Soil Response to Forest Clearing in the United States and the Tropics: Geological and Biological Factors

Twenty-six cross-sectional and time series studies of soil properties under natural forest and altered vegetation in the United States and ten tropical countries were examined to determine the changes associated with forest clearing. Organic C, total N, exchangeable Ca, Mg, and K, cation exchange capacity, available P, bulk density, and pH were considered. After forest clearing, only bulk density and available P tended to return to predearing levels. Losses of organic C, total N, and cation exchange capacity were 50 percent larger in soils developed on highly weathered parent materials in the tropics than in the same soils in the United States or in soils developed on young parent materials. Differences in soil response to clearing were related to local effects of temperature, rainfall, vegetation type, and soil acidity on organic matter decomposition. THIS STUDY TESTED THREE HYPOTHESES: 1) The removal of natural forest vegetation causes changes in soil properties; 2) changes in soil properties associated with changes in land use are long term; and 3) changes in soil properties associated with changed land use are more severe in the tropics or in soils developed on old parent materials. The study arose from the continued debate among soil scientists, ecologists, and land use planners over the adverse and beneficial effects of forest dearing, especially in developing countries in the tropics. On one side of this controversy are those who emphasize the drastic environmental changes associated with forest clearing in the tropics, frequently implying that they are irreversible (GomezPompa et al. 1972; Eckholm 1976, 1982; Myers 1976; Hecht 1982; N. J. H. Smith 1982). On the other side are those who have emphasized the variability of soils and therefore of the consequences of forest dearing (Jordan and Herrera 198 la, b), the potential for productive management of even very poor soils after deforestation (Sanchez et al. 1982), or the benefits from forest clearing, especially food production. At the heart of this debate is the question of whether soils in tropical climates respond more drastically to forest dearing than soils in temperate climates. This paper attempts to quantify the changes in soil properties that are associated with forest clearing and to identify factors associated with dramatic soil responses to clearing.