Abstract:In order to understand understory plant composition and diversity characteristics and their influencing factors in mature plantations with long-term vegetation restoration, four types of plantations (30 years old) with similar environmental conditions (slope, direction and altitude) in the Heshan National Field Research Station of Forest Ecosystem were investigated, including mixed Eucalyptus plantation (EE), Acacia mangium plantation (AM), mixed native species plantation (NS), and mixed coniferous plantation (MC), the compositions and diversities (Shannon-Wiener index, Simpson index and Pielou index) of understory vegetation were analyzed. The results showed that the types of understory plants are abundant in the plantation with vertical structure of tree-shrub-herb layers. At the shrub layer, the community composition in EE was similar to MC, so did in AM and NS. At the herb layer, the community composition was similar between EE and AM, and between NS and MC, while the community composition was significantly different between EE and MC. Stand type affected understory plant diversity. The diversity of shrub and herb in AM plantation was significantly lower than that in other three mixed forests (P<0.05), and the number of shrub species and individuals in AM plantation were the lowest. The MC plantation had the highest understory species richness. There was a positive relationship between canopy coverage and understory diversity (P<0.001). The distribution of understory vegetation was related to contents of soil nutrients, contents of different forms of nitrogen were related to the understory vegetation in EE and AM. Available phosphorus and total phosphorus contents correlated to species distribution in NS, while species distribution in MC was related to pH and total potassium. The NS plantation had the highest diversity and evenness, also the lowest dominance, performed best in maintaining the understory diversity. Therefore, the mixed forest of native tree species was more suitable for the construction of ecological public welfare forest or the stand transformation of some coniferous forest and monocultural forest of exotic tree species.