Structural thresholds regulate forest demographic processes across plantation and natural forests

Stand structure regulates forest recovery through demographic processes, yet thresholds controlling growth, recruitment, and mortality remain poorly quantified across forest types. Using long-term observations (1979–2017) from 1369 permanent plots (1225 natural forest plots and 144 plantation plots) in the mountain forest ecosystems of Sichuan, China, we separately analyzed growth, recruitment, and mortality dynamics in natural and plantation forests and identified their structural thresholds in response to stand age and basal area (BA). Plantation forests exhibited higher growth, recruitment, and mortality, with abrupt declines occurring at earlier thresholds than in natural forests (growth drops at 15.0 and 45.0 yr vs. 39.0 and 120.0 yr; recruitment declines at BA 12.0 and 28.0 m 2 ha −1 vs. 23.0 and 50.0 m 2 ha −1 ). Mortality showed contrasting trajectories: it increased at both early and late stages, indicating higher demographic risk in plantations, whereas it declined with age and only rose slightly in late stages in natural forests, reflecting more stable turnover. The influence of BA on demographic rates varied with stand age. In both natural and plantation forests, recruitment showed the highest sensitivity to BA from young to mature stands, whereas mortality became increasingly important in over-mature stands, indicating a general shift from recruitment limitation to competition-driven filtering during forest development. This transition occurred substantially earlier in plantations than in natural forests. These patterns highlight critical windows for intervention: managing stand structure before the onset of threshold declines in growth and recruitment—particularly in young, dense plantations—can accelerate transitions toward stable, resilient forest dynamics and promote more desirable forest recovery trajectories under ongoing restoration.