Aspects Of Highaltitude Habitats

High-altitude environments vary geographically, meteorologically, geologically and also historically. Despite this heterogeneity, they nevertheless have certain features in common. Mountain summits are generally the coldest habitats in their particular region, prone to erosion (Fig. 10.9), with poor soils and widely fluctuating day and night temperatures as well as being exposed to strong winds and high UV radiation, and in the temperate, boreal and arctic zones a short growing period. The problem of adjusting to the constant alternation of freezing nights followed with intense sunlight by day demands a greater physiological tolerance of temperature extremes than is normal in most flowering plants.

The atmosphere at high altitudes has a low water vapour content and this aridity factor has clearly imposed a selection for drought-resistant foliage in high mountain species. The low temperature and the stone covered soils of high mountains nevertheless protect the soil moisture from evaporation, which is a compensating factor for the dry atmospheric conditions (see also Chapter 3).

Whether or not high-altitude locations help plants avoid the dangers of herbivory is still an open question. The plants which have evolved under the limiting climatic and edaphic conditions on mountain tops share the characteristics that are associated with stress tolerance, namely slow growth, extended longevity, resource limitation and low palatability to herbivores. Under Grime's C-S-R theory (Grime, 2001) it would be expected that repeated biomass removals by herbivores would be a threat to plant persistence in alpine environments (see Section 3.4.2). Testing this hypothesis on populations of the alpine buttercup (Ranunculus glacialis) along an altitudinal transect in the Central Alps of Austria found that between 15% and 26% of the R. glacialis plants in each population showed signs of grazing injury primarily by snow voles (Microtus nivalis) (Diemer, 1996). Only a small population, isolated by glaciers, at the highest site (3310 m) showed no traces of herbivory. Quantitative assessment of the extent of the herbivory at two high-altitude sites (2600 m and 3180 m) showed that there was considerable damage: on average nearly 25% of a plant's total leaf area was removed in one year as well as 65-85% of all flowering plants having their inflorescences removed. Despite the magnitude of these losses neither reproductive investment nor the number of leaves initiated per plant changed appreciably in the subsequent year.

It has been claimed that as R. glacialis populations and other similar grazed species (e.g. Oxyria digyna) are able to support populations of herbivores at the alti-tudinal limits for plant growth, without obvious reductions in vigour, then these plants cannot be considered as fitting the stress-tolerator, competitor and ruderal scheme proposed by Grime (see Section 3.4.2). The 'Grime theory' asserts that having to endure both extreme stress and disturbance is not compatible with survival (Diemer, 1996). However, this objection can be countered by the fact that glacier buttercups and similar high-altitude species are not stressed in their high-altitude sites. There is even the possibility that burrowing and grazing by small microtines can actually enhance plant diversity at high-altitude sites by increasing sites for regeneration.

In the high-altitude rangelands of the TransHimalaya, pastoralists consider that small mammals act as competitors for their livestock, causing rangeland degradation, and in many places actively eradicate a substantial portion of the vegetation. A study which investigated the effects of small herbivores like pikas (e.g. Ochotona princeps, the American pika) and voles (Microtus spp.) found that soil disturbance due to small mammals was associated with higher plant diversity without causing any dramatic decline in overall vegetation cover (Bagchi et al., 2006).

Many high-altitude species are clonal and this can provide stability with access to reserves in high-altitude grasslands (Erschbaner et al., 2003). It would appear therefore that high-altitude sites are not a refuge from herbivory and that despite the low growth of the high-altitude plants they nevertheless possess sufficient

reserves to recover even from frequent grazing and can even benefit regeneration in these harsh exposed landscapes from the disturbance caused by burrowing herbivores.

Physical disturbance on mountains varies with the geology. The mass wasting of the habitat increases with the more friable sedimentary rocks and decreases where the rock is igneous. Disturbance from erosion, however, can have benefits as well as disadvantages for plant survival. Although erosion destroys vegetation it also provides fresh sites and nutrients, which promote colonization and growth. A sharp contrast in this effect can be seen when comparing mountain vegetation on the hard mountain rocks of the Caledonian system as seen in Norway, Scotland, and Newfoundland with the softer sedimentary rocks of the Southern Alps of New Zealand (Fig. 10.9). In the hard granites and metamorphic Caledonian rocks the soils of the upper mountain slopes are leached and nutrient poor, while the lower slopes are flushed and enriched with nutrients washed down from above. By contrast, in the hyperoceanic climate of New Zealand, the fast-weathering friable rocks provide available nutrients in the upper regions of the mountains. However, in contrast to the example of the hard Caledonian rocks the high rainfall and rapid erosion rates experienced in New Zealand rapidly wash out fine soils and nutrient from the valley bottoms leaving only gravel-filled river basins (Fig. 10.10; see also Fig. 11.30).

10.3.1 Geology and mountain floras

The very marked influence of geology on mountain vegetation has been noted ever since the beginnings of the systematic study of alpine Botany. In 1749 a thesis by H. F. Link was presented to the University of Gottingen entitled Goettingensis specimen sistens veget-abilis saxo calcareopropria, which described, probably for the first time, the difference between the flora of calcareous and siliceous rocks (Walter, 1960). Initially, this ecological distinction was interpreted as due to the physical rather than the chemical differences between the rocks. Calcareous rocks weather to produce soils that are warm and dry while soils based on siliceous rocks are colder and wetter; a distinction that is an important discriminatory feature in most mountain floras. More specifically plants differ in their reaction to soil chemistry as affected by pH. In acid soils aluminium (Al3+) becomes increasingly soluble below pH 5. Species that live in acid soils are termed calcifuges and are able to sequester or exclude the potentially phytotoxic aluminium ions, together with iron and manganese in their soluble reduced state as ferrous and manganous ions.

Calcicole plants that live in soils where the pH values are high (> pH 7.0) escape the potential toxicity of Al3+, Fe2+, Mn2+ as these are only soluble in low pH soils. As calcicoles lack the chelating characteristics of the calcifuge species they are unable to survive in acid soils. The adaptations appear to be mutually exclusive. The highly efficient means for excluding iron and

Fig. 10.10 Upper waters of the Waimakariri River, South Island, New Zealand, illustrating extensive deposition of gravel eroding from fragile mountain ranges in the Southern Alps.

Table 10.1. Examples of European alpine flowering plants which exist as contrasting vicarious species (calcicoles and calcifuges) occurring respectively on basic and acidic mountain soils

Calcicole

Achillea atrata Carex curvula ssp. rosae Eritrichium nanum ssp. jankkae Gentiana acaulis ssp. clusii Hutchinsia alpina Minuartia verna Primula auricula Ranunculus alpestris Rhododendron hirsutum Salix retusa Saxifraga aizoon Saxifraga oppositifolia Sedum atratum Sesleria caerulea Silene uniflora ssp. petraea Soldanella alpina

Calcifuge

A. moschata C. curvula ssp. curvula

E. nanum

G. acaulis ssp. kochiana

H. brevicaulis M. sedoides P. hirsuta

Ranunculus glacialis Rhododendron ferrugineum Salix herbacea Saxifraga cotyledon Saxifraga rupestris Sedum montanum Sesleria disticha Silene rupestris

Soldanella pusilla

Sources: various, including Walter (1960); Reisigl & Keller (1994); Jermyn (2005).

Fig. 10.11 Rhododendron ferrugineum, the calcifuge alpine rose. Also called the rusty alpine rose due to the iron-laden hairs in the momentum on the abaxial leaf surface (see inset). The removal ofiron to the outside ofthe leafpresumably serves as an effective method for avoiding excessive accumulations of toxic ferrous iron within the leaf.

Fig. 10.11 Rhododendron ferrugineum, the calcifuge alpine rose. Also called the rusty alpine rose due to the iron-laden hairs in the momentum on the abaxial leaf surface (see inset). The removal ofiron to the outside ofthe leafpresumably serves as an effective method for avoiding excessive accumulations of toxic ferrous iron within the leaf.

manganous ions in calcifuge species prevents them physiologically from accessing the necessary quantities of these ions. This together with the effects of high bicarbonate causes their foliage to become chlorotic -the condition generally referred to as lime-induced chlorosis (Marschner, 1995).

Many closely related species and subspecies differ genetically as to their preferences for calcareous or siliceous soils. This is particularly noticeable in mountains such as the Alps (Table 10.1) where the relationship between the underlying geology and the soils is not masked by deep soil development or overlying peat or glacial deposits.

One of the best known examples is the different forms ofalpine rhododendron, the 'alpine roses', where Rhododendron ferrugineum occurs on acid soils while the hairy alpine rose (R. hirsutum) is confined to calcareous soils. Even specific sites such as snow patches differ in the species present depending on whether the underlying rocks are calcareous and siliceous (Figs. 10.11-10.12).

By contrast, in the very highest mountains, as in the Himalaya, geology appears to be less important than altitude, precipitation, and aspect. It is possible to pass from one rock formation to another without having any noticeable change in the vegetation. Thus the long-leaved pine (Pinus roxburghii) grows on acid or basic rocks without any apparent discrimination (Polunin & Stainton, 1984).

The basis for the differences between the floras of acidic and calcareous soils can be due in part to the very real physical divergence between calcareous and siliceous rocks in terms of their physical properties, as well as to the chemical properties of the soil.

Fig. 10.12 Rhododendron hirsutum, the calcicole alpine rose. (Photo Dr A. Gerlach.)

Calcareous soils are generally warm and dry and siliceous soils are cool and wet. This together with the different reactions to both soil chemistry and plant physiology in terms of pH, calcium and bicarbonate ions causes a pronounced distinction in plant distribution. This is particularly noticeable in mountain floras where altitudinal temperature limitations differentiate sharply between early and late sites for the resumption of growth in spring. Consequently, there is a marked phenological dimension to the advantages and disadvantages of warm dry calcareous soils as opposed to the generally cooler and wetter environments associated with siliceous soils.

10.3.2 Adiabatic lapse rate

The cooling effect of increasing altitude is influenced by the adiabatic lapse rate, which is the negative vertical gradient of temperature maintained by the vertical motion of air through surroundings in hydrostatic equilibrium (Calow, 1998). When the air is unsaturated it is known as the dry adiabatic lapse rate and has a value of 9.8 °C km , provided the moving and ambient air are at nearly the same temperature (which is usually the case). If, however, saturation is maintained by the condensation of water vapour in rising air, then the release of latent heat reduces the adiabatic lapse rate (Fig. 10.13).

In the warm low troposphere (the lowermost portion of the Earth's atmosphere and the one in which most weather phenomena occur), the saturated adia-batic lapse rate may be as little as half the dry adiabatic lapse rate which prevails in the cold high troposphere (Calow, 1998). A global mean temperature lapse rate of 5.6 °C km-1 has been reported for mountains (Körner, 2003). However, where there are strong temperature inversions, as along the edges of the major ice sheets in Greenland, values as high as 12 °C km-1 have been recorded. Mixing of different air masses can modify the rate of temperature change with altitude. Oceanic regions of western Norway and Scotland frequently experience weather dominated by polar-maritime air. This air mass, which has been chilled by the Greenland ice cap, will be warmed in its lower levels as it passes over the North Atlantic Drift. Nevertheless, the upper air mass still remains cold. The mixture of low level warm air and high level cold air brings to these lands bordering the eastern shores of the North Atlantic a more rapid fall in temperature with altitude than would be normally predicted. In western Scotland, a decrease in temperature of 8-9 °C km-1 of altitude is not uncommon.

Ecologically, these different adiabatic rates have a profound effect on the zonation of mountain vegetation. In the hyperoceanic conditions of the Northern and Western Isles of Scotland the montane vegetation zonation is compressed due to the high adiabatic rate. It is possible in the Orkney Islands (59° N) to stand in montane tundra type vegetation on a summit at only 420 m, and view below crops of barley growing at sea level (Fig. 10.14).

10.3.3 Mountain topography and biodiversity

The potential floristic diversity of any particular mountain can often be predicted at a distance from its shape, geology and rock structure. Despite what might appear daunting prospects for plant colonization it is frequently surprising how many high mountains support a rich

Fig. 10.13 Adiabatic lapse rate as a function of altitude. (Based on Strahler, 1963, pp. 246—252.)

Temperature °C

Fig. 10.13 Adiabatic lapse rate as a function of altitude. (Based on Strahler, 1963, pp. 246—252.)

alpine flora. Slopes, screes and terraces may look unstable but they can provide, depending on aspect and geological structure, varied and favourable habitats that are adequate for a range of suitably adapted species. The general roughness of mountain topography also makes available abundant microsites where localized environmental conditions can provide temperature regimes that can counteract the effects of altitude and latitude.

Mountain chains and their orientation are also very important biogeographically in providing scope for migration, particularly during periods of significant climate change. A north-south orientation is generally considered the most favourable for alterations in temperature. However east-west orientations as in the

European Alps can accommodate changes from oceanic to continental climates. Not surprisingly it is often possible to predict the diversity of vegetation on many high mountains depending on whether the mountains can be described as concave or convex. Concave mountains provide reservoirs for soil, water, and nutrients, while convex mountains tend to be deficient in these resources. Consequently descriptors of relief curvature and roughness explain more of the variability in species distribution than 'classical' terrain attributes, such as elevation or exposure (Gottfried et al., 1998).

The negative features of being an isolated mountain are to some extent counteracted when the mountain is both large and high. Conversely, on small mountains,

Fig. 10.14 View from Ward Hill, Hoy, Orkney (59° N) illustrating the rapid change in climate and vegetation with increasing altitude in oceanic climates. Note the tundra vegetation in the foreground and active agriculture in practice only 305 m (1000 feet) below.

the bioclimatic vegetation zonation is compressed (low, middle and high alpine, see Section 10.5.1) as compared with larger mountains.

German-speaking ecologists refer to this phenomenon as Massenerhebung or the Massenerhebung effekt (mass elevation effect), where the size of the mountain can ameliorate the physical environment. The effect is most noticeable where large mountains are massed together. In the centre isotherm levels rise and create a more continental climate, usually with reduced cloud cover, which causes the nival zone (permanent snow)

level to retreat to a higher altitude. The uppermost alpine vegetation can therefore be found at higher altitudes on large mountains than on small mountains. The front ranges, however, do not differ greatly from smaller mountains.

As a consequence of this Massenerhebung effect there is also a greater production of seed at high altitudes, which provides a larger quantity of propagules to blow and wash down the mountainside and this in turn enhances the presence of alpine plants at lower elevations (Ellenberg, 1963).

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