Creep movement and soil moisture stress in forested vs. cutover slopes: results of field studies

Evidence for a cause and effect relationship betweenclear-cutting and accelerated mass erosion is reviewed. Clear-cutting is a timber harvesting procedure commonly employed on forest lands in many parts of the United States.The report describes the results of 8-years of creep monitoring and soil moisture stress measurements in forested and clear-cut slopes. The field data was obtained frominstrumented sites located on steep mountain slopes of the Cascade Range in Central Oregon; the Klamath Mountains of Northern California; and the North Cascade Range of Washington.
Trees enhance the strength and stability of soil onsteep slopes mainly through mechanical reinforcement by theroot system and through soil moisture depletion by transpiration. Large scale removal or cutting of trees on steepslopes, on the other hand, tends to increase rates of mass erosion or mass-soil movement. An emerging concensus in the published literature supports this conclusion as do theresults of the study reported herein.
Two types of monitoring studies were established at the field sites, viz., "side-by-side" comparisons of forested and adjacent cutover areas and "before-and-after" comparisons at a single site, initially forested and subsequently clear-cut. Creep rates were generally higher in cutover or clear-cut areas as opposed to adjacent forested areas. Creep rates also tended to increase at sites which were initially forested and were subsequently clear-cut. Soil creep func-tions as a primary or initial link in the natural transportof soil and rock downslope. Acceleration in creep ratesultimately results in increased rates of mass-erosion andsedimentation. Removal of slope vegetation tended to produce wetterconditions in the instrumented slopes. Wetter soil conditionsare consistent with observed increases in creep rate. Creep rates are also strongly influenced by precipitation patternswhich likewise affect the moisture regime in the soil. Inthis regard, the far West has been afflicted with consecu-tive years of drought which coincided with the post clear-cutting monitoring period at several of the sites. Even so,increases in creep rate were still detected. It is highlyrecommended, however, that monitoring of these sites con-tinue in order to observe the creep response under similarclimatic conditions following clear-cutting.