Cerrado and Caatinga in the Picos Area Antonio Alberto Jorge Farias Castro, Andreas Printz, Maura Rejane de Araújo Mendes, Francisco de Assis Rodrigues Soares, José Orlando Soares Oliveira, Ri- goberto Sousa Albino, Frank-Michael Lange and Ruth Raquel Soares de Farias Universidade Federal do Piauí (UFPI), Núcleo de Referência em Ciências Ambi- entais do Trópico, Ecotonal do Nordeste (TROPEN), CEP 64049-550 Teresina- Piauí, Brazil, aajfcastro@uol.com.br lntroduction Defining ecosystem vulnerability is a central focus o f the W AVES program, and the ecology o f plant systems is a key component in addressing this issue. Accor- dingly, the WAVES working group in landscape ecology has concentrated its re- search on the vegetation ecology o f the Piauí region. Research concerning the caatinga biome, especially within the regional context of the Piauí district, is still in its initial stages. For example, until now, only three botanical studies have been undertaken within the entire Picos region: a comple- mentary floristic survey (Goergen 1983); a botanical economy study (.Tenrich 1989); and a floristic and phytosociological study concemed only with the cerrado ,sensu lato" (Castro 1994), ali of which were conducted in the municipality of Tanque de Piauí (until 1996, part ofthe municipality o f Oeiras). A pioneer study concerning the floristic and phytosociological aspects of arbo- real-shrubby caatinga is needed for a better understanding of this ecosystem and its vulnerability. Therefore, four sampling areas within the W AVES focus region of Picos, concentrated around the Bocaina Reservoir, were selected. The results o f this survey are presented here. In addition, this research contributes to the assem- bly of the herbarium (TEPB) of the Federal University of Piauí (UFPI), an im- portant collection comprising 429 species (Tropen 2000). Material Studied, Area Description and Methods The WAVES focus region in Picos comprises an area o f about 3000 km 2 (1 . 2% o f the total area of the State of Piauí). It is composed of four municipalities (as of 1996), with a population o f about 91,000 and with a demographic density o f 30.5 inhabitants/km 2 . This can be considered very high when compared with the mean of Piauí (11.25 inh./km 2 ). However, population and agriculture are concentrated