No discussion of resource utilization by plants in marginal areas can be considered complete without some speculation as to the possible consequences of climatic warming. Particular attention has been given recently to this aspect of environmental change in relation to polar regions, which may be subject to substantial climatic warming. One of the most likely consequences of any substantial change will be an alteration in mineral nutrient availability (Chapin & Shaver, 1996; Wookey & Robinson, 1997). In general, experiments on the effects of climatic warming are difficult both to carry out and interpret. Generalized conclusions are elusive as the responses of plants vary between species and from one part of the Arctic to another. Populations of arctic heather (Cassiope tetragona) from the subarctic regions (Fig. 3.39) are known to respond positively to additions of nutrients while those in the High Arctic are little affected (Havstrom et al., 1993). Experiments on nutrition in cold climates are also complicated by the need for prolonged observations. Factorial environmental manipulation of growing season temperature, soil nutrient and water status conducted over three years at a polar semi-desert community in Spitsbergen have shown that Dryas octopetala can respond rapidly to an amelioration in the availability of inorganic nutrients (N, P and K) by an expansion in leaf area and biomass. Sexual reproduction and seed viability were also markedly improved by elevated temperatures or soil nutrient availability (Wookey et al., 1995).
Although short-term observations often record immediate nutrient-induced growth stimulation, as
in Dryas octopetala, in the long-term this may serve only to increase frost sensitivity. It follows therefore that the eventual outcome of additional nutrients is unlikely to depend solely on nutrient supply. The ultimate consequences of greater nutrient availability will depend on competitive interactions between plants, which may be modified substantially by other factors such as herbivory, disturbance, and overwintering survival.
In the Arctic the problem of nutrient supply is compounded by questions of low input rates and accessibility. Nutrients may be present in the ecosystem in organic deposits, but in short, cold, growing seasons they are not as readily available as they would be in soils with an active microbial flora. In addition to low input rates, the Arctic has the lowest nutrient cycling rates of any ecosystem with typically only 1% of the ecosystem's nutrients occurring in the living biomass. In this 'locking up' of nutrients in the nonliving material, the Arctic reaches yet another extreme end-point in comparison with other world ecosystems. Where there is an adequate supply of moisture as on wet-mossy sites there is usually an active growth of blue-green algae which accounts for the bulk of nitrogen fixation in most arctic habitats. Although some nitrogen comes into the system through snowfall and rain this is more than lost by the run-off that takes place with snowmelt in the spring. In the low temperature conditions of the High Arctic, nitrogen fixation, which is largely carried out by cyanobacteria, is severely limited by temperature and the input even in favourable coastal sites in Alaska is only 5% of the nitrogen that cycles through the vegetation annually (Chapin et al, 1980).
In the High Arctic plants are usually of dwarf stature with low growth rates and diminutive size. The frequent assumption that nutrients are limiting in arctic sites is suggested by the common observation that plant growth is often stimulated when additional nutrients are available, as around a rotting carcass or where they have been recently added to experimental plots (Wookey et al., 1995). However, this same stimulation can be found in temperate and lowland plant communities and it is always a false assumption to equate greater productivity with an increase in fitness, without corroborating demographic data. Although the concentrations of readily available nutrients in arctic soils are low there is no consistent evidence from analyses of leaves, either in the Arctic or in alpine regions, that there is any general deficiency in the major nutrients. Addition of nutrients can even have an adverse effect. With some arctic species additional nutrients can reduce fitness in the Darwinian sense (i.e. reducing the ability of an individual to contribute genetically to a subsequent generation) by causing detrimental phenological changes such as promoting early spring growth and delaying the onset of dormancy in autumn, consequently risking frost damage. Frequent augmentation in carbohydrate levels from extra nutrients may also be a contributing cause to increased fungal infections (Körner & Larcher, 1988).
Whether or not low growth rates are caused by a lack of resources in terms of carbon and mineral nutrients or are due to other environmental constraints is therefore open to question. Arctic soils are usually low in available nutrients but whether demand exceeds the ability to liberate nitrogen and phosphorus from organic sources is still a debatable point. As long as the axis of the Earth maintains its tilt in relation to the sun, the long, dark winters of the Arctic will persist and nutrient availability may be only a minor aspect of plant survival.
Fig. 4.1 A female plant of the arctic willow (Salix arctica) growing at Mesters Vig, East Greenland (73° N). Generally in the Arctic female willows outnumber the males. In Salix arctica (the only arctic willow with measurable growth rings) the females also outperform the males in radial stem growth (see Section 4.11).
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