population dynamics, trophic level composition, energy flow, and the disturbances caused to lower trophic levels by

consumers from higher trophic levels. In terrestrial cold environments, low-growing, often sparse, vegetation producing little (usually 100–800 g dry mass m−2 y−1 ) is easily overexploited, leading to die-backs of consumer populations, and is easily disturbed by burrowing and other activities. Where present, large mammals and small rodent herbivores affect both primary production and decomposition, and are more important than invertebrates or birds (Tikhomirov, 1959; McKendrick et al., 1980; Batzli et al., 1981). In the Antarctic, where native terrestrial herbivores are all invertebrates, most of the primary production is utilized by decomposers. At high elevations and latitudes, predators are supported mainly by detritivores, and herbivores may be absent. Some mammals have developed hooves that are effective for digging through soft snow (caribou and reindeer (Rangifer tarandus): Leader-Williams, 1988) or climbing rocks (Hoffmann, 1974). Animal populations in cold regions are highly variable spatially, and fluctuate strongly in time, both within and between years, particularly at higher latitudes (Arctic herbivorous insects: Haukioja, 1981). Caribou herds appear to be regulated by the availability of food (Messier et al., 1988; Crˆete and Huot, 1993; Crˆete and Manseau, 1996). The fact that fluctuations of distant populations of Ad´elie penguin (Pygoscelis

STRESS AND DISTURBANCE IN COLD REGION ECOSYSTEMS

adeliae) occur simultaneously suggests that they reflect wide-ranging environmental disturbances affecting the whole Ross Sea (Taylor et al., 1990). Overpopulation occurs only in a few productive areas and during climatically favorable years. In cold-region margins, populations of small mammals reach excessive levels every 3–10 years (Schultz, 1968; Kalela et al., 1971; Tast and Kalela, 1971). Disturbances occur to both vegetation and herbivore populations, especially when primary production drops following such highly favorable summers. Excessive populations of lesser snow geese destroy Arctic wetlands, which erosion then changes into mudflats (Jefferies, 1988; Kerbes et al., 1990), and overuse by caribou caused persistent suppression of shrub birch (Betula glandulosa) in the northern Labrador Peninsula (Crˆete and Doucet, 1998). Few alternative food sources are available to predators when their food source is destroyed by climatic fluctuations, which perhaps contributes to the relatively low number of terrestrial predators in cold regions. Low-level damage by grazing, browsing, and passage of animals may be typical for most of the productive ecosystems at cold-region margins. Repeated browsing by caribou and girdling by collared lemmings (Dicrostonyx torquatus) has damaged willows (Salix spp.) in tussock Arctic tundra, and microtine grazing has been shown to damage graminoids and mosses, particularly during peak population years (Hansson, 1969; Kalela and Koponen, 1971; White and Trudell, 1980). By winter grazing at the snow/ground interface, microtines damage mosses and decrease primary production during the summer in the Arctic (Batzli and Jung, 1980; Batzli, 1993); in alpine regions, intensive shredding by microtines produces patches of dead vegetation. In Alpine grasslands, grasshoppers removed between 19 and 30% of the aboveground phanerogam biomass in 1993 (Blumer and Diemer, 1996). Leaves which live for more than one growing season need to be protected by secondary compounds from herbivores and overwintering stress, and have lower concentrations of nitrogen and phosphorus than deciduous leaves, which are preferred by herbivores (Archer and Tieszen, 1980; Chapin, 1980; Chapin et al., 1980a). At least some secondary compounds are toxic to microtines (Jung et al., 1979; Batzli and Jung, 1980; Oksanen et al., 1987), to snowshoe hares (Lepus americanus) (Reichardt et al., 1990), and to some herbivorous insects (Niemel¨a et al., 1979). Retention

55

of standing dead leaves for one or two years may be a defense effective against ungulate grazers, as caribou do not eat evergreen shrubs (White and Trudell, 1980). Bitter chemicals protect some dicotyledonous herbs (Gentiana lutea, in the European Alps) and lichens. There seem to be few structural adaptations against herbivores, which apparently play a role smaller than in some other biomes. By periodically cutting plant material, disrupting the snow layer, and depositing feces, Arctic lemmings (Dicrostonyx, Lemmus) increase decomposition rates and plant production; flowering may increase and root regrowth may be depressed (Schultz, 1968; Batzli and Jung, 1980; Batzli, 1981). Arctic ground squirrels (Spermophilus parryii) prefer plants with high water content, which may be more palatable (Batzli and Sobaski, 1980), and various Arctic grazers prefer flowers and young plant parts, which contain more nutrients (White, 1983). Flower removal may favor vegetative reproduction. In the Colorado Rocky Mountains, pikas (Ochotona princeps) store plants rich in allelochemicals which increase the length of time for which the plants can be preserved, and decay by the time they are consumed (Dearing, 1997). Migratory herds of Arctic caribou and reindeer compact soil on permanent trails along old ridges and streambanks where it is easier to walk, and trample other vegetation (Pegau, 1970a; LeResche and Linderman, 1975; McKendrick et al., 1980). Trampling by introduced reindeer has created persistent holes in moss carpets on South Georgia (Kightley and Smith, 1976). Microtine trails and nests and dens of other animals also disturb vegetation. Reindeer and muskoxen (Ovibos moschatus) destroy lichens and cause erosion by digging for food in winter in places with shallow snow (Østbye et al., 1975; Raillard and Svoboda, 1989; Wielgolaski, 1997). Resting and molting seals compact and damage peat and vegetation, and flick sand and loose soil over themselves, which causes erosion, and affects populations of invertebrates and birds along the Antarctic and Subantarctic coasts (Gillham, 1961; Croxall et al., 1984; Bonner, 1985). Burrowing animals (Hoffmann, 1974) create bare patches, which may be enriched in nutrients, but also increase erosion and lower the soil carbon content (Thorn, 1978, 1982; Hall and Williams, 1981). They include pocket gophers (Thomomys talpoides), which feed on underground plant parts in the southern Rocky Mountains (Fig. 3.13; Thorn, 1978, 1982), introduced rabbits (Oryctolagus cuniculus) in Macquarie Island

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(Griffen, 1980; Selkirk et al., 1983; Scott, 1988), Arctic ground squirrels (Spermophilus parryii), which sometimes create round hill burrows several meters high, conspicuous in the surrounding flat landscape (Mallory and Heffernan, 1987), and rock hyrax (Procavia capensis) and groove-toothed rat (Otomys orestes) on Mount Kenya (Mahaney and Boyer, 1986). On the Tibetan Plateau, there are a few burrows (at densities of 10 per ha−1 ) of pikas (Ochotona spp.) and zokors (Myospalax spp.) which graze above the ground, while the surface is covered by a mosaic of burrow patches in varying stages of recovery at a density of 100 or more per hectare (Liu et al., 1982; Xia, 1986). High burrow densities sometimes found over areas up to 1500 m2 may result in bare, wind-eroded patches in which vegetation recovery is difficult (Ekvall, 1968; Xia, 1986). HUMAN DISTURBANCES

The first use of cold regions by humans was probably on hunting and gathering trips from adjacent areas. Camps and settlements were necessary in remote, extensive regions where humans were almost exclusively dependent on local resources. Most important were the availability of animal prey and animal products used in shelter construction, clothing, and heat production, and the preservation of edible animal products over winter when prey was scarce (Tibetan Plateau: Ekvall, 1968; Arctic: McGhee, 1974). At the edge of the Siberian Arctic, settlements occurred as early as 300 000 B.P. (Waters et al., 1997). In most Arctic and alpine areas, they flourished particularly during warmer and drier periods (e.g., 9000–5000 B.P. and 1200–1000 B.P.). Mountain ridges and ice prevented interchanges with neighboring populations, and unique gene pools, cultures, and languages developed. Small native populations and their limited technology damaged cold-region margins through hunting, fishing, grazing of domesticated animals, mining, plant litter collection, cutting grass and firewood, and burning forests, and they modified the landscape by terraces, irrigation systems, trails, corrals, encampments, gamedrive walls, monuments, shelters, and other structures. Limited resources led to customs such as primogeniture, polygamy, polyandry, mandatory emigration, celibacy, and prohibition of marriage, and to various systems of private and community ownership, transportation, and cooperation (Husted, 1974; McGhee, 1974; Simchenko, 1976).

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Today, mountain settlements are common at altitudes of 4000 m, and up to 2.5×107 people live above 3000 m, most of them on the less densely populated cold plateaus of Asia (Tibet, Pamir–Tien Shan) and on the warmer and more densely populated plateaus of South America and Africa (mainly Bolivia, Peru and Ethiopia) (Hedberg, 1964, 1978; Coe, 1967; Holdridge, 1978). Defended by mountains, some highlanders have independent states (Switzerland), but most natives were subjugated by lowlanders (Tibet by China) or split up among different states (Arctic). On account of population and political pressures, some of these cultures are now disappearing, and some are engaged in armed conflict (Davidson, 1993). Some mountain agricultural landscapes, usually including extensive terraces, are deteriorating due to emigration of rural labor (Vogel, 1988). Other native populations are still supported by traditional agriculture, animal husbandry, hunting, and fishing. Other subsistence or cash-generating activities include mining (up to 6000 m in the Andes), rock and gravel extraction, trading, home manufacturing, collection of medicinal plants, guiding, load carrying, and transportation. Most populations are becoming sedentary, utilize resources from lower latitudes and altitudes, and mix traditional and modern pursuits (Quigley and McBride, 1987; Usher and Wenzel, 1987; Smith and Wright, 1989). Increasing population pressures from lower altitudes in the mountains of less developed countries cause environmental overuse, a breakdown in patterns of land use, cooperation, and trading, and eventually emigration of native populations. Natives may also emigrate or settle in marginal habitats when immigrants overwhelm them, marginalizing their economy and instituting ill-advised policies (Chinese in Tibet: Goldstein and Beall, 1989; Goldstein et al., 1990; Tsering, 1992). Low-elevation land is being used for housing, military bases and installations, and other purposes, thus restricting croplands and lower pastures. Firewoodcutting, terracing, irrigation, and upper pastures are extended upward into unsuitable areas, resulting in erosion and loss of habitat, vegetation cover, and native organisms (King and Jackson, 1982; Harden, 1988; Longrigg and Rowe, 1990). The removal of mountain soils leads to increased runoff, floods, and water shortages; siltation of river channels, dams, and canals; and to decreasing the build-up of fertile soil in the lowlands (Byers, 1991; Bandyopadhyay and Gyawali, 1994).

STRESS AND DISTURBANCE IN COLD REGION ECOSYSTEMS

57

Fig. 3.14. Zones of ecosystem damage on the Kola Peninsula in the Russian Arctic. Zone I: completely destroyed ecosystems around industrial centers, especially nickel–copper smelters (Nikel’, Monchegorsk); this zone receives 25–30 tons of sulfur and 5–6 tons of nickel, copper, manganese, and zinc per square kilometer annually. Zone II: highly disturbed ecosystems receiving 3–5 tons of sulfur and 2–5 tons of metals per square kilometer annually. Zones III and IV represent intermediate stages, and zone V the initial stages of ecological damage. Undisturbed ecosystems occur only on the fringes. From Doiban et al. (1992).

In the densely populated mountains of more developed countries, where the tree-line has been depressed by centuries of wood-cutting and farming (Mikkelsen and Høeg, 1979; Tranquillini, 1979), the maintenance of attractive forest meadows needs to be supported by government aid to farmers, who often emigrate to lowlands or shift to more profitable, now almost year-round, tourism and recreation, e.g., European Alps (Lukschanderl, 1983; Brugger et al., 1984; Broggi, 1985), Carpathians (Kotarba, 1992), mountains in general (Allan et al., 1988). Most labor and resources must be supplied to cold regions from lower latitudes and altitudes, and development is expensive. Warfare, pollution, and storage of military materials have affected some areas (parts of the Arctic, Kashmir, South America, the European Alps, Tibet). Throughout the Arctic, a sparse network of airfields, rivers, ports, roads, and railroads supports military activities, research, and regional resource and economic development (Rempel, 1970; Walker, 1970; West, 1976). The environmentally destructive exploitation of rich mineral and fossil fuel resources in the margins of the Russian, North

American, and Scandinavian Arctic has been, and may still be, profitable. The vast Russian Arctic has assumed a great strategic importance since the days of the Soviet Union. Nearly 4×106 people, mostly immigrants, live there: 40% of them work in mining districts (Kola, Vorkuta, and Noril’sk); 25% fish, or work in the military; and 15% are aboriginal people. The Kola Peninsula contains Murmansk, the largest city north of the Arctic Circle, Russia’s main naval base, and the largest military base complex in the world (Bergesen et al., 1987; Osherenko and Young, 1989; Doiban et al., 1992); see also Fig. 3.14. Russia’s northern population has continued to grow to service the extractive industries (Armstrong, 1990). Development often conflicts with the interests of native peoples and native organisms (Kennedy, 1988; Chance, 1990). In the Western Hemisphere, native land claims (M.E. Thomas, 1986) and environmental impact studies have postponed and even prevented some projects (National Foreign Assessment Center, 1978); this is now occurring also in other countries (Vitebsky, 1990a). Sustainable development is more difficult to achieve than in temperate biomes (Pretes

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and Robinson, 1989; Lyck, 1990; Arikaynen, 1991). Some native peoples have benefited from land grants and government cash settlements or support, e.g., Alaska (Morehouse, 1989; Flanders, 1989), Greenland (Lyck and Taagholt, 1987; Poole, 1990), which could, along with natural-resource trust funds administering the current income from non-renewable resources, provide financial stability to native populations (Pretes and Robinson, 1989; Robinson et al., 1989; Knapp and Morehouse, 1991). Expeditions, research, and tourism at high latitudes and altitudes mostly occur during the summer. Antarctica supports transient populations of tourists and personnel operating research stations, ships, and aircraft (Parker et al., 1978; Parker and Angino, 1990). Landbased, year-round activities are more damaging than ship-based ones, because they further encroach upon the limited ice-free areas (Lipps, 1978; Harris, 1991; Olech, 1996). Antarctic deposits of minerals and fossil fuels are administered under the international Antarctic Treaty (Shapley, 1985; Gulland, 1987; Vicu˜na, 1988), which also bans military presence. The extraction of these resources would cause serious environmental problems (Gregory, 1990). Harvesting of organisms Humans have long utilized organisms, primarily animals, native to cold regions, and hunted many terrestrial animals to extinction during the Ice Ages. Caribou and reindeer, which are much easier to kill than polar bears (Ursus maritimus), probably supported most ancient Arctic populations, along with hares (Lepus spp.) and ptarmigans (Lagopus spp.). In the Russian Arctic, reindeer accounted for 49.7% and polar bear for 43.8% of the prey of hunters at 7800 B.P. (Makeyev et al., 1992); hunters followed seasonal reindeer routes (Simchenko, 1976). In the Rocky Mountains 5000 to 9000 years ago, game-drive walls directed bighorn sheep (Ovis canadensis) to concealed hunters. A tradition of hunting by Ute and other Native American tribes (Ives, 1942) continued after the arrival of Europeans; by the 1960s, the numbers of vicu˜na (Vicugna vicugna) in the Andes were reduced to 1% of previous levels – to a total of 10 000 individuals (Rabinovich et al., 1985). In the Alps, only a small herd of ibex (Capra ibex) survived (Grabherr, 1997). The greatest damage occurred during the 19th and early 20th centuries (Rossnes, 1991), when improved technology and communication increased

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the trade in animal products (fur, skin, meat, eggs, down, fat), and there were few measures to protect the animals. In populated areas today, the interests of native populations preserving their traditional way of life (Nuttall, 1990, 1991), the settler populations practicing subsistence, the animals and the institutions protecting them, and commercial sport hunting and fishing, are often in conflict (Marquette and Bockstoce, 1980; Riewe, 1981; Freeman, 1985). Large-scale collection of medicinal and ornamental plants, hunting for subsistence purposes, for medicinal and ritual items, and for trophies (Murphy et al., 1990), and fishing, still decimate the gene pools and may lead to extinctions. In Arctic Alaska, 50% loss of bowhead whales (Balaena mysticetus) that have been struck during the annual Inuit subsistence hunt (Marquette and Bockstoce, 1980; Mitchell and Reeves, 1980) is being reduced by radio telemetry (Follmann and Manning, 1989). On the Tibetan Plateau, rare and protected wild animals are hunted under permit; others are killed for meat, wool, musk, and other products sold commercially. In some areas, wild animals are not tolerated because they are thought to compete with livestock for food (Palber, 1992; Tsering, 1992; Schaller, 1993); a population of 70.4 pikas per ha−1 consume more than one Tibetan sheep (Ovis aries) (Pi, 1982). In some areas, animals that have been almost extinct are being reintroduced, e.g., the bearded vulture (Gypaetus barbatus) in the Alps, by a nongovernmental organization “Alp Action”. The damage is most systematic in the oceans, which in cold regions are easier to exploit than the land, despite local environmental fluctuations (Sanger, 1980, 1988, 1991; Sanger and Dickinson, 1989). For centuries, Arctic peoples fished and hunted seals sleeping on the ice, and whales surfacing in narrow leads. Commercial sealing and whaling, which began off Svalbard in the 17th century, reduced most populations to 25% or less of their initial stock size (Braham, 1992). One thousand human residents of St. Lawrence Island in the Bering Sea starved in the winter of 1878–79 after commercial whalers, finding bowhead whales (Balaena mysticetus) scarce, shot thousands of Pacific walrus (Odobenus rosmarus divergens for their ivory (Bockstoce, 1986; Huntington, 1992). Steller’s sea cow (Hydrodamalis gigas) of the western Bering Sea was exterminated in 1768, within 30 years of its discovery (National Foreign Assessment Center, 1978), the bowhead whale was reduced to less than

STRESS AND DISTURBANCE IN COLD REGION ECOSYSTEMS

10% of its population of about 12 000 before the start of commercial whaling (Reeves and Leatherwood, 1985; McCartney and Savelle, 1993). Sealing, penguin harvesting (Swithinbank, 1993), and whaling have caused perhaps irreversible damage to the ecosystems of the Southern Ocean since the late eighteenth century (Hempel, 1985; Kriwoken and Williamson, 1993); some populations have been reduced to a small fraction of their original size. Today, Arctic seas account for about 10% of the current recorded fish catch of the whole world, but overfishing using modern technology is causing the disappearance of some fish stocks (National Foreign Assessment Center, 1978; Wielgolaski, 1990). Overfishing in the Antarctic is threatening krill (Euphausia superba and other species), and with them the base of the Antarctic food webs, consisting of tens to hundreds of millions of tonnes of krill in the Southern Ocean (Hempel, 1985; Kock, 1985; Jouventin and Weimerskirch, 1990). Some krill fisheries may still be competing with birds and mammals, particularly during their breeding season – the more so because the stocks of many fin-fish, potential alternative prey, have been depleted. Some populations of birds and mammals, and their contributions of nutrients to terrestrial ecosystems, may be decreasing also because of the collateral mortality during fishing (Weimerskirch and Jouventin, 1987; Weimerskirch et al., 1987; Croxall et al., 1990). Southern giant petrels (Macronectes giganteus) suffer high mortality from fishing operations and from shooting at sea (Jouventin and Weimerskirch, 1990). Crabeater seals (Lobodon carcinophagus) may have declined in the Weddell Sea as a result either of increasing competition by recovering populations of baleen whales or of commercial krill harvesting, now at 4×105 metric tons per year (Erickson and Hanson, 1990). Increased fishing has also led to declines in elephant seals, southern giant petrels, wandering albatrosses (Diomedea exulans) in the Crozet Islands (Weimerskirch and Jouventin, 1987), and blackbrowed albatrosses (D. melanophris) in Kerguelen (Weimerskirch et al., 1987). Elephant seal populations have remained stable in South Georgia (McCann and Rothery, 1988). Trawling seriously damages benthos, which has low resilience (White, 1984; Dayton, 1990; Jones, 1992).

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Agriculture and animal husbandry Only some cold-region margins and relatively warm plateaus offer opportunity for limited crop production, and human residents usually depend on trading to obtain crop products. Cold and climatic fluctuations make cropping unreliable, and limit the effects of fertilization and irrigation. On the Tibetan Plateau, crops can usually be cultivated only in protected valleys (Fig. 3.15) where the number of days where the temperature reaches 10ºC exceeds 50 annually (Lin and Wu, 1981; Yu and Sun, 1981). In Tibet, barley (Hordeum vulgare) grows at elevations up to 4700 m in the south, potatoes (Solanum tuberosum) and oats (Avena sativa) to 4500 m, and wheat (Triticum aestivum) to 4100 m; corn (Zea mays) grows up to 4000 m on the warmer plateaus (Cern´ık and Sekyra, 1969). Highlanders of Central and South America were the first to grow potatoes, corn, squash (Cucurbita pepo) and beans (Phaseolus vulgaris), which perhaps made possible the expansion of other highland populations, e.g., potatoes in the Himalaya (Price, 1981). The diversity of mountain environments is reflected in the numerous varieties of mountain crops, which are endangered by current population and other pressures (Zimmerer, 1992). Sedentary societies in more densely populated mountainous areas (like the European Alps) combine farming at lower elevations with hay production and transhumance higher up (Price, 1981; Werner, 1981; Ellenberg, 1986). Farmers use fences to restrict domestic grazers to summer allotments of communally owned highlands. Animals spend snowy winters in barns at lower elevations eating stored feed, often hay from lower-elevation meadows that are not grazed, or from slopes too steep to pasture. In Europe, domestic animals control many alpine ecosystems and meadows cleared at the tree-line, all of which are now populated by weedy plants (Fenton, 1937, 1940). Lowering of the tree-line has increased avalanches, landslides, and erosion (Stern, 1983; Holtmeier, 1989), which are decreasing again now that some areas have been reforested (Bunza, 1984; Grabherr, 1997). Mountain farming and altitudinal movements of livestock are now declining, particularly in countries other than Switzerland, Germany, and Austria (Wielgolaski, 1975, 1978; Price, 1981; Greif and Schwackh¨ofer, 1983). Except for brief periods, the use of the Rocky Mountains by domestic animals was seldom uniform or intensive (Paulsen, 1960; Marr, 1964).

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Fig. 3.15. Crop agriculture in a lower-alpine river valley near the border with Nepal, above the village Zhong Zhang, Tibet. Terraced fields are limited to the relatively moist and protected river valley, and bordered by walls built of stones picked out of the fields.

Low-level, traditional grazing of domestic animals, which does not lead to vegetation changes, is the only sustainable agricultural use of the productive resilient margins of the cold regions. On highland plateaus and in the Arctic, both wild and domesticated yak (Bos grunniens), sheep (Ovis spp.), goats, antelopes, deer, llama (Lama glama), alpaca (Lama pacos), guanicoe (Lama guanicoe), vicu˜na, caribou and reindeer harvest the sparse vegetation over large plains unsuitable for crops (Ekvall, 1968). Herders build up the herds as insurance against weather or disease (Ekvall, 1968). Some of the animals have been interbred with their more productive, lowland domestic relatives which are unable to live at high altitude, and the few remaining wild populations are in danger of extinction and genepool changes, e.g., hybrids between yak and cattle. On the Tibetan Plateau, complex herd movements and pasture rotation prevent overuse, some pastures being reserved only for dry or cold season use or for hay. Overgrazing, erosion, and vegetation changes occur along migration routes and semi-permanent camp sites (Fig. 3.16). Long-term overgrazing of winter pastures leads to pasture degradation and loss of weight in animals (Xia, 1986; Zuo et al., 1986). Severe erosion (Fig. 3.17; King and Jackson, 1982) is usually related to increased population pressure from the lowlands, to military exclosures, or to past

attempts of the Chinese authorities to modify the native system by instituting collective ownership of animals and changing the pasture rotation (Goldstein and Beall, 1989, 1990; Goldstein et al., 1990; Reiter, 1991). This drives pastoralists up higher and puts greater pressure on summer pastures (Tsering, 1992), where weedy taxa replace the dominant sedges (Zuo and Le, 1980). From their recent maximum extent, high-altitude grasslands in Qinghai province had decreased by about one-fifth (7.3 million hectares) by 1986 because of overgrazing, desertification, and conversion to crop fields (Chinese sources quoted by Smil, 1993). In the Andean p´aramo, grazers overturn stem rosette plants by rubbing (Hofstede et al., 1995). Grazing is often combined with burning of natural vegetation, which has low productivity, turnover rate, belowground phytomass, and nutrient availability, and contains up to 80% of easily burnable standing-dead phytomass. Burns are colonized by palatable grasses, which replace stem rosettes not consumed by cattle (Grubb, 1970; Williamson et al., 1986; Lægaard, 1992). Reliance on animal dung for heating and cooking removes fertilizer from pastures. In the Andes, dung of llamas and cattle is used for fuel, and sheep dung for fertilizer; a family requires 25 sheep and 75 llamas for maintenance (Price, 1981). Seasonal movements with animals are not necessary on warmer plateaus. In desert mountains,

STRESS AND DISTURBANCE IN COLD REGION ECOSYSTEMS

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Fig. 3.16. Tibetan nomadic pastoralists in the Qilian Shan (Gangkar Chogley Namgyal) on the northern edge of the Tibetan Plateau. Most nomadic pastoralists have this home base near winter pastures where some family members stay, while others take yaks and sheep to higher-altitude summer pastures (up to 4800 m in the Himalaya), a few to several hundred kilometers distant. View toward Mt. Kan She Ka, Gangkar Chogley Namgyal (Nan Shan). The skyline ridge reaches an altitude of 5000 m, and the snow-line is about 4700 m. The lower alpine belt, covered primarily by Dasiphora fruticosa, has an upper boundary at about 4000 m. Most meadows are dominated by Kobresia humilis. Weedy vegetation in the foreground is controlled by dung fertilization and trampling by yaks.

Fig. 3.17. Overgrazed and trampled hillside along the road between Golmud and Lhasa, Tibet. Vegetation is probably dominated by the sedge Kobresia pygmaea. Possibly irreversible damage is seen on a steep slope where drought and erosion may prevent recovery even if overutilization ceases.

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they may follow the growth of vegetation emerging in storm-tracks. Centuries of reindeer browsing have diminished the area of Subarctic pine and birch forests in Eurasia (M¨uller-Wille, 1974; Emanuelsson, 1987). Nomadic pastoralism is disappearing in more developed countries, as pasturage and access to it shrink as a result of development. Reindeer herding by about 1600 Saami is declining in the Kola Peninsula (Kol’skiy Poluostrov; Doiban et al., 1992), and Saami in northern Europe seek employment in farming, handicraft, and other fields (National Foreign Assessment Center, 1978). Collectivization has diminished the family life of reindeer herders of Yakutia (Vitebsky, 1989), and reindeer milking has been declining in the Russian Arctic (Fondahl, 1989). Winter grazing by reindeer may cause damage (Helle and Aspi, 1983), especially to the lichens that are their main or sole winter food (Pulliainen, 1971; Virtala, 1992); lichen overgrazing is common (Kautto et al., 1986; Klein, 1987). In the European Arctic, reindeer cause damage by trampling, particularly along fencelines. Transportation and installations Transportation of humans (roads, railroads, tunnels, airports, landing strips, parking lots, ski lifts), materials and energy (oil or water pipelines and powerlines), and associated development (construction, housing, oil fields, mines, tourism) are all common disturbances at the margins of cold-region ecosystems. Many of these are accompanied by noise, pollution, and solid waste. In the mountains, water reservoirs permanently inundate valleys; they destroy habitats for plants and animals, change stream flow (Messier et al., 1989), attract tourism and, like transportation corridors, restrict animal migration routes (Hanson, 1981; Smith and Cameron, 1985; Curatolo and Murphy, 1986). Drainage changes cause ponding, ice jamming, and flooding that may lead to changes in land-forms and ecosystems (Thorhallsdottir, 1993). Power stations at lower elevations divert water from mountain streams (Wielgolaski, 1978). In Antarctica, penguin colonies have been destroyed and their predator skuas disturbed in order to free space for airfields (T. Thomas, 1986; Culik et al., 1990; Hemmings, 1990) and research stations (Wilson et al., 1990). Vehicles and solid waste take over habitats of Ad´elie penguins and Antarctic skuas, which decline (Johnston, 1971; Hemmings, 1990). In mountains, buildings and transportation

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corridors must be protected by fences, walls, roofs, and other structures from avalanches, rockfalls, and landslides. Slope bases may have to be reinforced by concrete and metal supports, and bare surfaces covered and treated chemically to prevent erosion and collapse of the structures the slopes support (Schiechtl, 1988; Stvan et al., 1991). In areas with ground ice, alterations to the insulating vegetation and surface organic layer, and heat input from power plants, buildings, and pipelines, lead to alterations of ground heat balance, melting of ground ice, thermal erosion, reduced albedo, additional heat input from the atmosphere, subsidence of the ground and collapse of conventionally-built structures supported by it. Meltwater accumulates in depressions, creating new ponds (thermokarst). These processes continue in some places for decades (Bliss and Wein, 1972; Price et al., 1974; Rickard and Brown, 1974). To avoid such damage, support structures for bridges, and pipelines for water supply, fuel, and sanitary purposes, are insulated and heated, or placed in heatdissipating, refrigerated structures, to keep the contents from freezing, or from thawing the ground ice around them (Hwang, 1976). After construction, continuing reclamation may be needed to prevent further spreading of terrain instability and environmental degradation (Wishart, 1988). Dirt and gravel roads must be elevated and graded continually, as paved surfaces quickly deteriorate; they are sources of dust and of spreading disturbances (National Foreign Assessment Center, 1978; Klinger et al., 1983; Hayley, 1988). In some environments, even one passage of an animal, a human, or a vehicle, whether during summer, or in the winter on roads and runways built on snow and ice, is sufficient to cause damage; most of the damage occurs during the first few passes (Ahlstrand and Racine, 1993). Both damage to dry surfaces resulting from traffic, and compression of the wet organicsoil surface layer in marshes, may be permanent (Hernandez, 1973). In polar deserts, less-weathered, angular soil material is brought to the surface, and weathered stones are overturned, leaving impressions (Campbell et al., 1993, 1998). Repeated compression on paths (Pounder, 1985) and vehicle trails, even by off-road vehicles (Slaughter et al., 1990; Forbes, 1992), crush and compact vegetation and the organic surfacesoil layer; both are destroyed by surface removal and subsurface compression. Results may include increased runoff, erosion, mass movement, reduction in taxonomic richness and permanent changes in the

STRESS AND DISTURBANCE IN COLD REGION ECOSYSTEMS

ecosystems (Challinor and Gersper, 1975; Gersper and Challinor, 1975; Bryan, 1977). Large-scale research using heavy equipment can be devastating, especially in marshes and polar deserts (Parker and Howard, 1977; Campbell et al., 1993). Extraction of minerals and fossil fuel Crystal collecting was one of the early human activities in cold regions. The effects of early mining were limited; in some areas, such as Tibet, it was banned for religious reasons. Primitive surface or shaft mining (Tibet, Bolivia, Rocky Mountains) or panning for gold (Tibet, Alaska) are still being carried out by natives, immigrants, or small-scale prospectors. Alpine mine spoils, roads, and solid waste are common in the Colorado Rocky Mountains, where mining for lead, silver, gold, tungsten, and other metals, and logging for mine supports, took place during the late 19th century. Some other mountains contain richly mineralized rocks, but large-scale mining operations have mostly been limited to accessible lower altitudes and to the Arctic. The Arctic is an important source of minerals and fossil fuels. The Kiruna mine in Swedish Lappland, which has been worked since the late 19th century, produces more than 25×106 tonnes of iron ore annually. Norway has a northern iron-ore mine (Kirken¨as, about 70ºN). Both Norway and Russia mine coal in Svalbard; the Norwegian mining is currently decreasing. Lead, zinc, and other metals (Sinding and Poole, 1991) are being mined in Greenland. Arctic sources account for 2% of Canadian mineral output. Estimates of Alaska’s coal reserves range as high as 9×1012 tons. Oil exploration there started about 60 years ago (Reed, 1958; Gryc, 1985), and a small natural gas field near Barrow has been producing local fuel since 1955. Alaskan oil production rose sharply in 1977 with the completion of the trans-Alaska pipeline from the Prudhoe Bay field. Most Arctic continental shelves may bear oil and gas (Rudkin, 1974; National Foreign Assessment Center, 1978; Atlas, 1985). In the Russian Arctic, the Kola Peninsula contains the world’s largest apatite deposit (2×109 tons), used for phosphate fertilizer. Two major metallurgical centers use copper–nickel ores, and there are many other mineral deposits and metal byproducts. Iron ore reserves may reach 109 tons. The Pechora Basin produces 107 tons of coal annually. Numerous mines, smelters, and refineries near the mouth of the Yenisey

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River yield more than half of the Russian production of nickel and a substantial part of its copper. Eastern Siberia supplies more than 90% of the Russian output of diamonds and tin, and the Far East supplies coal and mercury. Most of Russia’s gas and some oil reserves are in the Arctic, some offshore (National Foreign Assessment Center, 1978; Vitebsky, 1990b; Doiban et al., 1992). Mining leads to ecosystem destruction, accumulation of unweathered and potentially toxic overburden or spoils, and pollution by acids and heavy metals (Cooke, Chapter 14, this volume), which can be spread in mine effluents, leached out by water, or scattered by wind. In the Kola Peninsula, irreparable environmental damage surrounds mining and smelting operations, advancing at a rate of 2 km per year (Doiban et al., 1992; Rees and Kapitsa, 1994); see also Fig. 3.14. Oil and gas fields, offshore drilling platforms, pipelines, tankers, and other oil and gas transport and processing are sources of oil spills, oil and gas fires, and other forms of oil pollution. Back-up systems for containing oil from leaking storage tanks are required (Mackay, 1985). Drilling muds and concrete help to prevent freezing and thawing, and drill casing has to be insulated to prevent ground collapse. Underwater structures are endangered by bottom scouring from ice ridges and icebergs unless well heads are countersunk in the sea-floor and protected by silos (National Foreign Assessment Center, 1978). Solid waste Solid waste is scattered even through some of the most remote cold regions, because its removal is expensive and seldom carried out, and its distintegration is slow in the cold conditions. Current solid waste is similar to that at lower latitudes and altitudes, except that wood is also an exotic material. On land, it commonly includes gravel roads and pads, machinery, supports, pilings, and concrete abandoned at mining and drilling sites, barrels, camp remains, food remnants, coal, plastic, metals, and glass. Solid waste covers vegetation, killing it along with other organisms, and buries the soil. It creates new habitats (Gregory, 1987, 1990), and may change ecosystems in its vicinity by shading the ground, accumulating moisture and snow (Kershaw, 1991). Solid waste may disintegrate into, or leak, exotic or polluting chemicals. At Antarctic stations, sites for disposal of solid waste are often a source of such chemicals (Harris, 1991; Rodgers et al., 1991,

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1992), as may be sites in the Arctic where solid waste is buried (Schindler, 1983). Sites for the permanent disposal of nuclear-fuel waste must be able to withstand continental glaciations (Sheppard et al., 1995). Plastic and other garbage floating in cold-region surface ocean waters, from both distant and local sources, is often washed ashore (Johnston, 1971; Gregory et al., 1984; Merrell, 1980, 1984), and it may serve as a means of dispersal for invaders such as rats (Gregory, 1990). Seals and other marine animals die from being entangled in fishing nets, packing bands, twine, and other debris (Bonner and McCann, 1982; Ryan, 1987; Croxall et al., 1990), or from being poisoned by toxic chemicals from plastics (Ryan et al., 1988). Pribilof Island fur seals (Callorhinus ursinus) suffer a 5% mortality rate from entanglement in plastic debris (Reed et al., 1989). Birds may die by striking aerial arrays in flight, or from choking on humandiscarded objects. On glaciers, solid waste is being slowly incorporated into ice. Aluminum survey markers drilled into the surface of a blue icefield were still standing 34 years later (Brunk and Staiger, 1986; Swithinbank, 1993). Pollution Most airborne pollutants in cold regions are anthropogenic (Boutron and Patterson, 1987), carried from lower latitudes and altitudes by variable winds and ocean currents (Holdgate and Wace, 1961; Llano, 1967; Nriagu, 1978). Winds and currents also reexport some allochthonous pollutants, and export some autochthonous materials, which are relativelly scarce. Natural autochthonous pollutants include volcano emissions, e.g., Mount Erebus, Deception Island (Siegel et al., 1980; Bargagli et al., 1993), aerosol particles produced by oxidation of reduced sulfur gases produced by marine microorganisms (Gras, 1992), and bromine compounds produced by ice algae following the loss of surface ozone (Oltmans and Komhyr, 1976; Berg et al., 1984; Barrie et al., 1988), which affects most cold regions (Fehsenfeld et al., 1983; Jaffe, 1991; Oltmans, 1991, 1993). Sulfur compounds released by marine algae to the atmosphere can be oxidized there to sulfur dioxide and other compounds, forming aerosol particles serving as cloud condensation nuclei (Gibson et al., 1989; Curran and Burton, 1992). High diversity of cold regions is reflected both in the amounts of incoming pollutants, which may be modified by local wind patterns, and in the

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degree of pollutant effects, which may be modified by local conditions such as bedrock chemistry, insolation, distribution of organisms, or environmental and population fluctuations (Dunbar, 1985). Orographic uplift or scavenging may deliver relatively large amounts of some pollutants, such as acid rain, to windward mountain slopes (Parrish et al., 1986a–c; Oltmans and Levy, 1994), while downwind areas may be unaffected. Pollutant concentrations are usually lower in the more remote south than in the north (Bennington et al., 1975; Risebrough, 1977; Schneider et al., 1985), where pollutants, mainly from Europe and Asia, create haze which persists between November and May (Rahn and McCaffrey, 1980; Rahn, 1981; Rasmussen et al., 1983). Surface snow in Greenland contains about 200 times more lead than prehistoric Greenland ice (Murozumi et al., 1969; Ng and Patterson, 1981), while the Antarctic snow contains only 2–3 times more lead than prehistoric Antarctic ice (Boutron and Patterson, 1983). Trace metals that occur in cold-region environments and organisms are usually correlated with wind, current, and migration patterns (Bohn and McElroy, 1976; Bohn and Fallis, 1978; Boutron and Lorius, 1979). Mercury bioaccumulates in polar bears (Eaton and Farant, 1982; Norstrom et al., 1988; Dietz et al., 1990) and their prey (Smith and Armstrong, 1975, 1978). Mercury content in lichens of Victoria Land was surprisingly high (Bargagli et al., 1993). Antarctic seawater contains high levels of lead (Lee et al., 1990), which is, along with mercury, also present in native peoples in the Arctic (Charlebois, 1978; Grandjean, 1989), as well as other organisms. Radioactive fallout from atmospheric explosions of nuclear weapons (from 1945 until the 1970s) and nuclear accidents (the Chernobyl power plant in 1986) is accumulated by slow-growing organisms of the cold regions, especially by fruticose lichens and cushion plants (Rickard et al., 1965; Hanson, 1967, 1971, 1982). Cs137 increases by a factor of two for each higher trophic level; its concentration in human consumers of reindeer meat, which may be over 100 times higher than in other people (Hanson, 1967, 1968; Bird, 1968), could be responsible for high cancer rates among northern natives in Russia (Vitebsky, 1990c). The areas closer to the sources are more contaminated than distant areas (Baeza et al., 1994) and extreme latitudes (Hardy et al., 1968; HutchisonBenson et al., 1985; Taylor et al., 1985).

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The atmospheric content of nitrous oxide, halocarbons (Hall et al., 1990), anthropogenic gaseous chlorine, other trace gases (Rasmussen et al., 1981; Khalil and Rasmussen, 1985–1987) and air pollutants (Molski et al., 1981) have been rising both at the South Pole and other locations. Adsorption of organic vapors of high molecular weight to particulate matter in the atmosphere is enhanced by low temperatures (Bidleman, 1988; Bidleman et al., 1989; Focardi et al., 1991). Dichloro-diphenyl-trichloroethane (DDT) and other organochlorine pesticides and their derivatives are present in air, water, snow, ice, and sediments, and some are preferentially accumulated in the lipids in organisms. Concentrations of some have been declining, probably in response to the discontinuation of their use in some countries (Sladen et al., 1966a; George and Frear, 1966; Brewerton, 1969). Exudates from brown algae may alter the bioavailability of polychlorinated biphenyls (PCBs) or other compounds to which they bind (Lara et al., 1989). Organic runoff from animal husbandry and warm water from power plants may lead to eutrophication of water reservoirs (Doiban et al., 1992). Sewage can contaminate limited water supplies, especially in areas with ground ice or shallow weathering. Chemical fertilizers and insecticides used in river valleys on the Tibetan Plateau lower soil and water quality (Goldstein and Beall, 1989; Tsering, 1992). Pollution and chemical changes in environment and organisms are also caused by emissions from cars, boats, ships, generators and aircraft, concrete batching plants (Adamson and Seppelt, 1990; Roser et al., 1992; Adamson et al., 1994), waste water from research stations (Risebrough et al., 1990; Harris, 1991; Railsback, 1992), municipal waste (Bourgoin and Risk, 1987; Haertling, 1989) and exotic bacteria, fungi, and viruses (Toyoda et al., 1985, 1986). Dust from roads, cement, or long-range transport causes earlier snowmelt, changes soil chemistry, moisture, and nutrient mineralization, reduces primary production, and causes changes in the composition of communities (Bodhaine et al., 1981; Spatt and Miller, 1981; Walker and Everett, 1987). Lead in batteries, anti-freeze compounds, or other wastes may poison animals such as skuas (Catharacta spp.), sheathbills (Chionis spp.), and polar bears, which then spread the toxins in their carcasses (Johnston, 1971; Lunn and Stirling, 1985; Amstrup, 1989). In Arctic Russia, mining and military wastes include acids, heavy metals (mainly nickel, copper, manganese, zinc), radioactive materials, oil, and organochlorines. In

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the Kola Peninsula, annual wastes include 410×106 m3 of effluents, 6.5×105 tons of atmospheric pollutants (mainly SO2 ), and 55×106 tons of mining solids. Concentrations of heavy metals in the top soil are increasing, lakes are mineralized, and the Murmansk harbor is contaminated by oil and phenols. Heavily contaminated areas, in which vegetation has been completely destroyed, are spreading, and pollution is reaching to Finland and Norway (Fig. 3.14). Between 1980 and 1990, total emissions of sulfur dioxide were reduced by 18%, mainly through technological modernization (Freydin, 1972; Strelkov and Freydin, 1973; Rees and Kapitsa, 1994). Increases in ultraviolet-B radiation A thinning of the ultraviolet-absorbing stratospheric ozone layer, caused by chlorine pollutants in the atmosphere originating primarily in the Northern Hemisphere, is most serious in the polar regions despite their relatively large total stratospheric column of ozone during the periods of maximum radiation. The lack of denitrification in the northern polar vortex currently limits the loss of ozone over the Arctic, but future cooling of the lower stratosphere could increase it (Santee et al., 1995). Some volcanic and other aerosols enhancing ozone depletion (Hofmann et al., 1992; Hofmann and Oltmans, 1993; Tsitas and Yung, 1996) originate in cold regions. The largest ozone “hole” (up to 60% loss) develops in Antarctica in the spring, because the catalytic reactions of anthropogenic pollutants on the surface of ice crystals in polar stratospheric clouds are promoted in this coldest region of the atmosphere. Long days, lack of heavy cloud cover, and high elevation contribute to high monthly totals of ultraviolet radiation reaching the surface at the South Pole during midsummer, despite the high angle of incidence and truncation of the shorter, biologically most active wavelengths at the poles. A small percentage of UV-B may even reach depths of 60 m in clear ocean water, and penetrate clear sea ice, but not snow, which protects some organisms during the greatest development of the ozone “hole”. During that time in open water, nutrients may be low at the surface, and herbivores may be at depths where ultraviolet irradiation is low. Ozone destruction is thought likely to intensify and spread even as the emissions of pollutants decrease, because their transfer into the stratosphere takes decades (Caldwell et al., 1980; Baker-Blocker et al., 1984; Farman et al., 1985).

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Increased levels of UV-B radiation may cause mutagenesis, an increase in photoinhibition and photooxidative destruction of chlorophyll, depression of physiological processes such as photosynthesis, other detrimental changes (such as reduction in mobility: Ekelund, 1992), and may result in death. There may be 4% or more reduction in primary production and nutrient flux as a result of enhanced UV-B radiation. This radiation is damaging during spring bloom when organisms with little photoadaptation approach the ocean surface to take advantage of higher light levels. Many aquatic herbivores and other organisms with little protection (for their eyes and other organs) are sensitive to UV-B radiation and to the potentially toxic products of its reactions with pollutant chemicals. However, increased photooxidation of toxic chemicals could stimulate aquatic productivity. A shift toward dominance by protected native organisms or immigrants could cause changes at higher trophic levels (Calkins and Thordardottir, 1980; Adamson et al., 1988a; El-Sayed et al., 1990). Irradiation by UV-B could also affect the terrestrial alga Prasiola crispa, which is poorly protected (Adamson and Adamson, 1992). Ultraviolet radiation decreases the cell-mediated cutaneous immune response in humans in Antarctica (Williams et al., 1986; Muller et al., 1988; Roy et al., 1994). Increase in carbon dioxide and climatic change The current climatic warming and lengthening of the growing season have been linked to an atmospheric increase in the concentration of trace gases (carbon dioxide, methane, nitrous oxide, etc.) originating at lower latitudes (Barnola et al., 1987; Etheridge et al., 1988; Raynaud et al., 1988). This increase may be enhancing natural climatic cycles related to ocean circulation (Bond et al., 1997; Broecker, 1997), and causing increased solar irradiance, decreased volcanic activity, and internal climate feedbacks (Overpeck et al., 1997). A decrease in ozone in the lower stratosphere and in tropospheric aerosols may slow down the warming. Continuing warming may be accompanied by changes in air and water circulation and by the movement of latitudinal atmospheric and oceanic boundaries poleward. The changes in oceanic circulation might, however, cause a sudden cooling (Kerr, 1998). Temperature increases will be enhanced at high latitudes (up to 5ºC in summer and 10ºC in winter) through

´ ´ and F.E. WIELGOLASKI V. KOMARKOV A

positive feedback, to which increased evaporation and cloudiness may contribute. Stratospheric temperatures will decrease, and the temperature of warm air at the surface will rise, as it will be confined by the stable cold polar air above. The amount of heat lost to space will decrease, because sea ice, which reflects most of radiation insulates the ocean surface and prevents evaporation, will decrease in extent; subsequently, higher temperatures of ocean water will reduce the deep ocean sink of carbon dioxide (Manabe and Wetherald, 1975, 1980; Kelly et al., 1982). Increases in the average surface temperatures and a net decrease in the mean extent of sea ice (0.23 degrees of latitude per decade between 1973 and 1988) have been observed at most Antarctic and Southern Ocean stations (Raper et al. (1984)), Heard Island (Allison and Keage, 1986; Jones et al., 1986), Macquarie Island (Adamson et al., 1988b; Gloersen and Campbell, 1988; Jones, 1988, 1990) and in the Arctic (Garfinkel and Brubaker, 1980; Chapman and Walsh, 1993; Oechel et al., 1993). Satellite observations indicate a decrease in the areal extent of sea ice by 2.9±0.4 per cent per decade in the Arctic, and an increase by 1.3±0.2 per cent per decade in the Antarctic (Cavalieri et al., 1997). A cooling trend could be occurring in other areas, e.g., in the Rocky Mountains in Colorado, by 0.07ºC per year (Barry, 1973; Greenland, 1989), and in Fennoscandia (Kullman, 1993). The effects of warming are evident in the melting and breakup of polar ice shelves (Orheim, 1988; Zakharov, 1988; Doake and Vaughan, 1991), in the retreat and thinning of many glaciers, e.g., in the Alps, the Arctic (Calkin, 1988; Koerner and Fisher, 1990), Central Asia, New Zealand, the Subantarctic, northern and West Antarctica (Rignot, 1998) and Patagonia (Aniya et al., 1997), and in the warming and melting of Arctic and alpine permafrost (Lachenbruch and Marshall, 1986; Kane et al., 1991b; Haeberli et al., 1993). The reduction in global glacier area due to retreat is calculated as 6–8×103 km2 , and the mass balance has generally been negative from 1961 to 1990 (Dyurgerov and Meier, 1997a,b). Newly deglaciated surfaces may be subject to widespread erosion (National Foreign Assessment Center, 1978; Koster, 1991; Ballantyne and Benn, 1994). Increased runoff during melting may increase the frequency of landslides and failures of moraines and ice-dams (Mahaney, 1986), and contribute to the spreading of pollutants (Kane et al., 1992; Sheppard et al., 1995). Isostatic rebound of the underlying land masses, following melting of

STRESS AND DISTURBANCE IN COLD REGION ECOSYSTEMS

the Antarctic and Greenland ice sheets, could result in the elevation of low-lying coasts and islands in polar regions; but worldwide most coastal land masses would be drowned, as the rise in sea level due to thermal expansion would be substantial (Frei et al., 1988). The current warming trend may be decreasing the amplitude, frequency, and duration of precipitation events and storms in some areas and during some seasons (Manabe and Wetherald, 1986, 1987). In such areas, lakes, streams, snow-patches and glaciers are receding, and droughts and fires may become more frequent. Average precipitation is decreasing on the western part of the Tibetan Plateau, where the desertification trend is aggravated by the logging of the adjacent forests (Reiter, 1991). A slow decrease of soil moisture has been established in the Rocky Mountains in Colorado (Taylor and Seastedt, 1994). In other areas, precipitation and stream volume may be increasing due to greater evaporation, evapotranspiration, and a higher rate of water cycling, e.g., in the Antarctic (Morgan et al., 1991), and in oceanic areas of northern Europe (Hagen, 1996). Mean annual temperature is decreasing and annual precipitation is increasing in the Rocky Mountains in Colorado (Barry, 1973; Greenland, 1989), where the increase in precipitation (14 mm yr−1 ) explains about half of the 200% increase in annual wet deposition of nitrate over the last decade (Williams et al., 1996). In some areas, snow may turn to rain. Some glaciers, e.g., in Norway, Greenland (Zwally, 1989; Zwally et al., 1989), the Arctic, Iceland and the Antarctic (Morgan et al., 1991; Peel, 1992) have been growing during the last decade, probably through increased winter snowfall (Hagen, 1996). Increasing precipitation over the Antarctic, partly as a result of evaporation from areas previously covered by ice, could lead to lowering of the sea level (Robin, 1986; Budd, 1988, 1991; Gloersen and Campbell, 1988). Acid rain Acid rain, caused by the solution of NOx and SOx emissions from power plants and cars in liquid water in the atmosphere, could lead to significant changes in the nutrient and base content of shallow, poorly developed soils in the cold region (Lewis and Grant, 1980; Lewis, 1982; Haselwandter et al., 1983) and in the composition of the ecosystems that these soils support. So far, the effects of acid rain are not seen in remote cold regions, and are slight even in some cold regions near industrial

67

centers. In mountains that have high rates of erosion and solution, a significant proportion of the imported nutrients may be exported; in other areas, they may not be utilized due to drought. In remote headwater basins of the Urumqi River in the Tien Shan (China), the solute composition of surface water and snow is dominated by dissolution of rocks with rapid weathering kinetics, such as calcite and chromite (Williams et al., 1995a). In the Sierra Nevada in California, mineralization and nitrification produce more inorganic nitrogen than is received in wet and dry deposition (Williams et al., 1995b). Acid rain could, within a decade, lead to a widespread acidification of surface water in the alpine zone of the Rocky Mountains in Colorado (Caine, 1995b), although increased snowfall may increase rates of decomposition, mineralization, and nitrate export in streams (Williams et al., 1998). The annual wet deposition of nitrate increased by 200% over the last decade and only half of this amount is explained by the increase in precipitation. The increase in precipitation and decreased energy flux to the snowpack between 1994 and 1995 resulted in a 4- to 5-fold increase in the magnitude of solute release from the snowpack in the form of an ionic pulse, causing episodic acidification in headwater catchments (Williams et al., 1996). The retention of nitrogen is nearly 1 mg m−2 per day, or 2.5– 3 kg ha−1 of new nitrogen annually, which approaches 50% of the net annual mineralization of nitrogen (Sievering et al., 1992, 1996). The annual inorganic loading in wet deposition of about 4 kg/ha/yr is about twice that on the Pacific coast and similar to the northeast, leading to N saturation as a result of anthropogenically fixed N in wetfall and dryfall (Williams et al., 1998). Hydrocarbon spills Cold slows down the rates of oil evaporation, dispersion, and oxidization. At high latitudes, severe weather makes the behavior of marine oil spills less predictable, and the use of chemical dispersants (by aerial application) or burning less predictable and effective. Oil spills may persist longer than in temperate or tropical ecosystems, and their fate and effects may be more diverse. Oil sticks to sea ice, and may become encapsulated in it, followed by release the following spring (Atlas and Bartha, 1972a,b; Haines and Atlas, 1982). Crude oils are usually less toxic at 15ºC than at 0ºC or 10ºC (Hsiao, 1978).

68

Oil kills aquatic organisms, inhibits primary production, and changes the composition of communities in ponds and lakes (Miller et al., 1978; Federle et al., 1979). It also may reduce heterotrophic activity over a period, especially in sediments [for 18 months according to Griffiths et al. (1981a,b, 1982a,b)], depress cellulase activity (Linkins et al., 1978), and lower the rates of nitrogen fixation (Griffiths et al., 1982b; Atlas, 1985). The acute lethal toxicity of oil dispersions and water-soluble fractions for zooplankton in waters of both the cold and the cold-temperate region is indicated by values of the 4-day LC50 ranging between 0.05 and 9.4 mg °−1 . Isopods seem to be more resistant to oil than amphipods and decapods, which often swim erratically before they die. Molluscs, other invertebrates, and fish become sluggish and disoriented, and suffer reduced growth, development and reproduction, and death (Percy, 1976, 1977; Busdosh and Atlas, 1977). Fish can avoid many oil spills, but natural stresses (fluctuations of salinity, temperature, and food abundance, disease, parasites) increase their sensitivity to toxic compounds. Low concentrations of hydrocarbons in fish tissues make them unpalatable to humans (Rice, 1985). Oil damages bird embryos, and sticks to the skin, fur, and feathers of marine endotherms, adversely affecting thermoregulation and metabolism and causing loss of fur and feathers. Animals insulated by blubber are resistant to heat loss indiced by oil coatings. Animals attempting to clean their fur or plumage and those eating contaminated food will ingest oil. This may result in reproductive impairment, stunted growth, metabolic stress, anemia, and increased mortality (Kooyman et al., 1977; Peakall et al., 1980, 1981). Oil tends to be concentrated in ice leads and breathing holes, and wind piles it up against ice edges (Ayers et al., 1974; Percy and Wells, 1984). These places are frequented by marine mammals, which may be coated by the oil, inhale volatile hydrocarbons, or ingest contaminated food. Oil matted in baleen may reduce the filtering efficiency of baleen whales, especially in cold water, where it is more viscous. Oil-covered seals have difficulty in swimming and may die of exhaustion, their breathing orifices may be plugged by oil, and eye lesions may occur. Pathological tissue changes and death due to renal failure occurred in polar bears exposed to oil (Engelhardt, 1985). After the grounding of MV Bah´ıa Para´ıso at Anvers Island in Antarctica on 28 January 1989, adult skuas, penguins, and other birds were covered with oil, fledglings were deserted or poisoned by oil, and

´ ´ and F.E. WIELGOLASKI V. KOMARKOV A

intertidal fauna and flora were damaged; other fuel spills have also damaged terrestrial ecosystems in the Antarctic and Subantarctic (Barinaga and Lindley, 1989; Karl, 1989, 1992). In 1977, the largest blowout in an offshore drilling field at Ekofisk in the North Sea (56º38 N, 3º12 E) spread 23 000 tons of crude oil over an area of 40 000 km2 , killing many seabirds but causing little damage to other marine organisms (Wielgolaski, 1989). On land, natural oil seeps and substances spilled during extraction, processing, or use of crude oil and its products stick to soil particles and kill or damage organisms, usually through their toxicity (Wein and Bliss, 1973b; Everett, 1978; Walker et al., 1978). Human disturbance through organismic invasions Within cold regions, the success of alien taxa and the complexity of successful taxa rapidly decrease with increasing latitude and altitude. Beyond cold-region margins, there are no successful exotic invaders, and the organisms present (mostly lower organisms such as lichens, but also some sedges) are common to different cold regions, especially the dominants among them. This relatively high resistance to invasions resembles that in tropical ecosystems, forests in particular, where high resistance to invasions may be due to both biotic and abiotic factors and does not seem to be related to taxonomic diversity (Rejm´anek, 1996a). Cold regions seem to be protected by their harsh environment and by the lack of taxa adapted to similar conditions elsewhere. Their protection may be mostly environmental, because they have few invaders despite their low taxonomic diversity and the low competitiveness of their few residents. At cold region margins, cosmopolitan weeds may occur on exotic materials (along gravel roads) but, unlike in temperate biomes, there are no cosmopolitan weed ecosystems maintained by trampling or other disturbances. Other alien taxa may colonize disturbances. Introduced plants may be common in animal-disturbed sites, e.g., Cerastium fontanum in rabbit-disturbed sites on Macquarie Island (Scott and Kirkpatrick, 1994); or outcompete or destroy native taxa, but again only at the margins of the cold regions (Corte, 1961; Longton, 1966; Walton and Smith, 1973). Most invasive taxa that colonize overgrazed areas and other disturbances are natives of cold regions (Druzhinina and Zharkova, 1979; Matveyeva, 1979, 1988). Such invasive taxa are characterized by rapid and efficient reproduction,

STRESS AND DISTURBANCE IN COLD REGION ECOSYSTEMS

dispersal, growth, development, and population growth (Rejm´anek, 1996b; Rejm´anek and Richardson, 1996). Organisms with certain strategies (Grime, 1979; Grime et al., 1988) or trophic or growth-form types probably never reached, or evolved in, isolated relatively warm Subantarctic islands, leaving an opening for alien organisms. Some of these organisms could have been spontaneous colonizers, but most of them probably were transported by humans (whalers, sealers, unsuccessful settlers, research personnel) or on floating solid waste, or escaped from shipwrecks. Microorganisms, plants, invertebrates, and vertebrates have become established in almost all island groups. Flightless invertebrates and plants, which had evolved in the absence of large predators and herbivores, were decimated by vigorous invaders on many islands (Costin and Moore, 1960; Wace, 1960; Holdgate and Wace, 1961). On South Georgia and the Iles Crozet, invaders constitute a high proportion of the flora (Selkirk et al., 1990), and on some islands they have changed the predator–prey relationships and ecosystem processes (Tr´ehen et al., 1990; Chevrier et al., 1997). Examples of well-established destructive mammalian invaders include cats (Felis catus) and house mice (Mus musculus (house mouse)) on Marion Island (van Aarde, 1979, 1986; van Rensburg and Bester, 1988a), rabbits (Oryctolagus cuniculus) and other animals on Kerguelen (Dreux, 1974; Lesel and Derenne, 1975; Chapuis, 1988), rabbits (Costin and Moore, 1960; Scott, 1988) and house mice (Pye, 1993) on Macquarie Island, and rats (Rattus norvegicus) (Pye and Bonner, 1980) and reindeer (Rangifer tarandus) on South Georgia (Lindsay, 1973; Leader-Williams et al., 1981, 1987; Vogel et al., 1984). Following their introduction on Arctic islands, reindeer greatly reduced the biomass of lichens, their important winter food in areas free of snow or where snow is shallow in winter (Scheffer, 1951; Ouellet et al., 1993). Some invader populations decline eventually, primarily through the reduction of their food source (Scheffer, 1951; Klein, 1968; Caughley, 1970). In most cases, ecosystem recovery depends on artificial removal of the invader (Johnston, 1973; Watkins and Cooper, 1986; Leader-Williams et al., 1989) – for instance, of rabbits by myxomatosis (Brothers et al., 1982; Chapuis and Bouss`es, 1989; Chapuis et al., 1994), and cats by panleucopaenia and hunting (Van Rensburg, 1986; Van Rensburg et al., 1987; Van Rensburg and Bester, 1988b).

69

Tourism and recreation Cold regions provide spectacular scenery, largely undisturbed nature, year-round snow for sports, recreation, and expeditionary travel, and include important religious sites. Visitors are becoming more numerous as leisure time, disposable income, and mobility increase, but mass tourism does not reach the remote parts that are difficult and expensive to explore. Tourism and recreation have a capacity to provide revenues for the protection of popular sites from trampling, pollution, and other threats, for research, and for the potentially expensive protection of visitors, e.g., in the Antarctic International World Park (Potter, 1970; Schweickart, 1988). Many national parks and wilderness areas in the United States, and governments in countries where there are popular destinations for expeditions, issue permits to avoid overuse, ban roads and vehicles, provide public transportation and lodging outside parks, and pave the most popular trails (Wielgolaski, 1971, 1978). In more developed countries, sports facilities, overcrowding, high-rise buildings, noise, pollution, roads, and cars are overwhelming the valleys and are spreading to the slopes above (Tsuyuzaki, 1994). Bulldozing of ski runs may cause the most severe impact above the treeline (Cernusca, 1977; Schauer, 1981; Klug-P¨umpel, 1988). Damage by visitors includes litter, erosion along trails, trampling (Willard and Marr, 1970; Grabherr, 1982; Gibson, 1984), camps and campfires, wood gathering, snowmobile damage (Greller et al., 1974), effects on wildlife (Cederna and Lovari, 1983; Hamr, 1988), and species introduction. Tens of thousands of visitors come through some national parks in the United States every summer day, e.g., Rocky Mountain National Park (Marr and Willard, 1970), even if 95% of them visit only 2% of the total area (Ives, 1974). Similar problems are now arising in popular, accessible areas in less-developed countries, where tourists burden local food chains. In Antarctica and parts of Svalbard, tourism is less destructive than research because most of it is shipbased, but polar tourism and other visits are increasing (Reich, 1980; England, 1982; Harris, 1991). Between the late 1950s and 1992, about 39 000 tourists have visited Antarctica (Reich, 1980; Enzenbacher, 1992, 1993) and trampling is becoming a problem (Scott and Kirkpatrick, 1994). Airborne tourism has a potential for reducing impact (Boswall, 1986; Swithinbank, 1988), because fuel residues are dispersed, inland operations

70

seldom encounter wildlife, and waste can be brought out, replacing spent fuel (Swithinbank, 1993). Visitor guidelines for polar regions have been developed (National Science Foundation, 1990; Stonehouse, 1990; Hall and Johnston, 1995), although they are not always followed (Davis, 1995).

´ ´ and F.E. WIELGOLASKI V. KOMARKOV A

habits, e.g., bears (Ursus spp.) (Follmann and Hechtel, 1990). Seals occur in leads associated with off-shore drilling rigs, and polar bears are attracted to them when sea ice is completely frozen; this increases the likelihood of conflicts between bears and humans (Stirling, 1988). On land, bears are attracted to garbage dumps, where they feed (Lunn and Stirling, 1985).

Contact with wild animals Cold-region animals are disturbed by noise, by vehicles, and by human presence. Pacific black brant geese (Branta bernicla nigricans) never habituate to helicopter noise and seek safety in water; they may be prevented from completing their molt by frequent overflights (Miller et al., 1994). The heart rate of adult penguins rose 15.5% near humans, and up to 350% near helicopters, which caused panic reactions from even 1500 m away (Thompson, 1977; Culik et al., 1990; Wilson et al., 1991; Swithinbank, 1993). Helicopters also caused their body temperature to rise (Sladen et al., 1966b). Human visits to penguin colonies are not sufficiently regular and frequent to lead to habituation (Culik et al., 1990), and the disruption can lead to declines in breeding success (Thompson, 1977; Wilson et al., 1990; Young, 1990) and populations size (T. Thomas, 1986; Jouventin and Weimerskirch, 1990). During a circumnavigation of Macquarie Island at an altitude of 250 m by a C-130 Hercules aircraft in 1990, 7000 king penguins (Aptenodytes patagonicus) stampeded and suffocated (Rounsevell and Binns, 1991). At Marion Island, overpasses by this aircraft caused king penguins to panic and run inland (Cooper et al., 1994). Aggressive and predatory skuas may be killed because they interfere with research or station operations, or with agriculture or rabbits on southern temperate and Subantarctic islands (Hemmings, 1990). Sealing and whaling may have helped to maintain a larger skua population on South Georgia than the island could support otherwise (L¨onnberg, 1906; Stonehouse, 1956; Hemmings, 1990). Skuas feed on station wastes, defend dumps, destroy petrels dazzled by station lights, may overwinter instead of flying north, may be tolerant of people (Young, 1990), and their populations may increase (Trivelpiece and Volkman, 1982; Hemmings, 1990). Food wastes and pet animals may pass avian and other diseases and parasites to wild populations (Parmelee et al., 1979; Harris, 1991). At the edges of cold regions, food wastes may support invasions of rats (Rattus spp.) and gulls (Larus spp.). Human activities may lead to changes in animal movements and feeding

ECOSYSTEM RESPONSES TO DISTURBANCE

Steep environmental gradients, environmental fluctuations, and high levels of stress and disturbance produce greater diversity of ecosystems in cold regions than within most temperate biomes (Fig. 3.18). It could even be argued that cold regions encompass coldclimate variations of many other biomes (scrublands, grasslands, deserts), which have more in common with their temperate variants than with most extensive cold-region ecosystems (snow and ice, rocks). The level of surface disturbance reflects the level of cold stress, the availability of water, gravity, and movement-prone or water-accumulating surficial materials. Most disturbances break up large patches into smaller ones, increasing patch heterogeneity. The new patches support a greater variety of environmental and organism combinations, but even exotic disturbances only exceptionally produce exotic ecosystems. Areas with more frequent surface disturbances have greater ecosystem diversity than those less frequently disturbed (Kom´arkov´a, 1993); see Fig. 3.19. Damage to populations and ecosystems increases and the probability of ecosystem recovery decreases with increasing intensity, scope, duration, diversity, predictability, and frequency of disturbances. The rate and direction of recovery are dependent on the characteristics of the disturbance and the responding unit (Vitousek et al., 1981). Some disturbances may short-circuit or eliminate certain ecosystem pathways, and others may accelerate certain processes (Lugo and Snedaker, 1974). This disturbance-specificity of ecosystem response is important where ecosystems are in equilibrium with particular disturbances, especially where the probability of disturbance increases with the time since its last occurrence (Holling, 1981; Vitousek et al., 1981). Recovery does not occur if disturbance intensity exceeds certain thresholds (D.M. Raup, 1981; Auerback, 1981; Westman, 1986). Such thresholds decrease as altitude and latitude increase, and the time needed

STRESS AND DISTURBANCE IN COLD REGION ECOSYSTEMS

71

Fig. 3.18. Cold-region landscapes ranked by disturbance frequency and patch size. At high latitudes and altitudes, year-round temperatures below 0ºC limit surface disturbances by ice and ecosystem diversity in areas with little water, such as polar deserts, most of which are deflated by wind. Snow and glaciers cover and abrade areas at high latitude and altitude with enough precipitation. Snow and ice ecosystems do not undergo freeze/thaw cycles as they do at lower altitudes or latitudes. Large energy transfers occur at middle altitudes and latitudes, where temperature fluctuations include frequent passes through 0ºC and changes of state of water. Wet, relatively warm mountains with steep, long slopes, soft bedrock, and high runoff have probably the highest disturbance levels. Flat, waterlogged plains with loose surface materials and large amounts of ground ice and seasonal freeze/thaw cycles are also frequently disturbed, as are ice-scoured coasts and now deglaciated mountains with free-fall rock faces. Dry, gently sloping mountains and dry plateaus are much less disturbed. The least disturbed are the well-vegetated areas in cold-region margins with shrub, tussock, and turf ecosystems.

for recovery increases. They are specific for different ecosystem types (Fig. 3.20). Because organisms seem to have relatively weak control of cold-region ecosystems, these thresholds may be located at lower levels of disturbance intensity in cold than in temperate regions. Resistance and resilience Specialized structures and biochemistry enable most organisms resident in the cold regions to tolerate freezing, pollution, and other disturbances which affect functions more than structures. However, short growing seasons, cold stress, and low availability of nutrients lead to slow growth, metabolism, and maturation (Clarke, 1980; K¨orner and Larcher, 1988; K¨orner, 1994 ), and reproduction often fails, as result of environmental stress and fluctuations along extreme

physical gradients. Most resident organisms, therefore, are poor at resisting disturbances and repair damage slowly, and avoidance of stress and disturbance is perhaps more important in cold regions than in many other biomes. Ecosystems, most of which also have poorly developed soils, are also less resistant and resilient (“fragility”; Webber and Ives, 1978) to disturbance than ecosystems in most other biomes. However, when they contain uniquely adapted organisms, they eventually recover to the pre-disturbance state, unless an environmental change intervenes during their prolonged recovery. Resistance and resilience increase with increasing inputs of heat, moisture, and nutrients, and the most resistant and most resilient ecosystems occur in the relatively warm, moist marginal areas, where the environment is relatively favorable to plant production, reproduction, and growth, and supports relatively large

72

´ ´ and F.E. WIELGOLASKI V. KOMARKOV A

Fig. 3.19. In areas of approximately similar sizes and vascular plant diversities, Braun-Blanquet types of mature vegetation unaffected by human disturbance are more numerous and defined at lower similarity levels in the more frequently disturbed Arctic Coastal Plain at Atqasuk, Alaska (A) than in the less frequently disturbed glaciated mountains of the Front Range of the Rocky Mountains in Colorado (B). From Kom´arkov´a (1993).

STRESS AND DISTURBANCE IN COLD REGION ECOSYSTEMS

73

Fig. 3.20. The relationship between resistance, resilience, and recovery in some cold-region ecosystems.

amounts of living organic matter and well-developed soils. The terrestrial ecosystems most resistant to mechanical disturbance (mesic scrub, tussocks, turfs) have a relatively large amount of tough, sclerenchymatous phytomass, silica-rich or woody, forming a more or less closed cover of slow growing plants which are relatively strong competitors. Thick, welldeveloped soil, hard rocks, flat surfaces, and consolidated, structured surface materials increase resistance. Belowground storage, stem flexibility, low growth form, and summer hardening diminish the injuries caused by aboveground mechanical disturbance, but their effectiveness is limited; after 150 passes of offroad vehicles in an Alaskan shrub tussock tundra, there was little difference in the degree of compression between any of the shrub taxa (Ahlstrand and Racine, 1993). Resistance seems to be unrelated to the number of taxa; it mainly reflects their properties. The relatively low resilience of these ecosystems reflects the slow rates of growth and organic-matter accumulation by the dominants, and the numerous small, slow-growing, drought- and freeze-tolerant organisms, such as lichens, that live between them. Resilience may be inversely related to the number of taxa and to ecosystem complexity. Frequently disturbed, simple ecosystems (marshes, shores, coasts, sand dunes) have low resistance but

high resilience, even at relatively high latitudes and altitudes. Usually, the few vascular plant taxa present are composed of soft tissues, reproduce vegetatively, and disperse, establish, and grow rapidly, some from belowground or near-ground storage reserves. They have wide ecological ranges (Kom´arkov´a, 1979), high nitrogen storage per unit leaf area (K¨orner, 1989), and highly resilient growth forms (Z¨ottl, 1951; Jefferies, 1988; Kom´arkov´a and McKendrick, 1988), enabling them to respond to disturbances rapidly even under cold stress. Landforms and soils are loose and easily colonized. Such ecosystems recover to almost their original state in a relatively short time. In Arctic Alaska, the large amount of water in the marshy landscape and frequent surface disturbances are perhaps the reasons for a high proportion of rhizomatous plants adapted for rapid vegetative reproduction, colonization, and growth (Tables 3.1A and 3.1B; Kom´arkov´a and McKendrick, 1988). Change and recovery The rate and direction of recovery are dependent on the characteristics of the disturbance and of the responding unit (Vitousek et al., 1981). To a greater degree than in temperate biomes, recovery after disturbance in cold regions is controlled by steep environmental gradients,

´ ´ and F.E. WIELGOLASKI V. KOMARKOV A

74 Table 3.1A Proportions of vegetation growth forms in three Arctic locations 1,2 Site

Growth form 3

Latitude evergreen shrubs

rhizomatous graminoids

caespitose graminoids

prostrate forbs

0

0

23.4

5.5

33.4

6.8

30.7

deciduous large dwarf shrubs shrubs

erect forbs cushion others

Edgeøya 4

78º05 N

Barrow 5

71º18 N

1.7

4.5

1.7

46.4

7.2

7.8

0.2

30.3

Atqasuk

70º29 N

14.6

15.0

7.8

31.1

6.9

2.9

0.8

17.4

0

1 Rhizomatous growth forms seem to have greater importance in the more frequently disturbed landscapes than in the less frequently disturbed ones. 2 After Kom´ arkov´a and McKendrick (1988). For each growth form, the importance of vegatation cover is expressed as a percentage of the total for all vascular plants at that site. 3 The definitions of growth forms approximate those in Barkman’s system (Barkman, 1983, 1987). 4 Data from Barkman (1987). 5 Data from Webber et al. (1980).

Table 3.1B Proportions of vegetation growth forms in different latitudinal belts 1,2 Site

Growth form 3

Latitude deciduous shrubs

evergreen shrubs

rhizomatous graminoids

caespitose graminoids

prostrate forbs

erect forbs cushion others

Northern Arctic Edgeøya, Spitsbergen

78º05 N

0

0

23.4

5.5

33.4

6.8

30.7

Devon Island, Canada

75º33 N

5.0

7.8

46.1

11.1

9.4

11.8

8.1

Tareya, Russia

73º30 N

7.0

2.3

28.5

21.2

2.0

27.0

11.7

Barrow, Alaska

71º18 N

9.8

2.2

48.3

9.4

6.0

0.3

24.0

Atqasuk, Alaska

70º29 N

29.6

7.8

31.1

6.9

2.9

0.8

17.4

Disko Island, Greenland

69º15 N

28.4

2.3

2.6

16.2

6.0

12.6

28.6

68º22 N

9.3

22.4

35.1

12.0

0

0

21.1

54º12 N

1.1

35.5

11.3

35.0

2.2

0.4

13.5

54–55ºS

31.8

0

16.8

49.6

1.8

0

0

0

0

0

91.1

0

8.9

0

Southern Arctic

Subarctic Abisko, Sweden Temperate Glenamoy, Ireland Northern Antarctic South Georgia Southern Antarctic Signy Island

60ºS

1 Rhizomatous growth forms seem to have greater importance in the more frequently disturbed landscapes than in the less frequently disturbed ones. 2 Data from French (1981). 3 The definitions of growth forms approximate those in Barkman’s system (Barkman, 1983, 1987).

STRESS AND DISTURBANCE IN COLD REGION ECOSYSTEMS

frequent environmental fluctuations, and cold stress. The direction of recovery and the structure of successional pathways are determined by the environment, including the disturbed landform, and not by the intensity or type of the original or newly triggered disturbance (Kom´arkov´a, 1983b, 1989), unless that disturbance is persistent and indigenous to cold regions (Kom´arkov´a, 1983a, 1989). Recovery rates and sequences are best documented where the age of the disturbance and the original landform are known. The recovery from natural disturbances, the age of which is usually not known, may involve slow changes in landform and climate as well. In cold regions, ecosystems can recover from surface disturbances of only low to medium frequency. The precise medium frequency of disturbance that keeps ecosystems permanently in successional stages by allowing them to recover only partially, seems to be rare or nonexistent. While slow recovery probably continues and complexity increases until the next surface disturbance, the similarity of most younger ecosystems to the older ones (determined by the taxonomic composition of the vegetation), after a few decades to thousands of years of recovery, falls within the limits of their variability. Because surface disturbances are more frequent, and there is probably a constant adjustment to changing climate and disturbances (Krajina, 1933; Dahl, 1956; Kom´arkov´a, 1979), coldregion ecosystems may reflect a slow environmental change or a trend (climatic warming, increase in UV-B radiation) more closely than temperate ecosystems. However, this appears unlikely. Despite their simpler structure, the rate of change and recovery of most coldregion ecosystems is probably slower than that of their temperate counterparts. Above certain levels of disturbance frequency, recovery does not occur and the original ecosystem shifts to one controlled by frequent or persistent disturbance, such as ground-material movement, which is tolerated only by organisms adapted to it, e.g., scree (Schr¨oter, 1926). This is also true for temperate ecosystems (Tilman, 1996), but the proportion of such ecosystems may be higher in cold regions. In the presence of frequent avalanches, the shift from vegetation dominated by trees to vegetation dominated by shrubs occurs where the average interval between avalanches is less than 15 to 20 years. At that frequency, recovery does not produce trees capable of competing with shrubs that have flexible stems and

75

can reach maturity even where avalanches are frequent (Johnson, 1987). Ecosystem response to surface disturbance Recovery rates reflect the high diversity of environments, ecosystems, and disturbances in cold regions. They are generally slower than in temperate climates. They decrease with increasing latitude (Figs. 3.21– 3.23; Forbes, 1992), altitude, severity of climate, patch size (Kom´arkov´a, 1989; Forbes, 1992), intensity of disturbance, and importance of successional taxa and stages, and with decreasing availability of moisture (Fig. 3.24). They are considerably slower in continental than in oceanic areas, e.g., northern Sweden (Stork, 1963), the Alps (Jochimsen, 1963), the Alaska Range (Viereck, 1966), the Wrangell Mountains, Alaska (Scott, 1974). Elevated sites in mesic areas, dry hygrophobic organic matter, dry and unweathered or coarse mineral materials, e.g., such as volcanic ash (R.I.L. Smith, 1984a,b; H.G. Smith, 1985), and warm dry slopes may still be bare decades after the disturbance even when propagules are available. On Subantarctic Macquarie Island, landslides revegetate rapidly (Scott, 1983) and large grass tussocks develop in 50 years, but the windiest sites may still be bare even after this period of time (Ashton, 1965). The life-span of human and other natural disturbances is considerably changed by the natural disturbance factors and by cold stress. Persistent natural disturbances (wind erosion, movement of material under gravity, freeze–thaw effects) obliterate tracks or disperse pollutants in some habitats (sand dunes, talus slopes) within days or weeks. In flat, waterlogged landscapes with ground ice, human disturbances may persist for centuries (on polygon topography) or thousands of years (on river terraces or next to thaw lakes: Everett, 1979), depending on the original landform and the type of disturbance. Most will recover before a new disturbance occurs. In cold continental and dry areas, where most surfaces are disturbed by glaciers at intervals of 20 000 years, only ecosystems on deposits 3000–5000 years old or more have complexity comparable to that of ecosystems undisturbed since the last glaciation, e.g., Colorado Rocky Mountains (Kom´arkov´a, 1979, 1993). Probably only in such areas may the dominants, zonal dominants in particular, be in a disequilibrium with the current climate, and new surface disturbances may bring about the establishment of different zonal ecosystems reflecting

76

´ ´ and F.E. WIELGOLASKI V. KOMARKOV A

Fig. 3.21. Simple regression of percentage cover of vegetation in plots recovering from surface disturbance and their controls of natural undisturbed vegetation along the latitudinal gradient of the Antarctic Peninsula. The recovery rates and vegetation cover decrease toward the south. Some recovery occurs on human-disturbed surfaces after 30–50 years at 62ºS (Fig. 3.22) and at 64ºS (Fig. 3.23); none was observed at 68ºS. Complete development of complex plant communities has not been observed anywhere along the latitudinal gradient. Kom´arkov´a, unpublished data.

the changed climate. In polar deserts, only small surficial changes occur during tens of thousands of years, and disturbances may be preserved by persistent cold. Ecosystem response to soil compression In the moist margins of the polar regions, mosses and surface organic matter do not rebound, and subsidence persists even after a passage by a single vehicle on

partially disturbed winter trails for several years (Adam and Hernandez, 1977; Felix and Raynolds, 1989a,b) – even as long as 30 years (Kom´arkov´a, 1989; Forbes, 1992, Emers et al., 1995); the dissimilarity to the original vegetation may increase during the first five years after the disturbance (Felix et al., 1992). The effect of a single passage by a human or vehicle in summer may persist for decades – even for more than fifty years (Bellamy et al., 1971; Abele et al., 1984; Wielgolaski, 1997). The injury to long-lived

STRESS AND DISTURBANCE IN COLD REGION ECOSYSTEMS

77

Fig. 3.22. Recovery at the United Kingdom Base G (Admiralty Bay), east side of Keller Peninsula (62º05 S, 58º26 W), King George Island, South Shetland Islands, northern Antarctic. An almost complete cover of rock lichens, primarily Caloplaca, on a concrete surface that probably dates to the time when the base was constructed (1948–1950). Trampling ceased in 1961 when the base was abandoned (Hattersley-Smith, 1991).

Fig. 3.23. Recovery at the Argentinian Primavera Station, Cierva Point (64º09 S, 60º57 W), Danco Coast, Antarctic Peninsula. Part of the station was established in 1954 and a new, frequently occupied part opened in 1977 (Hattersley-Smith, 1991). Colobanthus quitensis and Deschampsia antarctica are colonizing concrete and wastes in the older part of the station. The largest cushions and tussocks are probably at least 15 years old. January, 1984.

78

´ ´ and F.E. WIELGOLASKI V. KOMARKOV A

Fig. 3.24. Comparison of the rates of recovery on disturbed surfaces of different landforms in a more or less mesic cold region, such as the Arctic Coastal Plain in Alaska, the Colorado Rocky Mountains, or the northeastern edge of the Tibetan Plateau. Recovery slows down with decreasing availability of moisture and as more taxa become established. Kom´arkov´a, unpublished data.

plants in multiple-pass trails is cumulative (Ahlstrand and Racine, 1993). Vegetation and the surface organic matter are compressed much more in marshes than in uplands (Bliss and Wein, 1972), but the melting of ground ice and thermokarst effects may change uplands into marshes. These disturbances favor herbs, including exotics (Poa annua on Macquarie Island), reduce cover by bryophytes and lichens (Bell and Bliss, 1973; Hernandez, 1973; Bayfield et al., 1981) and at high intensities may eliminate woody plants (Bliss and Wein, 1972; Chapin and Shaver, 1981). Recovery is rapid in wet, depressed areas with late-lying snow (Hernandez, 1973; Moskalenko, 1980, 1983), where the saturated compressed fibrous organic material may regain its original structure rapidly (Abele et al., 1984). Lichen recovery may be delayed (Bliss and Wein, 1972; Forbes, 1992; Roxburgh et al., 1988), especially where moisture accumulates in compressed areas (Kershaw, 1983; Herbein and Neal, 1990; Truett

and Kertell, 1992). Where habitats do not change, lichens do recover even from substantial damage (Kershaw and Kershaw, 1987; Adamson et al., 1990; Harper and Kershaw, 1996). In partially disturbed sites, soil temperature and thaw depth increase, releasing organic matter, seeds, and nutrients which increase plant productivity and biomass (Bliss and Wein, 1972; Chapin and Shaver, 1981). On the other hand, biomass may decrease, along with moisture and nutrient availability, when organic matter is removed (Bliss and Wein, 1972; Bishop and Chapin, 1989). In the absence of freeze/thaw cycles in the arid Antarctic, tracks and other slight disturbances were visible after 33 years; sparse vegetation developed only in moist spots (Campbell and Claridge, 1982; Campbell et al., 1993). In the dry Rocky Mountains of Colorado, colonization of alpine disturbances was not complete after 10 years (Kiener, 1939) or 26 years (Griggs, 1956); sites damaged during 25 seasons of visitor use

STRESS AND DISTURBANCE IN COLD REGION ECOSYSTEMS

may require hundreds of years to recover. However, the recovery of vegetation was almost complete in four years following a one-year trampling period (Willard and Marr, 1971; Bell and Bliss, 1973). The vegetation may recover rapidly from light damage in mesic habitats, but from serious damage only in wet habitats (Willard and Marr, 1970). In the northern Arctic, 18– 20 years after vehicle disturbance, recovery was almost complete only in the wettest meadows (Forbes, 1992). In Norway, after five years of trampling, damage was more pronounced in a lichen heath and a wet meadow than in a relatively nutrient-rich dry meadow, where the recovery was faster (Table 3.2; Wielgolaski, 1997). In Subantarctic South Georgia, no regeneration has been observed in areas devastated by the increasing populations of fur seals (Bonner, 1985).

79

Table 3.2 Study of succession over 22 years following trampling 1 Vegetation

1973

1976

1981

1986

1991

1995

Lichens

0.08

0.12

0.19

0.48

0.80

0.90

Bryophytes

0

12.50

9.60

7.68

2.82

2.25

Monocotyledons

0.18

0.48

0.14

0.20

0.57

0.03

Forbs

−

−

−

−

−

−

Woody plants

0.13

0.20

0.32

0.39

0.43

0.46

Lichens

0.05

0.23

0.84

2.30

2.11

2.70

Bryophytes

0.11

0.91

1.05

1.45

3.78

1.60

Monocotyledons

0.36

0.58

0.75

0.86

0.85

0.64

Forbs

0.46

0.65

0.79

0.71

0.73

0.65

Woody plants

0.13

0.31

0.47

0.49

0.57

0.66

Lichens

−

−

−

−

−

−

Bryophytes

0.27

1.06

1.27

1.46

1.40

1.59

Monocotyledons

0.12

0.39

0.57

0.60

0.54

0.50

Lichen heath

Dry meadow

Wet meadow

Ecosystem response to removal of surface soil After removal of vegetation and partial or total removal of soil organic matter, recovery usually begins with slow colonization by the original dominants, by the dominants of landforms similar to those newly created by the disturbance (e.g., gravel roads are colonized by species from river gravel bars), or by taxa inhabiting microdisturbances of adjacent mature, otherwise undisturbed vegetation. In dry sites, the removal of dead vegetation and the surface organic soil speeds up recovery by increasing moisture availability; in mesic sites, the propagules contained in this layer aid recovery, and removal of this material will cause a delay (Van Cleve, 1977; McGraw, 1980; Chambers et al., 1990). Vascular plants are usually the first colonizers, sprouting from live propagules in the remaining soil, new seeds, and rhizomes or roots (S¨oyrinki, 1938, 1939; Hernandez, 1973); lichens may also colonize, reducing needle-ice disturbance and facilitating seedling establishment (Bell and Bliss, 1980; Grulke and Bliss, 1988). Burned surfaces, and bare soil compacted by trampling (Table 3.2), are usually colonized by bryophytes; bryophyte recovery may be delayed in sites where soil has been removed (Harper and Kershaw, 1996). Mycorrhizal fungi, which may be reduced or lost by disturbance, colonize rapidly (Allen et al., 1984, 1987).

Forbs

0.31

0.31

0.73

0.39

0.37

0.47

Woody plants

0.33

0.13

0.38

0.47

0.53

0.74

1 The data are ratios of cover inside and outside trails used during the International Biological Program (1969–1973) at Hardangervidda, southern Norway. Values on six line transects were averaged. Data from Wielgolaski (1997).

Vegetative reproduction predominates among colonizing taxa in the northern Arctic, where the seed supply is low, plant growth is particularly slow, and where artificial revegetation by native taxa, useful elsewhere in cold regions (Gams, 1940; Younkin, 1973; Brown et al., 1978), may not speed up recovery (Bliss and Grulke, 1988; Forbes, 1992). In the margins of cold regions, artificial revegetation by non-native taxa may delay the establishment of native taxa, as planted grasses and colonizing bryophytes reduce moisture, space, and nutrients available to native propagules, and possibly disperse inhibitory chemicals (Younkin and Martens, 1987; Densmore, 1992). Colonization is followed by a gradual increase of the number of organisms and vegetation cover (Shaver et al., 1983; R.I.L. Smith, 1985b,c, 1987, 1993, 1995). This process appears to be simpler than in temperate regions. Successional taxa are limited to natives of cold regions, where they are present, and the number of successional stages to one or two.

80

´ ´ and F.E. WIELGOLASKI V. KOMARKOV A

Fig. 3.25. Ordination by detrended correspondence analysis of data for cumulative percentage cover of vascular plants at the Fish Creek Test Well 1 (70º18 36 N, 151º52 40 W) drilled in 1949 on the Arctic Coastal Plain, Alaska. Mature plant communities unaffected by human disturbance are paired in the outlined landform–vegetation categories (solid line) with communities on surfaces recovering from human disturbance. A group of undisturbed communities in the center of the ordination space (dashed line) indicates that disturbance is one of the principal factors ordering the communities on the ordination planes. Moisture, duration of snow cover, and depth of thaw are other important factors ordering them. The levels of similarity at which the composite undisturbed communities and communities that had recovered for thirty years were clustered decreased in the following order: marsh (82%), lowland (52%), snow-patch (26%), ridge (15%) and upland (14%). From Kom´arkov´a (1983a).

Marshes may form or almost recover in a few decades, even following blading1 , as long as the dominants are rapid vegetative colonizers, usually with rhizomes (Figs. 3.25 and 3.26; Kom´arkov´a, 1983a, 1989). Even marshes recover slowly, and support successional communities, when the original dominants are relatively strong competitors but poor colonizers. Zonal uplands are usually dominated by such taxa (Eriophorum vaginatum ssp. spissum in most of Arctic Alaska, Kobresia pygmaea on the Tibetan Plateau, Kobresia myosuroides in the Rocky Mountains of Colorado), which spread by seeds (Gartner et al., 1983, 1986) produced after a maturation delay (Grime et al., 1988) and form tightly closed turf or tussocks where organic matter accumulates (Fig. 3.27). However, Deschampsia antarctica, the tussock-forming zonal dominant along the Antarctic Peninsula, is an 1

efficient colonizer, spreading by seeds and tussock fragments into disturbed areas. Eriophorum vaginatum ssp. spissum is a successful colonizer in the southern, relatively warm parts of its geographical area (Hopkins and Sigafoos, 1951; Chapin and Chapin, 1980). Zonal uplands, ridges dominated by dwarf shrubs and lichens, and snow patches dominated by dwarf shrubs all support successional communities of grasses and other weedy colonizers which occur in microdisturbances of undisturbed vegetation. Such successional communities develop within 2–15 years after the disturbance (Fig. 3.28; Johnson, 1969; Hernandez, 1972, 1973). Seedlings and small patches of the original dominants are usually present after 25 years and after 70 years the similarity of recovering vegetation to the controls is about 25–35% (Tibetan Plateau: Kom´arkov´a, unpublished data). After 36 years, a

“Blading” means scraping of the surface vegetation and some soil by a metal blade dragged behind a vehicle.

STRESS AND DISTURBANCE IN COLD REGION ECOSYSTEMS

81

Fig. 3.26. A drainage ditch at the Fish Creek Test Well 1 (70º18 36 N, 151º52 40 W) drilled in 1949 on the Arctic Coastal Plain, Alaska. After the site was abandoned the same year, the ditch probably deepened through melting of ground ice and thermokarst effects. About 30 years after the disturbance, recovering marsh vegetation has 75% similarity to an undisturbed marsh. The rhizomatous dominants Carex aquatilis ssp. stans and Eriophorum angustifolium ssp. subarcticum recover directly, without a successional stage (Kom´arkov´a, 1983a, 1989).

Fig. 3.27. Mature zonal upland unaffected by human disturbance dominated by Eriophorum vaginatum ssp. spissum near the Fish Creek Test Well 1 (70º18 36 N, 151º52 40 W) site on the Arctic Coastal Plain, Alaska.

82

´ ´ and F.E. WIELGOLASKI V. KOMARKOV A

Fig. 3.28. Recovering zonal upland dominated by rhizomatous successional grasses including Arctagrostis arundinacea and Poa arctica on a bladed surface of the Fish Creek Test Well 1 (70º18 36 N, 151º52 40 W) drilled in 1949 on the Arctic Coastal Plain, Alaska. About 30 years after the disturbance, the upland vegetation has 28% similarity to the undisturbed tussock, whose dominant, Eriophorum vaginatum ssp. spissum, is present only in small quantities. Barely disintegrated and vegetated solid waste remained on the site in 1977 (Kom´arkov´a, 1983a, 1989).

spaded plot in Alaskan tundra was only 40% vegetated with Dryas and moss (Pegau, 1970b). When the disturbance ceases in animal-controlled areas (burrows, trails, carcasses, corral, camp, and shelter sites), the original dominants eventually outcompete the nutrientsubsidized grasses and dicotyledons dependent on persistent disturbance, and the overall phytomass decreases (Rikhari et al., 1993). On the Tibetan Plateau in the Qinghai province, a failed Chinese attempt to establish grain fields covering several square kilometers in the lower-alpine region at an elevation of about 3200 m, on zonal surfaces about 150 m above oat fields in protected valley bottoms, was followed by aeolian erosion and subsidence; the recovery of vegetation cover was almost complete after 25 years, but the original dominants were present only in small quantities (Kom´arkov´a, unpublished data). Successional dicotyledons (Ajania tenuifolia) may inhibit the return of the original dominants through the release of toxic substances. In artificial grasslands, productive rhizomatous C4 grasses (Shi, 1985; Xia, 1986) are within several years outcompeted by weedy dicotyledons, and eventually by sedges (Zhang et al., 1986a,b). Extensive artificial grasslands could result in

the depletion of nutrients, loss of soil, reduction in moisture-holding capacity, and desertification. Willow dominants of zonal surfaces in relatively warm and humid Arctic regions also occur in successional stages in areas where they do not dominate mature undisturbed vegetation. They spread from individuals or parts that survived the disturbance, but also from germinating seeds (Bliss and Wein, 1972; Hernandez, 1972, 1973). In areas where they dominate mature vegetation, woody plants may slowly recover from mechanical disturbance (Bliss and Wein, 1972; Babb and Bliss, 1974; Rickard and Brown, 1974). Soil effects in ecosystem responses The type of soil influences the process of recovery, even when plant propagules stored in the soil are discounted. On the Tibetan Plateau, burrowing by small mammals (zokors, pikas, pocket gophers) is shallow, and the nutrient-rich soil with high water-holding capacity is colonized by vascular plants 1–2 years after the disturbance. The mounds of subsurface material next to deep marmot (Marmota spp.) burrows contain unweathered fine particles with small stones, and are not colonized for several years after the disturbance

STRESS AND DISTURBANCE IN COLD REGION ECOSYSTEMS

ceases, even after the mounds are flattened by erosion (Kom´arkov´a, unpublished data). While partial disturbances usually damage vegetation more than soil, recovery of vegetation may lag behind the recovery of soil properties also for other reasons. However, when soils become warmer and ground ice melts during or after the disturbance, only the recovery of the insulating vegetation cover leads to the recovery of pre-disturbance soil properties and the depth of the ground-ice table (Haag and Bliss, 1974; Gersper and Challinor, 1975; Harper and Kershaw, 1996, 1997). In Arctic Alaska, soil properties did not return to the levels of the undisturbed controls in six years in the case of vehicle tracks (Challinor and Gersper, 1975; Gersper and Challinor, 1975), or in 30 years following blading; base saturation, nutrients, and depth of thaw were higher, and organic matter, available water, silt, clay, and cation exchange capacity were lower in the disturbed areas (Kom´arkov´a, 1989). The life span of dominants may be less important for total ecosystem recovery in cold regions than in temperate areas with more rapid soil formation. The development of vegetation cover after glacier retreat and in rocky habitats during primary succession may take hundreds of years or even longer (Frenot et al., 1995; Harper and Kershaw, 1996), and soils may take thousands of years to develop. Twentysix years after its appearance, only 1–2% of the island Surtsey near Iceland was vegetated (Fridriksson, 1987, 1989). In northwestern Canada, forty-eight years after disturbance, primary-succession soils were more coarsely textured, drier, warmer, and less acidic, and accumulated much less organic matter than undisturbed soils (Harper and Kershaw, 1997); shrub birch recovered only 10% of its cover, and a substantial part of the site remained bare (Harper and Kershaw, 1996). Soil development is also slow on glacier outwash (Viereck, 1966) and moraines distant from a nutrient source (Mahaney, 1974; Messer, 1988; Matthews, 1993). On moraines in Kerguelen, in the absence of manuring by marine birds or elephant seals, the organic content of soils was low even after 200 years, whereas leaching was rapid under the relatively high precipitation (Frenot et al., 1995). Ecosystem response to grazing Damage and recovery both depend on the intensity and frequency of grazing, and are related to the growth form and other plant grazing defenses, and to the

83

manner of plant utilization by the herbivores. In an alpine meadow in the central Himalaya, tussock grass was grazed less deeply and showed better regrowth than a more deeply grazed forb and a sedge; sheep bites were significantly deeper than those of a horse, and horse-grazed plants showed better regrowth than sheepgrazed plants (Negi et al., 1993). Grazing generally reduces lichens and shrubs, and increases herbs. The ability of deciduous shrubs and herbs to survive defoliation by low-intensity, repeated herbivory depends on nutrient reserves built up in favorable years (Archer and Tieszen, 1980; Chapin, 1980; Henry and Gunn, 1991). In graminoids, these reserves for replacement growth after grazing, die-back or mechanical damage such as abrasion by snow and ice are usually located below ground. Unlike shrubs, they also have meristems protected by long-lived, non-green, stiff stem bases, and recover rapidly once grazing or other low-intensity mechanical disturbance ceases (Babb and Bliss, 1974; Archer and Tieszen, 1980; Chapin, 1980). Clipping significantly affects their growth only if it is repeated during the season (Mattheis et al., 1976; Archer and Tieszen, 1980; McKendrick, 1981). When grazed equally, caespitose graminoids may not recover rapidly (Jefferies et al., 1992), although in Subantarctic South Georgia Poa flabellata recovers well from disturbance and severe grazing by reindeer (R.I.L. Smith, 1985a). Some rhizomatous or stoloniferous Arctic graminoids produce new leaves continuously during the grazing season and have a greater capability of regrowth than other graminoids, in which regrowth may be delayed until the next season. This is true for Carex subspathacea and Puccinellia phryganodes, which are grazed by geese along coasts, and for some other grasses (Arctophila fulva, Dupontia fisheri) (Mattheis et al., 1976; Jefferies et al., 1992). Rapid recycling of goose feces and old dead leaves provides the nutrients for regrowth (Kotanen and Jefferies, 1987; Bazely and Jefferies, 1989a,b). Herbaceous dicotyledons usually have a smaller proportion of biomass below ground than graminoids, grazing decreases their flowering and plant size, and it takes them longer to reach pregrazing levels after being grazed by muskoxen (Ovibos moschatus) (Mulder and Harmsen, 1995). Under intermediate frequencies and intensities of vertebrate browsing, deciduous shrubs produce leaves with progressively lower nutrient content, and growth forms that are less likely to be regrazed, whereas

84

´ ´ and F.E. WIELGOLASKI V. KOMARKOV A

Fig. 3.29. Recovery on different disturbance types in a dry, nutrient-poor fellfield on Niwot Ridge, Front Range of the Rocky Mountains in Colorado, in an area of relatively continental alpine climate. Unweathered subsurface material with little water-holding capacity delays recovery. Kom´arkov´a, unpublished data.

defoliation in evergreen shrubs decreases the concentration of protective secondary metabolites in leaves and increases their food value (Tuomi et al., 1984; Bryant et al., 1988, 1989). Some bryophytes are favored by grazing, but vascular plants may be favored over bryophytes (Batzli, 1975; Batzli et al., 1980; Oksanen and Ranta, 1992). Lichens may be slow to recover (Klein, 1968, 1987; Pegau, 1968). Twenty-two years following a crash in the reindeer population on an island in the Bering Sea, lichens had recovered, but only to 10% of the standing crop of lichens on an adjacent island with no history of grazing (Klein, 1987). To support migrating animals in the long term, the return intervals must be long enough, sometimes years, for vegetation to recover between visits (Andreyev, 1988). After being introduced to Alaska at the turn of the century (1892–1914), reindeer increased to 594 000

animals by 1936, then decreased to 20 000 animals. Most areas have recovered within 50 years after being overgrazed in the late 1930s, although overgrazing from the 1950s and 1960s is still evident on Bering Sea islands (Hanson, 1952; Collins, 1986). Intensive grazing or trampling by muskoxen creates microdisturbances invaded by weedy native colonizers, which may become dominant; overgrazed muskox pastures are rapidly recolonized once grazing ceases (McKendrick, 1981). On the Tibetan Plateau, overgrazing is damaging during spring and fall when the capacity for regrowth is low (Wang et al., 1986). On the relatively humid northeastern edge of the Tibetan Plateau, the original dominants have a low cover, and weedy taxa dominate both grazing exclosures (Zhou and Zhang, 1986) and 25-yr-old upland surface disturbances (Kom´arkov´a, unpublished data).

STRESS AND DISTURBANCE IN COLD REGION ECOSYSTEMS

85

Fig. 3.30. Recovering hydrocarbon spill on a bladed surface of the Fish Creek Test Well 1 (70º18 36 N, 151º52 40 W), drilled in 1949 on the Arctic Coastal Plain, Alaska. About 30 years after the disturbance, most of the spill is not vegetated; areas were the spill intensity is lower have 14% similarity to the control mesic surface. Solid waste has disintegrated little. The similarity of the vegetation recovering on solid waste to the corresponding vegetation unaffected by human disturbance is about 18%, but more on drier landforms (Kom´arkov´a, 1983a, 1989).

Ecosystem effects of solid waste In cold environments, disintegration of solid waste and pollutants and the release of toxic chemicals occur much more slowly than in most temperate environments. Potentially high photooxidation rates in some cold regions do not seem to contribute to higher rates of disintegration. Disintegration is faster in terrestrial environments (beaches), with relatively high temperature, humidity, salt spray, and ultraviolet radiation (denaturing plastic), than in the ocean (e.g., Ryan, 1987). Recovery rates from pollution are slower than from surface disturbances (Figs. 3.28, 3.29), but they vary with the same factors, primarily with the degree of cold and the availability of moisture. At high latitudes and altitudes, solid waste and pollutants persist and often accumulate, and the ecosystems may not recover unless the pollutant or solid waste is removed (Schindler, 1983). Surfaces of solid waste are abraded, but most remain bare of macro-organisms. Even paper in cold, relatively dry environments (south of the Antarctic Peninsula) and soft materials absorbing moisture (rope, tarpaulins, wood) in more humid cold regions (northern Alaska) persist for decades (Figs. 3.30 and 3.31) (Kom´arkov´a,

1983a, 1989). Colonization is delayed on unweathered solid-waste materials with low water-holding capacity (concrete, asphalt), especially on unweathered mining waste which may contain acids and toxic materials (Fig. 3.29), and on surfaces affected by chemicals (oil, paint, rust) and away from a substrate which can conduct moisture and supply abrading particles. Revegetation of solid waste is more rapid when it is covered by windblown mineral and organic particles that absorb moisture. The communities colonizing solid waste are similar to those on the surrounding landform. Solid waste of natural exotic materials (wood, gravel, concrete) may support adventive cold-region taxa in areas without wood and rocks. In its vicinity, solid waste creates disturbances such as shading, reduced wind, reflected heat, and prolonged duration of snow cover. Bryophyte-dominated communities in such environments are outcompeted by grasses or sedges soon after shading is removed. Ecosystem effects of pollution Many slow-growing organisms in the cold regions accumulate pollutants (Focardi et al., 1992), and some of the accumulations may reach harmful levels

86

´ ´ and F.E. WIELGOLASKI V. KOMARKOV A

Fig. 3.31. Solid waste, surface disturbance, and diesel fuel spills near Chaplinski hot springs on the Ul’khum River, Chukotski Peninsula, Russian Arctic. Plants colonizing surface disturbances, both here and elsewhere in cold regions, include mostly rhizomatous apophyte grasses (Arctagrostis arundinacea, Leymus interior), adventive grasses (Elytrigia repens) and ruderal plants (Artemisia tilesii). Photograph by A.E. Katenin.

Fig. 3.32. Unweathered subsurface mining waste on Niwot Ridge, Front Range of the Rocky Mountains in Colorado is not completely vegetated even after perhaps 70 to 100 years, despite being located in an area with late-lying snow and relatively high vegetation cover. The dominants of the recovering vegetation and the vegetation unaffected by human disturbance are the same (Trifolium parryi ssp. parryi, Acomastylis rossii ssp. turbinata).

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(Bargagli et al., 1993). Yet the dominance of cold and environmental fluctuations combine to limit the effect of low-level pollutants. Arctic marine invertebrates (e.g., oyster larvae, crustaceans, and annelid worms) are tolerant of high concentrations of zinc and lead, high salinity, and other extremes (Jones, 1975; Chapman and Brinkhurst, 1984; Lewis and Horning, 1991). Because the solubility of metal salts and the rate of movement of water and solutes across cell membranes decrease with temperature, metal toxicity may also be reduced (MacInnes and Calabrese, 1978; Chapman and McPherson, 1993). Cold and long winters, when soils are frozen and the ocean is covered by ice, and summer cold temperatures, delay the breakdown of pollutants and their entry into food chains, so that pollutants may be especially persistent in cold regions. Slow rates of production, decomposition, and chemical reactions account for the low self-purification and buffer capacity of cold ground and water. The rates of chemical reactions and pollutant breakdown decrease with increasing latitude and altitude, and in dry areas, where growth of organisms and turnover rates are slower (Horowitz et al., 1978; Hutchison-Benson et al., 1985; Adamson and Seppelt, 1990). Although the levels of contamination are less than at lower latitudes, Cs137 deposited over the last decades does not turn over readily in Arctic ecosystems, and they retain elevated levels for many years without further additions – longer than temperate ecosystems. The persistence of Cs137 in ecosystems increases with increasing latitude (Svoboda and Taylor, 1979; Hutchison-Benson et al., 1985; Taylor et al., 1985). Ecosystem effects of ultraviolet-B radiation The present impact of enhanced UV-B radiation is relatively small, as compared to the photoinhibition resulting from UV-B and UV-A radiation under normal conditions of stratospheric ozone. Up to thresholds for UV irradiance that occur at certain water depths, some Antarctic phytoplankton can adequately respond by synthesis of UV screening compounds (mycosporine-like amino acid pigments) and by repair mechanisms. It does not appear that such thresholds have been exceeded. The degree of increased protective pigmentation may be determined by the balance between increasing protection and decreasing effectiveness of photosynthesis (El-Sayed

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et al., 1990; Karentz et al., 1991; Holm-Hansen et al., 1993). When exposed to supplementary UV-B radiation in the field, significant accumulation of UV-absorbing leaf pigments occurred in vascular plants from higher latitudes, even if it was insufficient to protect them completely; most alpine plants were apparently protected without additional pigments (Barnes et al., 1987). Synthesis of photoprotective flavonoids in the moss Bryum argenteum is positively correlated with the levels of UV-B irradiation and negatively correlated with levels of ozone in the Antarctic stratosphere (Markham et al., 1990). Protective pigments did not change in cyanobacterial mats in the Antarctic under four UV regimes in the field, but pigmented cells might have migrated to the surface of the mat (Quesada et al., 1998). Ecosystem responses to increase in carbon dioxide and climatic change In cold regions, the effects of higher atmospheric concentration of carbon dioxide alone will be limited. Levels of carbon dioxide in dense moss and lichen carpets, turfs, in moist areas where decomposition is high, and under spring snow may be many times higher than in the surrounding air, even with the projected increase in carbon dioxide (Peterson and Billings, 1975; Billings et al., 1984; Oberbauer et al., 1986a). Long-term experimental exposure to elevated atmospheric concentration of carbon dioxide increased the root:shoot ratio or tiller production in some taxa (Oberbauer et al., 1986b; Tissue and Oechel, 1987), but it did not have a fertilizing effect except under higher temperatures (Oechel et al., 1994). Small alpine plants fix more carbon dioxide per unit area than lowland plants, possibly because of low temperatures and a greater length in the diffusion path in their thicker leaves; even under moderate climatic warming and enhanced atmospheric nitrogen deposition, they do not show positive phytomass responses to the enrichment of atmospheric carbon dioxide (K¨orner and Larcher, 1988; K¨orner, 1994; Sch¨appi and K¨orner, 1996). Most Arctic plants may have inherent physiological limitations to their ability to use additional carbon dioxide (Oechel and Billings, 1992). Nardus stricta showed a reduced photosynthetic and flowering capacity near cold springs in Iceland giving off carbon dioxide (Oechel and Vourlitis, 1996). The current initial response to climatic warming is

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slight; but, as in the past, persistent warming will cause changes in the extent of sea ice and in atmospheric and oceanic circulation, leading to ecosystem shifts upward and poleward. If the amplitude of change rapidly exceeds the current marginal temperatures of mountain and boreal forests, then trees will quickly outcompete the low-growing cold-region vegetation in areas with enough moisture. Ecosystems and organisms that do not have anywhere to retreat to will disappear (Tast, 1991; Grabherr et al., 1994), and coldadapted taxa that cannot acclimatize will be eliminated (Marchant, 1992). In some areas, the migration could be haphazard, as some slow-growing plants may not be able to track the rate of environmental change (Grabherr et al., 1978; K¨orner, 1994). Some treeless areas will become isolated, and trees will not be able to colonize rock outcrops and marshes. However, most ecosystems will probably migrate as units, and the present dominants will maintain the role determined by their competitiveness in new, equivalent environments. It seems unlikely that migrating ecosystems will be influenced by non-inherent herbivores, fires, or insect outbreaks. Changes in taxonomic composition will be reflected in changed demand for nutrients and water, decomposition, herbivory, and predation (Smith and Steenkamp, 1990; Jefferies et al., 1992; Seppelt, 1992). Infectious diseases and microorganisms which are rare in cold regions today will spread from lower latitudes and elevations, and people will have to change their practices – for instance, in agriculture and food preservation. Crop agriculture will be possible in more habitats than today. Some ecosystem changes are taking place already, although the replacement of dominants, zonal dominants in particular, is rare if it occurs at all. In the recent past, forest advanced only in a few places, and it is probably not advancing today. In Scandinavia, it expanded mainly through recruitment between 1950 and 1964 (Kullman, 1979, 1981, 1988, 1993); in the Rocky Mountains in Colorado, tree invasion of forest openings at the tree-line probably dates from the same, warmer and wetter period (Hessl and Baker, 1997). In the European Alps, taxonomic richness is increasing on summits, and nival plants are migrating upward at rates of 0–4 m per decade – rates far lower than the hypothetical movement of temperature isolines (Hofer, 1992; Gottfried et al., 1994; Grabherr et al., 1994, 1995). On Hopen Island in the Svalbard Archipelago, during the past 110 years, the proportion of vascular taxa with seed dispersal has increased at

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the expense of taxa with only vegetative reproduction (Skye, 1989). The number of vascular plant taxa is positively correlated with summer temperatures both there and in Arctic Canada (Rannie, 1986). On the Antarctic Peninsula and the Subantarctic islands, glacial retreat has exposed new areas for colonization, but only dominants or colonizers from adjacent areas are moving in (R.I.L. Smith, 1982; Hayward, 1983; Allison and Keage, 1986). However, the reported spread of Colobanthus quitensis to Gamage Point in Anvers Island, Antarctic Peninsula (Grobe et al., 1997) apparently concerns an extensive experimental garden where Colobanthus seeds were sown out and to which mature Colobanthus plants with seeds were transplanted in 1983–1984 (Kom´arkov´a, unpublished data). Some deglaciated areas cannot be colonized because snow and ice edges are moving away from ecosystems previously supplied by melt-water. Along the Antarctic Peninsula, some mossbanks are not recovering from decades-old disturbances, because their supply of moisture is drying out as a result of recession of glaciers and snow-patches (Kom´arkov´a, 1983b) caused by climatic warming in that area (Morrison, 1990; Stark, 1994). On the drying Tibetan Plateau, recovery may be slowing down, and may not occur, especially in arid central and western Tibet. Some invading taxa seem to be slowly outcompeting minor resident taxa, especially those that have been weakened by climatic warming, drought, or pollution; others are colonizing microdisturbances created by animals or by freeze–thaw processes between dominant plants. Lichens are overgrowing bryophyte dominants, e.g., on the Antarctic Peninsula (Kom´arkov´a, 1983b); where the moisture supply has decreased, fungal infections of the bryophytes are spreading (Longton, 1973). Shifts in atmospheric and oceanic circulation may be responsible for a steeply increasing proportion of exotics in the pollen rain on Marion Island, and easier invasion by exotic taxa (Scott and van Zinderen Bakker, 1985; Smith and Steenkamp, 1990). Other recent invasions include a terrestrial isopod, possibly introduced by human transport, on Macquarie Island (van Klinken and Green, 1992), additional vascular plants on Heard Island (Scott, 1989, 1990), algae (Broady and Smith, 1994), and bipolar lichen taxa (Smith and Øvstedal, 1994; Øvstedal and Gremmen, 1995). The current success of cosmopolitan weeds at cold-region margins, e.g., Poa annua on King George Island, South Shetland Islands (Olech, 1996), and of vigorous invaders on Marion Island in the

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Subantarctic – Agrostis stolonifera, Sagina apetala, herbivorous aphids and the diamondback moth Plutella xylostella (Crafford and Chown, 1987, 1990; Bergstrom and Smith, 1990; Chown and Smith, 1993) may be attributed to climatic warming (V.R. Smith and Steenkamp, 1990). There is little growth response to increasing temperatures among current cold-region plants, as a result of genetic constraints, low availability of nutrients (primarily nitrogen), or drought (Oechel and Billings, 1992; Havstr¨om et al., 1993; Wookey et al., 1993). The temperature optima of many alpine and arctic plants for uptake of carbon dioxide and for respiratory losses are similar to those of lowland plants (Tieszen, 1973; Tieszen et al., 1980; K¨orner, 1982). The acclimation to higher temperatures of dark respiration by leaves and of the photoperiodic control of development are different in different taxa (K¨orner and Diemer, 1987; K¨orner, 1994; Larigauderie and K¨orner, 1995), and the importance of photoperiod increases with increasing latitude and altitude (Heide, 1985, 1992; Prock and K¨orner, 1996). Temperature-opportunistic taxa may benefit from warming, especially if more water and nutrients become available, whereas taxa strongly controlled by photoperiod will not (Prock and K¨orner, 1996). Elevated night temperatures may affect tissue development (K¨orner and Pelaez MenendezRiedl, 1989; K¨orner, 1992). Until recently, some wet cold-region margins have been accumulating soil carbon, mostly in the form of dead organic matter slowly being incorporated in insulated, rising ground ice (Coyne and Kelley, 1978; Chapin et al., 1980b; Billings et al., 1982). Probably because of increasing temperatures and consequently decreasing site moisture, tussock tundra has become a source of carbon dioxide (Grulke et al., 1990; Oechel and Billings, 1992; Oechel et al., 1993, 1994), while wet marshes continue to be a source of methane (Sebacher et al., 1986; Whalen and Reeburgh, 1990a,b). Migration of carbon-accumulating ecosystems to higher latitudes and altitudes where little carbon is accumulated today, and the movement of forests into the current tundra areas, would significantly increase the removal of carbon dioxide from the atmosphere (Pastor and Post, 1988; Melillo et al., 1990; Oechel and Billings, 1992). In areas which remain treeless, warming temperatures and increases in precipitation, thaw depth, and the activity of anaerobic microorganisms could lead to increased emission of methane and

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renewed accumulation of atmospheric carbon dioxide in dead organic matter; if warming were moderate, the upper limit of permafrost could rise (Billings et al., 1982, 1983, 1984; Oberbauer et al., 1986a). Alternatively, if peat in waterlogged marshes dries out, their methane emissions would decrease, but positive feedback loops for carbon dioxide could decrease the thickness of the organic layer (M. Smith, 1990; Waelbroeck, 1993), carbon currently stored in dead soil organic matter could be depleted, and climatic warming could intensify (Billings et al., 1982; Whalen and Reeburgh, 1988, 1990a,b). Reduction of sea ice associated with a climatic warming would diminish the sea ice ecosystems and the nutrients in the Antarctic Bottom Water and their transport north (Marchant, 1992). The southern seals, penguins, and other sea birds could benefit from closer feeding and breeding areas, but the foraging areas of northern animals may already be more remote (Croxall et al., 1988; Smith and Steenkamp, 1990; Budd, 1991). Some feeding areas and population distributions of marine birds and mammals may also be affected by the El Ni˜no Southern Oscillation (Vergani and Stanganelli, 1990). Areas abandoned by penguin colonies during cool intervals (Stonehouse, 1970; Baroni and Orombelli, 1994; Emslie, 1995) could be reoccupied. Early melting and breakup of sea ice may result in the loss of the entire year’s brood of emperor penguins (Aptenodytes forsteri) through the break-up of colonies or drowning of chicks (Spindler 1994; Spindler and Dieckmann, 1994). Other penguins may suffer from too much heat, lack of snow patches on which they cool down after fishing, or greater snowfall (Kaiser, 1997). Ecosystem responses to hydrocarbon spills The numbers of cold-environment bacteria and other organisms (fungi, yeasts) utilizing hydrocarbons are similar to more temperate areas, but their numbers usually rise more slowly after a spill than in the temperate zone, especially in the absence of sufficient nutrients, oxygen, or moisture. The number of microbes usually decreases, because only hydrocarbon metabolizers can tolerate some of the toxic compounds present (Cundell and Traxler, 1974; Horowitz and Atlas, 1977; Sexstone and Atlas, 1977a,b). Bacterial communities may appear unchanged after 14 days of contamination, following an initial enhanced growth (Delille and Siron, 1993). Planktonic primary production slowly recovers but

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the taxonomic composition of the plankton changes, with an increase in relatively oil-resistant algae (Miller et al., 1978; Federle et al., 1979; Atlas, 1985). Heterotrophic activity may recover rapidly, especially in the water column (Jordan et al., 1978; Griffiths et al., 1981a,b). Copepods and other aquatic organisms may be tolerant of high levels of dispersed crude oils, but suffer internal and external contamination for weeks. Hydrocarbon concentrations in filter-feeders are usually lower than in deposit-feeders, which continue to take in oil from the sediment even a year after a spill (Wells and Percy, 1985). Oil-coated seals were clean after intervals ranging from one day to one month. Seals were efficient in excreting hydrocarbons from their bodies, and showed little pathological damage whether exposure was by immersion or by oil ingestion (Geraci and Smith, 1976; Engelhardt, 1985). On land, hydrocarbon spills are usually more damaging in dry sites than in wet ones, where they may disperse quickly; but they may persist for decades, as hydrocarbon-degrading microorganisms may be limited by available nitrogen and phosphorus (Atlas, 1977). Hydrocarbon spills remained bare even after 30 years in Arctic Alaska (Kom´arkov´a, 1983a, 1989), and under a 28-yr-old spill of diesel fuel there was still a significant depression of permafrost and a toxic component in the soil. Refined petroleum products are considered more toxic to plants than crude oil, and their spills are colonized more slowly (Plice, 1948; Cundell and Traxler, 1974; Linkins and Antibus, 1978). The recovering communities are similar to those on the surrounding landform. Recovery of oil spills may be accelerated by bioremediation in fertilized soils (McKendrick and Mitchell, 1978a,b; McKendrick, 1987; Kerry, 1993). Ecosystem effects of fire Because cold-region vegetation is seldom completely destroyed by fire and high root phytomass may increase resilience (Payton and Mark, 1979), recovery following fire may be relatively rapid (Hall et al., 1980; Fetcher et al., 1984). In the wet Arctic, Carex spp. and Eriophorum vaginatum tussocks often survive, and their recovery may take only two years (Wein and Bliss, 1973a). Rapidly growing herbs and grasses which occur as minor components in the original vegetation may increase their cover when the dominants are destroyed. Bryophytes colonizing burned surfaces may persist for decades (Johnson and Viereck, 1983;

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Tvorogov, 1988), or reach maximum cover 5–6 years after fire and then decline as the vascular overstory develops (Racine et al., 1987). Lichen recovery is much delayed (Wein and Bliss, 1973a; Wein, 1975; Hall et al., 1978). The proportions of the original taxa did not recover in Eriophorum vaginatum tussock tundra 13 years after fire (Fetcher et al., 1984). In New Zealand, recovery of burned Chionochloa tussocks may take 14 to 15 years (Payton and Mark, 1979). Empetrum evergreen dwarf-shrub and lichen vegetation on Amchitka Island in the Aleutians may take 20 years to recover (Shacklette et al., 1969). After a fire in South American p´aramo, dicotyledons with stem rosettes increased growth, but had a higher mortality rate (Verweij and Kok, 1992). Meristems survived burning, but trampling and consumption by cattle fragmented burned grass tussocks. Under increased frequency and intensity of burning and grazing, tussocks are replaced by shorter, more grazing-tolerant grasses and herbs, and the productivity, turnover, and belowground biomass increase. Further disturbance decreases biomass by up to 66%, and causes poor recovery of stem rosettes and large patches of bare ground (Cleef, 1981; Schmidt and Verweij, 1992; Verweij and Budde, 1992). Increased growth, flowering, and production after fire have been attributed to higher surface and soil temperatures, a surplus of moisture, increased nutrient availability as a result of increased mineralization and microbial activity in the warmer soils (Rowley, 1970; Wein and Bliss, 1973a; Weber, 1975), litter removal, and mobilization of reserves (Williams and Meurk, 1977). The nutrients in ash are generally not considered significant (O’Connor and Powell, 1963; Mark, 1965; Wein and Bliss, 1973a). The recovery of insulating vegetation leads to a decrease in the depth of thaw and ground uplift (Mackay, 1995). Fires may rejuvenate ecosystems in cold regions (Wein and Bliss, 1973a), but there appear to be no ecosystems dependent on fire. Changes in animal populations International and national agreements, and moratoria on commercial exploitation of cold-region animals, have helped to arrest or reverse the past steep declines of many populations of Greenland fur seal (Callorhinus ursinus), walrus, bowhead whales, and gray whales (Eschrictius robustus) (National Foreign Assessment Center, 1978). In Arctic Canada, populations of muskoxen (Ovibos moschatus), reduced by hunting for

STRESS AND DISTURBANCE IN COLD REGION ECOSYSTEMS

meat, hides, and furs to a few hundred individuals by 1917, have recovered to over 1.3×104 animals. However, commercial exploitation continues, and some populations are delayed in returning to previous levels. Populations fluctuating with climate (fur seals, penguins) may be more resilient to harvesting than whales. Populations of cold-region animals are dependent on finite food resources, so that declines in one species may lead to increases in others. For example, harvesting of whales apparently favored penguins and fur seals. The Antarctic biomass of baleen whales (Mysticeta), excluding the minke whale (Balaenoptera acutorostrata), decreased from 22×106 metric tons in the early 1920s to 2×106 metric tons in the mid-1960s. They previously consumed 100–150×106 metric tons of krill; this biomass then became available to other foragers (Mackintosh, 1970). Macaroni penguins (Eudyptes chrysolophus) on the Kerguelen Islands, and chinstrap penguins (Pygoscelis antarctica) and Ad´elie penguins (Pygoscelis adeliae) in Ad´elie Land and the Ross Sea region may be increasing as a result of a reduction in the numbers of whales (Sladen, 1964; Conroy, 1975; Croxall et al., 1984, 1988). Some populations of Ad´elie penguins have been increasing, even in human-disturbed rookeries, since 1982 – more than 15 years after the collapse of baleen whale stocks. This could be related to greater availability of winter food, caused by the reduction in sea ice which reflects the recent warming trend (Adamson et al., 1988b) in the Ross Sea area (Taylor et al., 1990), or to a combination of factors related to winter or spring sea ice, whale decline, krill availability, and breeding success (Wilson, 1990). This may also be true for chinstrap penguins (Fraser et al., 1992; Emslie, 1995), king penguins on Macquarie Island (Rounsevell and Copson, 1982), and Antarctic fur seals (Laws, 1977; Payne, 1977). Under the current climatic warming, the amplitude of fluctuations in populations of marine birds and mammals could diminish, and some populations could reach higher numbers in one part of their area than the carrying capacity of another part, leading to disturbances. Since the 1930s, from a few tens of animals (Payne, 1977), fur seal populations in South Georgia and the South Shetland Islands have increased to 9×105 animals (Bonner, 1985; Bengtson et al., 1990). They import more nutrients than the Antarctic terrestrial ecosystems can absorb, destroy them by trampling and lethal concentrations of nutrients

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(Bonner, 1985; Smith, 1988), and cause eutrophication of lakes (Ellis-Evans, 1990; Hawes, 1990). Fur seals have eradicated and damaged large expanses of Poa flabellata tussock grassland on South Georgia (Bonner, 1985) and Deschampsia antarctica meadows on Bird Island (Payne, 1977). Destruction of tussocks by lactating females feeding their pups deprives birds such as burrowing petrels of breeding habitat and exposes them to predation by Antarctic skuas (Catharacta antarctica) (Bonner, 1985). Young wandering albatrosses (Diomedea exulans) are disturbed by seals; their reproductive success has become lower, and the population is declining (Croxall et al., 1984, 1990).

CONCLUSIONS

Natural disturbances The gradient of surface heat, gradually decreasing beyond the marginal conditions for life, characterizes all cold regions. High altitudes and latitudes covered by ice, snow, and barren rock are dominated by the deep cold; temperatures above 0ºC are rare or nonexistent, and there are no resident higher organisms. At lower altitudes and toward the equator, and on the margins of the cold regions, a remarkable spatial and temporal diversity of natural stresses and disturbances is produced by fluctuations along steep environmental gradients, probably both in the oceans (light, nutrients, salinity) and on land (temperature, water, nutrients, duration of snow cover, wind, ice, gravity, animals). Natural stresses and disturbances are more frequent, variable, and intense than in most other biomes, and the steep environmental gradients control both their outcome and that of human-related disturbances to a greater degree. Ecosystem characteristics that contribute to the continuation of the original disturbance, or enable it to trigger new, different disturbances, are also more common in cold regions than in most other biomes. Near-surface ground ice, wind and moving water, freeze–thaw activity, unconsolidated surface deposits, and steep slopes all lower both ecosystem resistance and resilience, and disturbance-triggered processes (melting of ground ice, ground collapse, increased movement of material, changes in moisture and snow regimes) may eventually cause the replacement of the original landforms and/or ecosystems by new ones. Organic matter and pollutants accumulate in certain

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sinks, but in most cold regions there is greater export and import of materials by more powerful disturbance agents acting over longer distances and greater altitudes than in most other biomes. There is probably also more interconnectedness between cold regions, especially the small alpine ones, and the respective surrounding areas, than between other biomes and their neighbors. Most cold-region ecosystems have fewer organisms and lower complexity and productivity than their temperate counterparts. Interruption of ongoing evolutionary processes which would lead to adaptations to coldregion stresses and disturbances may dominate over an increase in evolutionary rates related to frequency of disturbances, and thus contribute to the organismic paucity of cold regions, in addition to their lack of resources. Most cold-region adaptations serve multiple purposes, e.g., a marine animal with a hard outer calcified shell has protection from abrasion, from being eaten, and from ultraviolet radiation (Karentz, 1994). Many cold-region organisms also have mechanisms by which to avoid severe disturbance and stress. Geographical limits of taxa may be determined by their tolerance of summer environmental fluctuations. If the harmful fluctuations become wider or more frequent, or move toward one or the other extreme, the geographical area of some organisms may diminish. Diversity of ecosystems is low in areas where deep cold predominates. In areas with ground ice, where surface disturbances recur at intervals between one thousand and several thousand years, ecosystems may be more diverse than those disturbed less often. Disturbances probably control the ecosystems that they disturb if they occur at least once every 20 years, or more often, depending on the characteristics of the disturbance and ecosystem. Only organisms capable of rapid growth, dispersal, and reproduction dominate in frequently disturbed areas. During recovery, successional stages are absent in stressed (the coldest) or frequently disturbed areas. They are usually present in zonal habitats at the margins of the cold regions. Most cold region ecosystems have relatively little resistance and resilience towards disturbance. More resistant ecosystems are usually less resilient, and more resilient ecosystems are less resistant. Low resilience stems from slow growth, failure of reproduction and establishment, and interruptions of the recovery process by new disturbances. Recovery rates are probably slower than in most other biomes, and some recovering ecosystems may not develop the maximum possible

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diversity of microtopography and complexity by the time of the next surface disturbance. This may happen when the interval between disturbances is between several decades and few thousand years long. Recovery may lead to ecosystems which differ from the original because of an intervening climate change; this probably occurs commonly in relatively stable cold regions where ecosystems are destroyed at intervals longer than several thousand years, and recovery may take even longer. Human disturbances Small native human populations live and practice agriculture in the cold-region margins. Their impact is limited, even if most have extensive contacts with lower latitudes and altitudes, the source of people and materials causing greatest disturbance. This influx and the disturbances are increasing, with growing human population, in most cold regions. In remote cold-region areas, human disturbances of relatively low intensity and extent occur only infrequently. In more accessible parts, they recur seasonally (tourism, research) or are persistent (construction, extraction of minerals and fossil fuel), but still affect only a small proportion of the total area. In contrast to lower latitudes and altitudes, human-controlled ecosystems are few, and limited in extent. Ecosystems controlled by domestic grazers are widespread in some marginal alpine regions. Suggestions as proposed by Laws (1984) and Gudmundsson (1986), that the secondary production of cold regions used by humans can increase, must probably be discounted, because the estimates of productivity and stocking rates used (Arnalds and Rittenhouse, 1986; Zhou et al., 1986) do not take into account many factors. The carrying capacities for people and livestock [2–3 Tibetan sheep per hectare (Pi and Zhao, 1986); 80–100 vicu˜nas per square kilometer (Rabinovich et al., 1985); see also estimates by Virtala (1992) for reindeer] may be diminishing due to overgrazing and other disturbances, especially in areas where climatic warming and drying trends coincide. Climatic cooling and grazing overuse probably led to a steady decrease of the primary productivity and the vegetated area in Iceland over the last 1100 years, and to the demise of medieval Norse settlements in Greenland (Fridriksson, 1972, 1986; Thorsteinsson, 1986; Whitaker, 1990).

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Stresses and disturbances related to human activity increase the naturally high levels of stress and disturbance, especially for cold-region organisms at their geographical and tolerance limits. The effects are more obvious than in most ecosystems of warmer climates, partly because organisms and ecosystems have little resilience and resistance to disturbance. The importance of where and when the disturbance occurs is perhaps greater in cold regions than in other biomes; the growing season is short, and the productive areas limited in size. Human disturbances which free space are tolerated in habitats with similar natural disturbances (trampling by humans in animaltrampled habitats, blading on ice-scoured shores), unless the new disturbances are more intensive, longlasting, or frequent (overgrazing). Some destructive human disturbances are equivalent to natural ones (rock blasting and rockfall). Some types of pollution initially at a low level may affect polar regions more than elsewhere because of stable polar air, the earth’s rotation, the magnetic poles, or the large masses of ice located there. They could cause a gradual die-off of taxa, and facilitate invasions by taxa from the surrounding areas. Simple cold-region ecosystems could be overwhelmed by smaller amounts of pollutants than might be tolerated in temperate ecosystems, but they are protected by slow chemical reactions, slow turnover, slow accumulation in sinks, long distances for pollutant dispersal, re-export of the pollutants by disturbance agents (wind, water, gravity, animals), and by organismic tolerance of environmental fluctuations. Existing adaptations may not enable the organisms to respond adequately to an increase in UV-B radiation that could destabilize polar food webs by decreasing primary production, but subtle ecosystem responses are more likely (Vincent and Roy, 1993). Rapidly reproducing organisms in exposed habitats may develop the minimum repair capacity needed to survive (Calkins and Thordardottir, 1980; Calkins, 1982; Frederick and Lubin, 1994). The impact of infrequent high-intensity point disturbances over limited areas (e.g., mining in parts of the Russian Arctic) are probably irreversible. Possibly greater irreversible damage has been done by partial non-point disturbances scattered over large areas (excessive harvesting of whales, overgrazing of high-altitude plateaus). Climatic warming or cooling related to the release of “greenhouse gases”, increase in ultraviolet radiation related to ozone layer destruction, and acid rain could also cause similar damage, but

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their demonstrated effects are limited. Cold-region organisms (fur seals, some vascular plants, lichens) and some invaders seem to be advancing slowly toward the centers of some cold regions. Rapid ecosystem response to the current climatic warming or cooling can probably be expected only where precipitation, meltwater, or ground ice conditions change rapidly. Indirectly through responses in the cold regions, organisms and ecosystems in the areas from which the stresses originated could also be affected. The most likely long-term effects of a climatic warming will be that the organisms now resident in the cold regions, and adapted to cold, will be outcompeted by more productive taxa invading from the adjacent temperate areas. There are few if any obligatory psychrophiles; but, if the rate of warming is rapid, the organisms best adapted to cold, with adaptations that cannot be changed rapidly (insulation, hibernation habits, and some migration patterns), may be the first ones to become extinct. The first ecosystems to disappear could be small alpine areas surrounded by advancing forests, and the Antarctic terrestrial ecosystems inhabiting the narrow strip of land between the oceans and glaciers; this land could be drowned before the thick glaciers retreat. Marine organisms in the Antarctic, where the Antarctic continent is in the cold center, and land organisms in the Arctic, where the Arctic Ocean occupies the same position, have few opportunities for long-term retreat. Subtle experimental manipulations of environmental factors may reveal the initial responses of organisms and ecosystems to long-term, gradual environmental change. However, long or intense experiments will only lead to changes in taxonomic composition and eventually will produce ecosystems similar to natural ecosystems controlled by the same environmental factors that are being manipulated. Such ecosystems can usually be found in the vicinity of the manipulated plots – ecosystems in neighboring warmer areas (climatic warming), on acid substrates (acid rain), on bird posts or in small mammal burrows (fertilization), or in snow-patches (prolonged duration of snow cover). Cold-region organisms and ecosystems should be protected from further human disturbances that may make them more vulnerable to slow environmental changes. Alpine regions, including the high plateaus (Tibetan Plateau), should be protected as nature sanctuaries or National Parks. Protection in the Southern

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Ocean Whale Sanctuary should be extended to all animals, because krill harvesting and fishing put both marine and terrestrial food webs in the Antarctic out of natural balance. A marine World Park in the north, both terrestrial and marine World Parks in the south (Herber, 1991, 1992), and national Arctic parks including undisturbed lands, should be established. Cold-region ecosystems should also be protected from excessive increases in the populations of indigenous animals, e.g., fur seals (Bonner, 1985), and from exotic invader organisms.

ACKNOWLEDGMENTS

The authors thank Drs. D.W. Goodall and L.R. Walker for their kind improvements of the manuscript. V.K. would like to thank all researchers who have sent reprints and bibliographies, and Drs. M.P. Hainard and M.H. Cl´emen¸con for the use of the facilities at the University of Lausanne. Her work was partly aided by U.S. National Science Foundation grants DPP-8201047, DPP-8611827, DPP-7825748, and OPP-7512949, and by a grant from the Committee on Scholarly Communication with the People’s Republic of China, and the U.S. National Academy of Sciences.

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