Stem Taper Functions For White Birch(Betula Platyphylla) In Different Regions Of Daxing' An Mountains In Northeastern China

Stem taper equations are essential tools for assessing the stem profile and volume in forest management studies.Such equations have long been investigated for different birch species in Canada,USA,Europe,and China.White birch(Betula platyphylla Suk.)is a valuable hardwood tree species and maintains a considerable share in the species composition of Northeast China.It is included among top ten national tree species.As fourth most valuable tree species in the region,it possesses significant economic,ecological,and medicinal values.However,a specific stem taper equation does not exist for white birch in China or abroad.This work presents stem taper and volume equations for white birch in different regions of Northeast China.In this study,the performance of eight well-known taper equations was evaluated to predict the diameter,total volume,and merchantable volume of white birch.The selected taper equations represented three major types of taper equations(simple polynomial taper equations,segmented taper equations,and variable form taper equations).Total 868 trees were selected for destructive sampling.The sample was collected from three regions of Daxing'an Mountains,Northeast China.Diameter and height data of sampled trees were utilized for fitting and validation of the taper equations.A first-order continuous-time autoregressive error structure was used to account for the inherent autocorrelation and to attain the valid estimation of the parameters.The condition number was used to detect the multicollinearity in the models.The evaluation of the taper equations based on the overall statistical criteria and box plots of diameter and volume residuals against relative height and diameter classes.The Models of Kozak(2004)-2,Bi(2000),Fang et al.(2000),and Max and Burkhart(1976)performed adequately among variable form and segmented taper functions,respectively.However,the models of Bi(2000)and Max and Burkhart(1976)behaved inconsistently depending upon the variables and regions.The models of Kozak(2004)-2 and Fang et al.(2000)showed similar results up to some extent.Overall,the model of Kozak(2004)-2 delivered the best results in predicting the diameter at any height of the stem,total volume,and merchantable volume.The model of Sharma and Oderwald(2001)appeared to be an inappropriate model for this dataset.The method of the non-linear extra sum of squares indicated differences in region-specific taper functions.Therefore,a specific taper function should be adjusted accordingly for each region in Daxing' an Mountains.