Cushion Presence and Species Pools Mitigate the Effect of Climate on Species Richness in Alpine Communities.

Abiotic and historical factors are major determinants of large-scale patterns of species richness, yet facilitative interactions can strongly influence diversity in low-productivity habitats such as alpine ecosystems. Cushion plants often promote the establishment of other species, but the relative roles of climate, species pools, and facilitation remain largely unknown. We analyzed 454 plots (4 × 4 m each) of vascular plants from the subnival belts of the Qinghai-Tibet Plateau, using generalized linear mixed models and structural equation modeling. Community species richness was shaped by climate, regional species pools, and cushion presence, with cushions exerting the strongest positive effect in the Hengduan Mountains and Tibetan Plateau. In the structural equation model, cushion presence exerted the strongest positive effect on species richness, whereas climate affected species richness mainly through indirect pathways: wetter conditions enlarged species pools, whereas colder conditions increased cushion presence, which in turn enhanced local species richness. Cushions also buffered the negative effects of aridity. In contrast, species richness variation in the relatively wetter regions of the Himalaya was primarily determined by abiotic factors but not by cushion presence, consistent with the dominant assumption that facilitation is not frequent under favorable climatic conditions. Our findings demonstrate that alpine species richness emerges from the combined effects of species pools and facilitation rather than direct climate effects alone, highlighting the need to integrate biotic and abiotic drivers when explaining biodiversity patterns in extreme environments.