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Additional disturbances are associated with extreme weather events such as hurricanes, tornadoes, unexpected drought or heavy rainfall, flooding, and icestorms that lead to extensive mortality and ecosystem change (e.g., Lugo et al., 1990; Walker and Waide, 1991). Such events generally are highly localized and take place in a relatively short period of time but have long-term economic impacts (Haight et al., 1995) and effects on ecosystems (Pontailler et al., 1997). There is some evidence of recent increases in damage from such extreme events (Berz, 1999; see also Chapters 8 and 9).
Future demand for industrial wood products depends on income growth, population growth, technological change, growth in human capital, changes in tastes and preferences, and institutional and political change (Solberg et al., 1996). Changes in other markets also can influence demand for wood products. For example, increases in the price of substitutes, such as steel and concrete building materials, would increase the demand for industrial timber. In light of these driving factors, recent timber market assessments have predicted that industrial harvests will increase by 1-2% yr-1 (Solberg et al., 1996; Brooks, 1997; FAO, 1997b; Sohngen et al., 1999). These results contrast with those in the SAR (Solomon et al., 1996), which concluded that global demand for industrial fiber would exceed global supply in the next century.
There is some debate about which forests are likely to be harvested in the future. Some authors contend that most supply will come from new industrial plantations, secondary growth forests, and enhanced management, rather than from native forests (FAO, 1997b; Sohngen et al., 1999). The proportion of global timber from subtropical plantations (presently 10%) may increase to 40% by 2050 (Sohngen et al., 1999). Non-native species, such as eucalypts and pines, are favored in these regions because the costs of management and harvesting are low compared to those in temperate and boreal forests (Sedjo and Lyon, 1990; Sedjo, 1999). Recent estimates use global timber market models that incorporate management responses to prices across a wide range of forests. Under most price scenarios, subtropical plantations with 5- to 20-year rotations are a financially attractive alternative (Sedjo, 1999). However, higher demand still may put pressures on native forests even if plantation establishment and forest management responds to price increases (Solberg et al., 1996; Brooks, 1997).
Policies that raise prices for substitute products, such as non-wood building materials made from steel or plastics, may increase timber demand, increase non-native plantation establishment, and cause additional pressures on native forests. Substituting non-wood products for wood products could increase carbon emissions as well (Marland and Schlamadinger, 1995; Schlamadinger et al., 1997).
Increased reliance on plantations may have positive and negative ancillary consequences. For example, most of the plantations established for industrial purposes involve nonindigenous species, and the environmental effects of these plantations are not fully evident. However, most plantations have been established on former agricultural lands, which begins the process of restoring forests (Lugo et al., 1993). Furthermore, plantations may reduce harvest pressures on natural forests. Despite increased reliance on plantations, however, industrial harvests in native forests along accessible roadways are likely to continue (Johns et al., 1996).
The relationship between income and fuelwood demand is nonlinear. As incomes rise and infrastructure grows, households substitute alternative fuel sources (e.g., natural gas, fuel oil). Brooks et al. (1996), for example, suggest that fuelwood harvests will increase by 17% by 2050 under a low-GDP-growth scenario, but only by 4% under a high-growth scenario. Fuelwood harvest depends on the extent of substitution by alternative methods of heating and cooking. Currently, use of wood for energy on a large scale does not appear to be cost-effective relative to other energy sources, but if future energy prices rise, the demand for wood as a source of energy could rise.
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