Erosion under forest cover and erosion in deforested areas in the humid tropical zone

Abstract

Although surface erosion and gullying are insignificant under dense tropical forest cover, severe erosion can occur locally as a result of land slippage in saturated soils. Examples may be observed in Madagascar, in the forests of Anamalazoatra and Marojejy. Another documented case of widespread erosion under forest cover may be found in the Peruvian Amazon, on a steeply sloping mountainside near the village of Merced. These are exceptions, however. During geological periods of tectonic transformation, intense erosion could have occurred under dense forest cover, forming a jagged relief or what are referred to as "badlands". But in all these cases, erosion under forest cover could only have occurred when the level of riverbeds rose in relation to the level of the sea, as during the Quaternary glaciations. In places where the forest cover has disappeared, the soil is sometimes spectacularly eroded. The same is true of previously forested areas that have recently been severely thinned. Extreme cases may be found in Madagascar, Congo, Gabon, Brazil and the temperate regions of the United States, particularly in the Tennessee copper basin. These examples clearly confirm the anti-erosive value of forests. Once erosion begins, it can only be halted at great cost and over limited areas. Forests are therefore a means to prevent erosion naturally, cheaply and over large areas.