Evolutionary diversity and species richness predict aboveground biomass better than tree size variation in local-scale tropical forest types of Nepal

Abstract

In forest ecosystems, it is debated which measure of biodiversity should better explain aboveground biomass (AGB). Here, we hypothesized that high species richness, evolutionary diversity and individual tree size variation drive higher AGB in tropical forests but dependent on local-scale tropical forest types. For this, we analyzed the data of 101 plots (each 0.05 ha in size) across three local-scale tropical forest types (i.e., tropical evergreen and deciduous forest, sub-tropical coniferous forest, and tropical evergreen and mixed hardwood forest) of western Nepal. We used structural equation models to examine the multiple multivariate relationships amongst elevation, species richness, evolutionary diversity [either phylogenetic species evenness (PSE), phylogenetic species variability (PSV) or phylogenetic species clustering (PSC)], individual tree diameter at breast height (DBH) variation, and AGB in all forests together and across forest types. We found that phylogenetically close species enhanced AGB in all forests together, particularly in tropical evergreen and deciduous forest, and sub-tropical coniferous forest. However, both phylogenetically close and distant species were responsible for higher AGB in tropical evergreen and mixed hardwood forest. The coexistence of phylogenetically close species at different abundances and high phylogenetic variability in species with higher species richness collectively enhanced AGB in the lowland tropical forests due to the niche diversification. However, AGB was mostly explained by phylogenetically close species with unequal abundances at higher elevation areas, which may be due to the selection effects. Interestingly, tree DBH variation possessed a negligible effect on AGB in studied tropical forests. We recommend to use evolutionary diversity indices in the future development of forest management plans where aboveground carbon sequestration is a priority objective.