Research On Construction And Application Of Polymorphic Site Index Model Including Fixed Effects

Site index model is the core and key of forest site quality evaluation.In the light of the existing site quality evaluation research,there is no comprehensive consideration of the impact of site type and stand type on the site quality,it leads to the same site type and the same tree species under the same conditions get different growth process,making the establishment of the site quality evaluation model is difficult to promote,limiting its applicability,and the site quality evaluation of existing research of natural mixed forests are rare.Through the research,in order to make the site index model is applicability and easy to promote and solve the problem of regional compatibility,and try to explore and solve the problem of natural forest site quality evaluation.Based on the Chinese fir plantation in Hunan province and subtropical oak natural mixed forests as the research object,using the survey data,based on height-age classification polymorphic site index model and site type of clustering polymorphic site index model,the construction and application of the polymorphic site index model of containing fixed effects on Chinese fir plantation in Hunan province and subtropical oak natural mixed forests were studied.The main research contents and conclusions are as follows:(1)Through the height-age grading to construct the fixed effects,the polymorphic site index model of Chinese fir plantation based on tree height-age classification was established.It is pointed out that the tree-age graduation set as five-year and the tree-height classified into nine classifications,and the graded result is the best,compared with the basic model,the determination coefficient of the model is increased from 0.79964 to 0.95303,which greatly improves the accuracy of the model.It is also verified the tree height-age grading method is applied to the plantation site quality evaluation is correct and feasible.(2)Through the site type of clustering to construct the fixed effects,the polymorphic site index model of Chinese fir plantation based on site type of clustering was established.Combine 606 sample neutrality factors into 240 initial site types,and the initial site type as the fixed effects add the parameters of a nonlinear regression model,get 240 initial site type coefficient value of the corresponding.Using k-means clustering in the R software,240 different site types are clustered into 12 categories,the combined factor level information contains 99%of the factor level information before the merger.The site type of each sample is transformed into the corresponding class,and 12 classes are added as fixed effects to the basic model for nonlinear fitting regression analysis,the coefficient of determination of the model R2 =0.98811,which further improves the accuracy of the model and confirms that the thinking and method of building the fixed effect of site type clustering is applied to the plantation site quality evaluation are feasible in practice.(3)Two kinds of site quality evaluation method was applied to oak natural mixed forests to explore research,set up two kinds of different oak natural mixed forests polymorphic site index model:1)Through the height-age grading to construct the fixed effects,the oak natural mixed forests polymorphic site index model of was established,the coefficient of determination of the model R2 = 0.98074,the model has high precision,and the modeling process does not need to add the site factor,and the operation is simple.2)Through the site type of clustering to construct the fixed effects,the oak natural mixed forests polymorphic site index model of was established,the coefficient of determination of the model R2=0.87584,the model has high precision,it can ensure that the same site type,the same species under the same conditions to get the same growth process.3)Compared with the simplex site index model can't fitting oak natural forest growth process,two methods all can apply to the oak natural forest site quality evaluation,and the model has high precision and strong applicability,which can solve the problem of quality evaluation of natural forest,and provide a new idea and reference method for the quality evaluation of natural forest.(4)The two methods of constructing the polymorphic site index model are suitable for both artificial forest and natural forest site quality evaluation.The model is applicable and can solve the problem of regional compatibility and the evaluation of the quality of natural forest.The polymorphic site index model including fixed effects provides a new idea and method for the evaluation of forest site quality in China,not only for Chinese fir plantation and oak natural mixed forest,but also can be used for other forest types of site quality evaluation.