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A study was conducted on two small watersheds in the Boise National Forest to determine the role of forest vegetation in maintaining more secure slopes in shallow, coarse-textured soils typical of the Idaho batholith. Both soil water piezometry and soil shear strength measurements were made in the watersheds. Results of the field studies and supporting analyses indicate that forest vegetation often provides a critical margin of safety. Woody vegetation growing on slopes of the batholith contributes to stability by root reinforcement, by soil moisture depletion from interception and transpiration, by regulation of snow accumulation and melt rates, and by soil arching restraint between tree stems. Conversely, removal of vegetation from a slope by timber harvesting or wildfire results in a loss or reduction of effectiveness of these stabilizing mechanisms. Loss of vegetative stabilization in turn can le^d to increased frequency of landslides as documented in this study. Management implications of the study are discussed. Suggested measures and approaches include more stringent controls on size and location of clearcut units, greater use of "vegetation leave areas" or buffer zones particularly along haul roads and next to streams, and construction of hydraulic structures that divert water away from critical areas.

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