Developing mixed-effects height-diameter model using stand and environmental factors for mixed forests in northern China

Understanding tree height (H)-diameter at breast height (D) allometric relationship is crucial for estimating biomass, carbon storage, and productivity. Environmental factors and species diversity strongly influence this relationship, yet remain understudied. Using data from 99 plots in Hulunbuir mixed forests, inner Mongolia, we developed an environment-sensitive nonlinear mixed-effects model for five tree species. The model incorporates variables representing stand-level attributes, soil properties, climate factors, and species diversity. Variations at the sample plot and species levels were accounted for by introducing random components into the H-D model. Quadratic mean diameter, total basal area greater than the target D (BAL), Simpson's diversity index (SIM), mean annual precipitation (MAP), and soil organic carbon significantly affected the H-D relationship. Height growth increased with BAL and MAP but declined with higher SIM. Species traits and environmental factors jointly shaped H-D scaling. The proposed model supports adaptive forest management under changing environments.