In this section, impacts studies on forests, grasslands, and protected areas are reviewed. Protected areas include mountains, wetlands, and coastal/marine areas.

15.2.2.1. Forests

We must consider two types of climate change effects on forests:

• Changes in the functions of existing forests relating to productivity, nutrient cycling, water quality, ecosystem carbon storage, trace gas fluxes, and biodiversity
• Changes in composition as forests regenerate under altered conditions. Fundamental changes in forest ecosystem structure can lead to very dramatic changes in functions. Climate change effects on catastrophic events (e.g., fire, insect outbreaks, pathogens, storms) that have marked effects on ecosystem structure are particularly important to consider.

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:

• Changes in the geographic range of different forest types
• Increases in the frequency of fire and insect outbreaks
• Changes in the carbon storage function of forests (i.e., from sinks to sources)
• Evaluation of the importance of multiple stresses (ozone, nitrogen deposition, land-use change) that work in concert with climate change
• Changes in human interactions with forests (e.g., risk to settlements, recreational use)
• Concern for the boreal forests of Canada because of their large extent, carbon reserves, and commercial value, combined with the fact that climate change is expected to be most severe at high latitudes

15.2.2.1.1. Changes in function of existing forests

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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