How do stand age and site quality shape productivity of Chinese fir plantations via stand structural pathways?

Stand productivity in plantation forests arises from the interaction among developmental stage, site potential, and structural organization, yet the pathways through which stand age and site quality regulate productivity remain insufficiently clarified in Chinese fir plantations. We examined pure Cunninghamia lanceolata plantations across four stand ages (5, 15, 20, and 30 years; 24 plots) in northern subtropical China. Productivity was quantified as total stand volume (TSV) and mean annual increment (MAI). Structural attributes-including stand density, canopy closure (CC), crown diameter ratio (CDR), and growth dominance coefficient (GDC)-were evaluated using multimodel regression and partial least squares structural equation modeling (PLS-SEM). Commercial thinning occurred only in the 20-year-old stands and was treated as a within-stage structural perturbation. Stand MAI followed a unimodal trajectory, peaking at 15 years, whereas TSV increased cumulatively with age, revealing scale-dependent productivity dynamics. Structural variables markedly enhanced explanatory power beyond stand age and site index. Among them, CDR exhibited the strongest association with MAI, while CC and GDC showed positive but moderate effects. Although stand age retained a significant direct association with productivity in SEM, its independent contribution declined after structural attributes were incorporated, indicating that apparent age effects operate primarily through structural succession rather than chronological accumulation. Thinning-induced density reduction at age 20 did not elevate stand MAI beyond the level observed in 15-year-old stands, suggesting that structural reorganization is developmentally constrained. Overall, stand age defines developmental opportunity, site index constrains growth potential, and stand structure mediates the realization of stand-level productivity, highlighting structural optimization as a key pathway for sustainable plantation management.