Alpine Ecosystem Processes

Almost everything gets slower when it gets cold, but slow production of biomass and slow recycling of dead biomass (litter) go hand in hand, so that the carbon and nutrient cycles remain in balance. Recycling of organic debris is responsible for most of the steady-state nutrient provision and thus controls vigor of growth. When mineral nutrients are added, all alpine vegetation tested had shown immediate growth stimulation, but this holds for most of the world's biota and is not specific to alpine ecosystems. On the other hand, nutrient addition had been shown to make alpine plants more susceptible to stress (softer tissue, reduced winter dormancy) and pathogen impact (e.g., fungal infections) and causes nitrophilous grasses and herbs to overgrow the best-adapted slow-growing alpine specialist species.

It comes as a rather surprising observation that alpine plant productivity - at least in the temperate zone - is only low when expressed as an annual rate of biomass accumulation, but is not low at all when expressed per unit of growing season length. In a 2-month alpine season in the temperate zone alpine belt, the biomass production (above-plus below-ground) accumulates to c. 400 gm- (range 200-600 gm- ). A northern deciduous hardwood forest produces 1200 gm-in 6 months and a humid tropical forest 2400 gm- in a 12-month season, all arriving at c. 200 gm-2 per month. Time constraints of growth are thus the major causes of reduced annual production in closed alpine grass- and shrub-land and not physiological limitations in what seems to a human hiker like a rather hostile environment. Acclimation to lowtemperature, perfect plant architecture, and developmental adjustments can equilibrate these constraints on a unit of time basis. It makes little sense to relate productivity to a 12-month period when 9-10 months show no plant activity because of freezing conditions and/or snow cover.

Similar to carbon and nutrient relations, alpine ecosystem's water relations are largely controlled by seasonality. During the growing season in the humid temperate zone, daily water consumption during bright weather hardly differs across altitude (c. 3.5-4 mm evapotranspiration). However, because of the short snow-free season at such latitudes, annual evapotransiration may be only 250-300 mm compared to 600-700 mm at low altitude, hence runoff is much higher in alpine altitudes. Given that precipitation often increases with altitude in the temperate zone (a doubling across 2000 m of altitude is not uncommon), annual runoff may be 3-5 times higher in the alpine belt, with major implications for erosion in steep slopes.

In many tropical and subtropical mountains, moisture availability drops rapidly above the condensation cloud

Figure 4 High-altitude semideserts (near Sajama, Bolivia, 4200 m) are often dominated by sparse tussock grasses, shrubs, and minor herbs in the intertussock space, all together preventing soil erosion, while being used for grazing. The wide spacing mitigates drought stress in an otherwise dry environment.

Figure 4 High-altitude semideserts (near Sajama, Bolivia, 4200 m) are often dominated by sparse tussock grasses, shrubs, and minor herbs in the intertussock space, all together preventing soil erosion, while being used for grazing. The wide spacing mitigates drought stress in an otherwise dry environment.

layer at 2000-3000 m altitude, causing the alpine belt to receive very little water, often not more than 200-400 mm per year (e.g., the high Andes, Tenerife, East African volcanoes). The resulting sparse vegetation is often termed alpine semidesert, but because of wide spacing of plants and very little ground cover, those plants which are found in this semiarid alpine landscape were found to be surprisingly well supplied with water even at the end of the dry season (Figure 4). As a rule of thumb, alpine plants are thus better supplied with moisture (even in dry alpine climates) than comparable low-altitude vegetation. True physiologically effective water stress is quite rare in the alpine belt, but moisture shortage in the top soil may restrict nutrient availablity periodically, which restricts growth.

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