Controls of Root-System Overlap on Hillslope Stability

Abstract Forests greatly influence the global vulnerability towards shallow mass movement, particularly in hilly and mountainous terrain. Vegetation root systems can reinforce soil structure which thereby stabilizes hillslopes. Hence, root contributions to slope stabilization (i.e., root reinforcement) have normally been evaluated from individual trees or shrubs. In a hillslope with vegetation competing for resources, root reinforcement depends not only on values from individual vegetation species but also on the overlap of multiple root systems, which we define as “root system overlap”. Here, we evaluated the effect of root-system overlap on slope stabilization by using a laboratory-scale flume with various densities of peas (Pisum sativum L), i.e., 50 (V-005), 200 (V-02), 400 (V-04), and 1100 (V-11) stems/m2, representing trees at a 1:70 experimental scale. Time from the onset of applied rainfall to landslide initiation was the longest for V-11, whereas landslides in lower density vegetation (V-02 and V-005) had similar timing to those in non-vegetated conditions. The smallest landslide size occurred at V-04 with a similar timing of landslides in V-11. Estimated root reinforcement (∆C) became 0.004 N/m2 for V-005 to 4.03 N/m2 for V-11. Although root reinforcement in dense vegetation (V-11) was greatest, the formation of saturated soil areas was most rapid, possibly associated with preferential flow within the overlapping roots. The optimum condition for sufficient lateral root binding, duration of hydrological processes, and minimizing landslide area occurred in V-04 with 0.37 N/m2 of root reinforcement. Findings reveal that vegetation management by controlling tree density may effectively mitigate hillslope instability.