Effects of the hydroedaphic gradient on tree species composition and aboveground wood biomass of oligotrophic forest ecosystems in the central Amazon basin

Amazonian forests in black-water floodplains (igap) and upon hydromorphic white-sand soils (campinarana) cover at least 500,000 km(2) of the area of the Amazon basin, but are poorly investigated ecosystems. We compared variation in tree species richness and composition (a parts per thousand yen 10 cm diameter at breast height), as well as forest structure and aboveground wood biomass (AGB) along hydroedaphic gradients in an igap and a campinarana in the central Brazilian Amazon, in an area totalling 6 ha. Inundation height (igap) and groundwater level oscillations (campinarana) were monitored during a one year period. Soil grain sizes and chemical variables were analysed. Variation in tree species composition was assessed using non-metric multidimensional scaling, and soil parameters using principal component analysis. The influence of hydroedaphic gradients on tree species richness, composition and AGB was investigated using partial and multiple regression analyses. Significant differences in soil texture, soil chemical variables, and tree species richness and composition were detected between both forest types, while AGB amounted to similar values, ranging from 141 +/- 62 Mg center dot ha(-1) in the igap to 164 +/- 121 Mg center dot ha(-1) in the campinarana. Although both forest types were floristically distinct, inundations in the igap and groundwater table oscillations in the campinarana influenced patterns of species richness and forest structure in similar ways, indicating decreasing species richness, forest stature and AGB in plots subjected to higher inundations and/or groundwater levels. Given the comparatively low AGB in the ecosystems studied, we call attention to the need for more studies in oligotrophic ecosystems of the Amazon basin with emphasis on their contribution to global carbon cycles.