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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.
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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