Intra-annual density fluctuations (IADFs) are wood cells formed in response to abnormal climatic events during the growing season. They are crucial for evaluating the relationship between extreme climatic events and radial growth, as well as for understanding wood quality. However, most existing research has focused on seasonally dry Mediterranean and semi-arid conifer forests, with limited studies conducted in other regions-particularly subtropical forests, where frequent and severe droughts constrain forest productivity and growth. Here, we investigated the occurrence patterns and triggering factors of IADFs in Pinus massoniana plantations along a climate gradient in southern China. We found that latewood IADFs (IADF-L) are the predominant type formed by P. massoniana, whereas earlywood IADFs (IADF-E) are relatively rare. The frequency of IADFs showed a clear spatial pattern, gradually increasing as climate conditions became warmer and wetter. IADF-L frequency was negatively correlated with elevation but positively correlated with tree-ring width. High precipitation in late summer and early autumn, as well as hot and dry conditions during summer, triggered the formation of IADF-Ls, while spring (May) droughts induced IADF-E. The inferred climatic drivers of IADFs were further confirmed by climate-growth relationships based on seasonal wood data and the VS-Lite tree-ring growth model. Our findings provide a valuable foundation for developing management strategies for drought-prone subtropical pine forests. For example, artificial rainfall or supplemental irrigation during summer-autumn dry spells could stimulate the formation of IADF-Ls, thereby enhancing forest growth and carbon sequestration capacity.