2 Study sites and methods

The study sites are situated between 7° and 8°S and between 32° and 38°E in the Brazilian North-East (fig. 12.1). Mean annual temperature and total annual precipitation are 20°C and 800-1,000mm, with a severe dry season from October to the following February (Nishizawa, 1976; Rizzini and Pinto, 1964). The soils are classified as solodized solonetz and regosols Ministério da Agricultura e Ministério do Interior, 1971).

The vegetation varies from place to place, according to land use, which includes ranching, cultivation, firewood collection, and charcoal Production.

The intensive study sites were located in the vicinity of Campina Grande and Patos, where the vegetation is typical for the region. Nine 10m x 10m quadrats were set out at sites dominated by Mimosa tenuiflora (synonym of Mimosa hostilis) and Caesalpinia pyramidalis. In each quadrat, I measured the diameter of tree stems at 130 cm high (DBH) for each species, and counted the number of shrubs less than 150 cm tall for each species.

For selected specimens of Caesalpinia pyramidalis, Mimosa tenuiflora and Aspidosperma pyrifolium, the weight of stems (Ws:kg), branches (Wb:kg), and leaves (Wl:kg) was taken after first measuring tree height (H:m) and stem diameter (D:cm) at 130 cm high.

The nitrogen and carbon contents of stems and leaves and of the surface soil were determined in the laboratory.