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Tropical forests represent about 40% of the world's forested area and contain about 60% of global forest biomass. Latin American tropical forests-which represent about 22% of the global forest coverage-have a strong influence on local and regional climate (Salati and Vose, 1984; IPCC 1996, WG II, Section 1.4), play a significant role in the global carbon budget (Dixon et al., 1994; IPCC 1996, WG II, Chapter 24), and contain a remarkably large share of all plant and animal species of the world (Annex D; see also Bierregaard et al., 1992; Mabberley, 1992; Myers, 1992; Riede, 1993). Moreover, Latin American tropical and subtropical forests are economically very important, providing commercial products for national and international markets. Large numbers of people base their activities in these trades; many others, particularly indigenous people, subsist mainly on the forests' nonmarket products. Temperate forests in South America are important, to a much smaller extent, for the export economies of Chile and, to an even lesser extent, Argentina (IPCC 1996, WG II, Section 15.4.3).
Forest cover in Latin America declined from 992 million ha in 1980 to 918 million ha in 1990, with an annual deforestation rate of 0.8% over this period. Average annual deforestation rose from 5.4 million ha in 1970 to 7.4 million ha in 1990 (FAO, 1993). Between 1980 and 1990, deforestation reduced tropical forest cover from 826 million ha to 753 million ha-a decrease of 0.9% (UNEP, 1992). The tropical forests of the Pacific coast of Central America once covered 55 million ha; less than 2% of this forest now remains, although countries like Costa Rica have preserved and protected some of their forests under national park or reserve status. Similarly, only 4% of the original 100 million ha of the Atlantic forest of Brazil (also marginally present in Paraguay and northeastern Argentina) remains as relatively pristine forest. In Argentina, forests covered 106 million ha in 1914, but less than a third of that surface (32.3-35.5 million ha) remained by the 1980s (Di Pace and Mazzuchelli, 1993). High deforestation rates also have been observed in the subtropical Paranaense and Great Chaco forests, as well as in the Andean-Patagonian and Austral forests.
Latin American forests would face an additional threat from climate change, as indicated in vulnerability studies by USCSP and GEF projects. Unless appropriate action is taken, mismanagement of these ecosystems will, in turn, make climate change impacts more severe. Wood harvesting is expected to increase, especially in tropical and subtropical countries; local communities will face serious shortages in forest products required for subsistence and traditional trading (IPCC 1996, WG II, Section 15.2.3). Forest clearing also is expected to increase in response to an increasing need for agricultural land (DENR-ADB, 1990; Starke, 1994; Zuidema et al., 1994).
Large-scale conversion of tropical forest into pasture will likely lead to changes in local climate through increased surface and soil temperatures, diurnal temperature fluctuations, and reduced evapotranspiration (Salati and Nobre, 1991; Cerri et al., 1995). A considerable proportion of the precipitation over the Amazon basin originates from evapotranspiration (Molion, 1975; Salati and Vose, 1984), which could be reduced by continued and large-scale deforestation. Such large-scale forest clearing could reduce the enormous runoff of the Amazon river system and result in other far-reaching, undesirable impacts beyond the cleared areas (Gash and Shuttleworth, 1991). According to projections by Shukla et al. (1990), if tropical forests were replaced by degraded pastures, there would be significant increases in surface temperature and decreases in evapotranspiration and precipitation in the Amazon basin. Furthermore, increases in the length of the dry season would make reestablishment of forests difficult.
The global carbon cycle also could be altered. The potential of Latin American tropical forests to act as carbon sinks has been considered high (da Rocha, 1996; Massera et al., 1996; Molion, 1996). According to Batjes and Sombroek (1997), however, the effect of climate change on carbon sequestration in tropical soils can be very complex; it depends on air temperature, CO2 concentration, seasonal rainfall distribution, nitrogen deposition, and fires. Forest-to-pasture conversion during a 35-year sequence in Amazonia resulted in the creation of a net source of methane (Steudler et al., 1996) and a net sink of CO2; 9-18% more carbon was stored in the soil under pasture than under the original forest. However, the conversion process involves a large net organic carbon mineralization (Cerri et al., 1995; Cerri et al., 1996).
Tropical forests are likely to be more affected by changes in soil water availability (e.g., from seasonal droughts or soil erosion and nutrient leaching resulting from heavy rainfall events) and possibly by CO2 fertilization than by changes in temperature (IPCC 1996, WG II, Section 1.4). Nutrient leaching, erosion, and timber harvesting also are likely to result in decreased biomass and biodiversity (Whitmore, 1984; Jordan, 1985; Vitousek and Sanford, 1986).
Global vegetation models do not agree on whether climatic change (in the absence of land-use change) will increase or decrease the total area of tropical forests in Latin America. These forests are projected to expand their geographical range in the MAPSS and BIOME3 models, although the results vary depending on the scenario (Annex C). When the IMAGE 2.0 model (Alcamo, 1994) has been used to simulate the joint effects of climatic and land-use changes (Leemans, 1992; IPCC 1996, WG II, Section 1.4), the projected net loss of tropical forest is not very large. This result may reflect the fact that, in that model, land-use changes are driven by the dynamics of human population growth and economic development. In reality, however, these variables are not necessarily directly related to natural resource exploitation in many Latin American countries, where harvests often are determined by needs outside the region (IPCC 1996, WG II, Chapter 15). Increased occurrence of droughts (as in the projections compiled by Greco et al., 1994) would affect natural forests and plantations in low-precipitation areas.
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