There exists little information about the vegetation of the sedimentary plains in the semi-arid of Brazilian Northeast. Considering this gap, a phytosociological survey of the caatinga vegetation was carried out in an area of the sedimentary plain with acid, sandy and deep soils situated in the Parque Nacional da serra da Capivara, Piauí State (08º26'50" - 08º54'23"S e 42º19'47" - 42º45'51"W), altitude of 600m, to characterize the floristic composition, physiognomy and structure of the woody component. All living or standing dead individuals with a stem diameter <FONT FACE=Symbol>³</FONT> 3cm at soil level and height<FONT FACE=Symbol>³</FONT> 1m present in 50 plots (10x20m) were recorded. 56 species and 19 families were registered. The area showed great similarity to other sedimentary areas, especially of the middle north basin. Mimosaceae, Myrtaceae, Fabaceae and Caesalpiniaceae included 47% of the species and totalized 59,8% of total importance value. Acacia langsdorffii Benth. (Mimosaceae), Campomanesia sp.(Myrtaceae), Pavonia glazioviana Gürke (Malvaceae) and Pterodon abruptus (Moric.) Benth. (Fabaceae) were the most important taxa. The physiognomy, similar with others vegetational types presents in the sedimentary plains, is predominantly shrubby and the individuals are rather branched at soil level, representing one of the highest total densities of the semi-arid (5.827ind.ha-1) and a total dominance of 31.9m²ha-1.
caatinga; semi-arid; sedimentary plains; Brazilian Northeast
