Aridity preferences alter the relative importance of abiotic and biotic drivers on plant species abundance in global drylands

1. Disentangling the interplay between species-specific environmental preferences and micro- and macroscale determinants of species abundance within plant communities remains challenging. Most existing studies addressing this issue either lack empirical data regarding species interactions and local abundances or cover a narrow range of environmental conditions. 2. We merged species distribution models and local spatial patterns to investigate the relative importance of key macro- (aridity) and micro(facilitation and competition)scale determinants of plant species abundance along aridity gradients in drylands world-wide. We used information derived from the environmental niches of species to evaluate how species-specific aridity preferences modulate the importance of such factors to drive species relative abundance. 3. Facilitation and aridity preferences were more important than competition to explain species local abundances in global drylands. The specialization of communities (i.e. their compositional shifts from species with a large range of aridity preferences towards only aridity specialists) also modulated the effect of aridity and plant-plant interactions on species abundances. The importance of facilitation to drive species abundances decreased with aridity, as species preferred arid conditions and did not need neighbours to thrive. Instead, competition showed stronger relationships with species abundances under high levels of aridity. As composition became dominated by aridity specialists, the importance of aridity in shaping dryland plant communities did not increase further from moderate to high aridity levels. 4. Synthesis. Our results showed that: (a) the degree of community specialization to aridity mediates the relative importance of plant-plant interactions in determining species abundances and (b) facilitation and competition were more strongly related to species abundance in communities dominated by generalists and specialists, respectively. We observed a shift from facilitation to competition as drivers of species abundances as aridity increases in global drylands. Our findings also pave the way to develop more robust predictions about the consequences of ongoing climate change on the assemblage of plant communities in drylands, the largest terrestrial biome.