Long-term effects of vegetation restoration on hydrological regulation functions and the implications to afforestation on the Loess Plateau

Vegetation restoration serves as a long-term and important strategic initiative in ecologically-fragile locations worldwide, while the ecological effects of long-term vegetation restoration must be thoroughly investigated to support ecological restoration in these regions. The Loess Plateau is a typical area where the eco-environment has been improved by vegetation restoration in China, wherein four representative sites: the P. tabulaeformis planted forest, P. orientalis planted forest, R. pseudoacacia planted forest and the natural Secondary forest were selected as examples of typical planted and natural forests in this study. To analyze the ecological benefits of different vegetation restoration types and discuss a more sustainable pattern of afforestation in the future, soil properties in 0–100 cm, vegetation attributes at four synusiae, and hydrological regulation functions (the rainfall distributions of 23 rainfall events and 10 years of soil water storage) were measured and compared. The results showed that: (1) the soil properties of natural Secondary forest, in particular, soil water-retention capacities, were better than those of the planted forests; (2) the biomass of arborous synusia in natural Secondary forest was less than that in the planted forests, whereas it was significantly higher at shrub and herbaceous synusiae; (3) there were differences in rainfall redistribution among the natural and planted forests examined in this study, and the natural forest exhibited the highest soil water storage (414.33 mm) during the last decade; (4) according to the structural equation model (SEM), the vegetation attributes at arborous and herbaceous synusiae had the most direct and total effect on the hydrological regulation functions in the ecosystems. The results indicate that afforestation should be conducted in a way of near-natural restoration, which can provide guidance for future vegetation restoration in the Loess Plateau and other ecologically vulnerable areas.