A matter of scale: Local biotic differentiation and potential regional homogenization of understory plant communities in a highly fragmented tropical landscape

Plant communities in highly fragmented tropical landscapes can undergo biotic homogenization and differentiation after land-use changes, such as forest fragmentation. Here we evaluated the beta diversity of understory plant communities in fragmented forest remnants and assessed species geographic distribution to infer potential local and regional scale dependency of homogenization and differentiation pathways in the Atlantic Forest in southern Brazil. More specifically we assessed: (i) community composition; (ii) the frequency of locally rare species, generalists, interior and edge specialists; (iii) the relative contribution of beta diversity components; and (iv) the influence of widely, intermediate and narrowly distributed species to the observed patterns. Understory communities were sampled in a nested design with plots allocated to the interior and edge of forest fragments, and geographic distributions were estimated based on occurrence records from herbaria collections. Our results revealed differences in species composition among fragments and habitat types (edge and interior). Most of the sampled species were classified as locally rare. The turnover component of beta diversity explained a higher proportion of variation in species composition among fragments and habitats. Species richness and abundance were higher in the interior of fragments and most of the recorded species in the study had a wide geographic distribution. Our results support a local community differentiation (high beta diversity) among fragments and habitat types and suggest a potential regional homogenization, given the predominance of widely distributed generalist species in a domain historically characterized by high levels of endemism. We recommend urgent protection and restoration actions on the few remaining fragments of the Atlantic Forest given that each remnant can support different plant assemblages that have been increasingly threatened. Future studies should explicitly consider multiple spatial and temporal scales across fragmented tropical landscapes to better understand links with biotic homogenization and differentiation of ecological communities.