To study the effects of competition and climatic factors on the growth of subtropical Chinese fir plantation, and construct a mixed-effect model with competition and climatic effects, so as to provide reference for the harvesting and management of subtropical Chinese fir plantation.

103 pieces of Chinese fir plantation in 6 subtropical provinces of China were used as research objects, and the random forest method was used to screen the competition and climatic factors with significant effects; the screened factors were graded and combined into competition and climatic types according to the standard, and then clustered into the competition and climate groups by K-means clustering; the optimal base model was screened out from five base models; the significance factors, significance factor, competitive type group and climatic type group were added as random effects to each parameter of the optimal base model to construct a mixed-effects model.

1) The competition factors screened for significant effects on stand break area were the ratio of the sum of the diameter at breast height of other trees not equal to the DBH of the target tree to the DBH of the target tree (CI1) and the Alemdag competition index (CI6); and he climatic factors are Hargreaves Climate Moisture Deficit (CMD), Mean Warmest Monthly Temperature (MWMT), Mean Summer Temperature (Tave_sm), Mean Minimum Summer Temperature (Tmin_sm); 2) The optimal base model obtained by screening was the Schumaker model M1 (R²=0.938 2); 3) The evaluation results of the constructed mixed-effects model showed that the coefficients of determination (R²) of the model increased to 0.954 3 and 0.955 8 when the significant competition and climatic factors were considered as random effects, and the coefficients of determination (R²) of the mixed-effects model further increased to 0.961 3 and 0.967 1 when competition and climate groups were considered as random effects. Considering that competition and climate have a common effect on stand break area, the coefficient of determination (R²) reaches 0.979 9 when the competition and climate groups are used as random effects.

Competition and climate have a significant effect on the growth of stand breaks, and the subtropical Chinese fir plantation stand break area model with competition and climatic factors has a better fitting effect and prediction accuracy, and provides a theoretical basis for the management of stand growth and management of subtropical Chinese fir plantation. It provides a theoretical basis for the growth and management of subtropical Chinese fir plantation.