Functional Diversity Metrics in Tropical Rainforest Ecosystems

Abstract

Functional diversity -- the variety of functional traits present in an ecological community and the degree to which they fill the available functional trait space -- is increasingly recognised as a more ecologically meaningful measure of biodiversity than taxonomic species richness, because it more directly reflects the range of ecological roles, resource use strategies, and ecosystem functions that a community can perform. This study quantified four complementary functional diversity metrics (functional richness FRic, functional evenness FEve, functional divergence FDiv, functional dispersion FDis) for tree communities across 42 one-hectare tropical rainforest plots in three major rainforest regions (Amazonia: n = 14; Congo Basin: n = 14; Borneo: n = 14), using 12-trait functional trait matrices compiled for 2,840 tree species from the TRY Plant Trait Database, BIEN (Botanical Information and Ecology Network), and field-collected measurements. Mean species richness per plot was 284 +/- 48 tree species per ha across all regions, with no significant regional difference (p = 0.42), while functional richness (FRic) was significantly higher in Amazonia (0.684 +/- 0.084) than Congo Basin (0.484 +/- 0.064) and Borneo (0.524 +/- 0.072) despite similar species richness -- suggesting that Amazonian communities fill a broader functional trait space per unit taxonomic diversity. Functional richness was positively correlated with aboveground carbon stocks (r = +0.74; p < 0.001) and negatively correlated with soil nutrient availability (r = -0.58; p < 0.001), consistent with niche differentiation under nutrient limitation driving functional trait diversification in more nutrient-poor Amazonian white-sand forests. A Functional Diversity-Ecosystem Function Index (FDEFI) combining all four FD metrics explained 74.4% of the variance in plot-level carbon stock estimates, substantially more than species richness alone (48.4%), confirming the superior predictive power of functional over taxonomic diversity for ecosystem function.