Fearnside and Laurance (2003) have criticized the Achard et al. (2002) data on the basis that it did not include dry forests; estimates on biomass were based on data from a single site; it failed to include palms, vines, and unders-tory vegetation; and erroneously assumed that secondary forests would regenerate 70% of their biomass in 25 years. This may account for the relatively low rate of deforestation reported in Table 4.3.

The World Resources Institute (2004) presents data from the US Geological Survey's Earth Resources Observation System on forest coverage in 1993 (Table 4.4). In their data, evergreen broadleaf forests are those that have a percent canopy cover greater than 60%, and almost all trees remain green throughout the year. Deciduous forests have canopy cover greater than 60% with annual cycles of leaf fall. Estimates diverge somewhat from those in Tables 4.2 and 4.3, but differences may be due to different systems of classification. The largest discrepancies are for Africa. The evergreen and deciduous forests for Africa shown in Table 4.4 do not include the open woodland shown in Table 4.2.

A further problem is simply defining what constitutes "deforestation". Does a trail made by loggers to extract one mahogany tree constitute deforestation? Does the clearing made by one shifting cultivator who enters the forest by that trail qualify? When loggers remove many of the trees, but leave some individuals still standing, is the area deforested? As the peasant farmers stream in and lay claim to the land, eventually it does become "deforested". What percentage of cover removal constitutes "deforestation?" Determining the point at which closed forest becomes deforested is quite arbitrary.

Further complicating the problem is the fact that there are many areas, as in the eastern Amazon region, where deforested land has been abandoned and is growing back into secondary forests (Dubois 1990). These stands are comprised of species different from those in closed, primary forests. Should the areas of these recovering forests be subtracted from the areas of primary forest that is cleared? Plantation forests growing on previously deforested land present the same question.

Laurance et al. (1997) pointed out that there is an increase in biomass loss in forest fragments, due to the fact that along fragment edges, microclimatic

Table 4.4. Tropical forest cover in 1993 characterized from 1-km advanced very high resolution radiometer data (USGS 2004). Values are in millions of hectares

Latin America

Sub-Saharan Africa


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