I. THE ROLE OF VEGETATION IN THE STABILITY OF FORESTED SLOPES

http://users.humboldt.edu/rziemer/pubs/ZiemerIUFRO1981.PDF

Robert R. Ziemer
Pacific Southwest Forest and Range Experiment Station
Forest Service, U.S. Dept. of Agric., 1700 Bayview Dr., Arcata, CA, USA

XVII IUFRO World Congress (INTERNATIONAL UNION OF FOREST RESEARCH ORGANIZATIONS)

Vegetation helps stabilize forested slopes by providing root strength and by modifying the saturated soil water regime. Plant roots can anchor through the soil mass into fractures in bedrock, can cross zones of weakness to more stable soil, and can provide interlocking long fibrous binders within a weak soil mass. In Mediterranean-type climates, having warm, dry

summers, forest evapotranspiration can develop a substantial soil moisture deficit which can reduce both piezometric head and slope mass.

Pore water pressures change seasonally in response to precipitation and are often the driving mechanism which ultimately leads to slope failure. When trees are cut, the root system begins to decay, and the soil-root fabric progressively weakens. The loss of root strength or increased soil moisture content or both after-tree removal can lower the slope safety factor sufficiently that a moderate storm and associated rise in pore water pressure can result in slope failure. After trees are removed, the frequency of landslides can increase.