Controls of root-system overlap on hillslope stability

More frequent extreme rainfall has heightened the need to understand vegetation’s influence on slope stability in steep terrain. Among vegetation-related factors, root-system overlap, the intersection of neighboring root networks, remains understudied despite its potential to influence shallow landslide processes. Here we present laboratory flume experiments testing whether root overlap modulates slope stabilization, using pea plants (Pisum sativum) at 50 (V-005), 200 (V-02), 400 (V-04), and 1100 (V-11) stems/m2 densities. Landslide initiation was most delayed at V-11, while V-02 and V-005 failed at times comparable to the unvegetated control. V-04 had the smallest landslide area, while lower and higher densities produced larger landslides. Higher plant density increased root reinforcement, yet accelerated soil saturation via preferential flow along overlapping roots. The optimal balance of reinforcement, delaying hydrological triggering, and minimizing landslide size occurred between V-04 and V-11. This study reveals how vegetation density influences slope stability through root-driven geotechnical and hydrological processes. Vegetation density influences slope stability with higher root-system overlaps delaying landslide initiation and smaller landslide areas at moderate densities, according to lab flume experiments using pea plants with different densities.