Long-term monitoring reveals differential climate and disturbance responses in Japanese forest biome composition

: Tree community responses to climate change differ markedly among biomes, and multiple interacting drivers complicate interpretation. This study investigated structural and compositional changes across major forest biomes (boreal conifers, deciduous broadleaved trees, temperate conifers, and evergreen broadleaved trees) using extensive spatial and temporal forest community data. The analyzed dataset included 191 plots across Japan, spanning decades since the 1950s and encompassing old-growth and secondary forest age classes. For each plot, biome types were classified, and changes in tree communities were quantified. Forest structural shifts were evaluated using the total basal area of all trees, four tree functional types, and dominant species. Compositional shifts were assessed based on the relative dominance of each functional type. Associations between these changes and climatic conditions, climate change trends, and other potential forest disturbance drivers were analyzed. Results showed that forest structure and functional composition changed across all biomes, with structural change significantly linked to climatic trends. Distinct response patterns to climatic variables emerged depending on biome and forest age. In old-growth forests, structural change rates were positively associated with the mean annual temperature change and negatively associated with the annual precipitation change. Structural change declined notably in southwestern regions with increased annual precipitation, likely due to intensified typhoon disturbance. Boreal conifer functional types declined markedly in old-growth forests and were replaced by deciduous broadleaved trees. Additionally, evergreen broadleaved types increased in some deciduous broadleaved old-growth forests. Overall, these findings align with thermophilization and indicate a broader, ongoing biome shift.