IMPACTS ON SOILS 3-3). Present undisturbed erosion rates on these lands is 35 tons per square mile per year. Tractor yarding would increase annual surface erosion rates 1.6 times (Megahan 1972), to 56 tons per square mile per year. Therefore, an additional 1,120 tons of soil would erode over the 4 years needed for the area to return to pre-yarding erosion levels. Approximately 17,775 acres would be disturbed and compacted by cable yarding (Table 3-3). A total of 1,460 additional tons would erode over the 4 years required to return to pre-yarding erosion levels. Analysis of the 3-year the 3-year timber sale plan shows that several sales are being planned on problem soils discussed in Section 2.4. The following sales lie partly or wholly upon unstable clayey soils: 80-1, 80-2, 80-3, 80-12, 80-17, 81-3, 81-12, 81-13, 81-15, 81-16, 82-2, 82-3, and 82-15. Slumping and/or mass failure of these soils could result in considerable soil erosion. Sales 80-19, 80-21, 81-19, 82-17, and 82-21 are planned on erosive granitic soils that have high erosion potential. Landslides and slumps may occur. On both types of sales, project design features and district specialists' recommendations would be used to minimize the possibility of failure. 3.3.1.3 Slash Disposal Slash disposal would involve gross yarding and/or slash burning on 56,700 acres. Gross yarding would compact and disturb about 30 percent of the area treated, or 9,900 acres. Since erosion rates would be similar to those following tractor yarding, an additional 975 tons of soil would erode over the 5 years needed to stabilize the areas. Slash burning, by removing protective vegetation and litter, would increase surface erosion, and if sufficiently hot, could cause surface soil structure to break down and reduce organic matter content (U.S. EPA 1976b). An estimated 5 to 8 percent of the areas, or 1,900 acres, would be severely burned (Tarrant 1956; Dyrness et al. 1957 Cited in U.S. EPA 1976b). Since burning would increase erosion by approximately one-half that of tractor yarding, about 90 additional tons of soil would erode over the 5 years needed to revegetate and stabilize. 3.3.1.4 Planting Planting burned, nonstocked, and poorly stocked areas would reduce long-term erosion losses and improve water infiltration in areas with poor vegetation cover (USDA, FS 1977). Soil loss from erosion would decrease in direct proportion to the increase in cover since spatter erosion and compaction of the surface by raindrops would be reduced (Lull 1959). Tree seedlings would hold more soil moisture through the dry season (June through September) and therefore reduce dry raveling (movement of soil downslope). More root penetration on steep slopes would increase slope stability and reduce chances of mass failure (Burroughs et al. 1976). 3.3.2 Soil Productivity Soil productivity would be adversely affected by compaction resulting from yarding and thinning operations and beneficially impacted by fertilization. Nutrients would be lost from the forest ecosystem as result of timber harvesting, thinning, and slash burning but amounts would be small in terms of the total amount of nutrients in the forest ecosystem (Sopper 1975), and site productivity would not be reduced significantly by losses. Impacts of these nutrient losses on water quality are discussed in Section 3.4.3. Other management activities would have insignificant impacts on soil productivity. Compaction would reduce soil productivity and result in reduced timber volume. About 9,700 acres would be compacted by yarding after timber harvest and commercial thinning (Table 3-3). Of this, about 7,100 acres would not be ripped and would be impacted by compaction. Soil compaction has been shown to reduce timber growth 14.8 percent (Froehlich 1978). This reduction would be a significant adverse impact. The soils would likely take 35 to 40 years to recover by natural forces. The proposed action would apply 200 pounds of nitrogen per acre, primarily in the form of urea pellets. Over the 10-year period, 2,320 tons of nitrogen IMPACTS ON WATER RESOURCES would be applied to 23,185 acres. Fertilization with nitrogen would impact soil productivity beneficially because nitrogen is not only the nutrient most susceptible to loss from the forest ecosystem, but is also most likely to limit maximum growth in the forest (Moore et al. 1974). The effects of increased nitrogen in the forest ecosystem would be to make the soils more acid, increase the rate of decomposition, and increase the solubility of organic matter. Little is known about the effect of nitrogen on the recycling rates of other nutrients in forest soils. The impact of an increase in the nitrogen content of the forest soil would be to increase the rate of growth of all plants in the ecosystem, including weeds, brush, microbes, and trees. 3.3.2.3 Conclusions About 64,000 tons of soil would erode as a result of the proposed management plan. By way of comparison, an acre of soil 1 inch deep weighs between 125 and 200 tons, depending on soil texture. The impacts of this soil erosion over the whole of the JKSYUs would be site-specific and vary in degree of significance. Loss of topsoil from low-fertility sites would impact soil productivity significantly and adversely. Soil productivity would be adversely and significantly affected on 7,100 acres by compaction from yarding and commercial thinning, and would not be expected recover naturally for 35 to 40 years. Fertilization would improve site productivity significantly on treated areas and result in increased vegetative cover. This would subsequently reduce soil erosion and water yield from these areas. 3.4 IMPACTS ON WATER RESOURCES 3.4.1 Water Yield Water yield would increase from those lands disturbed by commercial thinning, shelterwood and clearcut harvest, slash disposal, yarding, and road construction and would be reduced by fertilization and planting. Other management practices would have insignificant effects. Table 3-4 shows the acreage proposed for treatment annually and the resulting increase in yield from these treatments. Since fewer acres would be affected under the proposed action than have been in the last 5 years, the increase in water yield caused by the disturbance would not be as great. The increased water yield as a result of treatments under the proposed action would be 4,500 acre-feet per year, assuming water yield from relatively undisturbed lands in equilibrium is 2.5 acre-feet per acre per year. |