Trade-Offs and Synergies between Plant Species Diversity and Water Retention Capacity of Pinus massoniana Plantation Community in Danjiangkou Reservoir Area

In order to quantify the plant species diversity characteristics of Pinus massoniana plantations with different stand densities in the Danjiangkou reservoir area of Hubei Province and the relationship of their trade-offs and synergies with the water retention capacity in the reservoir area to give full play to the forest ecosystem services in the reservoir area and improve the level of sustainable management of plantations, we used the typical plot method, selected 35-year-old Pinus massoniana with low density (925–1000 plants·ha), medium density (1425–1625 plants·ha), and high density (2375–2525 plants·ha), and its community structure, species composition, and understory plant species diversity were studied, respectively, and the relationship of the trade-offs and synergies between the two services of water retention capacity and plant species diversity in Pinus massoniana plantations were calculated. We found that: (1) According to the survey statistics, there were 69 species of plants in the shrub and herb layers under forest including 32 species of shrubs and 37 species of herbs, belonging to 33 families and 62 genera. (2) The species richness of the shrub layer increased with the increase in the stand density, which was opposite in the herb layer. The Shannon–Wiener diversity index and Simpson dominance index of the shrub layer showed the regularity of high density > low density > medium density, while the herb layer decreased gradually with the increase in density. The Pielou evenness index of the shrub layer and herb layer was the highest in the high-density and medium-density stands, respectively. (3) Trunk flow, soil layer water storage and plant species diversity under medium and high density conditions, litter layer water storage and plant species diversity under low-medium density conditions showed synergistic relationships in the shrub layer and herb layer, everything else were trade-off relationships. As far as the Danjiangkou reservoir area is concerned, the low-density Pinus massoniana plantations have higher understory plant species diversity and more stable community structure, and there is a trade-off relationship between the water retention capacity and understory plant species diversity.