authors may delimit each forest type differently. Most studies have limited their estimates to "moist" forests, probably upland moist forests (Table 4.1). Other types of moist forest such as swamps, given in Table 3.3 (Chap. 3), would not be included if some of the data in Table 4.1 were limited to upland moist forests. Exclusion of open forests or dry forests results in further discrepancies. The pre-1990 estimates for forest cover in Table 4.1 are low compared to the estimates in Table 4.2, because the latter includes dry deciduous forests (forests with greater than 4 months' dry season and that lose all their leaves during that time).

Another problem with data on extent of forest cover and rates of deforestation is that, in many cases, values for individual countries are obtained from documents prepared by local authorities who often rely on maps drawn by field workers with little training. The authorities themselves may have political motivations for either exaggerating or under-reporting the extent of forest cover. Often maps are not available for all tropical countries or all regions within a country and estimates may be based upon extrapolations from available sources.

The more recent data of Achard et al. (2002; Table 4.3) were derived on a global basis from satellite imagery, but with varying degrees of resolution.

Table 4.2. Estimate of areas and rates of deforestation of all tropical closed forest, all open tropical woodlands, and the sum of the two, in 1980. Values for area are in millions of hectares. Rate of deforestation is in millions of hectares per year. (Adapted from Grainger 1993)

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