Metabolomics provides an unprecedented window on diverse plant secondary metabolites that represent a potentially critical niche dimension in tropical forests underlying co-existence. Here, we used untargeted metabolomics to evaluate the chemical composition of 358 tree species and its relationship to phylogeny and variation in light environment, soil nutrients, and insect-herbivore leaf damage in a tropical rain forest plot. We found that tree species that co-occur locally are less chemically similar than random, and that local chemical dispersion and metabolite diversity reduce herbivory, especially that of specialist insect herbivores. Our results suggest that plant secondary metabolites have the potential to mediate plant-herbivore interactions in a manner consistent with diversity maintenance at the community scale.

Phytochemicals are greatly ignored in trait-based ecology. Especially, the adaptations of phytochemicals to abiotic and biotic pressures in the rhizosphere are less understood. Here, we measured the metabolomics of fine roots and their rhizosphere microbiome along a climatic gradient (tropical, subtropical, and subalpine forests), to explore phytochemical diversity patterns and phytochemical-microorganism interactions. We found that high phytochemical diversity but low phytochemical endemism in subalpine species favor coping with high abiotic pressures. High phytochemical variation and phytochemical endemism in tropical species favor greater species coexistence and adaptation to complex biotic pressures. Moreover, there was evidence of widespread chemical niche partitioning of closely related species in all regions. Our findings support the Latitudinal Biotic Interaction Hypothesis, i.e., the intensity of phytochemical-microorganism interactions decreases from tropical to subalpine regions, which promotes greater multi-trophic coexistence in the tropics than in higher latitude forests. Our study provides novel insights into biotic interactions and species coexistence.

Understanding the mechanisms that drive community assembly in species-rich tropical forest remains a fundamental challenge in ecology. Here, we integrated trait dimensions, metabolomics, and phylogeny to test whether interspecific variation over multivariate trait dimensions contribute to coexistence among Euphorbiaceae species. We measured 41 functional traits related to resource acquisition, photosynthetic capacity, hydraulic safety and efficiency, and defense in all 26 Euphorbiaceae species in a 20-ha forest dynamics plot in tropical southwestern Yunnan, China. Network analysis revealed that a small number of traits with high network centrality reflected variation in ecological strategy among the Euphorbiaceae. Further, we observed significant turnover with respect to these high-centrality traits over environmental gradients at distinct spatial and temporal scales. Whereas resource-utilization traits and the habitat associations they mediate exhibited consistent phylogenetic signal. Phylogenetic divergence in chemical defenses likely represents an additional trait dimension that enhances local diversity of closely related Euphorbiaceae in southwestern China.