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Land-use change, including deforestation, is still considered to be a large pressure on global forests (Alcamo et al., 1998). Several factors contribute to deforestation in tropical regions, including income and population growth, road-building policies, and other government policies (Southgate, 1998). Conversion to agriculture is impractical in boreal systems because of low productivity and high access costs, but some boreal forests continue to be converted to second-growth, managed forests. In temperate regions, conversion of agricultural lands back to forest has increased with agricultural productivity and falling prices (Kuusela, 1992; UN-ECE/FAO, 2000a). There is some evidence that these trends will continue, although at a lower rate (UN-ECE/FAO, 2000a). Factors affecting these trends include urbanization, agricultural yields and prices, timber prices, access and conversion costs, and subsidy programs (such as those that promote afforestation for environmental reasons, including mitigation of climate change impacts).
Fragmentation of forest landscapes as a result of climate change, land-use practices, and disturbance is expected to take place in advance of larger scale biome shifts (Fahrig and Merriam, 1994; Shriner et al., 1998). Fragmentation can change biodiversity and resiliency (Sala et al., 2000). Fragmentation of the landscape can occur as a result of disturbance (natural or anthropogenic) or more gradually from successional responses to environmental changes (NBIOME SSC, 1992).
Pressures from air pollution and air quality: There is evidence of decline in forest condition as a result of air pollution, especially in areas adjacent to industrial areas and large cities (e.g., deposition of heavy metals, sulfur, nitrogen, and ozone) (Nilsson et al., 1998). Of major concern is increased nitrogen deposition caused by industrial processes and agriculture (Vitousek et al., 1997b). Nitrogen deposition is higher in northern Europe than elsewhere (Vitousek et al., 1997b). Low-level increases in nitrogen deposition associated with air pollution have been implicated in productivity increases over large regions (Schindler and Bayley, 1993; Vitousek et al., 1997b). Temperate and boreal forests, which historically have been nitrogen-limited, appear to be most affected (Townsend and Rastetter, 1996; Vitousek et al., 1997b). In other areas that become nitrogen-saturated, other nutrients are leached from the soil, resulting in forest dieback (Vitousek et al., 1997b)—counteracting, or even overwhelming, any growth-enhancing effects of CO2 enrichment.
Tropospheric ozone has been shown to impact the structure and productivity of forest ecosystems throughout industrialized countries (Chameides et al., 1994; Weber et al., 1994; Grulke et al., 1998) and is likely to increase in extent with further industrial development and agriculture management (Chameides et al., 1994). It has been suggested that the impact of ozone damage is reduced, but not eliminated, by increasing CO2 (Tingey et al., 2001).
In developed countries, the major impacts of air pollution on forest services are likely to be on recreation and non-wood products. Air pollution has been shown not to have significant impacts on industrial timber markets in the United States (Haynes and Kaiser, 1990), although European timber market studies suggest potentially larger local effects (Nilsson et al., 1992). Although increasing industrialization in developing countries that have less restrictive air pollution requirements could have effects on local industrial or fuelwood markets, these increases are not expected to have major effects on global timber supply.
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