Current and future carbon stocks of a dominant forest plantation species, Cryptomeria japonica, throughout Japan

Estimation of forest carbon stocks is essential in ecological and political contexts to understand material cycles and achieve a carbon-neutral society. Because old-growth forests are vital for determining forest growth capacity, and recent climate change has significantly influenced forests worldwide, integration of the most recent data sets covering a broad range of forest ages and geographical variation is needed to construct accurate growth models and to estimate current and future forest resources. Japan's National Forest Inventory (NFI) provides nationwide information regarding forest ecosystems, in which Cryptomeria japonica forests dominate and are indispensable for forest resources and the wood industry. This study aimed to establish accurate carbon-based growth models for C. japonica forests using NFI and other literature surveys and to reveal their geographical variation with a hierarchical model. We then used this model to predict future forest carbon stocks under several management scenarios. The model estimated that the maximum forest carbon density was 1.8 times higher than previously predicted based on outdated yield tables. However, the growth capacity varied across the country, likely due to environmental gradients and forestry practices. Our predictions indicate that C. japonica forests in Japan still have the capacity to accumulate substantial carbon resources consistently exceeding 750 TgC with sustainable wood supply under suggested management efforts to increase the reforestation rate. We conclude that updated forest growth models are required, especially for old-growth forests under ongoing climate change, and that appropriate forest management is necessary to maximize the potential for forest carbon accumulation.