Temporal Turnover Dynamics of Secondary Castanopsis eyrei-Schima superba Forests with Different Stand Ages in Subtropical Climate Zones

Based on monitoring data from 2017 to 2024, this study systematically analyzed biomass accumulation, productivity dynamics, and dominant species shifts in the secondary Castanopsis eyrei-Schima superba forest across three stand ages (20 ~ 30 a, 30 ~ 40 a, and 50 ~ 60 a). The results showed that community biomass increased as stand age raised, with the greatest increment observed in the 20 ~ 30-stands (40.43 t·hm⁻²), indicating a rapid growth phase during the early middle-aged stage. Productivity differed significantly among stand ages, being highest in the 20 ~ 30-year stands and lowest in the 50 ~ 60-year stands; nevertheless, the evident functional compensation and substitution effects within the community mitigated the decline of productivity with the increasing stand age. The contribution rate of dominant species to productivity gradually decreased with the increasing stand age (20 ~ 30 a: 87.84%; 30 ~ 40 a: 65.78%; 50 ~ 60 a: 37.42%), reflecting a successional shift from the community dominated by a few constructive species to a more stable structure with multi-species contributions. At the individual level, the supplemental rate peaked in the 30 ~ 40-year stands (1.36%), while the mortality and turnover rates were highest in the 50 ~ 60-year stands (2.08% and 3.39%, respectively), indicating that species replacement dominated in middle-aged stands, individual mortality dominated in old-aged stands. Overall, this forest community exhibited relatively stable productivity and a more balanced species contribution during the process of succession. These findings revealed the productivity allocation patterns of subtropical secondary evergreen broad-leaved forests at different age stages, providing a scientific basis for degraded forest restoration and sustainable management. Based on the stand age gradient and long-term dynamic monitoring, this study elucidates the stable functional compensation mechanism of community productivity during the succession of subtropical secondary C. eyrei-S. superba forests, identifies the stage characteristics of species and individual turnover, and fills the knowledge gap in successional dynamics of secondary forests in the region.