Effects of stand structure on leaf biomass and productivity in Larix kaempferi plantation

Stand age, competition‑induced mortality, and structural differentiation are key drivers of dynamic changes in leaf biomass and productivity in forest plantations, with important implications for stand development and sustainable management. In Larix kaempferi plantations in Hubei Province of central China, stand leaf biomass and productivity follow a synchronized unimodal pattern with increasing stand age. These dynamics regulate density‑dependent mortality, tree-size differentiation (Gini coefficient), and canopy leaf biomass, which collectively determine stand‑level productivity. In this study, an optimal mixed‑effects model for individual‑tree leaf biomass was developed using diameter at breast height and tree height as predictors. Based on data from 30 plots and 90 sample trees, a path analysis framework was employed to model six key variables (i.e., stand age, number of living trees, self-thinning rate, Gini coefficient, stand leaf biomass, and stand productivity) as an interactive system. A sequence of nested path models was used to quantify direct and indirect effects. Their results show that stand age exerts a strong negative indirect effect on productivity through reduced tree reduced tree density (path "stand age - number of living trees - stand productivity", effect = −0.590). Concurrently, stand age generates indirect effects by promoting tree-size differentiation (stand age - Gini coefficient - stand productivity, effect = 0.105) and increasing stand leaf biomass (stand age - leaf biomass - stand productivity, effect = 0.183). The opposing pathways of competition-induced mortality and canopy optimization highlight the need for a systems-modeling approach such as path analysis to unravel how stand productivity can be maintained despite decreasing density with stand age.