**3. Soil erosion and land degradation**

#### **3.1. Soil erosion in humid tropical environments**

Over the last four decades, deforestation and human interference with the environment have increased in nearly all tropical rain forest environments around the world [39]. The im‐ pact has caused increasing land degradation and is often accompanied by changes in the hy‐ drologic regime, severe soil erosion and a declining productivity of cultivated areas [87, 36]. Recent developments in agricultural techniques, the increased use of agricultural machinery and the replacement of subsistence-orientated agriculture by export-orientated agriculture have resulted in a rapidly increasing and unfavourable change in environmental conditions.

Most studies on the role of soil erosion in rain-forest environments indicate that soil erosion in undisturbed rain forests rarely exceeds rates of 1t ha-1 a-1 as the canopy and understorey protect the soil from the impact of raindrops [59, 38]. In rainforests, much of the rainfall is intercepted and evaporated in the canopy and understorey, and permeable litter layers sup‐ port high infiltration rates. Consequently, only a small fraction of the rain water remains available for overland flow [73]. The litter cover on the surface on the other hand, tends to dampen the forces of the impact of heavy raindrops. This cover is highly permeable. The permeability results from macro-pores provided by roots, which reduce the generation of erosive runoff [59]. Under natural conditions, with a continuous cover of litter layers, the water movement occurs as over litter-layer flow and as root litter flow in pores and in shal‐ low subsurface pipes within the root-litter carpet [19]. This water flow is mostly highly dis‐ continuous shallow unconcentrated overland flow with a low erosive power, except in hillslope hollows, where the convergence of surface water flow lines tends to promote a con‐ centrated overland flow.

Disturbance of vegetation in rain forest environments appear to have serious effects on ero‐ sion rates as the spatial variation in the intensity and frequency of large rainfall events tends to be higher than in savannah environments [87]. The loss of ground vegetation and litter reduces the amount of soil organic matter, which diminishes the aggregate stability and increases the vulnerability of the soil to raindrop impact and the likelihood of soil crusting [29, 36]. The destruction of the soil aggregates by raindrop impact and the formation of a fine grained crust on the soil surface tend to impede infiltration. During rain bursts, this causes a rapid increase in overland flow and favours the development of rills and gullies. Soil ero‐ sion and changes in the physical characteristics of the upper soil horizon are not the only effects of vegetation disturbance. The nutrient cycle is markedly changed as nutrients are lost by soil erosion, by leaching of the soil and by the removal of nutrients which were formerly stored in the vegetation [59]. As tropical rainforests are unable to sustain their nutrient base without sufficient vegetation, the combined effects of vegetation destruction and soil erosion tend to result in a marked depletion of the soils and in a reduction in the biodiversity [86]. The complex relationships between vegetation destruction, agricultural use, soil erosion and loss in soil fertility has been documented from several areas in the tropics, and the interaction between socioeconomic and ecologic factors appears to be of major importance.
