Self-Thinning Trajectories of Chinese Fir Plantations in Southern China

Chinese fir (Cunninghamia lanceolata [Lamb.] Hook.) is the most important conifer species for timber production with a wide geographic distribution in southern China. Self-thinning is a dynamic equilibrium between forest growth and mortality in fully stocked stands. In this study, the segmented model was used to analyze the trajectory of stand density and quadratic mean diameter of Chinese fir plantations. Results showed that a linear-quadratic-linear model could simulate the self-thinning trajectories of Chinese fir plantations well. For a given province, the self-thinning slope was invariant with changes in planting densities. The self-thinning trajectories in Jiangxi and Fujian provinces were similar and were different from those in Guangxi and Sichuan provinces. The slope of the self-thinning line of Chinese fir plantations in Sichuan province was much steeper than that of the other three provinces (−4.2169 versus −2.4453). These slopes were steeper than the value of −1.605 as proposed by Reineke for many species.