Construction of Nonlinear Mixed Model of Stand Growth Rate

The simultaneous equations of diameter at breast height and mixed growth rate were established based on the data of 2 periods of fixed sample plots in Lishui, Zhejiang. The model takes the average diameter at breast height as the independent variable, and the annual growth rate of stand volume as the dependent variable. First, simultaneous equation method was used to model fitting and primary selection, then the nonlinear mixed model method was used to analyze the fixed effect and random effect of the selected model. The results showed that the model and parameters constructed by simultaneous equations reflect the average growth rate of the model. However, without considering the random effects of origin, age group and tree species group, the fitting effect was not good, further analysis was needed using the mixed model method. The model parameters fitted by simultaneous equations method could explained the fixed effect of the mixed model well. The random effect of mixed model had significant effect on the growth rate, and the difference of parameters was significant. Due to the sum of random effect parameters of random variables of mixed model was zero, the random effect parameter was used to construct the simultaneous equations model to establish the growth rate mixed model, and construct the simultaneous equations of DBH model and growth rate mixed model. The applicability test of the test samples shows that the predicted and measured values of the simultaneous equations have no systematic deviation, and the system of equations has strong applicability. Based on the data of the same period, the growth of the whole city was predicted by the model. Compared with the same period of monitoring results of fixed sample plots, forecast accuracy of the number and rate of growth is 91.5%, 98.7% respectively.