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<ul><li><p>Biol. Rev. (1968), 43, p p . 481-529 </p><p>THE ECOLOGY OF ARCTIC AND ALPINE PLANTS </p><p>BY W. D. BILLINGS AND H. A. MOONEY Duke University, Durham, North Carolina, and Stanford University, </p><p>Stanford, California, U.S.A. </p><p>(Received I 5 May I 968) </p><p>CONTENTS </p><p>I. Introduction . . . . . . . . . . . . 481 484 </p><p>(I) Present and past distributions of tundras . . . . . . 484 (2) Environmental characteristics . . . . . . . . 485 (3) Vegetational characteristics . . . . . . . . 489 </p><p>11. Geographic extent and general characteristics of arctic and alpine vegetation </p><p>111. Adaptations of plants to arctic and alpine environments . . , . (2) Physiological ecology of the life-cycle in arctic and alpine vascular plants </p><p>Seedling establishment . . . . . . . . . </p><p>( I ) Life forms and general morphology . . . . . . . Seed dormancy and germination . . . . . . . Chlorophylls and other pigments . . . . . . . Photosynthesis and respiration . . . . . . . . . . . . . Annual cycle of growth . . . . . . . . . </p><p>IV. Primary productivity . . . . . . . . . . . Effects of water availability and use </p><p>V. Summary . . . . . . . . . . . . . 522 VI. References. . . . . . . . . . . . . 524 </p><p>I. INTRODUCTION </p><p>Among the earth's terrestrial environments, none has less biologically usable heat or has fewer kinds of adapted plants than the tundras and barrens above and beyond the alpine and arctic timberlines. Here, plant phenotypes are environmentally selected by a climatic severity unknown in the mild, moist tropical environments where vascular plants originated and remain in such variety today. Nor does so severe a selection operate in temperate regions where warm summers help to compensate for cold winters by supplying a season of tropical heat to the metabolic activities of plants emerging from dormancy. Only a few kinds of phenotypes have passed this low-temperature screening successfully and have added their gene-enzyme systems to the floras and vegetations of polar and alpine regions. It is the purpose of this review to bring together in brief form the principal facts and theories concerning plant adaptations to these cold environments. </p><p>In short space, we cannot improve on the detailed coverage of certain aspects of plant adaptation to arctic or alpine conditions provided by Holm (1922), Schroeter (1926), Sarensen (1941), Pisek (1960), Tikhomirov (1963), Bliss (1962b), Tranquillini (1964), and others. Here, we shall attempt to bring such information up to date and to discuss the problems of physiological adaptation to low-temperature environments. </p></li><li><p>482 W. D. BILLINGS AND H. A. MOONEY In attacking this problem, it is appropriate to ask What can we learn from studying </p><p>arctic and alpine plants that cannot be learned from other kinds of plants? Before answering this question, we must define our terms. </p><p>We shall define an arctic plant as one growing beyond the arctic timberline or belonging to a species whose main distribution is beyond this timberline. An alpine plant has the same relationships with the alpine timberline. Many arctic or alpine species occur also in meadows or open areas below timberline. Because of this fact and because timberlines are often broken, indistinct, or even lacking, timberline is only a rough boundary between the severity of a tundra environment and the relative protection of a forest or a subalpine meadow. </p><p>Timberline is a relatively reliable guide in the Northern Hemisphere but it is much less so in the southern part of the world. For example, in the alpine regions of New Zealand the Nothofagus timberline is at too low an elevation to indicate the real boundary between alpine and subalpine conditions. The situation is carried to an extreme in such places as the western slopes of the Andes in central Chile where there is no montane forest, thus no timberline, and the lower edge of the alpine zone is left solely to the ecologists judgement. In the last analysis, what constitutes arctic or alpine conditions always is a matter of judgement. However, the effect of a forest on microclimate rules out the radiation conditions of an open tundra, and thus the timberline boundary has some reality. (See Brockmann-Jerosch (1919) for a more complete discussion of timberline and its climatic relationships.) </p><p>In both arctic and alpine situations there are environmental gradients, genetic clines, and vegetational continua which cut across any sort of arbitrary boundary. It is thus impossible to delineate arctic conditions from subarctic, alpine from sub- alpine, in an absolute manner. Arctic and alpine conditions are primarily a matter of degree. However, such gradients and clines are usually steepest at timberline; thus, timberline provides us with an approximate and reasonably acceptable lower limit to arctic and alpine conditions. Unfortunately, this may not be true for much of the Southern Hemisphere for a variety of environmental and genetic reasons which result in a low timberline or none at all. While we shall be concerned primarily with true arctic and alpine plants in this review, we shall not exclude information derived from subarctic, subalpine, or subantarctic plants if this can be helpful in understanding the adaptive ecology of plants of cold regions. </p><p>While some plant species are endemic to the Arctic, and a great many more are endemic to the alpine regions of one mountain range or another, a relatively large group of common tundra species are widespread and occur in both arctic and alpine locations. Such taxa are commonly called arctic-alpine species. They provide impor- tant links between the arctic flora and the numerous alpine floras; an understanding of their adaptive mechanisms can provide answers to questions concerning evolution within and migrations between these similar, but different, low-temperature environ- ments. Among the more widespread of these arctic-alpine species are Trisetum spicatum,* Oxyria digyna and Silene acaulis, which are almost cosmopolitan in arctic and alpine regions of the Northern Hemisphere. Trisetum spicatum and a few other </p><p>* Nomenclature follows Polunin (1959) in so far as the species are included by him. </p></li><li><p>The ecology of arctic and alpine plants 483 species of arctic-alpine vascular plants also occur in the mountains of the cold- temperate parts of the Southern Hemisphere. They are, thus, ' bipolar arctic-alpine '. A number of species of lichens and mosses are also bipolar arctic-alpine. </p><p>Terrestrial plants of arctic and alpine regions are mainly flowering plants (Angio- sperms), bryophytes, and lichens; ferns are also represented but with fewer species. Almost all the Angiosperms are herbaceous perennials or very low shrubs ; annuals are very rare. The perennial herbs are of four principal life-forms: cushion or polster plants, rosette plants, leafy-stemmed plants, and grass-form plants. </p><p>All tundra plants, woody or herbaceous, show rapid shoot growth after melting of the snow-cover in spring or early summer. Such extremely rapid growth, in a matter of a week or two, is one of the unique characteristics of such vegetation. The energy and materials for this fast shoot growth are supplied by carbohydrates and lipids stored in roots, rhizomes, or bulbs. Tundra plants also have the ability to metabolize and to reproduce at low growing-season temperatures which are not far above the freezing mark. Under such conditions, sexual reproduction is often replaced by apomixis, vivipary, or various kinds of vegetative reproduction. All polar and alpine Angio- sperms, except for the few small and delicate annuals, have these characteristics in common. Additionally, many species, especially those of exposed, windy habitats, also can withstand extremely low temperatures and desiccation during the dormant season. In an already severe regional environment, windy ridge crests which are snow-free in winter provide one extreme in local severity, while the late-melting snowbank is the cause of an environmental severity of a quite different sort-the very short growing season. Angiosperms adapted to either extreme are relatively few; some lichens are well-adapted to the wind-swept environments and a few moss species are abundant in some but not all late snowbank sites. Really extreme situations of both types are without any plants. </p><p>The uniqueness of polar and alpine plants lies in the fact that they are the only plants adapted to metabolizing, growing, and reproducing at low temperatures. Many other kinds of plants from the forests, grasslands, and cold deserts of the middle latitudes can tolerate and survive extremely low temperatures during the dormant season but require higher temperatures for growth and development than do the plants of arctic and alpine environments. In subarctic or subalpine environments, such higher tempera- ture requirements are met by higher daytime temperatures since subalpine nights are often colder than alpine nights because of cold-air drainage. Similarly, subarctic nights often are colder than those of the Arctic because the sun goes below the horizon for several hours in contrast to the continuous daylight of higher latitudes. Higher daytime temperatures are closely allied with photosynthetic processes and thus it is daytime temperature that marks the real boundary between true arctic or alpine tundra and subarctic or subalpine meadows. </p><p>To answer our earlier question: by studying arctic and alpine plants we can hope to learn how this relative handful of species in the world's flora has succeeded not only in surviving low temperatures during dormancy but in manufacturing relativeIy large amounts of food at low temperatures in very short periods of time. We can get additional dividends by applying the question to alpine plants in particular. Alpine </p></li><li><p>484 W. D. BILLINGS AND H. A. MOONEY plants are subject not only to the rigours of low temperatures day and night but also to the other environmental stresses inherent in high altitudes: low partial pressures of oxygen and carbon dioxide, strong winds, and intense solar radiation including ultra- violet. The answers we get to these questions of low-temperature adaptation of populations and ecosystems will be worth the quest not only scientifically but also from the economic standpoint of food production in cold climates. </p><p>11. GEOGRAPHIC EXTENT AND GENERAL CHARACTERISTICS OF ARCTIC AND ALPINE VEGETATION </p><p>( I ) Present and past distribution of tundras Because of the moderating influence of the Arctic Ocean, tundra vegetation occurs </p><p>as far north as Peary Land on the northern tip of Greenland, reaching its limit with Saxifrage oppositifoliaat KapMorris Jesup in latitude 83" 39' N. (Holmen, 1957). South of this extreme, tundra extends around the seaward edges of the Greenland ice-cap and in a circumpolar band across northern Eurasia. Wherever mountain ranges enter the Arctic from the south, as in Alaska, Scandinavia, and the U.S.S.R., arctic tundras merge almost imperceptibly with alpine tundras and mountain meadows. Farther south, Northern Hemisphere types of alpine tundra and fell-fields exist in many mountain ranges of the middle latitudes and reach almost to the tropics in Nepal and south-western China. </p><p>Along the equator, in the high mountains of Africa and South America, are alpine regions very different from those farther north in environments, floras, and vegetation. The small alpine areas of the Southern Hemisphere in the southern Andes, New Zealand, and Australia superficially resemble those of the North in environments and in the life forms of the plants but, except for a few bipolar species, have a flora of quite different derivation. Subantarctic tundra occurs on some of the oceanic islands south of 55' S. and, sparsely, even on the shores of Palmer Peninsula in Antarctica to 64' S. </p><p>Good (1964) provides some figures on the approximate area of land covered by arctic or alpine vegetation. By far the largest amount is in the Northern Hemisphere: almost 9.1 million square miles compared to less than 0.5 million square miles in the Southern Hemisphere. In the Northern Hemisphere about 60 yo of this vegetation is north of 60" N. and can be considered as arctic or subarctic. Northern Hemisphere alpine areas are relatively small, isolated, and floristically diverse as compared with the circumboreal belt of arctic tundra. Conversely, in the Southern Hemisphere almost all of the tundra vegetation is alpine, with less than 0.1 yo being south of 60" S. In the Arctic, the land is peripheral to the Arctic Ocean, which moderates the climate and, in summer allows thawing and temperatures high enough for plant growth. In the Antarctic the high continent is central and the ocean peripheral; the result is ice- covered land with little or no chance for plant establishment. </p><p>The present distribution of tundra ecosystems with the greater area occurring at high latitudes in a circumpolar zone has been characteristic only of the warm inter- glacials of the Pleistocene and perhaps also of the later Tertiary. During much of the Pleistocene, tundra climates and biota have migrated back and forth with the cycles of </p></li><li><p>The ecology of arctic and alpine plants 485 continental and alpine glaciations. While little is known of the distribution of tundra during full-glacial times, continental ice-sheets and alpine glaciers covered much of the tundras present range. In North America and Europe, except for the unglaciated parts of Alaska and nunataks in the Scandinavian mountains and in the Canadian Cordillera, arctic plants could survive only south of the ice-front. Alpine plants grew on the lower mountain slopes, in the lowlands, and probably in mountain ranges such as the southern Appalachians, which are completely forest-covered today. In many places there must have been considerable mingling of the arctic and alpine floras. The resultant tundra in middle-latitude North America may have been only a relatively narrow zone between ice and forest. However, because much of Eurasia was both unglaciated and very cold, full-glacial tundra there could have covered a very large area, particularly in Russia and Siberia. An unusual situation existed in north-western Alaska, which was unglaciated and separated at full-glacial from the rest of the North American continent by 2000 miles of ice. Here, a large sample of North American tundra vegetation remained in the Arctic during the height of glaciation and for many thousands of years had better migrational connexions with the Eurasian tundra than with the remnants of the American tundra, which had retreated far to the south. The effects of this isolation are still evident in tundra floras and species structure in western and north-western North America. </p><p>( 2 ) Environmental characteristics Arctic and alpine tundra ecosystems may be described briefly as treeless areas beyond </p><p>timberline characterized by cool or cold summers and occupied by low herbaceous or shrubby vegetation, often with extensive mats of lichen...</p></li></ul>


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