Effects of competition, age and climate on tree slenderness of Chinese fir plantations in southern China

Tree slenderness coefficient (TSC) is an important measure of tree stability. Generally, trees with higher TSC values are prone to snow, icing, and wind damage than trees with smaller TSC values. Chinese fir (Cunninghamia lanceolata (Lamb.) Hook.) a native and fast growing species, is widely distributed in southern China. Yet our understanding of how TSC of Chinese fir plantations is affected by biotic (including size, competition, and age) and climatic variables in different sites is limited. A data set of 60 remeasured plots consisted of five spacing trials and distributed in in Fujian, Jiangxi, Guangxi, and Sichuan provinces in southern China were used to explore TSC of Chinese fir relating to biotic and climatic variables using nonlinear mixed effects model (NLME). Results showed that TSC values increased with increasing stand age (Age), stand basal area (BA), and mean annual temperature (MAT). In contrast, it decreased with increasing tree density index (TN), relative diameter (RD = DBH/quadratic mean diameter), mean warmest month temperature (MWMT), and annual heat-moisture index (AHM). The relative importance of RD influencing TSC was the most followed by Age, Spacing A, TN, Spacing D, MAT, BA, Spacing C, Spacing B, AHM, and MWMT through hierarchical partitioning analysis. MAT was the most important climate factor that contributed to TSC variations, followed by AHM, and MWMT, which indicated that temperature was a critical climate variable modulating TSC of Chinese fir in southern China. Additionally, trees grown in less dense stands of 1667, 3333, and 5000 trees ha−1 were more stable (lower TSC) than those grown in dense stands of 6667 and 10 000 trees ha−1. In a word, TSC variance explained by biotic variables was 59% much larger than climate variables 2.8%. Our results will help with further assessments of tree/stand stability of Chinese fir plantations under climate change.