Degradation of Tropical Geo-ecosystems and Formation of White Sand in Northeast Brazil

Abstract

The typical tropical geo-ecosystems in Brazilian Northeast or Nordeste are characterized by: 1) the Mar de Morros on deeply weathered crystalline rocks under the humid climate; 2) the Tabuleiros, consisting of low sedimentary upland (tabuleiros) under the humid climate; and 3) the Pediplain, being an undulating erosional plain covered by thorn bush, called caatinga, under the semi-arid climate. The characteristic degradation processes accompanied by the destruction of vegetation cover in each geo-ecosystem are: laterization or formation of a lateritic crust in the red-yellow latosol on the slopes of rounded hills, formation of white sand among the red-yellow podzolic soil on the tabuleiros, and the erosion of the originally thin non-calcic brown soil resulting in extensive bare lands. The formation of extremely infertile white sand on tabuleiros, predominately composed of quartz sand, is one of the most characteristic degradation processes in the humid tropics. The white sand is the result of shallow, acid groundwater action maintained or flowing over the shallow-lying hardpan formed by the precipitation of iron and aluminum oxides from subsurface water. Considering the fact that, on sandy soils like the ones on tabuleiros, deforestation decreases evapotranspiration and increases soil water, humans are believed to accelerate the process of white sand formation, for the increased subsurface water and elevated water table should favour the formation of such shallow hardpan and white sand. © 1994, The Association of Japanese Geographers. All rights reserved.