Taper And Volume Models For Dahurian Larch Plantations

This article take planted larch forest with different ages, densities and site conditions in Dailing Forest Bureau in Heilongjiang Province as the research object. Four taper equations were fitted using statistical principles and non-linear regression in SAS software based on 84 stem analysis data from 21 plots. Max and Burkhart (1976) taper equation was selected as best segmented model: x=h/H, ai=inflection point.i=1,2, Three taper and volume systems with different inflection points were constructed based on Max and Burkhart segmented taper model. All parameters were shared by both the taper and the volume systems. To concurrently minimize error in endogenous variables, the Seemingly Unrelated Regression (SUR) of PROC MODEL procedure in SAS was used to obtain parameter estimates of equation systems. Coefficient determination, absolute bias, relative absolute bias, root mean square error and relative root mean square error were employed to evaluate the precision of different model systems. The results showed that Max and Burkhart segmented taper and volume model system with two inflection points provided best precision of model fitting. Model validation further confimed that this model system provided best precision of diameter and volume prediction. Then, a nonlinear mixed-effects modeling approach was used to model stem taper based on the base model. The best performances were obtained for this model with parameters b1 and b2 as mixed effects when considering plot effects and with parameters b2 and b4 as mixed effects when considering tree effects. The mixed-effects models provided better model fitting than original model, moreover, the precision of mixed-effects model when considering tree effects is better than that when considering plot effects. Validation confirmed that the mixed model with calibration of random parameters could provide more accurate and precise prediction...