Facilitation as a driver of plant assemblages in Caatinga.

Nurse plants reduce the environmental severity experienced by neighboring plants by providing shade, enabling nutrient accumulation or protection from herbivores within patches of vegetation. Nurse plants should preferentially promote the coexistence of ecologically dissimilar species with little niche overlap, and if ecological traits are conserved within evolutionary lineages, this should result in phylogenetic overdispersion. In contrast to competition, facilitation is expected to increase species richness. Therefore, to examine the role of facilitation as a driver of plant assemblages in Caatinga, we quantified the functional traits of nurse species and compared species richness, phylogenetic diversity, phylogenetic structure of the tree layer and of the herbaceous layer between patchy Caatinga and Caatinga with segregated plants. Results show that functional traits related to resilience and resistance against herbivory seem to be crucial for facilitation in Caatinga. Autochory occurs at a higher frequency in nurse plants than in Caatinga in general. The herbaceous layer of patchy Caatinga is richer in species than of Caatinga with segregated plants, and facilitation is the suggested cause. As the whole community of the 196 patches is phylogenetically overdispersed compared to the null expectations, facilitation seems to predominantly promote the coexistence of dissimilar species with little niche overlap. [ABSTRACT FROM AUTHOR]