Objectives The study aims to investigate the effects of different stand ages on the species composition and diversity of understory vegetation in Pinus densiflora plantations, providing a theoretical basis for vegetation diversity conservation, stand structure optimization, and sustainable forest management.

Methods Taking the Pinus densiflora plantations in the central mountainous areas of Shandong Province as the research object, the influence of stand age on indicators such as understory species compositions, important values, diversity, and similarity was analyzed.

Results 1) A total of 13 understory plant species belonging to 12 genera and 9 families were recorded in the Pinus densiflora plantations, comprising 3 species, 2 genera, and 2 families, in the shrub layer, and 10 species, 10 genera, and 7 families in the herbaceous layer. With increasing stand age, both the total number of understory species and their habitat suitability exhibited an upward trend. 2) As stand age increased, the important values of Lespedeza virgata and Vitex negundo var. heterophylla in the shrub layer decreased, whereas that of Vitex negundo increased, becoming the dominant species. In the herb layer, Carex tristachya achieved an importance level of 63.27% in the young plantation, and Arthraxon hispidus attained 52.18% in the mature plantation, representing the dominant herbaceous species in their respective age stages. 3) The Margalef richness index of the shrub layer increased with stand age, with the young plantation exhibiting significantly lower values than the middle-aged and mature forest (P≤0.05). In the middle-aged plantation, the Simpson dominance index (0.63 for shrub layer; 0.74 for herb layer) and the Shannon-Wiener diversity index (1.04 for shrub layer; 1.36 for herb layer) were both significantly higher than those in the young and mature plantations (P≤0.05). Stand age did not exert a significant effect on the Pielou evenness index of either the shrub or herb layer. 4)The species composition similarity of understory vegetation among different stand ages was high, with the species similarity coefficient of the shrub layer being consistently higher than that of the herb layer across all age classes.

Conclusions The stand age exerted a certain effect on the pecies composition and diversity changes of understory vegetation in Pinus densiflora plantation. Improving the stand age structure is conducive to enhancing the diversity level of understory vegetation.