10.4.1 Water availability at high altitudes
Water retention at high altitudes is highly dependent on topography and soil development, as well as on exposure and the seasonal distribution of precipitation. In common with other skeletal soils, such as sand dunes and deserts (see Section 7.1.2) the top surface is prone to desiccation and plants in these areas are dependent on being able to extract water from the deeper soil layers which are frequently absent in mountain habitats. The so-called dry calcareous scree-slopes on the southern flanks of the Alps have been shown to be well supplied with water as the coarse substrate prevents evaporation and traps moisture.
In tropical mountains soils above the cloud zone are progressively less well supplied with water with increasing altitude (see the current situation on Mt Kilimanjaro, Section 10.5.2). Fortunately, in cooler, high-altitude temperature regimes, despite the lower atmospheric pressure increasing evaporation, the ratio of evaporation/precipitation (E/P ratio) is reduced due to the lower temperatures with a consequent reduction of the risk of drought injury. Calculations based on predicted water needs in a temperate alpine zone, assuming a growing season length of 100 days and supposing a maximum soil depth of one metre, suggest that in this type of habitat there is no need for additional rainfall during the growing season, provided that the soil pore space is fully saturated in spring, which is usually the case immediately after snowmelt (Körner, 2003).
Not all mountains are endowed with a deep soil, or even with a starting point of total water saturation at the beginning of the growing season. Wind exposure, particularly on mountain ridges, not only increases water loss, it also denudes exposed ridges and slopes of soil particles and reduces soil fertility. Drought can also be a permanent feature of many high-altitude sites. When mountains are subject to warm dry catabatic winds (fohn, chinook, mistral, etc.) the lower slopes will suffer additional desiccation as the descending winds warm as they fall. Fohn, or valley wind, as it descends on the northern side of Mt Everest has a desiccating effect in the valleys of the north slope, creating an extreme asymmetry in the altitudinal vegetation belts between the south and north slopes, with heavy rainfall on the former and extensive drought on the latter (Miehe, 1989). Also, in tropical and subtropical mountains, at altitudes that are above the main cloud zone, desert-style conditions can prevail for many months of the year.
Areas where snow accumulates and lies late into spring, although potentially limiting the length of the growing season, can nevertheless provide several advantages for plant survival and many mountain-plant species are known to be characteristic of snow patches. One of the principal compensations of snow patches for both plants and soils is the protection that is provided from winter winds, ablation and nutrient leaching. However, snow shelter on melting also ameliorates the water supply. The nature of the relief on the mountainside that determines where snow lies is usually coincidental with natural drainage patterns so that the same areas that gather snow in winter also benefit from summer precipitation. Much of the benefit that is derived by alpine plants from snow patches comes from the topography of the snow-patch area. Snow-accumulating hollows favour flushing rather than leaching, and therefore provide a natural reservoir for both water and nutrients. In addition, snow also traps dust and nutrients, which then settle in the patch as the snow melts. The same topography that gathers snow will also collect erosion sediments and as a result snow patches frequently have a greater soil depth with more favourable water and nutrient supplies than the more exposed parts of the mountain. In the modern world snow banks also accumulate air-borne pollutants and when they thaw release a surge of pollutants. Most noticeable is the lowering of pH in mountain streams as snow melts in polluted areas.
The temperatures on high mountains and plateaux fluctuate widely. In the Bolivian Altiplano the diurnal range can frequently be 20 °C by day and —5 °C by night. Throughout the world adaptation to these potentially stressful changes of temperature has resulted in remarkable examples of convergent evolution where plants of very different origin have evolved similar life-forms as they provide the optimal solution to this widespread phenomenon. The most striking example of this has been the evolution of the pachycaul plants (thick-stemmed species) in both South American and African alpine habitats. In the Afro-alpine region Dendrosenecio and Lobelia are typical examples as are Puya and Espeletia in South America. The pachycaul (thick stem) construction is also evident in desert environments where there are also large diurnal temperature fluctuations. Thick stems, such as are found in the large desert cacti, retard both heating and cooling by the mass of water they contain. However, the alpine pachycaul plants have an additional adaptation that is not found in the cacti through the nyctinastic movements of the leaves and bracts (closing at night) that envelope the stems.
Resisting heat by day is but one aspect of the adaptations needed for high mountain survival. Freezing tolerance that allows the plants to resume metabolic activity during the warm day is a highly specialized adaptation that has received considerable study. Not surprisingly, comparisons between different species in relation to relative use of different freezing-tolerance mechanisms can detect a diversity of responses which can be related to the size of the plants.
A study carried out on plants growing from two different elevations (3200 m and 3700 m) in a desert region of the high Andes (29° 45' S, 69° 59' W) found that all ground-level plants showed cellular freezing tolerance to be the main mechanism for resistance to freezing temperatures (Squeo et al., 1996). Tall shrubs avoided freezing temperatures, mainly through supercooling, the phenomenon whereby a liquid can be cooled below its freezing point without freezing taking place. Alternatively, a saturated solution can be cooled without crystallization taking place, to form a supersaturated solution. In both cases supercooling is possible because of the lack of solid particles around which crystals can form. Crystallization rapidly follows the introduction of a small crystal (seed) or agitation of the supercooled solution. Supercooling was only present in plants occupying the lower elevation (3200 m).
In another study, arborescent forms (i.e. giant rosettes and small trees) showed avoidance mechanisms mainly through supercooling, while intermediate-height plants (shrubs and perennial herbs) exhibited both tolerance and avoidance mechanisms. Insulating tissues, which help to avoid temperature extremes, were present in both arborescent and cushion life-forms. It was therefore surmised that for high tropical mountain plants, a combination of freezing tolerance and avoidance by insulation is less expensive than supercooling alone in relation to resource utilization and also provides a more secure mechanism for avoiding cold injury (Squeo et al, 1991).
High mountain plants are frequently highly pubescent. Dense well-developed leaf hairs can serve more than one purpose. They can shield vulnerable tissues against radiation and reduce the risk of heat injury. They can also dissipate heat into the atmosphere like flanges on a radiator. In addition, they have an important role in preventing stomata from becoming occluded by droplets of water that frequently condense on leaves in the cloud zone. Alpine plants that are adapted to living in the dry cold mountain air are very susceptible to moist conditions and the dense hairs give protection to the leaf and prevent water and potentially pathogenic fungal spores from settling on the epidermis (Figs. 10.15-10.16).
10.4.3 Protection against high levels of radiation at high altitudes and latitude
Cellular damage from high levels of radiation has been often suggested as a potential danger to both alpine and arctic plants. Early in the growing season the roots are still in a cold environment while the leaves may be exposed to high sunlight. Any limitation of growth at low temperatures while the leaves are exposed to full sunlight might lead to an accumulation of carbohydrates in leaves and could cause a feedback inhibition of carbon fixation. There could then follow a build-up in reducing power and an increase in light-generated reactive oxygen species with the potential for photodamage to the chloroplasts.
At high altitudes increased levels of sunlight pose a risk of photo-oxidation damage. A study of a selection of alpine plants that occur at different altitudes in the subnival and nival zones of the Obergurgl (Otztal, Austria) has shown marked increases in antioxidant content, principally ascorbic acid with increasing altitude (Wildi & Lutz, 1996). The contents of most compounds were found to follow a diurnal rhythm, with the maximum occurring at midday and the minimum during the night. This enhancement was mainly due to ascorbic acid contents. Each plant species displayed a specific reaction to the increase in stress
Fig. 10.15 An extreme example of hair development in a high-altitude specimen of Tanacetum gossypinum growing at 5000 m in the Himalayan Goyo valley. (Photo Professor R. M. Cormack.)
that accompanies an increase in altitude, resulting in a broad adaptation spectrum for these plants, which suggests that the combined effect of lower temperature and higher light intensity induces higher antioxidant contents (Figs. 10.17-10.18).
The overriding importance of plant form in relation to adaptation to high altitude conditions has been demonstrated in a series of studies carried out on the American marsh marigold (Caltha leptosepala) and the yellow glacier lily (Erythronium grandiflorum; Fig. 10.19) which can be found in Utah at altitudes up to 3120 m (Germino & Smith, 2001). Both plants are perennials that commonly emerge from alpine snow banks where there is a combination of cool temperatures and strong reflected sunlight. Caltha leptosepala occurs in microsites where colder air accumulates, and has larger, less inclined and more densely clustered leaves compared with E. grandiflorum which has two steeply
inclined leaves. These differences in microsite and plant form make for differences in leaf temperature. Caltha leptosepala, which has the larger leaves, was observed to have leaf temperatures below 0 °C in 70% of nights during the summer growing season as compared with only 38% in E. grandiflorum, the species with smaller and more densely clustered leaves. In addition, the leaves of C. leptosepala warmed more slowly on mornings following frosts compared with E. grandiflorum, due to less aerodynamic coupling between leaf and air temperature, and also to a 45% smaller ratio of sunlit to total leaf area due to mutual shading among leaves. As a result, night frost did not affect subsequent CO2 assimilation in E. grandiflorum, while in C. leptosepala frostless nights and warmer mornings led to a 35% greater CO2 assimilation in the early morning. Greater daily carbon gain probably occurs for E. grandiflorum because of its plant form and microclimate, rather than through differences in photosynthetic efficiency (Germino & Smith, 2001).
10.4.4 Effect of UV radiation on alpine vegetation
The depletion of the stratospheric ozone layer in recent years exposing the Earth's surface to increased levels of ultraviolet radiation has prompted considerable research into the potentially harmful effects of ultraviolet-B (UV-B, 280-320 nm) on plant tissues. High-altitude vegetation may be expected to be pre-adapted to this stress, particularly in mountains where the summits are frequently above the cloud zone. For plants, excessive UV-B radiation could, in theory, damage the photosynthetic apparatus and nucleic acids in the leaf mesophyll. Fibre-optic microprobes have been used to make direct measurements of the amount of UV-B reaching these potential targets in the mesophyll of intact foliage.
A comparison of foliage from a diverse group of Rocky Mountain plants showed that the foliage of some plant life-forms was more effective than others at screening UV-B radiation (Day et al., 1992). The leaf epidermis of herbaceous dicots was found to be ineffective at attenuating UV-B, with epidermal trans-mittance ranging from 18% to 41% and UV-B reached 40-145 im into the mesophyll or photosynthetic tissue. In contrast to the herbaceous dicots, the epidermis of one-year old conifer needles filtered out essentially all incident UV-B and virtually none of this radiation reached the mesophyll. Although on high elevation krummholz trees the epidermal layer was appreciably thinner in older needles (7 years), the epidermis still attenuated essentially all incident UV-B. The same epidermal screening effectiveness was observed after the removal of epicuticular waxes with chloroform. Leaves of woody dicots and grasses appeared to be intermediate between herbaceous dicots and conifers in their UV-B screening abilities with 3-12% of the incident UV-B reaching the mesophyll.
Evidence from other areas where UV-B may pose a risk to plants also suggests that little damage appears to take place. The southern part of Tierra del Fuego (Argentina, 55° S) is an area strongly affected by ozone depletion due to its proximity to Antarctica, and several investigations have been initiated to determine the biological impacts of the natural increase of solar UV-B on natural ecosystems in this region. Ambient UV-B has been found to have subtle but significant inhibitory effects on the growth of herbaceous and graminoid species, whereas no consistent inhibitory effects have been detected in woody perennials. The species investigated in greatest detail, the herbaceous Gunnera magellanica, showed increased levels of DNA damage in leaf tissue in the early spring which was correlated with the dose of weighted UV-B measured at ground level (Ballare et al., 2001). However, an opposite effect has been noted in relation to herbivory of the Antarctic beech (Nothofagus antarctica) where it was found that insects consumed at least 30% less area from branches exposed to UV-B than from those with reduced UV-B exposure (Rousseaux et al., 2004).
Further south in Antarctica, the performance of the only two Antarctic vascular plants, Antarctic pearlwort (Colobanthus quitensis) and Antarctic hair grass (Deschampsia antarctica), has shown that UV-B leads to reductions in leaf longevity, branch production, cushion diameter growth, above-ground biomass, and thickness of the non-green cushion base and litter layer. However, exposure to UV-B accelerated the development of reproductive structures and increased the number of panicles in D. antarctica and of capsules in C. quitensis, when calculated in terms of per unit of ground surface area covered by the mother plants. However, this effect was offset by a tendency for these panicles and capsules to produce fewer spikelets and seeds. Ultimately, UV-B exposure did not affect the numbers of spikelets or seeds produced per unit of ground surface area. In relation to vegetative growth the relative reductions in leaf elongation rates increased over four seasons, suggesting that UV-B growth responses tended to be cumulative over successive years (Day et al., 2001).
Not all these examples come from studies at high altitudes. Nevertheless, they illustrate the range of plant responses to UV-B radiation. Such is the range of the many individual effects that can be observed and investigated, it has to be concluded that the possibility of coming to any general conclusions as to their ultimate biological significance remains elusive.
The most common general effects noted so far of UV-B on plant growth is a reduction in plant height and possibly in severe cases a decrease in shoot mass and foliage area (Caldwell et al., 2003). Viewed as a whole, therefore, it would appear that the survival capacity of terrestrial plants is usually unaffected by enhanced UV-B, even though reduced growth has been observed and may increase in magnitude over successive years. It is quite possible that survival of alpine species may be enhanced by reduction in growth and this should therefore not be regarded as a negative reaction.
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