[Modeling the diameter distribution of Larix olgensis plantation based on thinning effects.]

In 1974, five permanent sample plots of Larix olgensis plantation were established in Mengjiagang forest farm in Heilongjiang Province, which included one control plot and four thinning plots with different thinning treatments (remove 3.7%-49.7% of the basal area). Diameter at breast height (DBH) for each tree (19-59 a) of the five plots was remeasured 31 times from 1974 to 2014. The three parameters Weibull function was used to describe the DBH distribution of different ages for the five plots. The dynamic changes and the effects of thinning on the diameter distribution curves and the three parameters of Weibull function were analyzed. Based on the thinning effects, the parameter prediction model (PPM) of simultaneous equations system was developed and the parameters were estimated by three-stage least squares method (3SLS) and error-in-variable method of simultaneous equations. The results showed that the diameter distribution curves with stand age changed from higher peak with narrow DBH range to lower peak with large DBH range for unthinned stand (control plot). For thinning plots, the skewness was more right-hand with higher peak, narrow DBH range, and increased asymmetry of the curves after thinning. Increasing with thinning intensity, more trees of small or middle sizes were cut and the curves of diameter distribution were obvious left-truncted higher peak with narrow DBH range than before thinning. The parameter a of the Weibull function increased with thinning intensity and the parameter b decreased. The parameter c was not influenced by lower intensity thinning, but decreased by higher thinning intensity. The fitting results for the PPM of simultaneous equations system showed that predicting models for the parameter b were best (R²>0.98) and the R² for model of the parameter c was larger than 0.91. The method of 3SLS was as good as the error-in-variable method, and there was no significant diffe-rence. The results of validation for the two methods showed that, the precision of the three prediction models was >97%, and the fitting efficiency (EF) was >0.92. The parameter prediction mo-dels developed based on the effects of thinning performed well and could be used to predict the dynamic change of diameter distribution and to provide suitable management of L. olgensis plantation.