Revised estimates of forest carbon sequestration reveal the true sink capacity of Japanese forests

An accurate estimate of total forest carbon sequestration is crucial to develop effective strategies for wood production and reduce CO₂ emissions. Japan's National Forest Inventory (NFI) collects data on stand stem volumes in forests across Japan. The inventory based on direct field measurements of individual tree sizes (m-NFI) is regarded as the most accurate and robust. Thus, the difference in m-NFI-derived carbon stock estimates between consecutive inventory surveys is considered the most reliable estimate of forest carbon uptake rates. Here, we show that net annual carbon uptake estimated from the m-NFI conducted in the periods 2009-2013 (third NFI) and 2014-2018 (fourth NFI) is 34.6 and 46.2 TgC yr^-1 for forests managed under the governmental management plan (m-NFI_1) and all forest categories (m-NFI_2), respectively. These are approximately double the estimates from the NFI based on the predictions from yield tables (p-NFI), which Japanese national and local forestry agencies have conventionally used. Notably, Cryptomeria japonica, the most important plantation tree species for timber production in Japan, has the highest area-based carbon uptake rate among the studied forest types and, despite being a single tree species, accounts for 43.1% of Japan's total annual forest carbon sequestration. Further investigation using a machine-learning algorithm revealed that the primary factor controlling total forest productivity is the forest type, followed by stem volume. Forest age and meteorological drivers are not significantly influential. We emphasize the importance of accounting for high forest productivity in planning for future forestry and carbon neutrality in Japan.