Phylogenetic Diversity Outperforms Functional Diversity in Predicting Aboveground Biomass Across Tree Strata in a Subtropical Forest.

Forest aboveground biomass (AGB) is crucial for maintaining global carbon balance, with its accumulation closely associated with biodiversity. However, the mechanisms by which these diversity effects take place, and which dimension of biodiversity that has the most prominent impact on AGB productivity remain insufficiently elucidated. We measured AGB, functional traits and evolutionary relatedness of the overstory and understory tree layers in a subtropical forest in Yunnan Province, China. We assessed predictive importance of functional and phylogenetic diversity indices on AGB across tree layers by using partial least squares (PLS) regression. Overstory exhibited greater AGB with tree species showing higher community-weighted means of resource-acquisitive traits (e.g., specific leaf area), as well as higher phylogenetic diversity. The understory AGB was mainly enhanced by conservative strategies (e.g., lower specific leaf area) as well as mean nearest taxon distance; and it was also increased by higher phylogenetic species richness and functional richness. Our results highlight the role of functional identity in regulating AGB. Notably, contrary to the results in previous Biodiversity and Ecosystem Function (BEF) studies where the influence of functional diversity was greater than phylogenetic diversity, we found that phylogenetic diversity is more capable of explaining the accumulation of AGB than functional diversity across tree strata. Our study thus underscores the significance of incorporating phylogenetic diversity into biodiversity and ecosystem function studies.