The structure of tree communities in tropical forests depends on environmental filters and biotic interactions such as competition and facilitation. Many ecotone forests in Northern Amazonia are intriguingly populated by tree assemblages characterized by distinct abundances of a single species, Peltogyne gracilipes (Leguminosae). It is unclear whether this pattern solely reflects environmental filters or also antagonistic interactions among species with similar habitat requirements. The aim of this study was to determine the response of species richness and composition to environmental filters, and analyze the role of P. gracilipes in structuring tree communities in ecotone forest areas of the Northern Brazilian Amazonia. We sampled 129 permanent plots along a hydro-edaphi gradient. All arboreal individuals with stem diameter ≥10 cm were measured and identified. Multiple regressions we...