Climate-sensitive self-thinning trajectories of Chinese fir plantations in south China

The self-thinning rule is fundamental in regulating maximum stocking and constructing stand density management diagrams. Chinese fir (Cunninghamia lanceolata (Lamb.) Hook.) is the most important tree species and widely distributed across subtropical China. Yet, our understanding of how the self-thinning line of Chinese fir relates to climate is limited. Longitudinal data from 48 plots distributed in Fujian, Jiangxi, Guangxi, and Sichuan provinces were used to describe self-thinning for Chinese fir in relation to climate through first-order autoregressive (AR(1)) and nonlinear mixed effects (NLME) models. Results showed that self-thinning lines had steeper slopes for Chinese fir growing in areas with larger annual precipitation and summer mean maximum temperature but flatter slopes with higher mean annual temperature, degree-days below 0 °C, and winter mean minimum temperature. Winter mean minimum temperature was the dominant climatic factor in shaping self-thinning lines, which suggests that temperature was the key climate driver that affects self-thinning of Chinese fir. In addition, differences of slopes for any two of the four sites were significant, except between the Guangxi and Sichuan sites. Our results will be useful for both the silvicultural practices and mitigation strategies of Chinese fir under climate change in south China.