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Results from plants of Nardus stricta growing adjacent to springs in Iceland indicate that they have been exposed to elevated levels of CO₂ for at least 150 years. There was a reduction in photosynthetic capacity of high-CO₂ grown plants of this grass from the vicinity of the spring, compared with plants grown at ambient CO₂ concentations—again linked to reduction in Rubisco content and the availability of nutrients (Oechel et al., 1997). Collectively, these results indicate that it is unlikely that carbon accumulation will increase markedly over the long term as a result of the direct effects of CO₂ alone (Oechel et al., 1997). A return to summer sink activity has occurred during the warmest and driest period in the past 4 decades in Alaskan arctic ecosystems, thereby eliminating a net summer CO₂ flux to the atmosphere that was characteristic of the early 1980s. The mechanisms are likely to include nutrient cycling, physiological acclimatization, and reorganization of populations and communities (Oechel et al., 2000), but these systems are still net sources of CO₂.

A compounding consideration for Arctic plants is the impact of increased UV-B radiation. In Arctic regions, UV-B radiation is low, but the relative increase from ozone depletion is large, although the ancestors of present-day Arctic plants were growing at lower latitudes with higher UV-B exposure. Over the past 20 years, there has been a trend of decreasing stratospheric ozone of approximately 10-15% in northern polar regions (Thompson and Wallace, 2000). As a first approximation, a 1% decrease in ozone results in a 1.5-2% increase in UV-B radiation. Damage processes to organisms are temperature-independent, whereas repair processes are slowed at low temperatures. Hence, it is predicted that Arctic plants may be sensitive to increased UV-B radiation, especially because many individuals are long-lived and the effects are cumulative. In a study of responses by Ericaceous plants to UV-B radiation, responses varied from species to species and were more evident in the second year of exposure (Bjorn et al., 1997; Callaghan et al., 1998). For unknown reasons, however, the growth of the moss Hylocomium splendens is strongly stimulated by increased UV-B, provided adequate moisture is available (Gehrke et al., 1996). Increased UV-B radiation also may alter plant chemistry that could reduce decomposition rates and nutrient availability (Bjorn et al., 1997, 1999). Soil fungi differ with regard to their sensitivity to UV-B radiation, and their response also will affect the processes of decomposition (Gehrke et al., 1995).

Climate change is likely to result in alterations to major biomes in the Arctic. Ecosystem models suggest that the tundra will decrease by as much as two-thirds of its present size (Everett and Fitzharris, 1998). On a broad scale—and subject to suitable edaphic conditions—there will be northward expansion of boreal forest into the tundra region, such that it may eventually cover more than 1.6 million km² of the Arctic. In northern Europe, vegetation change is likely to be more complicated. This is because of the influence of the geometrid moths, Epirrita autumnata and Operophtera spp., which can cause large-scale defoliation of boreal forests when winter temperatures are above 3.6°C (Neuvonen et al., 1999). Boreal forests are protected from geometrid moths only during cold winters. Empirical models estimate that by 2050, only one-third of the boreal forests of northern Europe will be protected by low winter temperatures in comparsion to the proportion protected during the period 1961-1991 (Virtanen et al., 1998). However, the northward movement of forest may lag changes in temperature by decades to centuries (Starfield and Chapin, 1996; Chapin and Starfield, 1997), as occurred for migration of different tree species after the last glaciation (Delcourt and Delcourt, 1987). The species composition of forests is likely to change, entire forest types may disappear, and new assemblages of species may be established. Significant land areas in the Arctic could have entirely different ecosystems with predicted climate changes (Everett and Fitzharris, 1998). However, note that locally, climate change may affect boreal forest through decreases in effective soil moisture (Weller and Lange, 1999), tree mortality from insect outbreaks (Fleming and Volney, 1995; Juday, 1996), probability of an increase of large fires, and changes caused by thawing of permafrost.

16.2.7.3. Changes in Arctic Animals

In the immediate future, the greatest environmental change for some parts of the Arctic is likely to result from increased herding of reindeer rather than climate change (Crete and Huot, 1993; Manseau et al., 1996; Callaghan et al., 1998). Winter lichen pastures are particularly susceptible to grazing and trampling, and recovery is slow, although summer pastures in tundra meadows and shrub-forb assemblages are less vulnerable (Vilchek, 1997). The overall impact of climatic warming on the population dynamics of reindeer and caribou ungulates is controversial. One view is that there will a decline in caribou and muskoxen, particularly if the climate becomes more variable (Gunn, 1995; Gunn and Skogland, 1997). An alternative view is that because caribou are generalist feeders and appear to be highly resilient, they should be able to tolerate climate change (Callaghan et al., 1998). Arctic island caribou migrate seasonally across the sea ice between Arctic islands in late spring and autumn. Less sea ice could disrupt these migrations, with unforeseen consequences for species survival and gene flow.

The decrease in the extent and thickness of Arctic sea ice in recent decades may lead to changes in the distribution, age structure, and size of populations of marine mammals. Seal species that use ice for resting, pup-rearing, and molting, as well as polar bears that feed on seals, are particularly at risk (Tynan and DeMaster, 1997). If break-up of annual ice occurs too early, seal pups are less accessible to polar bears (Stirling and Lunn, 1997; Stirling et al., 1999). According to observational data, recent decreases in sea ice are more extensive in the Siberian Arctic than in the Beaufort Sea, and marine mammal populations there may be the first to experience climate-induced geographic shifts or altered reproductive capacity (Tynan and DeMaster, 1997).

Ice edges are biologically productive systems, with diatoms and other algae forming a dense layer on the surface that sustains secondary production. Of concern as ice melts is the loss of prey species of marine mammals, such as Arctic cod (Boreogadus saida) and amphipods, that are associated with ice edges (Tynan and DeMaster, 1997). The degree of plasticity within and between species to adapt to these possible long-term changes in ice conditions and prey availability is poorly known and requires study. Regime shifts in the ocean will impact the distribution of commercially important fish stocks. Recruitment seems to be significantly better in warm years than in cold years, and the same is valid for growth (Loeng, 1989). The distribution of fish stocks and their migration routes also could vary considerably (Buch et al., 1994; Vilhjalmsson, 1997).

For other species, such as the lesser snow goose, reproductive success seems to be dependent on early-season climatic variables, especially early snowmelt (Skinner et al., 1998). Insects will benefit from temperature increases in the Arctic (Danks, 1992; Ring, 1994). Many insects are constrained from expanding northward by cold temperatures, and they may quickly take advantage of a temperature increase by expanding their range (Parmesan, 1998).

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