Temporal changes in taxonomic and functional alpha and beta diversity across tree communities in subtropical Atlantic forests

Biodiversity is changing globally, but how these changes impact the structure of local ecological communities remains uncertain and debated. Understanding whether biodiversity increases, decreases, or remains stable across different scales is essential for predicting and managing these shifts. Here we assessed temporal taxonomic and functional alpha and beta diversity of adult and juvenile tree communities across 11 sites in the subtropical Brazilian Atlantic Forest to infer about trends and drivers of biodiversity change. The tree communities were evaluated for temporal changes in: 1) taxonomic and functional alpha diversity, 2) taxonomic and functional composition (beta diversity), and 3) identifying potential abiotic and biotic drivers of these changes, considering three censuses across a period of 10 years. Our results revealed few changes and little directionality in temporal alpha diversity but evidenced directional changes in temporal beta diversity. Furthermore, the rate of change was slightly similar for adult and juvenile tree communities, considering both temporal alpha and beta diversity. Beta diversity decreased over time, which was more pronounced for taxonomic than functional dissimilarity, suggesting a taxonomic homogenization within these forests. Our findings offer important insights that help clarify the mixed results on biodiversity trends, showing that temporal beta diversity is a more sensitive indicator of community change than temporal alpha diversity. The study highlights species loss or gain, which may influence ecosystem functioning, providing crucial information for conservation and management efforts. With most global time series datasets geographically skewed toward the Northern Hemisphere, studies like ours are increasingly important for providing a more balanced understanding of global biodiversity changes.