Questions: The objectives of this study were to clarify the extent to which environmental factors and geographical distance account for tropical floristic composition, and propose a methodology for delimiting the boundaries of floristic types based on species similarity.

Location: The Panama Canal watershed.

Methods: To assess which factors (climate, topography, geology and geographical distance) account for floristic composition, we performed Mantel tests on distance matrices and partitioned variation in species composition using canonical analysis. We used a permutation-based regression model computed on distance matrices and a hierarchical clustering of the tree composition to construct a predictive map of forest types of the Panama Canal Watershed.

Results: We found that spatial variation alone explained 22–27% of species variation, while the fraction of species variation explained by environmental variables was smaller (10–12%); 13–19% of the variation was accounted for by the joint effect of environmental variation and geographic distance. The similarity-based map emphasizes the principal division in tree flora between the drier Pacific side and the wetter Caribbean slopes.

Conclusions: The distribution of Panamanian tree species appears to be primarily determined by dispersal limitation, then by environmental heterogeneity. ‘Environmental segregation’ processes do play an important role. Maps of broad-scale vegetation patterns based on thorough tree inventories can be used in conservation planning in the tropics.

Abbreviation: PCW = Panama Canal Watershed.