Understory shading exacerbated grassland soil erosion by changing community composition

Abstract The importance of understory herbs for soil erosion controlling in forests is well accepted, but its effectiveness was rarely reported. Understanding how understory shading affects herbs growth and soil erosion controlling benefits is essential for better policy management. Here, we investigated grassland characteristics and soil erosion process before and after grasslands placed beneath the forest canopies. Four typical grasslands (Trifolium repens, Medicago sativa, Elymus dahuricus and Bromus inermis) of the Loess Plateau were selected, and bare land was used as the control. Total fifteen plots (three replicates for each treatment) with a slope of 20° were constructed. All plots were placed outdoor (open areas) in the first year, and were moved to the undergrowth and remained in the Sophora japonica and Populus cathayana forests over the next five years. Time to runoff, runoff volume and sediment yield were examined by two simulated rainfall experiments before and after understory shading. Results showed that understory shading promoted the disappearance of the original grass species, increasing weeds with poor roots. Further, these changes led to more soil loss in all understory grassland treatments. After understory shading, the runoff volume (67.0–125.5 L) and biological soil crust coverage (1.0–15.7%) significantly decreased for all plots, whereas the sediment yield (5.0–1650.5 g), species richness (>5) and litter mass (75.8–241.0 g) significantly increased. Therefore, understory shading changed the community composition and structure of understory grasslands. Besides, we found that herb species difference, but not species richness difference, more determined understory soil erodibility. Our findings indicate that the shade-tolerant herb species application could effectively reduce soil erosion in forest-grassland complex ecosystem will help to achieve the sustainability of understory grassland during vegetation restoration.