Background/Question/Methods Experimental studies assessing the role of diversity in ecosystem functioning manipulated diversity level to measure the response of selected ecosystem function. However, in the natural forest, such direct manipulation of diversity is not possible. To address this issue, we used stratified random sampling to create different diversity levels in the 50 ha plot of Barro Colorado Island to examine the relative importance of functional diversity and species diversity on carbon storage as the ecosystem function. We identified 15 subplots with high functional diversity and 15 with low functional diversity, based on the hierarchical clustering of plant traits (diaspore dry mass, maximum height, crown diameter, leaf size, and specific leaf area). The carbon stock of each of the 30 subplots was estimated using allometric equations based on diameter at breast height and wood density. The stratified sampling was repeated to select subplots with high and low species richness. Results/Conclusions Functional diversity was most strongly related to aboveground carbon stocks than species diversity. However, a large proportion of variation to unexplained factors suggesting that spatial patterns and stochasticity plays a role in determining patterns of above ground biomass.