The health and productivity of Sierra Nevada ecosystems are strongly affected by the potential and condition of their forest soils. Silvicultural activities alter soil productivity as they alter soil volume, soil porosity, and soil organic matter. Soil volume is reduced by soil loss from erosion, displacement, and mass wasting. In the Sierra Nevada, soils developed from granitic bedrock are the most susceptible to rill and gully erosion and to dry ravel; soils developed from metasedimentary bedrock are the most stable. The biggest threat to soil productivity is direct mechanical displacement of the surface soil. Practices that manipulate the top layer of soil, and particularly those that remove it, degrade productivity and inevitably lead to a decline in site quality. Mass wasting has not had a major impact on soil productivity in the Sierra Nevada, but shallow debris flows and other forms of mass wasting could become more common if activity on steeper slopes increases. Soil compaction can be considered as a decrease in porosity with an increase in density and strength, the result of reduced pore space as air is expelled. When soil porosity is lost because of compaction, less soil volume is available for roots to occupy, and plant nutrients are relatively immobile. Avoiding or minimizing disturbance of the litter layer and surface soil high in organic matter is key to solving soil compaction problems. Nutrients are lost from forested sites by leaching, biomass removal, volatilization, and soil loss. Management practices that maintain the integrity and structure of surface organic matter will have the least impact.

Because much of the nutrient capital in Sierra Nevada forest ecosystems is contained in the upper soil layers and in the forest floor, and not in the standing biomass, activities that displace or compact the surface soil have the greatest potential to alter site productivity. The potential for altering the surface soil is greatest during site preparation, and somewhat lower during timber harvest. Intermediate silvicultural treatments generally have low impacts on soil productivity.

Fires can have physical, chemical, and biological effects on soils. Physical effects include loss of soil organic matter, loss of soil structure, hydrophobicity, erosion, and, in extreme cases, destruction of soil clay minerals. Chemical effects include an increase in pH, a loss of cation exchange capacity, and the loss of nutrients by volatilization, in fly ash, or by leaching. Biological effects include direct mortality of soil organisms and loss of their habitat. Wildfire has a much greater impact on soil productivity than prescribed fire, especially during postwildfire salvage and recovery operations. Although there is anecdotal evidence of locally severe losses in soil productivity, the extent of degraded soils in the Sierra Nevada is unknown.

Related SNEP chapter:
Poff, R. J. Effects of silvicultural practices and wildfire on productivity of forest soils. Vol. 2, chapter 16, pp. 477-495.