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.

Foundational cushion plants sustain a prominent proportion of alpine biodiversity, but they are quite sensitive to climate warming hence their population dynamics have important implications for biodiversity. The potential biodiversity changes with the population dynamics of cushion plants in alpine ecosystems remain, however, unclear. Using eight communities along a climatic and community successional gradient, we assessed ecological drivers of population dynamics and associated plant diversity changes in alpine communities dominated by the foundational cushion plant Arenaria polytrichoides. The population degeneration of Arenaria is attributed to ecological constraints, including temperature, water and light availability, extreme climate events, and interspecific competition, at a series of life history stages. Once Arenaria populations completely degenerate, previously cushion-dominated communities shift to climax communities that are overwhelmingly dominated by sedges. Future degeneration of foundational cushion populations induced by climate warming will therefore induce a biodiversity collapse in alpine ecosystems.