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Alpine Ecosystems☆
Alpine Ecosystems☆ Ch Körner, University of Basel, Basel, Switzerland r 2017 Elsevier Inc. All rights reserved.
Glossary Alpine Refers to the life zone above the climatic high-elevation treeline, irrespective of latitude. Though originating in the Alps (a pre-Roman word for high mountains), the term is applied globally. The reader should be aware that this term is often used in a much wider sense in common language, and is also applied to regions with mountains in general, including settlements and resorts, which is not the scientific meaning here. Apomixis A very common but “hidden” mode of clonal propagation by seeds, the embryos of which are 100% genetic copies of the source plant. Seeds are produced without fertilization, but often pollination is required to induce apomictic seed production. Apomictic plants also reproduce sexually, but these are very rare events. Clonal growth A vegetative mode of propagation and expansion of plants by runners, tillers, or plant fragment dispersal, which is very important in the alpine life zone (see also apomixis). Clonal plants also produce sexual offspring by seed, but their clonal propagules often show higher survival. All clonal offspring of one source plant have the same genome and hence belong to the same “genet.” Ecotype or ectotypic Refers to genetic (evolutionary) differentiation within a given species (a specific “race”) that reflects an obvious advantage in a given environment.
Ecotypic traits are retained when individuals are transplanted into a different environment where these traits have no advantage. Life-form The size and stature of a plant under natural life conditions. Environmental constraints can cause the life-form to differ substantially from that of a plant that develops under more favorable conditions, where genotype morphology, the “growth form,” finds full expression. For instance, the growth form “tree” may be modified to the life-form “shrub” in the alpine zone. Microclimate The climate that plants, small animals, and soil microbes experience, and that differs substantially from the “macroclimate” reported by weather stations. This difference is related to surface warming by the sun or cooling at night, as well as wind shelter effects, and is largely driven by relief, exposure, ground cover, and plant stature. Treeline Also known as the forest line, this describes the high-elevation limit of (usually fragmented) forest. Most often there is no “line” visible in the landscape, so treeline position represents a convention. “Outpost” tree individuals may occur at higher elevations (the tree species line), and the boundary of closed, tall forest with timber-size stems (the timber-line) commonly occurs at an elevation that is 50–150 m lower. The whole transition from timber-line to the tree species line is called the “treeline-ecotone.”
The Alpine Life Zone The lower boundary of the alpine life zone is, by definition, the natural climatic high-elevation treeline. Where a treeline is missing, as is the case in some dry continental areas or because of deforestation, the treeline-isotherm is still applied, irrespective of the presence or absence of trees. This is nothing more than a practical convention. The actual natural treeline and hence the lower end of the alpine zone do not form a sharp boundary, for patches of stunted trees and alpine plants often intermingle. This transition zone is termed the “treeline-ecotone.” The common climatological denominator of this boundary is a mean temperature during the growing season of 6.41C, with a minimum length of the growing season of 94 days (Körner, 2012; Paulsen and Körner, 2014); this definition applies worldwide (see the discussion in Körner, 2003). The length of the growing season within the alpine life zone varies from 12 months in the tropical alpine zone to merely 4–6 weeks in alpine snowbed vegetation at higher latitudes. About 4 million km2 fall into the alpine life zone (c. 2.6% of the global land area outside Antarctica). Because not all of this alpine area is covered by vegetation – some consists of bare rock, rock fields, scree, and glaciers – the vegetated alpine land area is estimated to be c. 3 million km2 (see Körner et al., 2011). Two-thirds of the global alpine area is situated in the temperate and subtropical zones and only 10% occurs in the tropics (Table 1). Partly this is because mountains in the tropics need to be very high, at least 3600–4000 m, to permit tropical alpine vegetation to occur, whereas at the polar circles mountain heights of only 600–800 m are required to support alpine habitat. These are the approximate treeline elevations of the respective climatic zones. At about 4000 m, treeline reaches its average highest elevations in the subtropics (local extremes up to 4900 m, Fig. 1). At high latitudes (465–701N), alpine vegetation merges with arctic tundra. Despite a number of common taxa and the overwhelming influence of low mean temperature, the arctic tundra life zone is very different from the alpine zone in terms of climate, land surface structure, and vegetation, hence several authors (eg, Löve and Billings) have recommended that alpine vegetation not be referred to as “alpine tundra.” The upper limits of vascular ☆
Change History: October 2015. Ch. Körner made minor edits to the text and added a few references.
Reference Module in Life Sciences
doi:10.1016/B978-0-12-809633-8.02180-4
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Alpine Ecosystems Table 1 The global area (million km2) of bioclimatic mountain belts (rugged terrain only), as defined for the Global Mountain Biodiversity Assessment (GMBA; Körner et al., 2011) Area million km2%
M%
T%
Thermal belts 1. Nival (o3.51C, GSo10 days) 2. Upper alpine (o3.51C, GS410 dayso54 days) 3. Lower alpine (o6.41C, GSo94 days)
0.53 0.75 2.27
3.24 4.53 13.74
0.40 0.56 1.68
The treeline 4. Upper montane (46.4o¼101C) 5. Lower montane (410o¼151C) 6. Remaining mountain area with frost (4151C) 7. Remaining mountain area without frost (4151C)
3.39 3.74 1.34 4.49
20.53 22.64 8.11 27.22
2.51 2.78 0.99 3.34
16.51
100.00
12.26
Total
Temperatures refer to season mean air temperatures: GS, growing season; M (%), percent of total mountain area (100% ¼ 16.5 million km2); T (%), percent of total terrestrial area outside Antarctica (100% ¼ 134.6 million km2). Total mountain area (16.51 million km2) is defined by ruggedness, ie, all terrain with at least 200 m of elevational contrast depicted among 9 cells of 3000 size in a 20 3000 pixel (4.6 km 4.6 km at the equator).
Fig. 1 The alpine life zone occurs at all latitudes, though at contrasting elevations. Reproduced from Körner, Ch., 2003. Alpine Plant Life. Heidelberg: Springer.
Table 2 The global area (million km2) of alpine belts (upper alpine belt (o3.51C, GS410do54d); lower alpine belt (o6.41C, GSo94d)) of rugged terrain only, as defined by Körner et al. (2011) Region (million km2)
All land area All rugged area Nival belt Upper alpine belt Lower alpine belt Total alpine and nival
Continent As
Eu
Af
N-A
Gld
S-A
Aus
Oce
Total
(%)
44.57 8.9 0.2 0.45 1.43 2.08
9.84 0.92 0.06 0.04 0.07 0.17
30.01 1.19 o0.01 o0.01 o0.01 o0.01
22.08 2.92 0.15 0.18 0.45 0.78
2.15 0.1 0.08 0.01 o0.01 0.1
17.82 2.16 0.03 0.07 0.31 0.41
7.7 0.13 o0.01 0 o0.01 o0.01
0.45 0.18 o0.01 o0.01 0.01 0.01
134.62 16.51 0.53 0.75 2.27 3.55
100.0 12.26 0.40 0.56 1.68 2.64
Temperatures refer to season mean air temperatures: GS, growing season; d, days; M (%), percent of total mountain area (100% ¼ 16.5 million km2); T (%), percent of total terrestrial area outside Antarctica (100%¼ 134.6 million km2). Rugged terrain as defined in Table 1. As, Asia; Eu, Europe; Af, Africa; N-A, North America; S-A, South America; GLD, Greenland; Aus, Australia and New Zealand; OCE, Oceania (including the large islands of SE Asia).
plant occurrence are commonly 1000–1500 m above the lower limits of the alpine zone (the treeline), but some extreme highelevation outposts of higher plants are found up to 4500 m in the temperate zone and up to 6400 m in the subtropics, where the uppermost individual of a higher plant was found in the Himalayas (Miehe, 1997). An important feature of alpine life is its isolation. Mountaintops with their high biological diversity represent islands or archipelagos surrounded by lowlands, where most alpine organisms cannot survive (Table 2). Various aspects of plant diversity in high mountain systems have been reviewed previously. A selection of such synthetic papers or volumes is included in the References. Full bibliographic references to original studies (mentioned in the text by authors’ names only) can be found in Körner (2003).
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Habitat Diversity, a Key to Alpine Plant Diversity Climatic Microhabitats There is a common belief that the alpine life zone is very hostile to plants and animals and that low temperatures restrict life activities, including productivity. This generalization is wrong for two reasons: (1) as will be shown, temperatures are not always that low, and (2) this belief reflects our human perspective of what is “cold.” For organisms adapted to alpine life conditions the temperatures they experience are not necessarily “cold.” In fact, if temperatures were higher, most of these organisms would suffer or even die, many of them because they would be outcompeted by other species that do better at such higher temperatures. The most important characteristic of the alpine life zone is its fragmentation into a multitude of microhabitats created by relief, exposure, and slope, which interact with solar radiation and wind to cause soil moisture, temperature, and substrate quality to vary enormously over very short distances (Scherrer and Körner, 2010). It is this diversity of microhabitats, the steepness of the terrain in particular, that makes the alpine life zone so different from arctic tundra, and that is responsible for its much greater organismic richness. Across a few meters one can easily find snowbed plants, just emerging from melting snow, and succulent plants on outcrops, which perform crassulacean acid metabolism (CAM) as do many hot desert plants. In fact, these succulents may regularly experience temperatures close to 501C on steep south slopes under full midday sun, even at high latitudes. In addition to these microclimatic determinants imposed by land surface structure, plants themselves influence their micro-environment (Fig. 2). Depending on life-form, plants may decouple themselves from ambient air conditions. Under direct insolation, compact cushion plants or prostrate dwarf shrubs have been shown to warm up to tropical temperatures. These lifeform-dependent microclimates largely disappear under thick clouds or at night, but storage of warmth in the topsoil is also strongly influenced by the type of plant cover, and so is radiative cooling during clear nights. Hence, the climate that alpine plants experience can be very different from what might be expected based on elevation alone or data measured at weather stations. Such weather data monitor the temperature, humidity, and windspeed that occur outside the calm boundary layer into which alpine plants (leaves in particular) and their animal and microbe partners are commonly nested. A widespread assumption is that alpine plants are small and prostrate because their growth is temperature limited. The reality is that, because alpine plants are small (and mostly stay small when grown at higher temperatures), they may periodically escape the cold and, at least during sunny hours, experience radiative warming, and thus are not always colder than lowland plants. This is well reflected in their thermal optima for photosynthesis, which (in the temperate zone) were shown to be similar to those of lowland plants. In contrast, night-time temperatures may be prohibitive for plant growth at alpine elevations, and thus limit structural investments of assimilates acquired during the day. Because of the strong link between plant life-form and microclimate, alpine biodiversity can be understood only if one accounts for plant structural diversity.
Diversity of Substrates Alpine microhabitats may belong to a suite of different land surface structures and soils, the 10 most important of which are common to all mountains:
• •
exposed rock terraces and rock crevices, block fields scree and mixed scree/rock slopes or flats,
Fig. 2 The diversity of plant morphologies creates diverse microclimates. Note the different temperatures in upright and prostrate leaves of two species in the Ecuadorian Andes. Reproduced from Diemer, M.D., in Körner, Ch., 2003. Alpine Plant Life. Heidelberg, Germany: Springer.
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drained ridges and plateaus, periodically wet depressions, gullies, or snowbeds, gentle slopes with relatively stable soil, steep slopes with creeping soil, flats with cryogenic structure (eg, hummocks, polygons), mires or other wet ground, and springs, water flows, or lakes.
These ten habitat types are not exhaustive. Special habitats not found everywhere include sand drifts or dunes and salt flats (in some semiarid subtropical mountains), disturbed surfaces due to animal trampling, rest places, or burrowing, avalanche tracks, and man-made landscapes such as pastures. Each of these types of habitats may be found at different exposures to sun and wind (so the number of substrate types multiplies with the number of microclimatic conditions, despite some redundancy), and each of these combinations in turn may be found on very different parent rock material such as calcareous or siliceous, mixed metamorphic, or volcanic material. Depending on wetness and elevation, plants may have converted the top layers of the substrate to an extent that it becomes chemically independent from the parent rock (eg, humic turf of pH 3.7 overlaying calcareous bedrock). Varying degrees of erosion and soil formation will then enhance the spatial heterogeneity of the substrate. The availability of soil nutrients and the extent of soil development are strongly relief driven, but plant life-forms also determine local nutrient retention. By forming dense cushions or tough tussocks, plant litter and thus the precious nutrients contained in it are prevented from being blown or washed away. When trapped beneath or strongly attached to the plant, litter can decompose and nutrients can recycle locally. Plant life-form, root and rhizome structure in particular, also influences substrate stability and soil formation (see Section Potential Functions of Alpine Biodiversity). To understand alpine biodiversity, it is important to be aware of this extensive microenvironmental patchiness of the alpine life zone, and the capacity of plants to influence their life conditions substantially. Taken together, the foregoing combinations of microenvironmental conditions yield hundreds of very specific niches, each preferred by a different combination of species.
Plant Diversity in the Alpine Life Zone Diversity of Morpho-Types Though the alpine life zone is not as rich in life-forms as a humid tropical forest, the diversity of morpho-types found here is surprisingly high. There are ten principal groups of life-forms, eight of higher plants and two of cryptogams, irrespective of whether individuals perform clonal growth. The first four groups are most important:
• • • •
low stature or prostrate woody shrubs, graminoids such as grasses and sedges, many forming tussocks, herbaceous perennials, often forming rosettes, and cushion plants of various types. Less common or of more regional importance are:
• • • •
giant rosettes of tropical mountains, geophytes, mainly confined to mountains with a pronounced seasonality, succulents, with both stem and leaf succulence, and annuals (sometimes biannuals), which become quite rare at high elevations. The remaining two life-forms are cryptogams, that is, desiccation-tolerant, nonflowering plants:
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bryophytes (“mosses”), in some areas also ferns and lycopods, and lichens (including fruticose, foliose, and crustaceous).
These life-forms, in mixtures of varying abundance of each, compose the “alpine vegetation.” In addition, algae and fungi play an important role. The diversity of plant structures is further enhanced by various modes of clonal propagation, which become increasingly important as elevation increases. The following is a short list of the diversity of clonal structures (Fig. 3):
• • • • • • • •
tussock graminoids, stoloniferous graminoids, mat- or cushion-forming forbs, stoloniferous or rhizomatous forbs, creeping dwarf shrubs, prostrate dwarf shrubs, viviparous plants, and accidential clonal plants (fragmentation by external forces).
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Fig. 3 Diversity of clonal growth in alpine plants. The examples shown here include (a) graminoid tussocks, (b) cushions with adventitious roots and potential later fragmentation, (c) mats of rhizomatous forbs or graminoids (d) mats of proliferating forbs that retain a primary root (e) stoloniferous plants, and (f) “viviparous” plants producing bulbils or floral plantlets. Reproduced from Hartmann, H., from Körner, Ch., 2003. Alpine Plant Life. Heidelberg, Germany: Springer.
These typologies do not account for plant height or degree of horizontal spreading, both of which vary considerably among species and microclimates. Overall, the diversity of plant stature in the alpine life zone is perhaps nearly as large as the taxonomic diversity (see Halloy and Mark, 1996). It is obvious that these structural features are significant functional attributes, strongly associated with microhabitat preference and microhabitat tolerance.
Diversity of Physio-Types The term physio-type is used here to circumscribe the physiological attributes of alpine plants, which may be as diverse as the variety of life-forms. Certain morphological attributes are functionally linked to physio-types. The following is only a brief summary. Readers with an interest in this field will find an extended treatment of the subject in Körner (2003). Rates of plant photosynthesis, how photo-assimilates are invested in the plant body, nutrient use, water relations, stress resistance, and secondary metabolites all vary substantially among alpine species. The predominance of physical limitations to growth, particularly at the uppermost limits where plants can grow and survive, may be expected to narrow the spectrum of possibilities, in the ultimate case by permitting only one way to survive. Surprisingly this is not so. Even in habitats that by all standards can be rated as “extreme,” one can find a suite of physio-types (often associated with specific morpho-types). This is a most important point for the study of alpine plant adaptation. The selection of a single species for study inevitably will produce data with no generalization potential. “The alpine physio-type” does not exist. One randomly selected species or small group of species may represent a very special case, and be all but “typical.” A good example is the way plants invest in biomass. Although this finds expression in morphology, the quantitative aspects of it are directly related to plant metabolism. Plants may favor roots, stems, storage organs (all three are net sinks for carbon), or leaves (the net source of carbon). How plants invest is key to the understanding of whole-plant carbon balance and to growth and reproduction. Co-occurring species, equally successful in terms of abundance and often found together in the same habitat, may represent the left and right tails of a frequency distribution of these traits at a common attitude (Fig. 4). There seems to be a multitude of ways to cope with the demands of life conditions even at extremely high elevations.
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Fig. 4 Frequency distribution of leaf mass fraction (LMF; the fraction of leaf dry matter versus total plant dry matter) in the Alps and the Andes. In the Alps, for instance, co-occurring species are found in the left (Ranunculus glacialis) and right (Cerastium uniflorum) tails, reflecting the great diversity of plant dry matter investment even at extremely high elevations. The same is true for any other mountain region, including the example for Argentina shown in the right diagram. Modified from Körner, Ch., 2003. Alpine Plant Life. Heidelberg, Germany: Springer.
Fig. 5 Frequency distribution of specific leaf area (SLA) in alpine species in the Alps and the Andes. Reproduced from Körner et al., 1989.
Another example is the way alpine plants construct their leaves in terms of leaf dry matter investment per unit leaf area. “Expensive” (thick) leaves contain a lot of dry matter per unit leaf area, whereas “cheap” (thin) leaves contain little. Again the diversity of this trait expressed as “leaf mass per area” (LMA, or its reciprocal specific leaf area, SLA) is very large. There is nothing like a typical alpine SLA (Fig. 5; for a full account of such leaf traits in a global comparison see Körner et al., 1989). The same applies to leaf nitrogen concentration and mobile carbohydrates, although there are slight overall trends with increasing elevation for SLA to decrease and nitrogen and mobile carbohydrate concentrations to increase (leaves becoming more “expensive”). However, such means across many species need to be treated with care, given the great diversity. It cannot be concluded that alpine plants have lower SLA when a substantial fraction of all studied alpine species does not fit this pattern. The species from different elevations that are included or excluded from such community subsamples will always affect the mean. The same applies to any other physiological trait that has been studied in more than one species. For example, freezing resistance varies enormously in alpine plant species. According to Larcher, some species can survive any low winter temperature that could occur on Earth (less than 701C), and others are killed by only 121C. Freezing resistance is one of the few traits where this diversity of responses could be explained by habitat characteristics, in this case the predictability of snow cover in winter. But for most other traits such clear-cut causal links have not been found. It rather seems there are many different ways to cope with similar problems. These different traits may be “nonfunctional” in the sense of not being critical for survival and reproduction under “normal” situations, but some may become decisive under very specific and rare conditions through which they might have been selected for as advantageous (see the discussion of the ecological function of diversity in Section Potential Functions of Alpine Biodiversity).
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Diversity of Reproduction Recruitment via seed is rather risky in alpine environments because the seedling needs to establish in an often short season and on a potentially hostile substrate. Hence the most common strategies for alpine plants to survive and persist are long life and clonal propagation. The production of seed itself does not seem to be a problem and alpine plants produce lots of viable seed in most years, as has been known from the beginning of alpine plant research in the 19th century (see review by Körner, 2003). For a functional understanding of alpine biodiversity, seed production is very important, as rare as successful seedling establishment might be. It is through sexual reproduction that the genetic diversity is retained and by which the ecotypic differentiation of traits, essential in the fragmented and ever-changing alpine environment, develops. The most important pollinators are flies; at lower alpine elevations, bumblebees, solitary bees, and butterflies become more prominent (with decreasing importance by order; a detailed analysis for the southern Andes was done by Arroyo). Roughly one-fourth of alpine plant species are wind pollinated (such as Poaceae, Cyperaceae, and Polygonaceae). Current evidence suggests that outbreeding is the dominant form of sexual reproduction, but most alpine plants can also self (which secures some seed production if pollen transfer fails because of bad weather), and some are even obligatory selfers. The most extreme forms of retaining the parental genome is apomixis, where embryos are genetic copies of the source plant. Surprisingly, both clones and apomicts exhibit high intraspecific genetic diversity, indicating that sexual reproduction also occurs in such plants. Through genetic fingerprinting (a recent review by De Witte and Stöcklin, 2010), it was shown that some of these obligatory clonal alpine plants can produce very large genets and be thousands of years old, but clones of different genets were often found to be intermingled. Yet clonal propagation must not be seen as a substitute to sexual propagation. Both the sexual and clonal modes occur simultaneously for most of the time. There is no indication of genetic depauperation within species with increasing elevation, but this is a field that needs more research. Current knowledge does not suggest that alpine plant diversity is limited by reproductive constraints. It is well established through classic transplantation experiments in California, the Rocky Mountains, and the Alps that ecotypic differentiation does occur among populations from different elevations, so that genotypes of the same species from high elevation differ in stature, phenology, and physiology from lower-elevation genotypes when grown in a common garden (see review by Clements et al., 1950), Hence, in addition to species diversity, discussed next, there is also a high degree of within-species diversity.
Taxonomic Diversity The total alpine flora of the globe consists of approximately 8000–10,000 species of flowering plants (Körner, 2003). These belong to about 100 (710) families and 2000 genera. Hence, one-fourth to one-third of all plant families of higher plants have representatives in the alpine life zone. Assuming a total known global flora of 250,000 species, alpine species contribute about 4% of the global plant species diversity. Given that only 2.6% of the global land area outside Antarctica falls in the alpine zone, global alpine plant diversity per unit land area is in fact higher than the global mean of all other biomes. Although such gross means need to be treated with great caution, one can at least conclude that plant species diversity in the alpine life zone is comparatively high, in view of the generally less luxurious life conditions found there. Whether high mountains are biodiversity hot spots (Barthlott et al., 1996) is a question of scale. The preceding numbers strictly refer to the alpine life zone as defined in the Section The Alpine Life Zone. If one selects a census area that includes the full spectrum of elevations (eg, a cross section through a whole mountain range), it may encompass almost all life zones on Earth from humid tropical forest at the bottom to glacier forefields at the top. High mountains, those in tropical latitudes in particular, indeed represent an incredible compression of biomes. Climatic and vegetation zones separated by several thousands of kilometers at sea level may be found within 50 km or less horizontal distance on the flanks of the major subtropical or tropical mountain systems. However, the topic of life zone compression in mountains exceeds the frame-work of this article, which is restricted to the discrete alpine part of mountain vegetation. A single mountain system such as the Rocky Mountains, the Alps, the Caucasus, the Venezuelan part of the Andes, or the mountains of New Zealand commonly has an alpine flora consisting of 600–1500 species. This needs to be considered in view of a total flora of the arctic tundra of about 1000–1500 species (depending on how sub-species are ranked). In this respect, biodiversity of the alpine life zone is outstanding. A distinct mountain region such as the Teton Range in Wyoming, the Snowy Mountains of Australia, or the central part of the Swiss Alps commonly contains roughly 200–400 alpine species, a number that is surprisingly constant across the globe (most ranges harbor around 250 species). On a single sample plot (eg, 100 m 100 m) one may find one-third of the total regional flora. An analysis of the Swiss Alps by Wohlgemut revealed that alpine plant species diversity increases with the size of the observation area up to 20 km2, but beyond that species numbers level off. Plant species diversity generally decreases with increasing elevation (Fig. 6), although there may be intermittent peaks where two elevational life zones merge. Again this is a question of scale. The decline of species diversity is particularly impressive in the uppermost part of the alpine life zone (Fig. 7). The diversity of cryptogams, mosses and lichens in particular, is relatively high in the alpine life zone. Species numbers may be of the same order of magnitude as for vascular plants, depending on humidity. With greater humidity one generally finds higher numbers of cryptogams. In some parts of temperate zone and subarctic mountains the biomass of fruticose lichens in alpine
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Fig. 6 Elevational trends in species diversity of higher plants in the Alps (after Theurillant, J.P. and Schlüssel, A., upper diagram) and bryophytes (after Geissler, P. and Velluti, C., lower diagram). Adapted from Körner, Ch., 2003. Alpine Plant Life. Heidelberg, Germany: Springer.
Fig. 7 Elevational trends in plant species diversity in the uppermost range of higher plant life in the Alps. Reproduced from Grabherr, G., from Körner, Ch., 2003. Alpine Plant Life. Heidelberg, Germany: Springer.
grassland is double that of higher plants. Since mosses and lichens are desiccation tolerant, they can occupy bare rock and scree and are among the first organisms to initiate humus accumulation on raw substrates. Although cryptogams have almost no elevation limits as long as there is some bare substrate (rock lichens are reported at 7200 m elevation in the Himalayas), the number of cryptogam species also declines with greater elevation, as is shown in Fig. 6 for bryophytes. Besides the major role of microhabitat diversity discussed earlier, there are also historical reasons for alpine plant diversity. While the lowland flora changed owing to global climatic changes, such as after the ice ages, a fraction of the then cold-adapted lowland flora migrated to high elevations and enriched the existing older stock of alpine species. This is how the edelweiss
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(Leontopodium alpinum) became a recent addition to the alpine flora of the Alps when the ice retreated. Agakhanjanz and Breckle (1995) suggested that in some mountains (eg, the Pamirs) tectonic uplift matched time constants of speciation and contributed to the autochthonous species richness. In summary, the alpine life zone is rich in plant species that belong to a multitude of morphological, physiological, and reproductive types and that inhabit a broad spectrum of microhabitats, reflecting both historical and even geological events. Because alpine plants are small, species diversity may exceed 50 species per 1 m2 of land area, which is among the highest in the world. Alpine plants also provide a varied diet of food for alpine animals and microbes, the diversity of which will be briefly touched on in the following section.
Diversity of Alpine Animals and Microorganisms Animal Diversity The great variety of animals from mites to birds, and the greater fragmentation of expertise in animal sciences, may explain why there has been no attempt at a global synthesis of animal diversity in the alpine life zone. Franz (1979), in his German book on high mountain ecology, referred to a great number of otherwise scattered observations, and Meyer and Thaler (1995) attempted a summary for invertebrates, with a focus on the Alps. The following brief account leans heavily on the latter publication. The only resident vertebrates in the uppermost part of the alpine zone (often called the nival zone) are voles, with the record finding by Halloy at c. 6000 m elevation at a geothermic vent on Sucompa volcano in the Argentinean Andes. Snow voles are very abundant and active even above 3200 m in the Alps. At these elevations birds are visitors, but only a few hundred meters lower they are resident as the plant cover becomes more regular. In the Argentinean Andes, ducks breed at 4250 m elevation (personal observation at an alpine lake in the Cumbres Calchaquies). Snakes, lizards, and frogs are reported to occur up to 1000 m above treeline. Large mammal grazers (eg, ibex in the Alps and guanacos in the southern Andes) occur at almost any alpine elevation, except for peaks surrounded by glaciers or inaccessible cliffs. These grazers profoundly influence the development of alpine plant diversity. In the lower and middle alpine belts, wild herbivores have been replaced by domestic herbivores in many areas (sheep, goats, yaks), which continue to exert selective pressure on vegetation. Some of these animals graze the highest fragments of vegetation, as was reported for yaks, which were found grazing above 5000 m in the dry part of the Himalayas (Miehe, 1997). Quantitative data for some important groups of invertebrates have been compiled by Meyer and Thaler (1995). The major losses of taxonomic groups as elevation approaches the upper limits of closed vegetation are those of earthworms, gastropods, grasshoppers, Hymenoptera, and beetles (Fig. 8), with the latter showing the greatest decline already below treeline. By contrast, flies, spiders, and springtails remain quite abundant (at 300 m above treeline, one-fifth of the total regional number of 200 springtail species were found in the Tirolian Alps). According to Swan, a salticid spider species was collected in the Himalayas at an elevation above any plant growth, possibly living on the aerial import of small arthropods from lower elevations. Only 2% of the 400 species of spiders of the central Alps of Tirol (Austria) are regularly found in the uppermost alpine zone. The more open vegetation becomes, the more invertebrate life is linked to the occurrence of compact plant forms, such as cushion plants. In their overview, Meyer and Thaler noted that among invertebrates the herbivore species numbers declined more rapidly than species numbers in higher trophic levels. The reduced species number at “extreme” habitats is often balanced by greater individual
Fig. 8 Examples of elevational trends in invertebrate species diversity in the Alps. Reproduced from Meyer, E., Thaler, K., 1995. Animal diversity at high altitudes in the Austrian Central Alps. In: Chapin III, F.S., Körner, Ch. (Eds.), Arctic and Alpine Biodiversity: Patterns, Causes and Ecosystem Consequences. Heidelberg, Germany: Springer, pp. 97–108.
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Fig. 9 Elevational trends in mycorrhizae occurrence in the Alps. Reproduced from Haselwandter, K., Read, D.L., from Körner, Ch. 2003. Alpine Plant Life. Heidelberg: Springer.
densities per species. Overall, animal species diversity in the alpine life zone follows similar elevational trends as in plant species diversity, but in terms of the total numbers of alpine species, animals may exceed plants by factors of 5–10 (this is a personal guess). Establishing such diversity ratios would be an important contribution to the understanding of alpine biodiversity in general.
Microbial Diversity As the elevation above treeline increases, animals make a smaller contribution to plant litter decomposition and soil formation and microbes become more important, even as their species numbers decline steadily. Diversity of soil fungi decreases with elevation and mycorrhization, the important plant root–fungus symbiosis, also decreases (Fig. 9). All known types of mycorrhizae occur in alpine soils: ectomycorrhizae (eg, on Salix, Dry as, Polygonum, and Kobresia spp.), ericoid mycorrhizae (Ericaceae), vesicular-arbuscular (VA) mycorrhizae (most forbs, grasses, and some sedges), and even orchid mycorrhizae. Nonmycorrhizal plant species can also be found (Gardes and Dahlberg, 1996). So-called arbuscular mycorrhizae (Glomeromycota) and darkseptate hyphae type species (belonging to Ascomycota) are found even in the highest rock and scree habitats, though at greatly reduced abundance (for references see Körner, 2003, 2011). Yet, even completely isolated plants above 4000 m in the Alps were found with intense mycorrhization (Oehl and Körner, 2014). The most robust of all organisms, the bacteria plus some unicellular fungi, retain a high diversity and abundance in the alpine life zone. Schinner and coworkers isolated 130 different strains of microorganisms in alpine environments of the western and eastern Alps that can survive and multiply at 01C. Of these, 77% were bacteria, 20% yeasts, and only 3% hyphal fungi. A very detailed analysis of bacterial diversity on Niwot Ridge, Colorado, by Mancinelli showed that Pseudomonas and Bacillus were the most abundant genera. Dinitrogen-fixing as well as nitrifying and denitrifying bacteria are abundant in alpine soils. Bacteria have no elevational limits as long as some organic dust and short spells with liquid water occur, and Swan reported a number of taxa isolated from substrate collected at 8400 m elevation on Mt. Everest, the environment on Earth that he thinks is most comparable with that of Mars. Taken together, these observations indicate that mycorrhizae are a common element of alpine plant life, being more prominent in lower alpine elevations and in infertile soils and becoming rare only with isolated plants on high mountain peaks, where soils have little carbon. Rich bacterial life is found even at the highest elevations, indicating a capacity for metabolism under the most extreme conditions.
Potential Functions of Alpine Biodiversity Among the theories that attempt to explain the functional significance of organismic diversity, the insurance theory seems to be most relevant for the alpine life zone. In simple terms it says that a species-rich, functionally partly redundant organismic “work
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force” ensures the functional integrity of ecosystems even if some of the organisms die out. A complete loss of species by extreme stress or through pathogens would be nearly irreversible in steep alpine terrain, because it is the presence of at least some species that prevents the soil from being washed away. The sustained functional integrity of alpine ecosystems is inevitably tied to the presence of soil, and this presence is inevitably dependent on the roots and rhizomes that hold it. Strong-rooted plants are the keystone elements for the preservation of the alpine ecosystem in steep terrain. A high diversity of species is commonly associated with a high diversity of rooting patterns, which in combination create the mechanical strength required to hold the soil (Pohl et al., 2009). Messerli and Ives estimated that 10% of the global human population depends directly, and 40% of the population indirectly, on mountain ecosystems, and thus the stability of their upper part, the alpine zone, is of critical significance to society (Körner and Ohsawa, 2006). Supplies of drinking and irrigation water, as well as the safety of hydroelectric schemes and transport routes, depend on intact upslope soil conditions, and alpine biodiversity helps to ensure ecosystem health. Of course alpine biodiversity also provides other things. Alpine meadows and fellfields are very attractive landscapes for human recreation, and with their high biodiversity they are of prime conservation value; in many regions they represent the last undisturbed natural areas. Because the alpine life zone is represented across the globe, it is also ideally suited for global monitoring of biological responses to atmospheric change.
Alpine Biodiversity and Global Change Global change has many facets, all related to the ever-increasing use of resources and land area because of human population growth and the increasing consumption of goods. The most severe impacts on alpine ecosystems are (1) land use practices (2) potential global warming with associated changes of snow cover and permafrost, and (3) increasing wet nitrogen deposition. Other aspects of global change, such as atmospheric CO2 enrichment by itself, increasing ultraviolet radiation due to ozone layer depletion, and air pollutants other than nitrogen loading, seem to be of minor significance on a global scale (see review by Körner, 2003). Land use, in particular the intensification of pasturing or the reverse, the abrupt abandonment of former, traditionally pastured alpine terrain, exerts the greatest influence. The steeper the terrain, the more critical these effects become. Overgrazing generally
Fig. 10 (a) Species responses to climatic warming. Mountains may be refugia (2, 4), traps leading to local exinction (3, 5), or a chance to escape climate warming by topography effects (6). Lowland species often have to move greater distances (1). (b) Upslope migration of species or niche filling? Both responses are likely with climatic warming and will alter local species abundance. Niche filling, that is, “horizontal reallocation,” will precede longer-term changes of the position of whole vegetation belts.
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diminishes plant diversity and ruins the protective plant cover within a few seasons, with self-repair of the ecosystem commonly occurring at a much slower rate than soil erosion. In contrast, moderate and well-managed grazing can increase biodiversity and create short and dense swards of vegetation that are extremely robust against erosion and increase catchment water yield. Some of the biodiversity hot spots of the Alps, the Caucasus, and the Himalayas are traditional pastures in the lower alpine belt. There is no way of maintaining these systems by “let alone” grazing strategies, because herds of domestic animals tend to crowd certain areas and ignore others. Maintaining these intact alpine grasslands is very important for at least three reasons: (1) their often very impressive biological richness, (2) their continued potential as a “clean” food source, and (3) their positive influence on water yield. The abrupt abandonment of pastures leads to an unstable transition phase that may last for a century before new, welladapted communities of species are able to return. A later reversal back to the previous biodiverse pasture-land is often impossible within a reasonable time and with affordable effort, because the biological structure and the soils of these pastures took many hundreds or thousands of years to develop, but are rapidly converted. Climate warming will affect alpine biodiversity in subtle ways because of the great variety of intermingled microhabitats (see Section Habitat Diversity, a Key to Alpine Plant Diversity). Although a number of authors documented some upslope migration of species, the more important changes possibly happen among microhabitats, with new niches filled by species and other niches abandoned (Fig. 10; Scherrer and Körner, 2010). These mosaics of life conditions represent a certain margin of safety against the loss of alpine species in a slightly warmer overall climate. However, the abundance of species will change as the abundance of their microhabitats changes. Effects on snow cover and snow duration will be more critical than temperature per se. Exceptions to these scenarios are mountains that are not high enough, in which case the current alpine biota will find no upslope escape if it gets warmer. Since alpine vegetation is well adjusted to cope with low soil nutrient levels, the regional increase of soluble nitrogen deposition will influence biodiversity. More vigorous and nitrogen-demanding species are likely to gain space over slowergrowing, smaller species. Since these more vigorous species are commonly also less resistant to stress, nitrogen deposition can increase the sensitivity of certain ecosystems, while others (eg, pioneer vegetation) may profit. It was shown that even minute additions of nitrogen fertilizer – less than is contained in many places in lowland rain-water – can create drastic changes in the alpine flora (Körner, 2003). In summary, the greatest risk of loss of biological diversity in the alpine life zone is human land use. However, land use can also contribute to the maintenance of highly diverse and stable ecosystems in the lower alpine belt if sustainable management practices are applied.
References Agakhanjanz, O., Breckle, S.W., 1995. Origin and evolution of the mountain flora in middle Asia and neighboring mountain regions. In: Chapin III, F.S., Körner, Ch. (Eds.), Arctic and Alpine Biodiversity: Patterns, Causes and Ecosystem Consequences. Heidelberg, Germany: Springer, pp. 63–80. Barthlott, W., Lauer, W., Placke, A., 1996. Global distribution of species diversity in vascular plants: Towards a world map of phytodiversity. Erdkunde 50, 317–327. Clements, F.E., Martin, E.V., Long, F.L., 1950. Adaptation and Origin in the Plant World. The Role of Environment in Evolution. Waltham, MA: The Chronica Botanica Company. De Witte, L.C., Stöcklin, J., 2010. Longevity of clonal plants: Why it matters and how to measure it. Annals of Botany 106, 859–870. Franz, H., 1979. Ökologie der Hochgebirge. Stuttgart: Verlag Eugen Ulmer. Gardes, M., Dahlberg, A., 1996. Mycorrhizal diversity in arctic and alpine tundra: An open question. New Phytologist 133, 147–157. Halloy, S.R.P., Mark, A.F., 1996. Comparative leaf morphology spectra of plant communities in New Zealand, the Andes and the European Alps. Journal of the Royal Society of New Zealand 26, 41–78. Körner, Ch., 2003. Alpine Plant Life. Heidelberg, Germany: Springer. Körner, Ch., 2011. The coldest places on earth for higher plant life. Alpine Botany 121, 11–22. Körner, Ch., 2012. Alpine Treelines. Basel: Springer. Körner, Ch., Neumayer, M., Pelaez Menendez-Riedl, S., Smeets-Scheel, A., 1989. Functional morphology of mountain plants. Flora 182, 353–383. Körner, Ch., Ohsawa, M., 2006. Mountain systems. In: Hassan, R., Scholes, R., Ash, N. (Eds.), Ecosystem and Human Well-Being: Current State and Trends, vol. 1. Washington, DC: Island Press, pp. 681–716 (Millennium Ecosystem Assessment). Körner, Ch., Paulsen, J., Spehn, E.M., 2011. A definition of mountains and their bioclimatic belts for global comparisons of biodiversity data. Alpine Botany 121, 73–78. Meyer, E., Thaler, K., 1995. Animal diversity at high altitudes in the Austrian Central Alps. In: Chapin III, F.S., Körner, Ch. (Eds.), Arctic and Alpine Biodiversity: Patterns, Causes and Ecosystem Consequences. Heidelberg, Germany: Springer, pp. 97–108. Miehe, G., 1997. Alpine vegetation types of the central Himalaya. In: Wielgolaski, F.E. (Ed.), Polar and Alpine Tundra: Ecosystems of the World 3. Amsterdam, The Netherlands: Elsevier, pp. 161–184. Oehl, F., Körner, Ch., 2014. Multiple mycorrhization at the coldest place known for Angiosperm plant life. Alpine Botany 124, 193–198. Paulsen, J., Körner, Ch., 2014. A climate-based model to predict potential treeline position around the globe. Alpine Botany 124, 1–12. Pohl, M., Alig, D., Körner, Ch., Rixen, Ch., 2009. Higher plant diversity enhances soil stability in disturbed alpine ecosystems. Plant and Soil 324, 91–102. Scherrer, D., Körner, Ch, 2010. Topography-controlled thermal-habitat differentiation buffers alpine plant diversity against climate warming. Journal of Biogeography 38, 406–416.
Glossary Alpine Refers to the life zone above the climatic high-elevation treeline, irrespective of latitude. Though originating in the Alps (a pre-Roman word for high mountains), the term is applied globally. The reader should be aware that this term is often used in a much wider sense in common language, and is also applied to regions with mountains in general, including settlements and resorts, which is not the scientific meaning here. Apomixis A very common but “hidden” mode of clonal propagation by seeds, the embryos of which are 100% genetic copies of the source plant. Seeds are produced without fertilization, but often pollination is required to induce apomictic seed production. Apomictic plants also reproduce sexually, but these are very rare events. Clonal growth A vegetative mode of propagation and expansion of plants by runners, tillers, or plant fragment dispersal, which is very important in the alpine life zone (see also apomixis). Clonal plants also produce sexual offspring by seed, but their clonal propagules often show higher survival. All clonal offspring of one source plant have the same genome and hence belong to the same “genet.” Ecotype or ectotypic Refers to genetic (evolutionary) differentiation within a given species (a specific “race”) that reflects an obvious advantage in a given environment.
Ecotypic traits are retained when individuals are transplanted into a different environment where these traits have no advantage. Life-form The size and stature of a plant under natural life conditions. Environmental constraints can cause the life-form to differ substantially from that of a plant that develops under more favorable conditions, where genotype morphology, the “growth form,” finds full expression. For instance, the growth form “tree” may be modified to the life-form “shrub” in the alpine zone. Microclimate The climate that plants, small animals, and soil microbes experience, and that differs substantially from the “macroclimate” reported by weather stations. This difference is related to surface warming by the sun or cooling at night, as well as wind shelter effects, and is largely driven by relief, exposure, ground cover, and plant stature. Treeline Also known as the forest line, this describes the high-elevation limit of (usually fragmented) forest. Most often there is no “line” visible in the landscape, so treeline position represents a convention. “Outpost” tree individuals may occur at higher elevations (the tree species line), and the boundary of closed, tall forest with timber-size stems (the timber-line) commonly occurs at an elevation that is 50–150 m lower. The whole transition from timber-line to the tree species line is called the “treeline-ecotone.”
The Alpine Life Zone The lower boundary of the alpine life zone is, by definition, the natural climatic high-elevation treeline. Where a treeline is missing, as is the case in some dry continental areas or because of deforestation, the treeline-isotherm is still applied, irrespective of the presence or absence of trees. This is nothing more than a practical convention. The actual natural treeline and hence the lower end of the alpine zone do not form a sharp boundary, for patches of stunted trees and alpine plants often intermingle. This transition zone is termed the “treeline-ecotone.” The common climatological denominator of this boundary is a mean temperature during the growing season of 6.41C, with a minimum length of the growing season of 94 days (Körner, 2012; Paulsen and Körner, 2014); this definition applies worldwide (see the discussion in Körner, 2003). The length of the growing season within the alpine life zone varies from 12 months in the tropical alpine zone to merely 4–6 weeks in alpine snowbed vegetation at higher latitudes. About 4 million km2 fall into the alpine life zone (c. 2.6% of the global land area outside Antarctica). Because not all of this alpine area is covered by vegetation – some consists of bare rock, rock fields, scree, and glaciers – the vegetated alpine land area is estimated to be c. 3 million km2 (see Körner et al., 2011). Two-thirds of the global alpine area is situated in the temperate and subtropical zones and only 10% occurs in the tropics (Table 1). Partly this is because mountains in the tropics need to be very high, at least 3600–4000 m, to permit tropical alpine vegetation to occur, whereas at the polar circles mountain heights of only 600–800 m are required to support alpine habitat. These are the approximate treeline elevations of the respective climatic zones. At about 4000 m, treeline reaches its average highest elevations in the subtropics (local extremes up to 4900 m, Fig. 1). At high latitudes (465–701N), alpine vegetation merges with arctic tundra. Despite a number of common taxa and the overwhelming influence of low mean temperature, the arctic tundra life zone is very different from the alpine zone in terms of climate, land surface structure, and vegetation, hence several authors (eg, Löve and Billings) have recommended that alpine vegetation not be referred to as “alpine tundra.” The upper limits of vascular ☆
Change History: October 2015. Ch. Körner made minor edits to the text and added a few references.
Reference Module in Life Sciences
doi:10.1016/B978-0-12-809633-8.02180-4
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Alpine Ecosystems Table 1 The global area (million km2) of bioclimatic mountain belts (rugged terrain only), as defined for the Global Mountain Biodiversity Assessment (GMBA; Körner et al., 2011) Area million km2%
M%
T%
Thermal belts 1. Nival (o3.51C, GSo10 days) 2. Upper alpine (o3.51C, GS410 dayso54 days) 3. Lower alpine (o6.41C, GSo94 days)
0.53 0.75 2.27
3.24 4.53 13.74
0.40 0.56 1.68
The treeline 4. Upper montane (46.4o¼101C) 5. Lower montane (410o¼151C) 6. Remaining mountain area with frost (4151C) 7. Remaining mountain area without frost (4151C)
3.39 3.74 1.34 4.49
20.53 22.64 8.11 27.22
2.51 2.78 0.99 3.34
16.51
100.00
12.26
Total
Temperatures refer to season mean air temperatures: GS, growing season; M (%), percent of total mountain area (100% ¼ 16.5 million km2); T (%), percent of total terrestrial area outside Antarctica (100% ¼ 134.6 million km2). Total mountain area (16.51 million km2) is defined by ruggedness, ie, all terrain with at least 200 m of elevational contrast depicted among 9 cells of 3000 size in a 20 3000 pixel (4.6 km 4.6 km at the equator).
Fig. 1 The alpine life zone occurs at all latitudes, though at contrasting elevations. Reproduced from Körner, Ch., 2003. Alpine Plant Life. Heidelberg: Springer.
Table 2 The global area (million km2) of alpine belts (upper alpine belt (o3.51C, GS410do54d); lower alpine belt (o6.41C, GSo94d)) of rugged terrain only, as defined by Körner et al. (2011) Region (million km2)
All land area All rugged area Nival belt Upper alpine belt Lower alpine belt Total alpine and nival
Continent As
Eu
Af
N-A
Gld
S-A
Aus
Oce
Total
(%)
44.57 8.9 0.2 0.45 1.43 2.08
9.84 0.92 0.06 0.04 0.07 0.17
30.01 1.19 o0.01 o0.01 o0.01 o0.01
22.08 2.92 0.15 0.18 0.45 0.78
2.15 0.1 0.08 0.01 o0.01 0.1
17.82 2.16 0.03 0.07 0.31 0.41
7.7 0.13 o0.01 0 o0.01 o0.01
0.45 0.18 o0.01 o0.01 0.01 0.01
134.62 16.51 0.53 0.75 2.27 3.55
100.0 12.26 0.40 0.56 1.68 2.64
Temperatures refer to season mean air temperatures: GS, growing season; d, days; M (%), percent of total mountain area (100% ¼ 16.5 million km2); T (%), percent of total terrestrial area outside Antarctica (100%¼ 134.6 million km2). Rugged terrain as defined in Table 1. As, Asia; Eu, Europe; Af, Africa; N-A, North America; S-A, South America; GLD, Greenland; Aus, Australia and New Zealand; OCE, Oceania (including the large islands of SE Asia).
plant occurrence are commonly 1000–1500 m above the lower limits of the alpine zone (the treeline), but some extreme highelevation outposts of higher plants are found up to 4500 m in the temperate zone and up to 6400 m in the subtropics, where the uppermost individual of a higher plant was found in the Himalayas (Miehe, 1997). An important feature of alpine life is its isolation. Mountaintops with their high biological diversity represent islands or archipelagos surrounded by lowlands, where most alpine organisms cannot survive (Table 2). Various aspects of plant diversity in high mountain systems have been reviewed previously. A selection of such synthetic papers or volumes is included in the References. Full bibliographic references to original studies (mentioned in the text by authors’ names only) can be found in Körner (2003).
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Habitat Diversity, a Key to Alpine Plant Diversity Climatic Microhabitats There is a common belief that the alpine life zone is very hostile to plants and animals and that low temperatures restrict life activities, including productivity. This generalization is wrong for two reasons: (1) as will be shown, temperatures are not always that low, and (2) this belief reflects our human perspective of what is “cold.” For organisms adapted to alpine life conditions the temperatures they experience are not necessarily “cold.” In fact, if temperatures were higher, most of these organisms would suffer or even die, many of them because they would be outcompeted by other species that do better at such higher temperatures. The most important characteristic of the alpine life zone is its fragmentation into a multitude of microhabitats created by relief, exposure, and slope, which interact with solar radiation and wind to cause soil moisture, temperature, and substrate quality to vary enormously over very short distances (Scherrer and Körner, 2010). It is this diversity of microhabitats, the steepness of the terrain in particular, that makes the alpine life zone so different from arctic tundra, and that is responsible for its much greater organismic richness. Across a few meters one can easily find snowbed plants, just emerging from melting snow, and succulent plants on outcrops, which perform crassulacean acid metabolism (CAM) as do many hot desert plants. In fact, these succulents may regularly experience temperatures close to 501C on steep south slopes under full midday sun, even at high latitudes. In addition to these microclimatic determinants imposed by land surface structure, plants themselves influence their micro-environment (Fig. 2). Depending on life-form, plants may decouple themselves from ambient air conditions. Under direct insolation, compact cushion plants or prostrate dwarf shrubs have been shown to warm up to tropical temperatures. These lifeform-dependent microclimates largely disappear under thick clouds or at night, but storage of warmth in the topsoil is also strongly influenced by the type of plant cover, and so is radiative cooling during clear nights. Hence, the climate that alpine plants experience can be very different from what might be expected based on elevation alone or data measured at weather stations. Such weather data monitor the temperature, humidity, and windspeed that occur outside the calm boundary layer into which alpine plants (leaves in particular) and their animal and microbe partners are commonly nested. A widespread assumption is that alpine plants are small and prostrate because their growth is temperature limited. The reality is that, because alpine plants are small (and mostly stay small when grown at higher temperatures), they may periodically escape the cold and, at least during sunny hours, experience radiative warming, and thus are not always colder than lowland plants. This is well reflected in their thermal optima for photosynthesis, which (in the temperate zone) were shown to be similar to those of lowland plants. In contrast, night-time temperatures may be prohibitive for plant growth at alpine elevations, and thus limit structural investments of assimilates acquired during the day. Because of the strong link between plant life-form and microclimate, alpine biodiversity can be understood only if one accounts for plant structural diversity.
Diversity of Substrates Alpine microhabitats may belong to a suite of different land surface structures and soils, the 10 most important of which are common to all mountains:
• •
exposed rock terraces and rock crevices, block fields scree and mixed scree/rock slopes or flats,
Fig. 2 The diversity of plant morphologies creates diverse microclimates. Note the different temperatures in upright and prostrate leaves of two species in the Ecuadorian Andes. Reproduced from Diemer, M.D., in Körner, Ch., 2003. Alpine Plant Life. Heidelberg, Germany: Springer.
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drained ridges and plateaus, periodically wet depressions, gullies, or snowbeds, gentle slopes with relatively stable soil, steep slopes with creeping soil, flats with cryogenic structure (eg, hummocks, polygons), mires or other wet ground, and springs, water flows, or lakes.
These ten habitat types are not exhaustive. Special habitats not found everywhere include sand drifts or dunes and salt flats (in some semiarid subtropical mountains), disturbed surfaces due to animal trampling, rest places, or burrowing, avalanche tracks, and man-made landscapes such as pastures. Each of these types of habitats may be found at different exposures to sun and wind (so the number of substrate types multiplies with the number of microclimatic conditions, despite some redundancy), and each of these combinations in turn may be found on very different parent rock material such as calcareous or siliceous, mixed metamorphic, or volcanic material. Depending on wetness and elevation, plants may have converted the top layers of the substrate to an extent that it becomes chemically independent from the parent rock (eg, humic turf of pH 3.7 overlaying calcareous bedrock). Varying degrees of erosion and soil formation will then enhance the spatial heterogeneity of the substrate. The availability of soil nutrients and the extent of soil development are strongly relief driven, but plant life-forms also determine local nutrient retention. By forming dense cushions or tough tussocks, plant litter and thus the precious nutrients contained in it are prevented from being blown or washed away. When trapped beneath or strongly attached to the plant, litter can decompose and nutrients can recycle locally. Plant life-form, root and rhizome structure in particular, also influences substrate stability and soil formation (see Section Potential Functions of Alpine Biodiversity). To understand alpine biodiversity, it is important to be aware of this extensive microenvironmental patchiness of the alpine life zone, and the capacity of plants to influence their life conditions substantially. Taken together, the foregoing combinations of microenvironmental conditions yield hundreds of very specific niches, each preferred by a different combination of species.
Plant Diversity in the Alpine Life Zone Diversity of Morpho-Types Though the alpine life zone is not as rich in life-forms as a humid tropical forest, the diversity of morpho-types found here is surprisingly high. There are ten principal groups of life-forms, eight of higher plants and two of cryptogams, irrespective of whether individuals perform clonal growth. The first four groups are most important:
• • • •
low stature or prostrate woody shrubs, graminoids such as grasses and sedges, many forming tussocks, herbaceous perennials, often forming rosettes, and cushion plants of various types. Less common or of more regional importance are:
• • • •
giant rosettes of tropical mountains, geophytes, mainly confined to mountains with a pronounced seasonality, succulents, with both stem and leaf succulence, and annuals (sometimes biannuals), which become quite rare at high elevations. The remaining two life-forms are cryptogams, that is, desiccation-tolerant, nonflowering plants:
• •
bryophytes (“mosses”), in some areas also ferns and lycopods, and lichens (including fruticose, foliose, and crustaceous).
These life-forms, in mixtures of varying abundance of each, compose the “alpine vegetation.” In addition, algae and fungi play an important role. The diversity of plant structures is further enhanced by various modes of clonal propagation, which become increasingly important as elevation increases. The following is a short list of the diversity of clonal structures (Fig. 3):
• • • • • • • •
tussock graminoids, stoloniferous graminoids, mat- or cushion-forming forbs, stoloniferous or rhizomatous forbs, creeping dwarf shrubs, prostrate dwarf shrubs, viviparous plants, and accidential clonal plants (fragmentation by external forces).
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Fig. 3 Diversity of clonal growth in alpine plants. The examples shown here include (a) graminoid tussocks, (b) cushions with adventitious roots and potential later fragmentation, (c) mats of rhizomatous forbs or graminoids (d) mats of proliferating forbs that retain a primary root (e) stoloniferous plants, and (f) “viviparous” plants producing bulbils or floral plantlets. Reproduced from Hartmann, H., from Körner, Ch., 2003. Alpine Plant Life. Heidelberg, Germany: Springer.
These typologies do not account for plant height or degree of horizontal spreading, both of which vary considerably among species and microclimates. Overall, the diversity of plant stature in the alpine life zone is perhaps nearly as large as the taxonomic diversity (see Halloy and Mark, 1996). It is obvious that these structural features are significant functional attributes, strongly associated with microhabitat preference and microhabitat tolerance.
Diversity of Physio-Types The term physio-type is used here to circumscribe the physiological attributes of alpine plants, which may be as diverse as the variety of life-forms. Certain morphological attributes are functionally linked to physio-types. The following is only a brief summary. Readers with an interest in this field will find an extended treatment of the subject in Körner (2003). Rates of plant photosynthesis, how photo-assimilates are invested in the plant body, nutrient use, water relations, stress resistance, and secondary metabolites all vary substantially among alpine species. The predominance of physical limitations to growth, particularly at the uppermost limits where plants can grow and survive, may be expected to narrow the spectrum of possibilities, in the ultimate case by permitting only one way to survive. Surprisingly this is not so. Even in habitats that by all standards can be rated as “extreme,” one can find a suite of physio-types (often associated with specific morpho-types). This is a most important point for the study of alpine plant adaptation. The selection of a single species for study inevitably will produce data with no generalization potential. “The alpine physio-type” does not exist. One randomly selected species or small group of species may represent a very special case, and be all but “typical.” A good example is the way plants invest in biomass. Although this finds expression in morphology, the quantitative aspects of it are directly related to plant metabolism. Plants may favor roots, stems, storage organs (all three are net sinks for carbon), or leaves (the net source of carbon). How plants invest is key to the understanding of whole-plant carbon balance and to growth and reproduction. Co-occurring species, equally successful in terms of abundance and often found together in the same habitat, may represent the left and right tails of a frequency distribution of these traits at a common attitude (Fig. 4). There seems to be a multitude of ways to cope with the demands of life conditions even at extremely high elevations.
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Fig. 4 Frequency distribution of leaf mass fraction (LMF; the fraction of leaf dry matter versus total plant dry matter) in the Alps and the Andes. In the Alps, for instance, co-occurring species are found in the left (Ranunculus glacialis) and right (Cerastium uniflorum) tails, reflecting the great diversity of plant dry matter investment even at extremely high elevations. The same is true for any other mountain region, including the example for Argentina shown in the right diagram. Modified from Körner, Ch., 2003. Alpine Plant Life. Heidelberg, Germany: Springer.
Fig. 5 Frequency distribution of specific leaf area (SLA) in alpine species in the Alps and the Andes. Reproduced from Körner et al., 1989.
Another example is the way alpine plants construct their leaves in terms of leaf dry matter investment per unit leaf area. “Expensive” (thick) leaves contain a lot of dry matter per unit leaf area, whereas “cheap” (thin) leaves contain little. Again the diversity of this trait expressed as “leaf mass per area” (LMA, or its reciprocal specific leaf area, SLA) is very large. There is nothing like a typical alpine SLA (Fig. 5; for a full account of such leaf traits in a global comparison see Körner et al., 1989). The same applies to leaf nitrogen concentration and mobile carbohydrates, although there are slight overall trends with increasing elevation for SLA to decrease and nitrogen and mobile carbohydrate concentrations to increase (leaves becoming more “expensive”). However, such means across many species need to be treated with care, given the great diversity. It cannot be concluded that alpine plants have lower SLA when a substantial fraction of all studied alpine species does not fit this pattern. The species from different elevations that are included or excluded from such community subsamples will always affect the mean. The same applies to any other physiological trait that has been studied in more than one species. For example, freezing resistance varies enormously in alpine plant species. According to Larcher, some species can survive any low winter temperature that could occur on Earth (less than 701C), and others are killed by only 121C. Freezing resistance is one of the few traits where this diversity of responses could be explained by habitat characteristics, in this case the predictability of snow cover in winter. But for most other traits such clear-cut causal links have not been found. It rather seems there are many different ways to cope with similar problems. These different traits may be “nonfunctional” in the sense of not being critical for survival and reproduction under “normal” situations, but some may become decisive under very specific and rare conditions through which they might have been selected for as advantageous (see the discussion of the ecological function of diversity in Section Potential Functions of Alpine Biodiversity).
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Diversity of Reproduction Recruitment via seed is rather risky in alpine environments because the seedling needs to establish in an often short season and on a potentially hostile substrate. Hence the most common strategies for alpine plants to survive and persist are long life and clonal propagation. The production of seed itself does not seem to be a problem and alpine plants produce lots of viable seed in most years, as has been known from the beginning of alpine plant research in the 19th century (see review by Körner, 2003). For a functional understanding of alpine biodiversity, seed production is very important, as rare as successful seedling establishment might be. It is through sexual reproduction that the genetic diversity is retained and by which the ecotypic differentiation of traits, essential in the fragmented and ever-changing alpine environment, develops. The most important pollinators are flies; at lower alpine elevations, bumblebees, solitary bees, and butterflies become more prominent (with decreasing importance by order; a detailed analysis for the southern Andes was done by Arroyo). Roughly one-fourth of alpine plant species are wind pollinated (such as Poaceae, Cyperaceae, and Polygonaceae). Current evidence suggests that outbreeding is the dominant form of sexual reproduction, but most alpine plants can also self (which secures some seed production if pollen transfer fails because of bad weather), and some are even obligatory selfers. The most extreme forms of retaining the parental genome is apomixis, where embryos are genetic copies of the source plant. Surprisingly, both clones and apomicts exhibit high intraspecific genetic diversity, indicating that sexual reproduction also occurs in such plants. Through genetic fingerprinting (a recent review by De Witte and Stöcklin, 2010), it was shown that some of these obligatory clonal alpine plants can produce very large genets and be thousands of years old, but clones of different genets were often found to be intermingled. Yet clonal propagation must not be seen as a substitute to sexual propagation. Both the sexual and clonal modes occur simultaneously for most of the time. There is no indication of genetic depauperation within species with increasing elevation, but this is a field that needs more research. Current knowledge does not suggest that alpine plant diversity is limited by reproductive constraints. It is well established through classic transplantation experiments in California, the Rocky Mountains, and the Alps that ecotypic differentiation does occur among populations from different elevations, so that genotypes of the same species from high elevation differ in stature, phenology, and physiology from lower-elevation genotypes when grown in a common garden (see review by Clements et al., 1950), Hence, in addition to species diversity, discussed next, there is also a high degree of within-species diversity.
Taxonomic Diversity The total alpine flora of the globe consists of approximately 8000–10,000 species of flowering plants (Körner, 2003). These belong to about 100 (710) families and 2000 genera. Hence, one-fourth to one-third of all plant families of higher plants have representatives in the alpine life zone. Assuming a total known global flora of 250,000 species, alpine species contribute about 4% of the global plant species diversity. Given that only 2.6% of the global land area outside Antarctica falls in the alpine zone, global alpine plant diversity per unit land area is in fact higher than the global mean of all other biomes. Although such gross means need to be treated with great caution, one can at least conclude that plant species diversity in the alpine life zone is comparatively high, in view of the generally less luxurious life conditions found there. Whether high mountains are biodiversity hot spots (Barthlott et al., 1996) is a question of scale. The preceding numbers strictly refer to the alpine life zone as defined in the Section The Alpine Life Zone. If one selects a census area that includes the full spectrum of elevations (eg, a cross section through a whole mountain range), it may encompass almost all life zones on Earth from humid tropical forest at the bottom to glacier forefields at the top. High mountains, those in tropical latitudes in particular, indeed represent an incredible compression of biomes. Climatic and vegetation zones separated by several thousands of kilometers at sea level may be found within 50 km or less horizontal distance on the flanks of the major subtropical or tropical mountain systems. However, the topic of life zone compression in mountains exceeds the frame-work of this article, which is restricted to the discrete alpine part of mountain vegetation. A single mountain system such as the Rocky Mountains, the Alps, the Caucasus, the Venezuelan part of the Andes, or the mountains of New Zealand commonly has an alpine flora consisting of 600–1500 species. This needs to be considered in view of a total flora of the arctic tundra of about 1000–1500 species (depending on how sub-species are ranked). In this respect, biodiversity of the alpine life zone is outstanding. A distinct mountain region such as the Teton Range in Wyoming, the Snowy Mountains of Australia, or the central part of the Swiss Alps commonly contains roughly 200–400 alpine species, a number that is surprisingly constant across the globe (most ranges harbor around 250 species). On a single sample plot (eg, 100 m 100 m) one may find one-third of the total regional flora. An analysis of the Swiss Alps by Wohlgemut revealed that alpine plant species diversity increases with the size of the observation area up to 20 km2, but beyond that species numbers level off. Plant species diversity generally decreases with increasing elevation (Fig. 6), although there may be intermittent peaks where two elevational life zones merge. Again this is a question of scale. The decline of species diversity is particularly impressive in the uppermost part of the alpine life zone (Fig. 7). The diversity of cryptogams, mosses and lichens in particular, is relatively high in the alpine life zone. Species numbers may be of the same order of magnitude as for vascular plants, depending on humidity. With greater humidity one generally finds higher numbers of cryptogams. In some parts of temperate zone and subarctic mountains the biomass of fruticose lichens in alpine
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Fig. 6 Elevational trends in species diversity of higher plants in the Alps (after Theurillant, J.P. and Schlüssel, A., upper diagram) and bryophytes (after Geissler, P. and Velluti, C., lower diagram). Adapted from Körner, Ch., 2003. Alpine Plant Life. Heidelberg, Germany: Springer.
Fig. 7 Elevational trends in plant species diversity in the uppermost range of higher plant life in the Alps. Reproduced from Grabherr, G., from Körner, Ch., 2003. Alpine Plant Life. Heidelberg, Germany: Springer.
grassland is double that of higher plants. Since mosses and lichens are desiccation tolerant, they can occupy bare rock and scree and are among the first organisms to initiate humus accumulation on raw substrates. Although cryptogams have almost no elevation limits as long as there is some bare substrate (rock lichens are reported at 7200 m elevation in the Himalayas), the number of cryptogam species also declines with greater elevation, as is shown in Fig. 6 for bryophytes. Besides the major role of microhabitat diversity discussed earlier, there are also historical reasons for alpine plant diversity. While the lowland flora changed owing to global climatic changes, such as after the ice ages, a fraction of the then cold-adapted lowland flora migrated to high elevations and enriched the existing older stock of alpine species. This is how the edelweiss
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(Leontopodium alpinum) became a recent addition to the alpine flora of the Alps when the ice retreated. Agakhanjanz and Breckle (1995) suggested that in some mountains (eg, the Pamirs) tectonic uplift matched time constants of speciation and contributed to the autochthonous species richness. In summary, the alpine life zone is rich in plant species that belong to a multitude of morphological, physiological, and reproductive types and that inhabit a broad spectrum of microhabitats, reflecting both historical and even geological events. Because alpine plants are small, species diversity may exceed 50 species per 1 m2 of land area, which is among the highest in the world. Alpine plants also provide a varied diet of food for alpine animals and microbes, the diversity of which will be briefly touched on in the following section.
Diversity of Alpine Animals and Microorganisms Animal Diversity The great variety of animals from mites to birds, and the greater fragmentation of expertise in animal sciences, may explain why there has been no attempt at a global synthesis of animal diversity in the alpine life zone. Franz (1979), in his German book on high mountain ecology, referred to a great number of otherwise scattered observations, and Meyer and Thaler (1995) attempted a summary for invertebrates, with a focus on the Alps. The following brief account leans heavily on the latter publication. The only resident vertebrates in the uppermost part of the alpine zone (often called the nival zone) are voles, with the record finding by Halloy at c. 6000 m elevation at a geothermic vent on Sucompa volcano in the Argentinean Andes. Snow voles are very abundant and active even above 3200 m in the Alps. At these elevations birds are visitors, but only a few hundred meters lower they are resident as the plant cover becomes more regular. In the Argentinean Andes, ducks breed at 4250 m elevation (personal observation at an alpine lake in the Cumbres Calchaquies). Snakes, lizards, and frogs are reported to occur up to 1000 m above treeline. Large mammal grazers (eg, ibex in the Alps and guanacos in the southern Andes) occur at almost any alpine elevation, except for peaks surrounded by glaciers or inaccessible cliffs. These grazers profoundly influence the development of alpine plant diversity. In the lower and middle alpine belts, wild herbivores have been replaced by domestic herbivores in many areas (sheep, goats, yaks), which continue to exert selective pressure on vegetation. Some of these animals graze the highest fragments of vegetation, as was reported for yaks, which were found grazing above 5000 m in the dry part of the Himalayas (Miehe, 1997). Quantitative data for some important groups of invertebrates have been compiled by Meyer and Thaler (1995). The major losses of taxonomic groups as elevation approaches the upper limits of closed vegetation are those of earthworms, gastropods, grasshoppers, Hymenoptera, and beetles (Fig. 8), with the latter showing the greatest decline already below treeline. By contrast, flies, spiders, and springtails remain quite abundant (at 300 m above treeline, one-fifth of the total regional number of 200 springtail species were found in the Tirolian Alps). According to Swan, a salticid spider species was collected in the Himalayas at an elevation above any plant growth, possibly living on the aerial import of small arthropods from lower elevations. Only 2% of the 400 species of spiders of the central Alps of Tirol (Austria) are regularly found in the uppermost alpine zone. The more open vegetation becomes, the more invertebrate life is linked to the occurrence of compact plant forms, such as cushion plants. In their overview, Meyer and Thaler noted that among invertebrates the herbivore species numbers declined more rapidly than species numbers in higher trophic levels. The reduced species number at “extreme” habitats is often balanced by greater individual
Fig. 8 Examples of elevational trends in invertebrate species diversity in the Alps. Reproduced from Meyer, E., Thaler, K., 1995. Animal diversity at high altitudes in the Austrian Central Alps. In: Chapin III, F.S., Körner, Ch. (Eds.), Arctic and Alpine Biodiversity: Patterns, Causes and Ecosystem Consequences. Heidelberg, Germany: Springer, pp. 97–108.
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Fig. 9 Elevational trends in mycorrhizae occurrence in the Alps. Reproduced from Haselwandter, K., Read, D.L., from Körner, Ch. 2003. Alpine Plant Life. Heidelberg: Springer.
densities per species. Overall, animal species diversity in the alpine life zone follows similar elevational trends as in plant species diversity, but in terms of the total numbers of alpine species, animals may exceed plants by factors of 5–10 (this is a personal guess). Establishing such diversity ratios would be an important contribution to the understanding of alpine biodiversity in general.
Microbial Diversity As the elevation above treeline increases, animals make a smaller contribution to plant litter decomposition and soil formation and microbes become more important, even as their species numbers decline steadily. Diversity of soil fungi decreases with elevation and mycorrhization, the important plant root–fungus symbiosis, also decreases (Fig. 9). All known types of mycorrhizae occur in alpine soils: ectomycorrhizae (eg, on Salix, Dry as, Polygonum, and Kobresia spp.), ericoid mycorrhizae (Ericaceae), vesicular-arbuscular (VA) mycorrhizae (most forbs, grasses, and some sedges), and even orchid mycorrhizae. Nonmycorrhizal plant species can also be found (Gardes and Dahlberg, 1996). So-called arbuscular mycorrhizae (Glomeromycota) and darkseptate hyphae type species (belonging to Ascomycota) are found even in the highest rock and scree habitats, though at greatly reduced abundance (for references see Körner, 2003, 2011). Yet, even completely isolated plants above 4000 m in the Alps were found with intense mycorrhization (Oehl and Körner, 2014). The most robust of all organisms, the bacteria plus some unicellular fungi, retain a high diversity and abundance in the alpine life zone. Schinner and coworkers isolated 130 different strains of microorganisms in alpine environments of the western and eastern Alps that can survive and multiply at 01C. Of these, 77% were bacteria, 20% yeasts, and only 3% hyphal fungi. A very detailed analysis of bacterial diversity on Niwot Ridge, Colorado, by Mancinelli showed that Pseudomonas and Bacillus were the most abundant genera. Dinitrogen-fixing as well as nitrifying and denitrifying bacteria are abundant in alpine soils. Bacteria have no elevational limits as long as some organic dust and short spells with liquid water occur, and Swan reported a number of taxa isolated from substrate collected at 8400 m elevation on Mt. Everest, the environment on Earth that he thinks is most comparable with that of Mars. Taken together, these observations indicate that mycorrhizae are a common element of alpine plant life, being more prominent in lower alpine elevations and in infertile soils and becoming rare only with isolated plants on high mountain peaks, where soils have little carbon. Rich bacterial life is found even at the highest elevations, indicating a capacity for metabolism under the most extreme conditions.
Potential Functions of Alpine Biodiversity Among the theories that attempt to explain the functional significance of organismic diversity, the insurance theory seems to be most relevant for the alpine life zone. In simple terms it says that a species-rich, functionally partly redundant organismic “work
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force” ensures the functional integrity of ecosystems even if some of the organisms die out. A complete loss of species by extreme stress or through pathogens would be nearly irreversible in steep alpine terrain, because it is the presence of at least some species that prevents the soil from being washed away. The sustained functional integrity of alpine ecosystems is inevitably tied to the presence of soil, and this presence is inevitably dependent on the roots and rhizomes that hold it. Strong-rooted plants are the keystone elements for the preservation of the alpine ecosystem in steep terrain. A high diversity of species is commonly associated with a high diversity of rooting patterns, which in combination create the mechanical strength required to hold the soil (Pohl et al., 2009). Messerli and Ives estimated that 10% of the global human population depends directly, and 40% of the population indirectly, on mountain ecosystems, and thus the stability of their upper part, the alpine zone, is of critical significance to society (Körner and Ohsawa, 2006). Supplies of drinking and irrigation water, as well as the safety of hydroelectric schemes and transport routes, depend on intact upslope soil conditions, and alpine biodiversity helps to ensure ecosystem health. Of course alpine biodiversity also provides other things. Alpine meadows and fellfields are very attractive landscapes for human recreation, and with their high biodiversity they are of prime conservation value; in many regions they represent the last undisturbed natural areas. Because the alpine life zone is represented across the globe, it is also ideally suited for global monitoring of biological responses to atmospheric change.
Alpine Biodiversity and Global Change Global change has many facets, all related to the ever-increasing use of resources and land area because of human population growth and the increasing consumption of goods. The most severe impacts on alpine ecosystems are (1) land use practices (2) potential global warming with associated changes of snow cover and permafrost, and (3) increasing wet nitrogen deposition. Other aspects of global change, such as atmospheric CO2 enrichment by itself, increasing ultraviolet radiation due to ozone layer depletion, and air pollutants other than nitrogen loading, seem to be of minor significance on a global scale (see review by Körner, 2003). Land use, in particular the intensification of pasturing or the reverse, the abrupt abandonment of former, traditionally pastured alpine terrain, exerts the greatest influence. The steeper the terrain, the more critical these effects become. Overgrazing generally
Fig. 10 (a) Species responses to climatic warming. Mountains may be refugia (2, 4), traps leading to local exinction (3, 5), or a chance to escape climate warming by topography effects (6). Lowland species often have to move greater distances (1). (b) Upslope migration of species or niche filling? Both responses are likely with climatic warming and will alter local species abundance. Niche filling, that is, “horizontal reallocation,” will precede longer-term changes of the position of whole vegetation belts.
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diminishes plant diversity and ruins the protective plant cover within a few seasons, with self-repair of the ecosystem commonly occurring at a much slower rate than soil erosion. In contrast, moderate and well-managed grazing can increase biodiversity and create short and dense swards of vegetation that are extremely robust against erosion and increase catchment water yield. Some of the biodiversity hot spots of the Alps, the Caucasus, and the Himalayas are traditional pastures in the lower alpine belt. There is no way of maintaining these systems by “let alone” grazing strategies, because herds of domestic animals tend to crowd certain areas and ignore others. Maintaining these intact alpine grasslands is very important for at least three reasons: (1) their often very impressive biological richness, (2) their continued potential as a “clean” food source, and (3) their positive influence on water yield. The abrupt abandonment of pastures leads to an unstable transition phase that may last for a century before new, welladapted communities of species are able to return. A later reversal back to the previous biodiverse pasture-land is often impossible within a reasonable time and with affordable effort, because the biological structure and the soils of these pastures took many hundreds or thousands of years to develop, but are rapidly converted. Climate warming will affect alpine biodiversity in subtle ways because of the great variety of intermingled microhabitats (see Section Habitat Diversity, a Key to Alpine Plant Diversity). Although a number of authors documented some upslope migration of species, the more important changes possibly happen among microhabitats, with new niches filled by species and other niches abandoned (Fig. 10; Scherrer and Körner, 2010). These mosaics of life conditions represent a certain margin of safety against the loss of alpine species in a slightly warmer overall climate. However, the abundance of species will change as the abundance of their microhabitats changes. Effects on snow cover and snow duration will be more critical than temperature per se. Exceptions to these scenarios are mountains that are not high enough, in which case the current alpine biota will find no upslope escape if it gets warmer. Since alpine vegetation is well adjusted to cope with low soil nutrient levels, the regional increase of soluble nitrogen deposition will influence biodiversity. More vigorous and nitrogen-demanding species are likely to gain space over slowergrowing, smaller species. Since these more vigorous species are commonly also less resistant to stress, nitrogen deposition can increase the sensitivity of certain ecosystems, while others (eg, pioneer vegetation) may profit. It was shown that even minute additions of nitrogen fertilizer – less than is contained in many places in lowland rain-water – can create drastic changes in the alpine flora (Körner, 2003). In summary, the greatest risk of loss of biological diversity in the alpine life zone is human land use. However, land use can also contribute to the maintenance of highly diverse and stable ecosystems in the lower alpine belt if sustainable management practices are applied.
References Agakhanjanz, O., Breckle, S.W., 1995. Origin and evolution of the mountain flora in middle Asia and neighboring mountain regions. In: Chapin III, F.S., Körner, Ch. (Eds.), Arctic and Alpine Biodiversity: Patterns, Causes and Ecosystem Consequences. Heidelberg, Germany: Springer, pp. 63–80. Barthlott, W., Lauer, W., Placke, A., 1996. Global distribution of species diversity in vascular plants: Towards a world map of phytodiversity. Erdkunde 50, 317–327. Clements, F.E., Martin, E.V., Long, F.L., 1950. Adaptation and Origin in the Plant World. The Role of Environment in Evolution. Waltham, MA: The Chronica Botanica Company. De Witte, L.C., Stöcklin, J., 2010. Longevity of clonal plants: Why it matters and how to measure it. Annals of Botany 106, 859–870. Franz, H., 1979. Ökologie der Hochgebirge. Stuttgart: Verlag Eugen Ulmer. Gardes, M., Dahlberg, A., 1996. Mycorrhizal diversity in arctic and alpine tundra: An open question. New Phytologist 133, 147–157. Halloy, S.R.P., Mark, A.F., 1996. Comparative leaf morphology spectra of plant communities in New Zealand, the Andes and the European Alps. Journal of the Royal Society of New Zealand 26, 41–78. Körner, Ch., 2003. Alpine Plant Life. Heidelberg, Germany: Springer. Körner, Ch., 2011. The coldest places on earth for higher plant life. Alpine Botany 121, 11–22. Körner, Ch., 2012. Alpine Treelines. Basel: Springer. Körner, Ch., Neumayer, M., Pelaez Menendez-Riedl, S., Smeets-Scheel, A., 1989. Functional morphology of mountain plants. Flora 182, 353–383. Körner, Ch., Ohsawa, M., 2006. Mountain systems. In: Hassan, R., Scholes, R., Ash, N. (Eds.), Ecosystem and Human Well-Being: Current State and Trends, vol. 1. Washington, DC: Island Press, pp. 681–716 (Millennium Ecosystem Assessment). Körner, Ch., Paulsen, J., Spehn, E.M., 2011. A definition of mountains and their bioclimatic belts for global comparisons of biodiversity data. Alpine Botany 121, 73–78. Meyer, E., Thaler, K., 1995. Animal diversity at high altitudes in the Austrian Central Alps. In: Chapin III, F.S., Körner, Ch. (Eds.), Arctic and Alpine Biodiversity: Patterns, Causes and Ecosystem Consequences. Heidelberg, Germany: Springer, pp. 97–108. Miehe, G., 1997. Alpine vegetation types of the central Himalaya. In: Wielgolaski, F.E. (Ed.), Polar and Alpine Tundra: Ecosystems of the World 3. Amsterdam, The Netherlands: Elsevier, pp. 161–184. Oehl, F., Körner, Ch., 2014. Multiple mycorrhization at the coldest place known for Angiosperm plant life. Alpine Botany 124, 193–198. Paulsen, J., Körner, Ch., 2014. A climate-based model to predict potential treeline position around the globe. Alpine Botany 124, 1–12. Pohl, M., Alig, D., Körner, Ch., Rixen, Ch., 2009. Higher plant diversity enhances soil stability in disturbed alpine ecosystems. Plant and Soil 324, 91–102. Scherrer, D., Körner, Ch, 2010. Topography-controlled thermal-habitat differentiation buffers alpine plant diversity against climate warming. Journal of Biogeography 38, 406–416.