Although phylogenetic distance between native and exotic species has a close link with their interactions, it is still unclear how environmental stresses and species interactions influence the effect of phylogenetic distance on biological invasion. Here we assessed the effect of invader-native phylogenetic distance on the growth of the invader (Symphyotrichum subulatum) under three levels of drought (no, moderate or intensive drought). We found the effect of communities with closely related natives on the invader shifted from negative (supporting Darwin’s naturalization hypothesis) to positive (supporting the pre-adaptation hypothesis.) with increasing drought intensity, which could be largely explained by the shift of species interactions from competition to facilitation by mutualism with arbuscular mycorrhizal fungi. Therefore, our results provide a new angle to resolve Darwin’s naturalization conundrum from the change of species interactions along a stress gradient, and provide important clues for invasion management in the changing environments.

Nitrogen (N) deficiency has been recorded in the top surface of Tibetan Plateau. However, the variation of soil N availability across the elevational gradient in alpine forests remains poorly understood. Here, the elevational patterns and determinants of soil N composition, key N transformation processes and functional microbes across three typical mountains on the southeastern Tibetan Plateau were characterized by multiple techniques. Our results showed that soil total N and ammonium were markedly enriched in high elevation zones where a stable N release via mineralization and extremely low net nitrification were observed. Further, the increasing biological N fixation rates along the elevation driven by abiotic (i.e., high organic carbon) and biotic (i.e., key diazotrophic taxa like Bradyrhizobium, Herbaspirillum and Klebsiella) factors greatly benefited N accumulation at high elevations. Our study offers new insights into the N dynamics in alpine forests on the Tibetan Plateau under scenarios of future climate change.