基于随机森林模型的天然林立地生产力预测研究

Study on prediction of natural forest productivity based on random forest model

从汪清林业局大荒沟林场、大柞树林场等11个林场的森林资源二类调查数据中选取优势树种为蒙古栎、阔叶混交林及针阔混交林的小班。以海拔、土层厚度、坡位、坡向、腐殖质层厚度、坡度6个立地因子以及年平均气温、月平均气温差等19个气候因子为输入变量,以树种年平均蓄积生长量为输出变量,应用随机森林回归算法分别建立蒙古栎、阔叶混交林及针阔混交林的立地质量评价模型,对不同立地条件下的造林地进行生产潜力预测。同时,分析了各环境因子对树种生长的影响权重。结果表明:1)所建立的3种回归模型的RMSE的值分别为:0.22、0.54、0.52,R^2值分别为:0.79、0.79、0.72,模型的拟合效果较为理想。2)研究区域内,对蒙古栎生长影响较大的因子依次为月平均气温差、温度季节性变化、坡度、年降水量、年平均气温差;对针阔混交林生长影响较大的因子依次为:坡度、腐殖质层厚度、月平均气温差、最湿季度降水量、最暖季度降水量;对阔叶混交林生长影响较大的因子依次为:坡度、坡位、坡向、温度季节性变化、最干旱季平均气温。3)通过对比同一立地3种类型的生产力,针阔混交林、阔叶混交林的年平均蓄积生长量均高于蒙古栎纯林,针阔混交林略高于阔叶混交林。4)因此,应客观考虑环境因子对于林木的影响程度,使其生长环境条件尽可能地处于最佳组合状态。

The data of dominant tree species were selected from the second-class survey data of 11 forest farms such as Dahuanggou Forest Farm and Damong Quercus Forest Farm of Wangqing Forestry Bureau as the subcompartment data of Mongolian Quercus, broadleaved mixed forest and coniferous-broad mixed forest. Taking 19 climatic factors, such as annual average temperature, monthly average temperature difference etc. and 6 site factors (altitude, soil thickness, slope position, slope aspect, humus layer thickness, slope degree) as the input variables, annual average increment as the output variable, three regression models such as site quality evaluation model for Quercus mongolica, for conifer-broadleaf forest and for broadleaf forest were set up respectively by adopting random forest regression algorithm, and productive potential of afforestation land under different site conditions was predicted. At the same time, the weights of environmental factors affecting tree species growth were analyzed. The results are as follows. 1) The RMSE values of the three regression models were 0.22, 0.54, and 0.52, and the R^2 values were 0.79, 0.79, and 0.72, respectively;the fitting effect of the model was ideal. 2) In the study area, the major factors affecting the growth of Q.mongolica were monthly average temperature difference, seasonal temperature change, slope, annual total water volume and annual average temperature difference, and the major factors affecting the growth of coniferous-broad-leaved mixed forest were slope, humus layer thickness, monthly average temperature difference, wettest season precipitation and warmest season precipitation, and the factors affecting the growth of broadleaf mixed forest were slope, slope position, slope direction, seasonal variation of temperature and average temperature in the driest season. 3) By comparing the productivity values of three kinds of non-forest land types, the average annual growth values of coniferous + broad-leaved mixed forest and broad-leaved mixed forest were higher than that of pure Mongolian oak, and that of the coniferous + broad-leaved mixed forest was slightly higher than that of broad-leaved mixed forest. 4) Therefore, the influences of environmental factors on trees growth should be considered objectively so as to make their growth environment in the best combination state as far as possible.