In this section, impacts studies on forests, grasslands, and protected areas are reviewed. Protected areas include mountains, wetlands, and coastal/marine areas.
We must consider two types of climate change effects on forests:
A general discussion of forest response to climate change appears in Chapter 5.
North America contains about 17% of the world's forests (Brooks, 1993),
and these forests contain about 14-17% of the world's terrestrial
biospheric carbon (Heath et al., 1993). Key climate change issues related to
forests in North America include:
There is strong evidence that there has been significant warming at high latitudes
(Jacoby et al., 1996) and that this warming has increased boreal forest productivity
(Ciais et al., 1995; Myneni et al., 1997). However, carbon balance is not necessarily
changed by increases in productivity. Net ecosystem carbon flux (or carbon storage)
is a product of changes in ecosystem production and decomposition. Keyser et al. (2000) used long-term meteorological records to drive the BIOME-BGC model
to evaluate changes in the carbon balance of North American high-latitude forests.
They conclude that increases in net primary production and decomposition were
roughly balanced and that net ecosystem production (i.e., total carbon storage)
was not likely to shift significantly with climate change. In contrast, Goulden
et al. (1998) and Lindroth et al. (1998) found that boreal forests could become
net CO2 sources. The key uncertainties in this area are the effects
of permafrost melting on release of previously frozen carbon, the ability of
more productive ecosystem types (aspen, white spruce) to expand in extent, and
the importance of soil moisture. Evaluating changes in carbon balance in northern
forests should be a priority topic for research.
There is consensus emerging that at mid-latitudes, site-specific conditions
as well as history, human management, air pollution, and biotic effects (e.g.,
herbivory) are much stronger controllers of forest productivity, decomposition,
and carbon balance than climate change or CO2 enrichment (Eamus and
Jarvis, 1989; Aber and Driscoll, 1997; Ollinger et al., 1997; Goodale et al.,
1998; Stohlgren et al., 1998).
There is general agreement that excess nitrogen deposition, which is most
pronounced in the mid-latitudes, has increased carbon storage in mid-latitude
forests by facilitating increases in production in response to elevated CO2
(Townsend et al., 1996). The ability of forests to continue to absorb excess
nitrogen and CO2 is not at all certain, however (Norby, 1998).
Evidence for climate change effects on forest ecosystem "services" (i.e., functions that are important to productivity, environmental quality, and other human concerns) are beginning to emerge in North America. Murdoch et al. (1998) suggest that climate warming increases soil acidification and stream nitrate (NO3-) concentrations, especially in forests with a history of high nitrogen deposition. Extreme climate events (e.g., soil freezing, which may increase as a result of warming-induced decreases in snow cover) also appear to lead to increases in soil and stream acidification and NO3- levels (Mitchell et al., 1996; Groffman et al., 1999). Evaluations of climate change effects on fluxes of trace gases other than CO2 [methane (CH4), nitrous oxide (N2O)] have been inconclusive (Prather et al., 1995).
Climate change effects on biogeochemical processes are likely to be small relative to site characteristics, land-use history, and atmospheric chemistry, especially in mid-latitudes (Aber and Driscoll, 1997).
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