Vertical stratification and treefall gaps define tree species composition and rarity in a neotropical forest

Vertical stratification of forest canopies has been hypothesized to create a variety of niches allowing for high levels of tree species diversity and rarity in tropical forests. We tested this hypothesis on a 50 hectare neotropical forest, Barro Colorado Island, Panama. Correlations of species richness, rare species richness, Shannon diversity, and rare species Shannon diversity against vertical stratification in one hectare plots were performed on three distinct forest types: slope, plateau, and slope-plateau transition. Negative relationships were observed in plateau hectares for species richness, rare species richness and Shannon diversity. The only positive correlation was observed in slope hectares for Shannon diversity. We attribute this variance in relationships across forest types to differences in topography. Species composition was well estimated by vertical stratification within each forest type; however this relationship was strong for common species and weak for rare species. Rare species locations were best explained by treefall gaps, as the proportion of individuals found in gaps in comparison to non-gaps was higher for rare species than common ones. We conclude that although vertical stratification poorly estimates species richness and Shannon diversity, it can be an effective tool for community level stand comparisons within a specific forest type. We also reevaluate the role that treefall gaps play in sustaining rare species in tropical forests.