Management strategies for shrinking and aging tree plantations are constrained by the synergies and trade-offs between carbon sequestration and other forest ecosystem services.

Planted forests have expanded globally over the last three decades and are expected to act as carbon sinks to mitigate further climate change. However, the planted coniferous forests in Japan are now predicted to shrink in area and age in the future. To quantify the impact of the shrinking and aging of Japanese cedar (Cryptomeria japonica D. Don) stands on the carbon sequestration rate in 2010, 2050, and 2090, we estimated net ecosystem production (NEP) at the national scale for Japan. We ran a process-based model under four forest management scenarios, in which 4-34% of the area of Japanese cedar stands were replaced by secondary broadleaf forests by 2050, and the average age of the remaining cedar stands increased from 44.4 to 59.4-80.8 years old. Contrary to our expectations, NEP was estimated to decrease from 2010 to 2050 or 2090 under all climate/forest management scenarios. The average decline in NEP from 2010 to 2050 under the shared socioeconomic pathway 1-2.6 (SSP1-2.6) was -21% across the four forest management scenarios. We also estimated the synergies and trade-offs of NEP values with two types of forest ecosystem service, landslide prevention and habitat provisioning for broadleaf trees. As a result, high rates of cutting and planting of Japanese cedar were presented as a forest management strategy in southern Japan, while low rates were more appropriate in northern Japan. The newly-developed framework for exploring climate change mitigation pathways is likely to be effective in the future, especially in developed countries where planted forests are projected to shrink and age in the coming decades.