Topography and Soil Moisture Regulate the Temperature‐Biodiversity Relationship of Forests

Aim Climate change poses a global threat to forest ecosystems. However, its effects are usually examined independently of local factors, assuming that functionally diverse forest habitat types within a single biome will react similarly. Here we evaluated how temperature influences the taxonomic and functional diversity of tree and shrub guilds, accounting for the regulatory effects of local factors across forest habitat types. Location Italy. Time Period From 1970 to 2020. Major Taxa Studied Trees and shrubs. Methods We integrated > 5000 forest vegetation plots from a national databases with data on seven functional traits. We fitted regression models to quantify the individual and interaction effects of temperature, solar radiation (a proxy for topography), and soil moisture on the taxonomic and functional diversity of tree and shrub guilds across four main forest habitat types. Results Temperature gradients similarly affected the taxonomic and functional diversity of both tree and shrub guilds, although with a stronger magnitude for trees. Topographic solar radiation regulated mainly the temperature-diversity relationship in trees, with a stronger positive effect on cold forest habitats compared to warm ones. Soil moisture exerted a stronger control on shrub guilds, especially in cold forest habitats, but with positive and negative effects on taxonomic and functional diversity, respectively. Assuming climate-diversity responses will hold under climate change, projections to 2100 suggest that the diversity of warm forest habitats might reduce, possibly due to intensification of summer drought stress, while the diversity of cold forest habitats might increase as winter frost stress lessens. Main Conclusion Temperature is a strong biodiversity determinant. Still, projections should account for local regulatory mechanisms and consider that grouping different forest habitat types into a broad category can obscure critical diversity responses. The distinct responses of forest habitat types to topography-mediated climate conditions suggest the need for targeted adaptive management strategies.