Tree size inequality and competition effects on nonlinear mixed effects crown width model for natural spruce-fir-broadleaf mixed forest in northeast China

Crown size is strongly correlated with tree growth. Crown width (CW) is affected by tree size, site quality and competition, but most CW models do not quantify the effects of stand structure, intra and inter-specific competition, especially in mixed-species forest. In this study, we quantified these effects by developing nonlinear mixed effects CW model for natural spruce-fir-broadleaf mixed forest in northeast China. Data from a total of 10,086 individual trees in the 192 plots were used for the purpose. Plot-level random effect parameters accounting for stand heterogeneity and randomness were included in the CW model. The results showed the optimal model could explain 63% of the variation of CW with no substantial residual heterogeneity. Gini coefficient by tree size (GC) was tested as a predictor denoting tree size inequality or stand structural diversity in the model. Among all predictor variables, diameter at breast height (DBH), GC, the cumulative basal area of trees larger than the subject tree (BAL) including intra-specific (BALintra) and inter-specific (BALinter) competition, and height to DBH ratio (HDR) contributed most significantly to the model. Tree CW increased with the increasing GC, but decreased with the increasing HDR, BALintra and BALinter. The effect of intra-specific competition on CW was greater than that of inter-specific competition. Our study highlighted the importance of tree size inequality in tree CW modelling for mixed forest.