Niche differentiation along multiple functional-trait dimensions contributes to high local diversity of Euphorbiaceae in a tropical tree assemblage

Understanding the mechanisms that drive community assembly in species-rich
tropical forest remains a fundamental challenge in ecology. Here, we
integrated multivariate functional trait dimensions, phylogeny, and
metabolomics to test fundamental predictions concerning the role of
differentiation with respect to abiotic and biotic niche axes in the
maintenance of high local diversity of woody plants in the Euphorbiaceae.
We measured 40 functional traits related to resource acquisition,
photosynthetic capacity, hydraulic efficiency, and secondary-metabolite
profiles generated using untargeted metabolomics in all 26 Euphorbiaceae
species in a 20-ha forest dynamics plot in tropical southwestern China. We
examined the correlation structure of 40 traits using a trait networking
approach. We coupled these traits with variation in soil nutrients, light
environment, soil water content and herbivore pressure within the plot to
assess niche differentiation in space. We compared phylogenetic signal
among multivariate trait dimensions and secondary metabolites to
assess niche differentiation in evolutionary time. Network analysis
revealed that a small number of traits with high network centrality
reflected variation in ecological strategy among the Euphorbiaceae. Using
these high-centrality traits, we observed significant functional turnover
along environmental gradients defined by light, soil moisture, soil
nutrients and leaf herbivory, respectively. Most resource utilization
traits showed significant phylogenetic signal, whereas almost all
defensive traits lacked phylogenetic signal, including species similarity
with respect to plant secondary metabolites. Synthesis. Our
results suggest that resource-utilization traits and the habitat
associations play a significant role in the niche
segregation of co-occurring woody plants in the
Euphorbiaceae. Secondary metabolites, however, may enhance
diversity at a finer spatial scale by allowing closely related species
with similar functional traits to partition biotic niche space within
shared habitats in tropical rainforest.