Washington State Department of Ecology

Vegetation and Slope Stability

Positive Mechanisms - Hydrological

Foliage intercepts rainfall, causing absorptive and evaporative losses that reduce rainfall available for infiltration.
Roots and stems increase the roughness of the ground surface and the permeability of the soil, leading to increased infiltration capacity.

Negative Mechanisms - Hydrological

Roots and stems increase the roughness of the ground surface and the permeability of the soil, leading to increased infiltration capacity.
Depletion of soil moisture may accentuate desiccation cracking in the soil resulting in higher infiltration capacity (uncommon around Puget Sound).

Positive Mechanisms - Mechanical

Roots reinforce the soil, increasing soil shear strength. Tree roots may anchor into firm strata, providing support to the upslope soil mainly through buttressing and arching.
Roots bind soil particles at the ground surface, reducing their susceptibility to erosion.

Negative Mechanisms - Mechanical

Vegetation exposed to the wind which transmits forces into the slope. (Degree of diverse effect is dependent upon exposure and health of vegetation. Typically a minor consideration for Puget Sound Inland Waterways).

Positive/Negative Mechanisms - Mechanical

Weight of trees surcharges the slope, increasing normal and downhill force components. (Tree weight in some situations is beneficial to slope stability. Trees should not be arbitrarily cut to "unweight" slopes).

Vegetation and Slope Stability (Source: Modified from Greenway, 1987).

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