| The Fragile Tropics of Latin America: Sustainable Management of Changing Environments (1995) |
|Part 4 : The semi-arid north-east|
|White sand soils in north-east Brazil|
6 The effect of deforestation
The formation of white sand may be mainly the outcome of natural processes under existing site conditions, such as geological, climatic, and vegetational ones. However, humans are also believed to accelerate this process through deforestation.
It may be clear from the preceding description that the abundance of subsurface water, maintaining shallow groundwater, should favour the formation of white sand. On sandy soils like the ones on tabuleiros, deforestation may decrease evapotranspiration and increase soil water. Such an outcome leads to the elevation of the groundwater table, which, in turn, results in the formation of the impermeable hard layer near the surface. In this manner, white sands may expand spatially.
The increase in soil water in the deforested terrain was ascertained through the comparison of water content in the soil profiles under the thicket and under the barren land on the Conde upland (fig. 11.9). The soil under the cleared land, both of white and brown sands, dried out only superficially, while, at depth, it contains more soil water than the soil under vegetation. Owing to poor capillary action in the coarse sandy soil, little soil water may evaporate from the soil surface, and the absorption and transpiration through the roots and leaves of plants is the dominant process for the output of water into the air.
This situation was reported also by Kashiwagi (1986) from the sandy soil area near Campina Grande, Paraíba where the soil of a nearly bare cropped field contained more than double the moisture present in the soil under the caatinga.
The zone da mate of North-East Brazil has a history of deforestation of some five hundred years, beginning with the extraction of Brazil-wood (pau brasil; Caesalpinia echinata) in the sixteenth century, followed by clearing for sugar cane plantations and the cutting of firewood for sugar production. Recently, after the establishment of the Pró-Alcool Project, which aims to produce fuel alcohol for automobiles from sugar cane, the sugar cane fields have been extended even onto the surface of tabuleiros. Through such a long continued and repeated deforestation, the soil on the tabuleiros must have been severely modified, especially in the form of the expansion of the white sand areas.
The zona da mata do Nordeste, having many similarities with Amazonia in terms of its natural environment, such as on geology, climate, vegetation, and so on, might be referred to as the region suggesting "Amazonia in the future after development."