SHALLOW LANDSLIDING, ROOT REINFORCEMENT, AND THE SPATIAL DISTRIBUTION OF TREES IN THE OREGON COAST RANGE

The influence of root reinforcement on shallow landsliding has been well established through mechanistic and empirical studies, yet few studies have examined how local vegetative patterns influence slope stability. Because root networks spread outward from tree trunks, the species, size and spacing of trees should influence the spatial distribution of root reinforcement. We documented the distribution and characteristics of trees adjacent to 32 landslides that occurred during 1996 in the Oregon Coast Range. Although broadly classified as a conifer-dominated forest, we observed sparse coniferous and abundant hardwood trees near landslide scars in an industrial forest (Mapleton) that experienced widespread burning in the 19th century. In industrial forests that were clearcut 100 years ago and not replanted (Elliott State Forest), swordfern was ubiquitous near landslides and we observed similar numbers of live conifer and hardwood trees proximal to landslide scarps. We demonstrate that root strength quantified in landslide scarps and soil pits correlates with a geometry-based index of root network contribution derived from mapping the size, species, health, and spacing of local trees, indicating that root reinforcement can be predicted by mapping the distribution and characteristics of trees on potentially unstable slopes. In our study sites, landslides tend to occur in areas of reduced root reinforcement, suggesting that slope stability analyses should include information regarding the diversity and distribution of vegetation in potentially unstable terrain.