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Tropical deforestation and its impact on soil, environment, and agricultural productivity

Introduction
TRF and its conversion
Soils of the TRF ecosystem
Forest conversion and soil productivity
Deforestation and the emission of radiatively active gases
Deforestation and hydrological balance
Sustainable use of the TRF ecosystem
Research needs
References

Rattan Lal

Introduction

The humid tropics comprise about 31 per cent of all tropical biomes, cover 11 per cent of the earth's total surface, occupy about 1.5 billion ha of land area, and are home to about 2 billion people (WRI 199091). Of the 1.5 billion ha of the humid tropics, 45 per cent lie in the Americas, 30 per cent in Africa, and 25 per cent in Asia and Oceania. Within the generic term "tropical rain forest" (TRF), there are three principal types of forest vegetation including: lowland rain forest (80 per cent of the humid tropical vegetation), premontane forest (10 per cent), and lower montane and montane forests (10 per cent). The TRF ecosystems are characterized by constantly high temperatures and relative humidity, high annual precipitation, highly weathered and leached soils of low chemical fertility, and high total biomass. High total biomass production, despite low soil fertility, is due to the effect of high temperatures and relative humidity, abundant rainfall, and low moisture deficit. The natural vegetation of the TRF is characterized by a high degree of biodiversity. The TRF ecosystem has global importance in terms of soil and climatic interactions and its impact on several processes. For example, local and global climatic patterns are influenced by the interaction of the TRF with the atmosphere (Salati et al. 1983; Myers 1989; Houghton 1990). An important aspect with global influences involves the impact of the TRF on biogeophysical cycles, e.g. C, N, S, and H2O. Conversion of the TRF to other land use disrupts these cycles, which are critical in regulating several global processes; e.g. emission of radiatively active gases into the atmosphere, change in the total water vapour present in the atmosphere. It is because of these local, regional, and global interactions that the TRF and its conversion are a major concern.

TRF and its conversion

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)

Sources: Richards (1991); WRI (1990).

Table 9.2 Esffmated area of deforestation, 1650-1978 ('000 km²)

Source: Williams (1991). a. H = high estimate; L = low estimate.

Table 9.3 Present estimates of TRF and the annual rate of deforestation

a. Includes wooded land area and closed forest.

Table 9.4 Different categones of TRF and their conversion rate

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.

Soils of the TRF ecosystem

The predominant soils of the humid tropics are oxisols, ultisols, and alfisols (table 9.5). Oxisols and ultisols comprise 63 per cent of soils of the TRF (table 9.6). These soils are highly weathered, leached, devoid of basic cations, and relatively infertile. Young soils of moderate to high fertility (mollisols, inceptisols, and entisols) occupy about 15 per cent of the total land area. There are several soil-related constraints on intensive food crop production in the humid tropics. The principal constraints are listed in table 9.7. Oxisols and ultisols have low nutrient reserves and are prone to toxicity owing to high concentrations of Al and Mn. In general, these soils have high P-fixation capacity. Alfisols are relatively more fertile than oxisols and ultisols. However, alfisols have weakly developed structure and are highly prone to accelerated soil erosion. The effective rooting depth for food crops and annuals is generally 20-30 cm owing to either physical (compacted, concretionary, or gravelly subsoil) or chemical (Al or Mn toxicity, low P) limitations. Coupled with low plant-available water reserves, water deficiency can be a problem for shallow-rooted annuals. In contrast, upland crops can be subjected to periodic inundation and anaerobiosis. With proper management, however, the agricultural productivity of these soils can be greatly improved while minimizing risks of soil and environmental degradation. An impor tent strategy in enhancing the productive potential of these soils is to reduce the adverse effects of forest conversion.

Table 9.5 Geographical extent and distribution of major soils of the humid tropics (ha million)

Source: N RC (1993).
a. Inceptisols include Aquepts, Tropepts, Andepts, and Entisols.
b. Entisols include Fluvents, Psamments, and Lithic Entisols.

Table 9.6 Soils of the humid tropics (% of the total area)

Source: NRC (1982).

Table 9.7 Soil-related constraints on intensive land use for food crop production in the TRF ecosystem

3 = severe; 2 = moderate; 1 = slight.

Forest conversion and soil productivity

Deforestation and conversion to arable land use have drastic impacts on soil properties, water and energy balance, and soil erosion hazard (Lal 1987). The worst-case-scenario local effects are outlined in table 9.8. The principal soil degradation effects include adverse effects on soil structure leading to crusting, compaction, and hardsetting. Reduction in infiltration, increase in surface runoff, and soil exposure to raindrop impact and to the shearing effect of overland flow accentuate soil erosion risks. Alterations in pore size distribution and reduction in the colloid content of the surface soil, owing to eluviation, and preferential removal of clay and organic carbon by erosion drastically reduce plant-available water reserves. High soil temperatures, often reaching 40-45°C at 0-5 cm depth for 4 to 6 hours a day, further aggravate the frequency and intensity of drought stress experienced by shallow-rooted crops.

The principal impact of deforestation on chemical and nutritional properties is related to a decrease in the organic matter content of the soil and to disruption in nutrient-recycling mechanisms owing to the removal of deep-rooted trees. The decrease in soil organic matter content is mostly due to the high rate of mineralization caused by high temperatures. The absence of actively growing roots in the subsoil horizon leads to leaching of bases (e.g. Ca, Mg, K, Na) and increase in soil acidity. In addition to leaching, loss of N and S also occurs owing to volatilization.

Table 9.8 Worst-case scenario regarding the adverse effects of deforestation on soil productivity

Table 9.9 Technological options to minimize the adverse effects of deforestation

The magnitude of these adverse effects depends on the method of deforestation and on the soil and crop management practices. In addition, there exists a strong interaction with soil type, rainfall regime, nature of the existing vegetation, ambient soil moisture content, and microclimate. Technological options for minimizing the adverse effects of deforestation are outlined in table 9.9. It is well known that the adverse effects of deforestation are more severe for mechanical than for manual land-clearing. The adverse effects of mechanical clearing (soil compaction and accelerated erosion) are generally less severe for shearblade than for treepusher or bulldozer blade methods of tree felling. Compaction and structural degradation are more severe when soil wetness is high at the time of landclearing (Ghuman and Lal 1992). Stumping and removal of roots, which is necessary only to facilitate mechanized farm operations, should preferably be done manually. Root ploughing is disruptive and causes considerable soil disturbance. Windrowing also scrapes topsoil and concentrates nutrient-rich ash in narrow strips. Management of the soil structure and erosion control can be achieved by sowing a quick-growing cover crop. The cover crop should preferably be a legume, e.g. Mucuna utilis, Pueraria phaseoloides, Centrosema spp., or Desmodium spp. Erosion control on sloping lands can be achieved by establishing vegetative hedges (e.g. Vetiver, Leucaena, Gliricidia) and other multi-purpose trees and woody shrubs. Adoption of agro-forestry practices also enhances nutrient recycling and minimizes leaching losses of bases.

Enhancing the nutrient capital of the soil is critical to increasing the agricultural productivity of these soils of low inherent fertility. Soil fertility is further depleted by deforestation and biomass removal and/or burning. Therefore, judicious application of fertilizer needs careful consideration. To some extent, nitrogen can be supplied through biological fixation. However, other nutrients, including Ca, Mg, and P, must be made available from off-farm sources. Admittedly, resource-poor farmers cannot afford capital-intensive inputs. None the less, essential nutrients must be supplied, through application of either organic manure or mineral fertilizer, if high yields are expected on a sustained basis.

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