Modeling Height-Diameter Relationships for Mixed-Species Plantations ofFraxinus mandshuricaRupr. andLarix olgensisHenry in Northeastern China

The mixture of tree species has gradually become the focus of forest research, especially native species mixing. Mixed-species plantations of Manchurian ash (Fraxinus mandshuricaRupr.) and Changbai larch (Larix olgensisHenry) have successfully been cultivated in Northeast China. Height-diameter (H-D) models were found to be effective in designing the silvicultural planning for mixed-species plantations. Thus, this study aimed to develop a new system ofH-Dmodels for juvenile ash and larch mixed-species plantations, based on competition information and tree and stand attributes. The leave-one-out cross-validation was utilized for model validation. The result showed that theH-Drelationship was affected not only by the tree attributes (i.e., tree size and competition information) but also by stand characteristics, such as site quality and species proportion of basal area. The best model explained more than 80% and 85% variation of the tree height of ash and larch, respectively. Moreover, model validation also confirmed the high accuracy of the newly developed model's predictions. We also found that, in terms of total tree height, ash in middle rows were higher than those in side rows, while larch in the middle rows were higher in the early growth period but then became lower than those in the side rows, as the diameter increased. The newly establishedH-Dmodels would be useful for forestry inventory practice and have the potential to aid decisions in mixed-species plantations of ash and larch.