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.

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Old, climatically buffered, infertile landscapes (OCBILs) have been hypothesized to harbour an elevated number of persistent plant lineages and are predicted to occur across different parts of the globe, interspersed with other types of landscapes. We tested whether the mean age of a plant community is associated with occurrence on OCBILs, as predicted by climatic stability and poor soil environments. Using digitized occurrence data for seed plants occurring in Australia (7033 species), sub-Saharan Africa (3990 species) and South America (44 482 species), regions that comprise commonly investigated OCBILs (Southwestern Australian Floristic Region, Greater Cape Floristic Region and campos rupestres), and phylogenies pruned to match the species occurrences, we tested for associations between environmental data (current climate, soil composition, elevation and climatic stability) and two novel metrics developed here that capture the age of a community (mean tip length and mean node height). Our results indicate that plant community ages are influenced by a combination of multiple environmental predictors that vary globally; we did not find statistically strong associations between the environments of OCBIL areas and community age, in contrast to the prediction for these landscapes. The Cape Floristic Region was the only OCBIL that showed a significant, although not strong, overlap with old communities.