 | The Fragile Tropics of Latin America: Sustainable Management of Changing Environments (UNU, 1995) |
 |  | Part 4 : The semi-arid north-east |
| | White sand soils in north-east Brazil |
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2 Site characteristics
Broadly speaking, the North-East consists of three distinctive geoecological regions: the zona da mata, the agreste, and the sertão (Andrade, 1980). They are arranged zonally in this order from the Atlantic coast to the inland (fig. 11.2).
The zona da mata do Nordeste is part of the extensive forest zone that stretches along the Atlantic coast from the north-east to the south-east of Brazil. It enjoys a sub-humid climate with marked seasonality. That is, the annual rainfall amounts 1,000 to 2,000 mm, but there is a weak dry season for two or three months when the monthly rainfall is less than 50 mm. Its original plant cover was generally a tropical evergreen seasonal forest, although because of long continued developments in the region, little original vegetation remains.
Figure 11.2 Schematic cross-profile of landform and geology, and distribution of white sand soils in the state of Paraíba, along the latitude of about 7°S. (After Machida et al., 1976.) (1) crystalline basement (Precambrian gneiss, granite, schist, etc.);(2) Cretaceous sedimentary rocks (sandstone, limestone, etc.); (3) Pliocene and Pleistocene sediment (Barreiras Group).
The geomorphology of the zone da mate is characterized, by low uplands called tabuleiros, although in some parts, such as southern Pernambuco, there are low, rounded hills ("half oranges" or colinas) of deeply weathered crystalline rocks. The tabuleiros are low uplands 30 to 200 metres above sealevel. They have been dissected to a greater or lesser degree by numerous valleys; consequently, some appear as extensive flat uplands; others are only residual fragments (fig. 11.3). They are composed of sandy, permeable, and unconsolidated sediments of Pliocene to Pleistocene age (Barreiras Group). The tabuleiros are largely grouped into three geomorphic surfaces: the higher and older erosional surface; the lower and later depositional surface; and the lowest fluvial terrace surfaces (Matsumoto, 1983). Geologically and geomorphologically, the tabuleiros in the North-East are correlated with the terra firme uplands that predominate in the Amazonian lowland.
The sertão is a semi-arid inland area, and a part of Brazilian plateau (Borborema highland), on which low-relief erosional plains (pediplains) have developed on the crystalline rocks. The annual rainfall in the sertão ranges 500 to 800 mm, and there is a six- to ten-months-long, almost rainless, severe dry season. A type of drought-resistant xerophytic vegetation called caatinga dominates.
The agreste is the transitional zone between the zone da mate and the sertão.
Locally, within the generally semi-arid sertão or the agreste, however, there are some small isolated sub-humid areas covered by forest. Such areas, called brejos, are formed generally on and around isolated heights standing above the plateau surface.
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