Northern environments are thought to be particularly sensitive to climate warming, owing to changes in surface albedo degradation of permafrost, increased active-layer thickness, and earlier snowmelt. Recent warming reflects partial replacement of dry Arctic air masses by wetter Atlantic, Pacific, and southerly air masses, particularly in the subarctic and southern Arctic (Rouse et al., 1997). Most of the increase is related to higher late-winter and early-spring air temperatures (+4.5°C), whereas summer temperatures in the region have increased by only about 2°C (Chapman and Walsh, 1993). In parts of Alaska, growing-season length has increased over the past 65 years as a result of reductions in snow and ice cover (Sharratt, 1992; Groisman et al., 1994). The annual amplitude of the seasonal CO2 cycle also has increased recently by as much as 40% (Keeling et al., 1996). There has been an accompanying advance in the timing of spring by about 7 days and a delay in autumn by the same period, leading to an expected lengthening of the growing season (Keeling et al., 1996). Such trends are likely to continue with projected climate change.
Precipitation will increase during the summer months, but evaporation and transpiration rates also are predicted to increase; this, together with earlier snowmelt, will lead to soil moisture deficits later in the summer (Oechel et al., 1997). These changes in moisture and the thermal regimes of arctic soils ultimately will determine the fate of soil carbon stored in permafrost and the active layer (Miller et al., 1983, Billings, 1987; Gorham, 1991; Schlesinger, 1991). Any change in physical properties in the active layer will have an impact on organisms and abiotic variables (Gates et al., 1992; Kane et al., 1992; Waelbroeck, 1993; Groisman et al., 1994). The effects of changed drainage patterns and active-layer detachments (Dyke, 2000)increasing sediment-nutrient loads in lakes and riverswill alter biological productivity in aquatic ecosystems considerably (McDonald et al., 1996).
Changes in the relative abundance of soil biota are predicted to affect soil processes, but only in a modest way. The rate of decomposition of organic matter is expected to rise with an increase in soil temperature and a longer growing season. This is likely to lead to enhanced rates of gross mineralization. However, the high carbon-to-nitrogen ratio of organic soils may still limit the availability of nitrogen to plants. If the thickened active layer extends into mineral layers, the release of mineral nutrients may be increased greatly, causing the upper organic layer to become more alkaline (Heal, 1998).
In summary, climate change probably will release more soil nutrients to biota, but nutrient limitationsespecially of nitrogen and phosphorusstill are likely to occur. Studies have identified that the combination of nutrient and temperature increases results in the increasing importance of dominant plant species and suppression of subordinate ones. Species richness increased in an Arctic site, as a result of invasion of nitrophilous species, but decreased in a subarctic site. The impact of higher nutrient supplies and temperatures are likely to exceed those of elevated CO2 and UV-B as measured in a related experiment (Press et al., 1998). Because little is known about interactions between species within the soil and hence overall community response, the effect of climate change on rates of tundra soil decomposition and carbon loss is hard to predict (Smith et al., 1998b).
There are a wide range of physiological responses of Arctic plants to climate change. Evidence indicates that changes will be at the level of individual species rather than groups of species (Chapin et al., 1997; Callaghan et al., 1998). Some responses to increased nutrient supply are specific to particular growth forms, such as the positive response of forbs, graminoids, and deciduous shrubs and the negative responses of mosses, lichens, and evergreen shrubs (Jonasson, 1992; Chapin et al., 1995, 1996; Callaghan et al., 1998; Shaver and Jonasson, 1999). The direct growth responses of evergreen dwarf shrubs to increased temperatures are small (Havström et al., 1993; Wookey et al., 1993; Chapin et al., 1995, 1996). This is in contrast to the growth rates of graminoids, forbs, and deciduous dwarf shrubs (Wookey et al., 1994; Henry and Molau, 1997; Arft et al., 1999). In the Arctic, where these growth forms are abundant, there is a significant increase in plant biomass in response to increased temperatures in summer (Henry and Molau, 1997). The response of reproductive and vegetative structures to warming will vary, depending on abiotic constraints (Wookey et al., 1994; Arft et al., 1999). Warming in the winter and spring may encourage premature growth, so subsequent frost can lead to damage in plants (e.g., as discussed for Vaccinium myrtillus by Laine et al., 1995, and for Diapensia lapponica by Molau, 1996). In northern Europe, processes that control the transition from boreal forest to tundra are more complicated than in North America because of infrequent large-scale defoliation of birch forest by geometrid moths. The eggs of the moths, which are killed at low temperatures, are likely to survive if winter temperatures increase (Neuvonen et al., 1999).
Increased CO2 concentrations in the atmosphere also could directly affect photosynthesis and growth rates of Arctic plants, but the effects are not easily predictable. For example, a rise in CO2 concentration in the air leads to an initial increase in photosynthesis in individual tussocks of Eriophorum vaginatum. However, this effect can disappear in as little as 3 weeks as a result of homeostatic adjustment, indicating a loss of photosynthetic potential has occurred (Oechel et al., 1997). Such short-lived responses are thought to be caused by limitations in nutrient (especially nitrogen) availability (Chapin and Shaver, 1985; Oechel et al., 1997). At the ecosystem level in tussock tundra plots, the homeostatic adjustment took 3 yearsby which time enhanced rates of net CO2 uptake had disappeared entirely, in spite of substantial photosynthetic carbon gain during the period (Oechel et al., 1994). Adjustment of the photosynthetic rate was thought to be caused by nutrient limitation. Körner et al. (1996) also have reported an increase in net CO2 accumulation of 41% in Carex curvula after 2 years of exposure to elevated CO2, but plant growth changes were minimal. Exposure of plants to increased temperatures and levels of CO2 indicated that the initial stimulation in net CO2 flux persisted, for reasons that are not entirely clear, although it may be linked to increased nutrient availability (Oechel and Vourlitis, 1994; Oechel et al., 1994, 1997).
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