Climate change as a driver of biotic homogenization of woody plants in the Atlantic Forest

Aim: The importance of anthropogenic activities in reshaping biodiversity is increasing fast. Interactive effects of climate change, biological invasions and species replacement are poorly understood, particularly at large scales and in megadiverse biomes. We aimed to assess the effects of climate change as a driver of spatio-temporal biodiversity patterns and homogenization of woody plants at multiple scales, in a hyperdiverse species assemblage. Location: The Atlantic Forest, Brazil. Time period: Present, future projections. Major taxa studied: Woody plants. Methods: We used ecological niche modeling to estimate geographic distributions of 2,255 plant species under present and future climates. Range-diversity plots based on species presence-absence matrices and null models were used to explore changes in alpha and beta diversity, range size and covariance in species composition across ecoregions, climatic scenarios and within protected areas. We also partitioned dissimilarity into turnover and nestedness components and explored expansions and retractions of species' ranges to infer invasive potential and implications for conservation in the future. Results: Despite a general increase in local richness, beta diversity decreased with time. Similarity among sites was accentuated in severe climate change scenarios enhancing biotic homogenization at large scales. Changes were not constant or homogeneous across ecoregions and at smaller scales, but a consistent pattern was the reduction of beta diversity accompanied by increments in the mean range size of widespread species. Likewise, subsets of assemblages within protected areas presented higher similarity, increased mean range size and invariability of richness through time, indicating potential compositional turnover promoted by the expansion of widespread species. Main conclusions: Expansion of current generalist and disturbance-tolerant species rather than extinction or retraction of local endemics may lead to woody plant homogenization in the tropics. The woody plant assemblage in the Atlantic Forest is prone to a structural reorganization due to climate change, threatening conservation of biodiversity and potentially leading to severe large-scale biotic homogenization in the near future.