基于两水平混合模型的杂种落叶松胸径和树高生长模拟

Simulating DBH and Height Growth of Trees for Hybrid Larch Plantation with Two-level Mixed Effect Model

以江山娇实验林场的16块杂种落叶松密度对比标准地为研究对象,使用非线性最小二乘法,对4种常用胸径和树高生长模型拟合优度进行比较,最终选择Schumacher方程作为混合模型的基础模型。考虑林分效应和林木效应,采用两水平嵌套混合模型的方法,通过R软件进行不同参数混合作用的模拟,选择收敛的AIC、BIC和对数似然值最小的混合模型作为最优模型,最后将其与传统模型进行拟合优度的比较。结果表明:胸径生长模型中将林分效应作用于参数a1、林木效应作用于参数a0和a1,树高生长方程中将林分效应和林木效应作用于参数a0时的混合模型最优,可将胸径和树高生长传统模型的确定系数从0.751 0和0.900 8提高到0.946 3和0.947 4,均方根误差也大幅缩小,并且较好地消除了异方差现象,说明通过构造方差协方差矩阵可以校正随机参数以反映树木之间生长的差异。

Four DBH and height growth equations were evaluated using nonlinear least square method to develop the model with the highest precision for Hybrid Larch stands of different density in Jiangshanjiao Experimental Forest Farm of Heilongjiang Academy of Forestry, and the Schumacher model was chosen for a higher precision. Considering the stand effect and the trees effect at the same time, the mixing effects with different parameters were simulated based on two-level mixed effects model using R software, and the best mixed model was developed for the smallest AIC, BIC and loglikelihood. When al with the stand effects and tree effects, a0 with the tree effects were considered, the DBH growth model was best. When a0 with the stand effects and tree effects were considered, the height growth model was best. The coefficients of determination could be reached 0.946 3 and 0.947 4 from 0.751 0 and 0.900 8, and RMSE were also significantly decreased. The mixed model showed more homogeneous residual variance than the traditional models. Therefore, the mixed model not only showed the mean trends of DBH and height, but also showed the individual difference based on variance-covariance structure .