A mixed-effect model of mean tree height was constructed with site and climate factors, provide an effective method for scientific management decision-making and site quality evaluation of fir plantations.

The average tree height and age of 406 groups of Chinese fir plantations in 95 Chinese fir plantation plots were used as the study subjects, using quantitative method I to screen the site factors and climatic factors that significantly affect the average tree height growth, the k-means clustering method was used to cluster into climate type group and site type group according to the standard. Based on the classification and screening results of climate factors and site factors, six commonly used tree height growth models were selected. The optimal combination after clustering was added to the optimal basic model as a random effect, the mixed effect method was used to explore the effects of climatic factors and site factors on the average tree height growth model.

The significance order of site factors was slope aspect > soil type > altitude > slope position, the significance order of climate factors was the average temperature differences > average annual temperature > annual average precipitation, the optimalbasic model is Schumark, and R² is 0.699 9, the tree height growth equations R² of Cunninghamia lanceolata plantations with climatic factors and site factors were 0.823 0 and 0.934 7, respectively; the tree height growth equation R² of Cunninghamia lanceolata plantation with site and climate factors was 0.936 9, which was 33.86% higher than that of the basic model. The results indicated that the average tree height model of Chinese fir plantation with climate and site factors had good fitting effect and prediction accuracy.

Therefore, the tree height growth model constructed provides reasonable support for regional site quality evaluation and for considering climate and site factors in the tree height growth model. At the same time, it is also of great significance for the prediction and management of forest stands.