Wind speed and crown class influence the height–diameter relationship of lodgepole pine: Nonlinear mixed effects modeling

Wind-induced mechanical perturbation is one of the mechanisms determining the height–diameter relationship of trees, but its effects have largely been neglected in past height–diameter models. In this study, we examined the effects of including wind speed as a regressor in the height–diameter model for lodgepole pine (Pinus contorta var. latifolia Engelm.). We also tested the hypothesis that the height–diameter relationship differs between trees of different crown classes (dominant and subordinate trees), and that a mixed effects height–diameter model developed at the crown class level should achieve a better prediction than that developed at the plot level. The results showed that including wind speed significantly improved the fit of the height–diameter model. The results also showed that the height–diameter relationships differ significantly between the two crown classes. The mixed effects model developed based on crown classes of individual trees had better fit with reduced residual variance and decreased Akaike's information criterion and the Schwarz's Bayesian information criterion than the plot-based mixed model. Evaluation of the developed model suggests the crown class-based height–diameter model has better prediction in terms of prediction error and precision. The developed crown class-based mixed effects model can be used to provide more accurate prediction of tree heights for lodgepole pine from their diameters.