Aim: Plant–plant interactions are an important factor in structuring plant biodiver- sity. However, most studies on the impacts of plant–plant interactions on biodiversity focus on species richness and to a lesser extent on other facets of diversity. When other facets of biodiversity are considered, studies often include a limited set of en- vironmental conditions. We aimed to quantify the impacts of nurse plants on taxo- nomic, functional and phylogenetic facets of plant biodiversity and how these impacts change across environmental gradients. Location: We report data on 28 alpine and 50 dryland sites, established in 11 countries. Time period: 1998–2019. Major taxa studied: Vascular plants. Methods: We analysed plant–plant interactions using co-occurrence analyses and obtained information on the functional traits and phylogenetic relationships of the beneficiary species. We calculated the change in taxonomic, functional and phyloge- netic diversity caused by the presence of nurse plants and environmental conditions. Results: Nurse plants enhanced taxonomic and phylogenetic diversity in alpine com- munities and enhanced functional and phylogenetic diversity in dryland communities. However, they had a positive effect on functional and taxonomic diversity only in moderate environments in alpine and dryland communities, respectively. The effects of nurse plants were larger in drylands for functional and phylogenetic diversity and in alpine communities for taxonomic diversity. Interestingly, nurse plants induced non- linear changes in all facets of biodiversity across environmental gradients, whereas biodiversity tended to increase linearly toward more benign conditions in open interspaces. Main conclusions: Facilitation is important in alpine environments to maximize taxo- nomic diversity and has a more consistent positive effect on the functional and phy- logenetic diversity of drylands. In general, effects of facilitation on biodiversity peak in moderate environmental conditions, which suggests that nurse plants can protect biodiversity from impacts of climate change mostly in those places currently under low environmental stress and that will be likely to experience moderate stress levels in the future.

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