Relative effects of climate, stand environment and tree characteristics on annual tree growth in subtropical Cunninghamia lanceolata forests

Tree growth is driven by various factors, including site climate, stand and tree variables. A thorough understanding of the respective roles of the main drivers is imperative to project or promote forest productivity in future scenarios. However, the complex relationships among the drivers and tree growth remained insufficiently understood. Using the structural equation modeling (SEM) approach, we examined the multiple and interactive relationships between annual tree growth and climate, stand- (site quality and competition) and tree-level (tree age and tree size) variables based on data from Chinese fir (Cunninghamia lanceolata) plantations, Southeastern China. To simplify the SEM structures, randomforest (RF) was employed initially to screen for factors with greater impacts on tree growth. SEMs showed that annual tree growth largely depended on climate and site quality, while competition had a smaller relative effect of less than 8%. Climate was the primary driver. Different climate factors influenced tree growth directly, and also indirectly via tree size and site quality. Tree growth increments decreased with the increase of maximum daily temperature (especially in current-year August), while increased with relative humidity (especially in previous-year October) and potential evapotranspiration (especially in current-year May). Site index (SI) and tree size were both of higher importance in predicting the annual tree increments and had a positive impact on tree growth, contributing about 1/3 and 15 % to the total effect, respectively. In contrast, both the direct and indirect effects of stand competition were negative. Basal area of larger trees (BAL) appeared to better capture competition than stand density. This study suggested that SEM combined with RF provided a promising way to understand the complexity of tree growth. Larger trees in forests with higher site quality and lower density grow faster, however, their annual increments seem to decrease with increasing temperature.