Objective Transforming Pinus massoniana coniferous forest into broadleaved forest is beneficial to improving the structure and function of forest ecosystem and has significant influence on the regeneration and succession of understory vegetation. The analysis of the diversity and community stability of understory vegetation after forest transformation can provide reference for sustainable forest management.

Method In this study, based on the four types of stand transformation (i.e., I thinning and promoting regeneration, II thinning and planting broadleaf trees, III clear cutting and promoting regeneration, clear cutting and IV clear cutting and planting broadleaf trees) of Pinus massoniana forests in Thousand-island Lake Forest area, Chun’an county, and control treatment (V no-transformation forest )), the species composition, diversity and community stability of understory vegetation were investigated. The relationship between understory species composition and stand factors was analyzed, and the relationship between species diversity and community stability was also explored.

Result The difference of understory dominant species among different stands was lower in shrub layer than in herb layer. The understory species diversity in control group was significantly lower than that in transformed stand (P < 0.05) and the species diversity of herb layer in type IV was significantly higher than that in other types (P < 0.05). The stand factors could explain about 40% of understory species variation and had higher effect on the species composition of herb layer than that of shrub layer. Among these, the canopy density was the most important factor. In the early stage of stand transformation, the understory community stability in mode V was the best, followed by I, II, III, and IV. The community stability of shrub layer had low correlation with species diversity, while the stability of herb layer was negatively correlated with species diversity (P < 0.05).

Conclusion In the early stage of broad-leaf transformation of P. massoniana forest in Thousand-island Lake area, the diversity of herbaceous layer was significantly improved mainly by reducing canopy density, but the stability of understory vegetation community was low under clear cutting treatments. Therefore, considering vegetation diversity and community stability, appropriate thinning and tending measures can promote the development of understory vegetation in P. massoniana forest.