摘要
基于章古台地区22块樟子松(Pinus sylvestris var.mongolica)人工纯林标准地的702棵樟子松立木数据,构建了樟子松固沙林冠幅—胸径关系的基础模型、广义模型及基于混合效应的基础模型和广义模型;比较了随机选择样本木、选择平均胸径树、选胸径较小树和选胸径较大树4种方案,计算混合模型随机参数时的混合模型预测精度;最后分析了不同林木因子和林分变量对冠幅—胸径关系的影响。模型评价指标包括决定系数(R~2)、平均绝对误差(MAE)以及均方根误差(RMSE)。结果表明:枝下高(HCB)、相对植距(RS)和林龄(A)对冠幅—胸径关系影响最为显著;混合模型拟合精度(基础混合模型R~2、MAE和RMSE分别是0.703 0、0.386 6和0.515 4;广义混合模型R~2、MAE和RMSE为0.705 1、0.382 2和0.513 6)高于最小二乘法回归(OLS)模型(基础模型R~2、MAE和RMSE分别为0.587 5、0.469 6、0.607 5;广义模型R~2、MAE和RMSE分别为0.661 8、0.415 5和0.550 0)。基础混合模型和广义混合模型差异较小(2模型R~2、MAE和RMSE均相差1%左右)。冠幅随HCB和A的增大而减小,随RS的增大而增大。进行冠幅预测时,推荐使用基础混合模型并从每块标准地选择2棵平均木冠幅计算其随机参数,或使用方法较为简单的OLS广义模型预测单木冠幅大小。
In this study,the data consisted of 702 individuals of Pinus sylvestris var.mongolica in 22 temporary sample plots of pure plantation in Zhanggutai of Liaoning Province,northeast China,were used to develop the canopy-DBH models,i.e.the basic model(fitted by ordinary least squares,OLS),generalized model(fitted by OLS),nonlinear mixed-effect basic model and nonlinear mixed-effect generalized model.The goodness-of-fits and prediction accuracy of the four models were compared.For the mixed-effect model,four sampling strategies,i.e.,random sampling,large-DBH tree sampling,small-DBH tree sampling and medium-DBH tree sampling,were designed to calculate the random parameters.The effects of individual factor and stand level variables on the canopy-DBH relationship were simulated.Model evaluation indices included the determination coefficient(R~2),mean absolute error(MAE)and root mean square error(RMSE).Results showed that the height to live crown base(HCB),relative spacing index(RS)and stand age(A)were the dominant factors in CW-DBH models.The goodness-of-fits of mixed-effect CW-DBH models(R~2,MAE and RMSE)were 0.703 0,0.386 6 and 0.515 4,and those of mixedeffect generalized model were 0.705 1,0.382 2 and 0.513 6,respectively,which were better than those of the OLS models.However,the difference in goodness-of-fit between the mixed-effect basic and generalized models was not significant(below 1%).Canopy decreased with the increase of A and height of HCB,but increased with the increase of RS.
引文
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