Elucidating loessal landslide initiation in wood- and shrub-land by hydro-mechanical heterogeneity

Abstract. Vegetation recovery on the Chinese Loess Plateau has markedly changed the hydrological and mechanical controls on hillslope erosion, shifting sediment production from runoff-driven erosion to gravity-driven processes such as rainfall-induced loessal landslides. Presently, few studies have clearly documented the differences in landslide erosion and initiation between shrubland and woodland. We conducted field investigations, rainfall soil-moisture observations, dye-tracer experiments, and soil-root tests, to examine landslide characteristics in terms of geometry and volume, excess soil-water ratio, preferential-flow pathways, and failure potential in the two stands. Rainfall-induced loessal landslides in the shrubland stand have shallower failure depths and smaller volumes but are wider than those in the woodland stand, and they are triggered under lower contributing area-slope conditions. Moreover, vertical infiltration in the woodland stand tends to be more stable and efficient, characterized by greater water penetration depth and enhanced pore connectivity. The relationship between the excess soil-water ratio and soil-water storage demonstrates that subsurface flow in woodland stand is triggered at relatively lower degrees of saturation. This behavior is attributed to well-developed preferential-flow pathways and reduced matric suction. The landscape dissection-rainfall index indicates lower landslide susceptibility on steep woodland slopes than on steep shrubland slopes, consistent with the lower landslide density in woodland relative to shrubland. Overall, these hydrological and mechanical contrasts indicate that woodland slopes, by combining deep root systems, stable preferential-flow pathways, and strong mechanical reinforcement, support an effective subsurface flow system that enhances infiltration and delays shallow saturation, thus improving slope stability. These results highlight the need to reassess sediment production on the Loess Plateau by explicitly accounting for landslides rather than attributing it solely to runoff-driven erosion.