Effects of Herbaceous and Shrub Species on Soil Erosion Resistance and Root–Soil Reinforcement of Arid Slopes

Vegetation restoration can reduce soil erosion and improve soil stability on erosion-prone slopes, but the contrasting roles of herbaceous and shrub species in surface erosion resistance and subsurface reinforcement remain unclear. This study compared two herbaceous species, Melilotus officinalis (L.) Pall (MOP) and Suaeda glauca (Bunge) Bunge (SGB), and two shrub species, Alhagi camelorum Fisch. (ACF) and Nitraria tangutorum Bobrov (NTB), with bare soil (CK) as the control. Soil detachment capacity (Dc), root–soil shear strength, root traits, and soil properties were measured using planting experiments, flume scouring tests, and direct shear tests. Compared with CK, the vegetation treatments showed lower bulk density and higher total porosity and water-stable aggregates content, while soil organic matter exhibited differences among treatments. Herbaceous species developed dense fine roots in surface soil, whereas shrubs had deeper, coarser root systems. Vegetation significantly increased root–soil shear strength, mainly by increasing cohesion; herbs provided stronger surface reinforcement, whereas shrubs maintained more stable reinforcement at depth. All vegetation treatments significantly reduced Dc, and MOP showed the best overall anti-erosion performance. Random forest analysis further showed that root traits dominated Dc variation, accounting for 84.9% of total variable importance, with root length density and root surface area density as the leading factors. A regression model based on shear stress, total porosity, bulk density, and root length density predicted Dc well (R2 = 0.86, NSE = 0.65, RMSE = 0.083). These findings support site-specific vegetation selection: herbs for surface erosion control, shrubs for slope-stability concerns, and shrub–herb combinations where both risks coexist.