Species Diversity of Canopy Versus Understory Trees in a Neotropical Forest: Implications for Forest Structure, Function and Monitoring

Abstract

Species composition and diversity of the canopy layer of tropical forests have rarely been described, yet they are important to many aspects of ecosystem structure and function. Species composition was compared among canopy trees (defined as sun-exposed crowns), understory trees, trees ≥10-cm diameter at breast height (DBH), and the tree community as a whole in a Neotropical moist forest. High-resolution stereophotographs were used to map all individual canopy tree crowns in 8.6 ha of a 50-ha forest dynamics plot on Barro Colorado Island (BCI), Panama. The canopy was found to have high species diversity in relation to the understory and the whole forest. Only 5% of the stems were found in the canopy, but it contained 70% of the species. Diversity, standardized by stem count, for the canopy (≈135 species per 1000 trees) was higher than that of the forest as a whole (≈108 species per 1000 trees), and species composition was different between the two communities. Although only 50% of trees ≥10-cm DBH, the typical size range used in many forest inventories, were in the canopy, the species diversity and composition of the canopy and trees ≥10-cm DBH were nearly identical. The percentage of gap species in the canopy increased with tree size, providing evidence of the dynamic nature of the BCI forest. To the degree that tree function, such as carbon uptake and transpiration, vary among species, the rarified species richness of the canopy will generate high functional diversity at local-to-landscape scales.