The Golfo Dulce region located in southwestern Costa Rica represents one of the remnant global biodiversity hotspots containing the largest Pacific lowland tropical forest in Central America. Recent biogeographical analyses revealed a close relation of taxonomic tree species composition to northern South America. However, the under- lying factors leading to the extraordinary taxonomic richness of the region have so far remained elusive. In this study, we analyzed the composition of tropical forest plant communities (trees, lianas and palms with a diameter at breast height ≥10 cm) occur- ring in one-hectare permanent forest inventory plots. To that end, we established 20 forest sites across the Golfo Dulce region, i.e. 5 geographic locations each comprising 4 forest habitat types in ridge, slope and ravine positions, as well as in secondary forest stands. Our study highlights the oligarchic dominance of tropical forest plant com- munities in the Golfo Dulce region, and further suggests that plant species composi- tion is associated with topography, disturbance history and edaphic properties. The finding that oligarchic and rare species are affected by environmental filtering has important implications for assessment and modeling of important ecosystem func- tions, such as carbon storage and productivity, as well as nutrient cycling and trophic interactions and thus could potentially improve biodiversity conservation and man- agement strategies.