Trait-mediated community assembly and species responses to environmental gradients in alpine plant communities across contrasting regional species pools

Abstract Background Mountain systems exhibit steep elevational gradients that shape plant communities, but community assembly also depends on the composition and size of the regional species pool and local climatic and environmental conditions. This study compares two contrasting mountain ranges, the Eurosiberian Pyrenees and the Mediterranean Guadarrama, to assess how functional traits mediate plant community responses to elevational and local environmental gradients. We hypothesize that in regions with a diverse species pool, such as the Eurosiberian Pyrenees, community assembly will exhibit strong trait-based filtering due to greater functional variability. In contrast, in the Mediterranean Guadarrama, where water stress interacts with low-temperature constraints, we expect stronger trait-based assembly despite the smaller regional species pool. This contrast offers a unique opportunity to disentangle the relative roles of regional species availability and local environmental filtering in shaping alpine plant communities. Results The Mediterranean mountain showed lower taxonomic and functional richness (48% and 66%), yet it exhibited higher functional diversity and greater interspecific variability in traits such as vegetative height and seed mass. Hierarchical Modeling of Species Communities (HMSC) revealed larger variation in species elevational responses in Guadarrama. Trait–environment relationships differed between mountains. In Guadarrama, species with higher leaf dry matter content (LDMC) tended to respond positively to elevation, while in the Eurosiberian Pyrenees, elevation selected for short species with small seeds. In both mountains, soil nitrogen had an overall positive effect on plant species, but it interacted differently with traits. In Guadarrama, tall plants with large seeds were more abundant in fertile soils, whereas in the Pyrenees small-seeded species prevailed. Solar radiation acted as an additional filter in the Pyrenees, promoting species with higher LDMC. Conclusions Our results demonstrate that community assembly is context-dependent, with local environmental filters modulated by regional climate and the size of the species pool. In the Mediterranean mountain, the simultaneous stressors, cold and drought, lead to higher functional diversity despite lower species and functional richness. These findings highlight the joint influence of regional species pools and local abiotic pressures in shaping divergent functional strategies among alpine plant communities.