The woody flora and soils of seven Brazilian Amazonian dry savanna areas

Abstract

This study compares the soils, floristic composition, phytosociological structure and history of seven Brazilian savannas in the Amazon basin: those at Alter do Chao, Amapa, Roraima and SE Humaita are islands in rain forest; and those at Chapada dos Parecis, Redencao, and Carolina are on the periphery (northern border) of the central Brazilian savannas (the so-called 'cerrado'). A total of 26 transects were sampled by the PCQ method (for trees with dbh greater than or equal to 5 cm) and additional tree species were recorded by 'wide patrolling'. A total of 101 species were recorded from the transects and another 43 species were recorded by 'wide patrolling'. Byrsonima crassifolia, B. coccolobifolia, Curatella americana, Salvertia convallariodora and Plathymenia reticulata occurred in most or all sites, but no species occurred in all transects. The number of species in the isolated savannas decreased with the distance from the central Brazilian core savanna area. Both cluster analysis (based on the S0rensen Similarity Index) and ordination (DCA), showed that the disjunct and non-isolated peripheral areas were in floristically distinct groups. Five surface soil samples (0 - 10 cm) were collected from each of the 26 transects. Two soil cores (up to 4 m depth), one located in the savanna and the other from the nearest forest present, were taken from each study site for carbon isotope analysis. All the sites had acidic soils (pH 4.5 - 5.2) and a wide range of concentrations of aluminium (0.12 - 1.49 meq 100g-1); most of the other soil properties varied significantly within study sites. An ordination (PCA) distinguished the soils from Amapa, Alter do Chao, Redencao and Roraima, but did not distinguish the disjunct sites from non-isolated peripheral ones. The soil delta 13C values of all the disjunct savannas indicated a vegetation change in the past from C3 to C4 plants, showing that forest (or at least a vegetation with few C4 plants) formely covered these sites. 14C dating indicated that the disjunct savannas are of relatively recent origin, e.g. Humaita was dated at about 2,000 years BP, and hence that they are not remnants of a more widespread Pleistocene savanna in the Amazon.