Tree Species Distribution along Environmental Gradients in an Area Affected by a Hydroelectric Dam in Southern Amazonia

We describe the patterns of composition and abundance of tree species and their relationships to environmental factors in an area affected by the Jirau hydroelectric dam on the Madeira River in the Brazilian state of Rondônia. Trees were sampled in 20 1‐ha forest plots distributed among areas affected and unaffected by the hydroelectric reservoir. Predictors of species distribution included in the analyses were soil fertility (sum of bases) and texture, slope, and the vertical distance from the nearest drainage. We sampled 8504 individuals belonging to 909 species/morphospecies. Floristic differences occurred throughout the reservoir area and between the sides of the river, and were strongly associated with edaphic and geological factors. The predictor with the greatest effect on the floristic changes was the sum of bases, coupled with the vertical distance from the nearest drainage, the latter reflecting the influence of the riparian zone. The forests sampled were heterogeneous and included transitional formations between unflooded terra firme and seasonally inundated (varzea, campinarana) forests. Although a considerable number of species (253) were restricted to plots affected by the reservoir, most of these species have a widespread distribution in the Amazon region. However, conservation measures are required in order to preserve species associations and ecosystems processes similar to those most affected by the reservoir such as the river floodplains. Our study reinforces the need to consider the heterogeneity of habitats affected by reservoirs when making decisions concerning the building of hydroelectric dams in the Amazon.