The variability of root cohesion as an influence on shallow landslide susceptibility in the Oregon Coast Range

Decades of quantitative measurement indicate that roots can mechanically re inforce shallow soils in forested landscapes. Forests, however, have variat ions in vegetation species and age which can dominate the local stability o f landslide-initiation sites. To assess the influence of this variability o n root cohesion we examined scarps of landslides triggered during large sto rms in February and November of 1996 in the Oregon Coast Range and hand-dug soil pits on stable ground. At 41 sites we estimated the cohesive reinforc ement to soil due to roots by determining the tensile strength, species, de pth, orientation, relative health, and the density of roots greater than or equal to1 mm in diameter within a measured soil area. We found that median lateral root cohesion ranges from 6.8-23.2 kPa in industrial forests with significant understory and deciduous vegetation to 25.6-94.3 kPa in natural forests dominated by coniferous vegetation. Lateral root cohesion in clear cuts is uniformly less than or equal to 10 kPa. Some 100-year-old industria l forests have species compositions, lateral root cohesion, and root diamet ers that more closely resemble 10-year-old clearcuts than natural forests. As such, the influence of root cohesion variability on landslide susceptibi lity cannot be determined solely from broad age classifications or extrapol ated from the presence of one species of vegetation. Furthermore, the anthr opogenic disturbance legacy modifies root cohesion for at least a century a nd should be considered when comparing contemporary landslide rates from in dustrial forests with geologic background rates.