Aging plantations are constrained by a biophysical ceiling on carbon and ecological co-benefits

Large-scale afforestation underpins climate mitigation strategies worldwide, but aging plantations face biophysical limits on what they can deliver. Using Japan as a national case, where plantations cover ∼40% of forest area and most stands have reached maturity, we integrate field measurements and multi-source remote sensing data to map two structural dimensions of needleleaf forests: stand size, a structural index reflecting cumulative growth, and structural complexity, a structural index of within-stand heterogeneity in tree size. We find that plantation stand size appears to plateau along the observed age gradient at around 70 years, whereas structural complexity is only weakly related to stand age and instead co-varies more strongly with topography and management legacies. In natural forests, by contrast, both stand size and structural complexity show a positive association with stand age across the sampled gradient. Comparing observed plantation states with the structural states of natural-forest analogs under similar climate and topography allows us to characterize the trade-off between stand size and structural complexity and to identify candidate areas for restoration prioritization. Of Japan's 3.50 Mha national conversion target, 51–59% (1.79–2.06 Mha, depending on reference forest mask dataset) falls within areas where natural-forest analogs show higher values in both dimensions than current plantation states, while the remaining area corresponds to configurations involving explicit trade-offs. Our findings point to a biophysical ceiling on what plantation-based restoration can deliver under current climate and topographic constraints and underscore the need for spatially explicit policy trade-offs.