In order to investigate the dynamics of intraspecific competition and the variations in leaf functional traits across different diameter classes, this study focused on the Pinus tabuliformis plantation located in Taiyue Mountain, Shanxi Province. The diameter classes of P. tabuliformis were categorized at intervals of 5 cm within designated sample plots. From each diameter class, three representative P. tabuliformis individuals were selected for leaf sampling to ensure that our measurements adequately captured the characteristics of each class.We measured several key leaf functional traits, including intraspecific competition intensity, specific leaf area, leaf dry matter content, leaf nitrogen concentration, leaf phosphorus levels, and the ratios of nitrogen to phosphorus in leaves across the differing diameter classes. To analyze the differences in intraspecific competition and the associated leaf functional traits among the various diameter classes, one-way ANOVA was employed, while Pearson correlation analysis was utilized to explore the relationships between the leaf functional traits of P. tabuliformis.The results indicated several significant findings: (1) Intraspecific competition for P. tabuliformis exhibited marked differences among the different diameter classes, showing a clear trend where competition intensity decreased as the diameter classes increased. (2) A strong power function relationship was identified between diameter at breast height (DBH) and the competition index of P. tabuliformis, with a statistically significant correlation (P < 0.01). Specifically, the competition index exhibited a gradual decline alongside increasing DBH, with a further pronounced reduction that stabilized at a low level once the DBH of the target individuals surpassed 20 cm. This suggests that larger individuals experience less competition, likely due to their established dominance. (3) In response to varying intensities of intraspecific competition, the leaf functional traits demonstrated distinct adaptive strategies across the diameter classes. Smaller-diameter P. tabuliformis trees tended to adopt a resource-acquisition strategy aimed at maximizing their leaf area and resource uptake, while larger-diameter individuals favored a resource-conservation strategy that allowed them to maintain their physiological resources. (4) The specific leaf area for both annual and perennial leaves was found to be negatively correlated with leaf dry matter content, leaf nitrogen, and the nitrogen-to-phosphorus ratio, Furthermore, leaf dry matter content exhibited a positive correlation with leaf nitrogen concentration. Notably, a negative correlation was observed between leaf area and leaf nitrogen in annual leaves, while a highly significant negative correlation was found in perennial leaves. It is worth mentioning that leaf nitrogen was positively correlated with the nitrogen-to-phosphorus ratio in annual leaves, whereas perennial leaves displayed differing relationships, further underscoring the varied constraints experienced by annual versus perennial leaves.These findings underline the necessity of considering intraspecific competition alongside the changes in leaf functional traits across different diameter classes when managing P. tabuliformis plantations. This comprehensive approach is essential for developing effective protection strategies and management practices that will ultimately enhance the sustainability and productivity of these forest ecosystems.