Objectives The natural Pinus massoniana–Castanopsis kawakamii mixed forest is a representative zonal vegetation type within the Fujian Sanming Castanopsis kawakamii Provincial Nature Reserve. It plays a crucial role in maintaining species diversity and holds significant value. Understanding the stratification and diameter distribution characteristics of this forest is essential for its sustainable development.

Methods Field investigations were conducted to explore the stratification, species diversity, and diameter distribution characteristics of the forest.

Results The findings are as follows: (1) The Pinus massoniana–Castanopsis kawakamii mixed forest exhibited distinct vertical stratification, comprising three sublayers and a maximum canopy height of 16.5 m. The tree layer was relatively species-rich, comprising 18 species, with Castanopsis kawakamii identified as the dominant species and Pinus massoniana as the subdominant species. (2) The third sublayer exhibited the highest species richness, diversity index, and evenness, while the first and second sublayers contributed the bulk of the overall timber volume in the tree layer. (3) Substratum III exhibited the highest coefficients of variation for both tree height and diameter at breast height. (4) The majority of Pinus massoniana and Castanopsis kawakamii individuals have reached the middle-aged forest stage. However, the prevalence of small diameter classes indicated that competition among trees was intense, suggesting that the forest was transitioning towards a stable state but has not yet reached full equilibrium.

Conclusions  These results provide scientific guidance for the sustainable management of the natural Pinus massoniana–Castanopsis kawakamii mixed forest.