Pinus taiwanensis is an important timber species in China and has significant ecological and regional economic value. Establishing an appropriate height-diameter at breast height (DBH) model for P. taiwanensis is essential for accurately predicting its growth and yield, thereby providing a theoretical foundation for the effective management and sustainable utilization of these forest resources. Taking the P. taiwanensis plantations in the State-owned Huangbaishan Forest Farm of Shangcheng County in Henan Province as the research object, based on the field-measured data, the research initially selected 30 commonly used empirical height-diameter models as candidate basic models. The most effective basic models were identified through a comprehensive evaluation using five statistical indicators of R a d j 2 , RMSE, MAE, RMAE and AIC. Subsequently, to enhance the predictive accuracy and biological realism of the models, three key factors known to significantly influence individual tree growth, namely the competition index (I), height to the first live branch (under-branch height, Z), and crown width (W), were introduced into the three best-performing basic models, and 21 modified models were constructed. A comparative analysis of the fitting performance of these modified models was then conducted to determine the most suitable height-diameter model specifically for P. taiwanensis plantations located in the climate transition zone. The results indicated that among the 30 initial basic models, parameters of 27 tree height-diameter models could be successfully estimated. The evaluation results revealed that the three basic models, designated as M02, M10 and M11, had superior fitting performance and were therefore selected as the optimal basic models for further modification. After the thorough comparison of the fitting effects and the significance test results of the parameters for 27 modified models, the study concluded that the M10-Z modified model (i.e., the M10 basic model introduced with the under-branch height (Z) factor) had the best fitting effect. Notably, all parameters within the M10-Z modified model were statistically significant differences. Compared with the M10 basic model, the R a d j 2 value of the M10-Z modified model increased by 70.46%, indicating that the introduction of Z factor significantly enhanced the explanatory ability for the relationship between tree height and DBH. This significant increase underscores the under-branch height, a morphological trait related to light capture and historical growth, has critical influence on the height-diameter relationship for P. taiwanensis in these environments. In conclusion, this study identifies the M10-Z modified model as the most appropriate height-diameter model for P. taiwanensis plantations in the climate transition zone. This model can effectively characterize the relationship between tree height and diameter at breast height, offering a robust tool for growth and yield prediction for P. taiwanensis plantations in the climate transition zone. The demonstrated importance of the under-branch height factor suggests that silvicultural practices influencing crown development and stem form may be particularly important for managing this species in such ecotonal regions. The application of this model can significantly enhance the precision of forest inventory, improve harvest forecasts, and inform strategic forest management decisions, ultimately contributing to the sustainable stewardship of P. taiwanensis plantation resources.
ZHOU et al. (Wed,) studied this question.