While plant diversity enhances community stability against invasions, its interplay with environmental stressors remains unclear. We tested how drought modulates diversity-stability relationships by establishing 300 experimental plots across a richness gradient (1-16 species), introducing invader Symphyotrichum subulatum to half, and applying three drought treatments (none, moderate and extreme). Results revealed a drought-dependent shift: species richness stabilized communities under no drought by intensifying light competition (via selection effects of Patrinia scabiosaefolia/Artemisia migoana), but this effect disappeared under drought stress. Instead, reduced root-shoot-ratio (RSR) emerged as the key driver of stability under drought stress, which was unaffected by species richness. The divergent roles of species richness in mediating light competition (no drought) versus RSR (drought stress) explain this context-dependent stability pattern. These findings provide crucial insights for predicting and managing invasion dynamics in the context of increasing drought extremes.

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.

Although herbivory and allelopathy play important roles in plant invasions, their roles in mediating the effect of plant diversity on invasion resistance remain unknown. We performed a two-year field experiment in which we manipulated plant-species richness, herbivory and allelopathy, and then experimentally invaded the plots with the invasive Solidago canadensis. One year after the start of the experiment, there was no significant effect of species richness on biomass of the invader, as a negative indirect effect of species richness via increase light interception was compensated by a positive direct effect. However, in the second year, the relationship was negative, in line with the diversity-invasibility hypothesis. The negative relationship was strongest when herbivory and allelopathy were not manipulated. Therefore, we conclude that joint effects of herbivory and allelopathic interactions between the native plants and the invader play a major role in the observed species richness-invasibility relationship.