The effects of soil phosphorus on aboveground biomass are mediated by functional diversity in a tropical cloud forest

Abstract

Soil phosphorus is a key driver of plant biodiversity and aboveground biomass (AGB) in tropical forests. A plant community’s ability to exploit such limiting resources may be better represented by functional diversity than species or phylogenetic diversity, and may therefore have the higher predictive power regarding how soil phosphorus influences AGB in tropical forests. However, nearly no studies have tested this in tropical high-altitude forest ecosystems. Here we aim to test: 1) the relative influence of three biodiversity metrics (functional diversity, species diversity, and phylogenetic diversity) on aboveground biomass in a tropical cloud forest and 2) the interrelationships among soil phosphorus, biodiversity, and AGB in this ecosystem. In a tropical cloud forest in Hainan Island, China, we measured 13 key functional traits for 195 species in 48 plots of size 20 × 20 m2 each. We also measured soil phosphorus in all plots and computed the community phylogeny. Using this dataset, we tested the interrelationships among soil phosphorus, species diversity, functional diversity, phylogenetic diversity, and AGB using Generalized Additive Modeling and Redundancy Analysis. Functional diversity was significantly positively related to AGB, whereas species diversity and phylogenetic diversity were not significantly related to AGB. Functional diversity explained 53% of AGB, while species diversity and phylogenetic diversity only explained 17% and 6%, respectively. Soil phosphorus explained 56% of the variation in functional diversity, but only explained 22%, 13% and 21% of the variation in species diversity, phylogenetic diversity and AGB, respectively. Functional diversity, rather than species and phylogenetic diversity, are the better predictors of AGB. The influence of functional diversity on AGB appears to be linked to how variation in soil phosphorus affects functional diversity. We suggest that functional diversity should be incorporated into models that are designed to test how soil resources influence ecosystem function in tropical forest ecosystems.