A new site-specific height-diameter model for Crimean pine stands in the Black Sea Backward Region, Türkiye

Background: Relationship between height (H) and diameter (D) highly depends on site conditions and stand structures. In this regard, this study aimed to build a new site-specific model based on the Chapman-Richards function, for the Crimean pine stands in the Black Sea Backward Region of Türkiye. Methods: The model was developed using the mixed-effects framework and its accuracy and performance were assessed using a validation data set. The model was then compared to two highly predictive models derived from the Chapman-Richards function through fit statistics, curvature and bias measures of nonlinearity, and biological principles. Results: The new model resulted in the most favourable fit statistics and nonlinearity measures, and also exhibited a suitable H-D curve that was compatible with the ecological conditions of the study area. While the alternative models in the current literature showed good fit statistics, they resulted in biased parameter estimates and produced inappropriate H-D curves. The proposed model, however, demonstrated a satisfactory accuracy when validated using the validation data set. Conclusions: (1) It is essential to assess H-D models by considering both fit statistics and curvature and bias measures of nonlinearity; and (2) it is crucial to examine whether H-D models are appropriate for the specific ecological conditions of a particular region.