|Sustaining the Future: Economic, Social, and Environmental Change in Sub-Saharan Africa (UNU, 1996, 365 pages)|
|Part 2: Environmental issues and futures|
|Tropical deforestation and its impact on soil, environment, and agricultural productivity|
In prehistoric times, the geographical area of undisturbed TRF was about 1.5 billion ha. It is estimated that 45 per cent of the original TRF has been converted to other land uses, with a regional loss of about 52 per cent in Africa, 42 per cent in Asia, and 37 per cent in Latin America (Richards 1991). Because of the wide diversity in vegetation type and in the mode and degree of conversion, however, there is a large variation in the estimates of the extent of the remaining TRF and rates of its conversion. The areal extent of TRF from 1700 to 1990 for three regions is depicted in table 9.1. Over the 290 years, the TRF decreased by 36 per cent in tropical Africa, 26 per cent in Latin America, and 30 per cent in Asia. The most drastic conversion happened between 1920 and 1950. The data in table 9.2 are an estimate of the total deforestation that occurred in different regions over a 328-year period. Low and high estimates of total forest conversion range from 484 million ha (32 per cent of the total) to 538 million ha (36 per cent of the total).
Present estimates of the remaining area of tropical rain forest and annual rates of deforestation are also highly variable and erratic (Myers 1991). Estimates of the total area of TRF for the year 1990 range from 1,282 million ha (FAO) to 1,715 million ha (WRI) (table 9.3). The principal discrepancy in the data in table 9.3 lies in the estimate of TRF for Africa. The WRI estimate of 600 million ha includes both closed forest and wooded areas. There are several categories of vegetation called TRF. These include closed forest, forest land, woodland, shrub land, forest land under shifting cultivation, and miscellaneous land (FAO 1981). The closed forest is the true TRF. The distinction between these categories is difficult to make, and estimates vary widely. Estimates of the area of closed forest and wooded land and the rate of conversion are shown in table 9.4.
Table 9.1 Global change in tropical rain forest and woodland, 1700-1990 (million ha)
|Tropical Africa||1,358||1,336||1,275||1,188||1,074||869||- 489|
|Latin America||1,445||1,420||1,369||1,273||1,151||1,067||- 378|
|South & South-East Asia||558||569||536||493||415||410||- 178|
Sources: Richards (1991); WRI (1990).
Table 9.2 Esffmated area of deforestation, 1650-1978 ('000 km2)
|Region||Levela||Pre-1650||1650-1749||1750-1849||1850-1978||Total deforestation 1650-1978|
Source: Williams (1991). a. H = high estimate; L = low estimate.
Table 9.3 Present estimates of TRF and the annual rate of deforestation
WRI (1992 93)
|Total area||Annual rate||Total area||Annual rate|
|(ha m.)||(%)||(ha m.)||(%)|
a. Includes wooded land area and closed forest.
Table 9.4 Different categones of TRF and their conversion rate
Total area (ha m.)
Conversion rate (ha m./yr)
|Closed forest||Wooded land||Closed forest||Wooded land|
Source: WRI (1988 89).
There are several problems with the available data. Original data based on recent and direct surveys are not available. Most estimates are 10-20 years old and obsolete. Furthermore, there are differences in the criteria used, and the accuracy of most estimates is questionable. As with the area, the rate of deforestation is also hard to estimate. However, reliable estimates of the areas of TRF and rate of conversion are needed for: (i) land-use planning, and (ii) predicting the impact of forest conversion on soil and environment.