When forests are planted on former non-forest lands, soils may become acidified through removal of base cations in harvested wood. In areas with high concentrations of polluted cloud water or reactive pollutant gases in the air (HNO3, HN3, and HCl), trees can also increase the transfer of those pollutants from the atmosphere to the ground (Fowler et al., 1989). This transfer can result in acidification of soil and waters and increase aluminum in waters, especially in areas with base-poor soils. This transfer can have detrimental effects on fish, invertebrates, vegetation, and perhaps trees themselves (Ormerod et al., 1989; Kreiser et al., 1990). The risk of enhanced acidification is a well-known constraint on afforestation in parts of Europe and North America. It may be a consideration in any country with high levels of acidifying air pollutants or in areas with soils of low buffer capacity where cation extraction exceeds mineralization (Last and Watling, 1991).
Water cycling is another major environmental service of forests. One of the expected impacts that would result from a significant expansion of the extent of deforestation in Amazonia and other parts of Brazil would be a reduction in rainfall, especially during the dry season (Lean et al., 1996). Similar effects have been calculated for the effects of forests on rainfall in the Indian subcontinent (Harding, 1992), and tropical forest protection has been shown to generate drought mitigation and flood mitigation benefits in Indonesia (Pattanayak and Kramer, 2000) and Madagascar (Kramer et al., 1997).
Anthropogenic wildland fires release significant quantities of GHGs and have considerable socioeconomic and ecological impacts. For example, the 1997-1998 wildland fires in southeast Asia resulted from extensive land clearing, exacerbated by unusually dry El Niņo conditions. These fires adversely affected the health of an estimated 20 million people and produced extensive damage to the region's forests and biodiversity, at a total estimated cost of US$4.4 billion (EEPSEA/WWF, 1998, as cited in Levine et al., 1999).
The relatively high water use by forests compared with non-forest lands transfers more water to the atmosphere, with potential effects on local and regional climate if forest areas are very extensive. Although the magnitude of these feedbacks is the subject of contention, extensive forestation may increase humidity, lower temperature, and increase rainfall in temperate and tropical regions (Harding, 1992; Blythe et al., 1994).
Deforestation can lead to decreased local rainfall and increased temperature-most notably in Amazonia, where about 50 percent of rainfall originates from within the Amazon basin and the predicted decrease in rainfall and warming (after loss of forest) could make conditions unsuitable for subsequent regeneration of many rainforest species (Gash and Shuttleworth, 1991). Similarly, adverse impacts of reduced rainfall from deforestation have been documented in Asia (Chan, 1986; Meher-Homji, 1992).
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