Centurial changes in forest structure and composition in southwestern Japan based on historical data

AimsThe impacts of anthropogenic activities, such as deforestation and land-use change, and climate change on forest ecosystems have intensified in recent years. To predict future dynamics and guide management, it is crucial to clarify forest structure and composition prior to these disturbances; however, few studies have analyzed data recorded before major human and climate-related impacts. Here we analyzed forest inventory data (467,809 trees ≥ 7.5 cm in diameter at breast height at 534 plots) from the 1920s–1930s to evaluate these long-term effects.LocationAlmost all forest vegetation plots in southwestern Japan.MethodsTo assess centurial changes in forest structure and composition, we calculated aboveground biomass (AGB), size structure, species richness, and species composition for historical and current forest plots. We further examined the effects of anthropogenic activity on the density of large-diameter trees.ResultsThe average density of large-diameter trees and species richness in historical forests were significantly 2.4 and 1.3–1.4 times higher, respectively, than in current forests, but average AGB was similar. The effect of anthropogenic activity was larger in historical forests than current forests. In historical forests, species typical of old-growth forests were dominant, while species typical of human-managed and early successional forests were dominant in current forests.ConclusionOverall, our results indicate that, over the past century, forests in southwestern Japan have been shaped more strongly by human activities than by climate change, resulting in pronounced changes in forest structure and species composition. The stability of AGB despite these changes suggests that AGB alone is insufficient to characterize forest functioning. The reconstructed forest status from a century ago offers valuable insights for future forest management and restoration efforts to achieve nature positive.