Site index for Chinese fir plantations varies with climatic and soil factors in southern China

Chinese fir [ Cunninghamia lanceolata (Lamb.) Hook.] has a large native distribution range in southern China. Here, we tested differences in productivity of Chinese fir plantations in different climatic regions and screened the main environmental factors affecting site productivity in each region. Relationships of a Chinese fir site index with climatic factors and the soil physiochemical properties of five soil layers were examined in a long-term positioning observation trial comprising a total of 45 permanent plots in Fujian (eastern region in the middle subtropics), Guangxi (south subtropics) and Sichuan (central region in the middle subtropics) in southern China. Linear mixed effects models were developed to predict the site index for Chinese fir, which was found to vary significantly among different climatic regions. Available P, total N, bulk density and total K were dominant predictors of site index in three climatic regions. The regional linear mixed models built using these predictors in the three climatic regions fit well ( R 2 = 0.86–0.97). For the whole study area, the available P in the 0–20-cm soil layer and total N in the 80–100-cm soil layer were the most indicative soil factors. MAP was the most important climatic variable influencing the site index. The model evaluation results showed that the fitting performance and prediction accuracy of the global site index model using the climatic region as the dummy variable and random parameters and the most important soil factors of the three climatic regions as predictors was higher than that of global site index model using the climatic variable and the most indicative soil variables of the whole study area. Our results will help with further evaluation of site quality of Chinese fir plantations and the selection of its appropriate sites in southern China as the climatic changes.