How soil and elevation shape local plant biodiversity in a Mediterranean hotspot

Elucidating how evolutionary and ecological factors drive the assemblage of communities in biodiversity hotspots remains an important challenge. This currently impedes our ability to predict the responses of communities to the ongoing global changes in these major world’s biodiversity reservoirs. Here, we focus on the Sierra Nevada mountain range, a core region of the Baetic-Rifan biodiversity hotspot in the western Mediterranean, and explore the relative importance of soil properties and elevation in shaping phylogenetic and functional diversity of shrub communities. We recorded the total number of each species in community transects across elevation gradients and contrasting soil conditions, and measured some ecologically relevant functional traits (specific leaf area, leaf carbon:nitrogen ratio, plant height and blooming duration). Phylogenetic distances among species were inferred from a genus-level time-calibrated molecular phylogeny. Elevation was the main factor predicting phylogenetic and functional alpha diversity of plant communities. Species in high-elevation communities were phylogenetically distant but functionally similar, being relatively smaller and having relatively short blooming durations, whilst species in low-elevation communities showed the opposite pattern. Beta diversity in SLA and leaf C:N ratio based on species incidences were positively correlated with a soil pH and micronutrient gradient. Specifically, communities that develop on soils of high pH and low micronutrient concentrations showed low SLA values and high leaf C:N ratios, whilst communities on soils of lower pH and high micronutrient concentrations showed the opposite pattern. We conclude that soil properties and elevation simultaneously shape the structure of Mediterranean shrub communities by differentially acting on the different dimensions of the species niches. Elevation seems to filter plant height and phenology-related traits whereas nutrient-related functional traits are more related to soil properties. Our study illustrates the primary role of environmental heterogeneity for the maintenance of diversity in Mediterranean mountain ecosystems.