2.1 Study area
The study area is located in the Zhifanggou watershed (36°48’ N, 109°15’ E) in the loess hilly and gully region of northern Shaanxi Province, China (Fig. 1). This area belongs to a typical forest-grass zone with complex terrain, ravine and crisscross, and an elevation ranging from 997~1731 m. The region is mainly affected by the semiarid continental climate in the warm temperate zone, with an average annual temperature of 8.8 ℃ and an annual rainfall of 500-600 mm. The natural vegetation of the area has been almost completely destroyed, and the restored plantations are composed of Robinia pseudoacacia ,Populus simonyi , Caragana korshinskii, and Hippophae rhamnoides . The abandoned slope is mainly composed of herbs at different stages of succession, such as Artemisia gmelinii ,Artemisia Giraldi , Stipa bungeana , and Bothriochloa ischaemum . The soil type is loessal soil, of which silt accounts for over 64%. In this area, the soil is loose with poor soil resistance, which leads to serious soil and water erosion.
In the watershed, typical land cover (forest, shrub, grass, crop, and bare land) was selected (Table 1). The same slope gradient site was chosen as the experimental plot (22°~25°), which was a 5×20 m standard experimental plot. In the forest plot, the major tree species was Robinia pseudoacacia . Many herbaceous plants grow here, such as Stipa bungeana , Setaria viridis , andLeymus secalinus . In the shrub plot, the main planted species wasCaragana korshinskii , with many undergrowth vegetation types, such as Artemisia gmelinii , Bothriochloa ischaemum , andHeteropappus altaicus . In the grass plot, the dominant grass species were Lespedeza daurica , Bothriochloa ischaemum ,Stipa bungeana , Artemisia gmelinii , andLeymus secalinus . In the crop plot, the major crop species wasSyringa oblata . The bare land plot did not have any plant growth and was weeded once a month.