Relative contributions of competition, stand structure, age, and climate factors to tree mortality of Chinese fir plantations: Long-term spacing trials in southern China

Tree mortality is an important process during forest succession, influencing forest structure, composition, and ecosystem services. Chinese fir (Cunninghamia lanceolata (Lamb.) Hook.), a native and fast growing species with high quality timber, is widely planted in southern China. Yet the mechanisms leading to tree mortality of Chinese fir plantations under climate change conditions is still not clearly understood. A long-term spacing trial including 60 permanent plots located in Fujian, Jiangxi, Guangxi, and Sichuan provinces in southern China were used to explore tree mortality in relation to competition, age, structural diversity, and climatic variables using a logistic mixed effects model. Results showed that tree mortality increased with increasing LnN (natural logarithm of number of trees per ha), tree density index (TN), Gini coefficient, age (A), LnAP (natural logarithm of annual precipitation), and mean warmest month temperature, and decreased with increasing relative density (RD), inverse of diameter (1/D), and mean annual temperature. The relative contributions of competition, age and structure variables to mortality were 88.82%, much larger than climate variables 11.18%. In addition, in view of the contributions of stand factors, competition indices including RD, LnN, TN were 50.57%, much larger than age (A, 1/D) contribution of 37.45%, and structural diversity contribution of 0.80%, which indicated that competition was the main driver of tree mortality. Our findings highlight the mechanisms of tree mortality of Chinese fir in southern China under climate change.