AimTree species diversity (TSD) is positively correlated with forest availability in both historical and current contexts, which, in turn, may influence ecosystem services. Both observed and dark diversity have been widely used for biodiversity conservation and ecosystem restoration. Therefore, exploring the drivers of both observed and dark diversity could contribute to ecosystem management and nature conservation. In this study, we examined whether historical and current forest availability drive observed and dark diversity in tree species.LocationGlobal.TaxonTrees.MethodsWe obtained distribution data of the tree species, including their geographical coordinates, from a variety of databases and forest availability maps across different periods from the Historical Database of the Global Environment 3.3 (HYDE 3.3). Then, we used a hypergeometric method to estimate the species pool and dark diversity at the ecoregional scale based on pairwise associations among species, using species x species co-occurrence matrices. Finally, a general linear mixed model coupled with general additive models was used to explore the relationships between TSD (i.e., observed diversity, dark diversity, species pool, and community completeness) and forest availability.ResultsWe found significant differences in TSD in different realms. Historical and current forest availability were drivers of TSD across different spatial and temporal scales. Observed diversity, dark diversity, and species pools showed a strong but lagged response to forest availability, whereas community completeness exhibited unlagged effects. Spatial differences in TSD could exist widely along the gradient of forest availability.Main ConclusionsForest management for TSD should be based on community completeness at appropriate temporal and spatial scales. Our study provides novel insights into the conservation of forest ecosystems using different TSD indices in relation to historical and current forest availability.