<p>Diameter distribution models are essential components of forest growth and yield models, providing valuable information for estimating timber volume, monitoring carbon stocks, and guiding forest management strategies. Diameter distribution depends on environmental and stand conditions. While numerous studies have examined the effects of stand attributes on diameter distribution, the regulatory role of climate and soil resources has received less attention. This study developed mixed-effect models to examine the effects of climate, soil, and stand variables on diameter distribution. Data were collected from 219 sample plots in Korean pine plantations in northeast China. The results showed that quadratic mean diameter (Dg), dominant diameter (DD), bulk density (BD), mean annual precipitation (MAP), or organic carbon density (OCD) are key factors which shape the diameter distribution. The inclusion of climate and soil variables significantly improved model performance. Increased soil bulk density was associated with a higher Weibull scale parameter, indicating a shift towards larger diameter classes and a higher frequency of larger trees. Higher precipitation and organic carbon density were associated with a decrease in the Weibull shape parameter, suggesting a more uniform diameter distribution. Further analysis revealed that the relative importance of soil factors is significant and far exceeds that of climate in influencing diameter distribution. These findings provide new insights into the environmental variability of diameter distribution in planted forests amid global environmental changes, which is crucial for improving regional and global estimates of forest biomass and carbon stocks.</p>