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Crown width (CW) is an important individual tree variable commonly used to assess tree vigor and the production efficiency of stands. However, our understanding of the effects of climate and the combined effects of climate with competition on the variation of CW remains unknown. Therefore, this study developed CW models by nonlinear mixed-effects (NLME) to explore these effects. Data were obtained from Korean pine (Pinus koraiensis Siebold and Zucc.) plantations in five forestry agencies in northeastern China. The results showed that stand basal area (BA), height to crown base (HCB), isothermality (BIO3), and annual precipitation (BIO12) were significantly related to the variations of CW. CW increased with increasing BIO12, while decreased with increasing BA, HCB, and BIO3. The hierarchical partitioning (HP) analysis showed that the relative importance of BIO3 was larger than BA, HCB, and BIO12. In addition, we found that competition altered the variations of CW responses to climate. Competition, tree size, and climate CW model (CC-CWM) performed the best performance in model fitting and prediction accuracy. Therefore, CC-CWM was selected to predict CW under future climate change. By comparing the eleven sampling methods with eighteen sample sizes, it was reasonable to select six medium-trees to estimate the random effect parameters. Our study provides evidence of the effects of climate and the combined effects of competition and climate on the variations of CW, potentially useful for the development of rational and scientific forest management decisions under future climate changes. [ABSTRACT FROM AUTHOR]