Facilitation beyond species richness

1. Facilitation studies have previously focused on the effects of plant-plant interactions on species richness and, more recently, on functional traits or phylogenetic aspects. Little is known, however, about the simultaneous effects that facilitation have on overall biodiversity, jointly considering taxonomic, functional and phylogenetic diversity. 2. In this study, we investigated shrub facilitation on herb communities in a Mediterranean grassland over a 9-year period (2007-2015), each year representing different water availability conditions. Taxonomic, functional and phylogenetic diversity indices were estimated using generalized Hill numbers within the integrated framework of attribute diversity. We used generalized linear mixed models to analyse the effect of shrub cover on the different diversity indices along a water availability gradient. 3. Shrub cover had a positive effect on all attribute diversity indices, yet greater for abundant species. Shrubs also favoured survival of certain species (26.5% of total species), but above all they affected relative abundances of common species, filtering growth of dominant species (e.g. Poa bulbosa) and promoting evenness. Diversity indices accounting for abundant (i.e. high coverage) species showed stronger facilitation effects under mid-range water availability conditions. 4. These results suggest a preeminent role of shrubs in the maintenance of all three dimensions of diversity in Mediterranean grasslands, promoting more even communities, and to a lower extent, also species richness. Facilitation effects on taxonomic, functional and phylogenetic diversity also increased at mid-range values of water availability, in accordance with the humped-back model. Our study illustrates the need to evaluate different biodiversity dimensions from an integrated perspective, such as attribute diversity, to disentangle the role of facilitation in community assembly.