Carbon stock in Japanese forests has been greatly underestimated.

An accurate estimate of total forest carbon (C) stock and C uptake is crucial for predicting global warming scenarios and planning CO emission reductions. Forest inventory, based on field measurements of individual tree sizes, is considered the most accurate estimation method for forest C stock. Japan's national forest inventory (NFI) provides stand-scale stem volume for the entire forested area based on (1) direct field measurements (m-NFI) and (2) prediction using yield tables (p-NFI). Here, we show that Japanese national and local forestry agencies and some research studies have used p-NFI and greatly underestimated the Japanese forest C stock (58-64%) and net annual C uptake (41-48%). This was because approximately 10% of the forest area was not counted in p-NFI and because the yield tables in p-NFI, which were constructed around 1970, were outdated. For accurate estimation of the forest C stock, yield tables used in p-NFI should be reconstructed or ideally field measurement campaigns for m-NFI should be continued. In the future, appropriate forest management plans are necessary to effectively use the high CO absorption capacity of Japanese forests and these should be compared with other industries' CO reduction plans from a cost-benefit perspective.