Table 1_How do stand age and site quality shape productivity of Chinese fir plantations via stand structural pathways?.docx

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.

人工林的林分生产力源于发育阶段、立地潜力与林分结构组成之间的相互作用，但在杉木（Cunninghamia lanceolata）人工林中，林分年龄与立地质量调控生产力的具体路径仍未得到充分阐明。本研究在中国北亚热带地区选取了4个林分年龄（5、15、20和30年；共24块样地）的纯杉木人工林作为研究对象。本研究以林分总蓄积量（total stand volume, TSV）和年均材积生长量（mean annual increment, MAI）作为生产力的量化指标。研究选取的林分结构特征包括林分密度、林分郁闭度（canopy closure, CC）、冠径比（crown diameter ratio, CDR）以及生长优势系数（growth dominance coefficient, GDC），并采用多模型回归与偏最小二乘结构方程模型（partial least squares structural equation modeling, PLS-SEM）进行分析。仅20年生林分实施了商品性抚育间伐，该操作被视为该发育阶段内的林分结构扰动事件。林分MAI呈单峰曲线变化，在15年生时达到峰值；而TSV则随林分年龄持续累积增长，这揭示了生产力动态存在尺度依赖性。林分结构变量显著提升了模型的解释效力，其贡献远超单独使用林分年龄与立地指数的模型。其中，冠径比（CDR）与MAI的关联强度最高，而林分郁闭度（CC）与生长优势系数（GDC）则对MAI产生正向但中等程度的影响。尽管在结构方程模型（SEM）分析中，林分年龄仍与生产力存在显著的直接关联，但当纳入林分结构特征变量后，林分年龄的独立解释贡献有所下降，这表明林分年龄的表观效应主要通过结构演替而非时间累积发挥作用。20年生林分通过间伐降低的林分密度，并未使林分MAI提升至15年生林分的水平之上，这表明林分结构重组受到发育阶段的限制。综上，林分年龄决定了林分的发育潜力，立地指数制约着林分的生长上限，而林分结构则介导了林分生产力的实现过程，这凸显出林分结构优化是实现人工林可持续经营的核心路径。