Susceptibility of volcanic ash-influenced soil in northern Idaho to mechanical compaction

Timber harvesting and mechanical site preparation can reduce site productivity if they excessively disturb or compact the soil. Volcanic ash-influenced soils with low undisturbed bulk densities and rock content are particularly susceptible. This study evaluates the effects of harvesting and site preparation on changes in the bulk density of ash-influenced forest soils in northern Idaho. Three different levels of surface organic matter were studied. Soil samples were taken before and after harvesting to determine the extent and depth of compaction. Soil bulk densities increased significantly aR.er extensive compaction from site preparation, especially when little logging slash and surface organic matter were left on the aoil surface. As site preparation intensity increased, bulk density increased significantly at greater depths in the soil profile. Although ash-influenced soils have naturally low bulk densities, they can easily be compacted to levels that limit growth. This experimental site has been designated as part of the Forest Service’s national long-term site productivity study into the impacts of organic matter depletion and soil compaction on stand development.