Ground-based logging typically produces well-defined skid trails on the forest floor during log extraction. With conventional logging, these skid trails cover from 18 to 36 percent of the soil surface after a single harvest entry (Dyrness 1965; Froehlich 1976). Subsequent entries can increase the area in skid trails to as much as 80 percent of the soil surface. The soil in the skid trails is almost always compacted. Increases in soil density during skidding generally range from 10 to 80 percent (Foil and Ralston 1967; Froehlich 1979), depending on soil strength, moisture content, organic matter, and machine variables such as ground pressure and vibration. Logging over frozen ground or a heavy cover of snow or slash can moderate or eliminate soil compaction (Froehlich 1978). In most areas, however, timber harvesting cannot always be scheduled to take advantage of those protective conditions; thus, soil compaction usually accompanies log skidding. Soil compaction alters soil structure to a condition usually less favorable to plant growth. The increase in soil density is accompanied by a decrease in pore space, which in turn reduces air exchange, water infiltration, and permeability. The increase in soil density also produces an increase In soil strength, which retards tree root extension. Soil compaction reduces the height growth of seedlings of several coniferous species by 10 to 50 percent (Foil and Ralston 1967; Froehlich 1979). Stem volume may be affected more severely than height growth. In a study with 16-year-old ponderosa pine, - soil compaction decreased height growth by 29 percent and stem growth by 68 percent (Froehlich 1979). Similarly, soil compaction reduced height growth of 26-year-old loblolly pine by 13 percent and stem volume growth by 53 percent (Perry 1964). The growth reduction of individual trees apparently is related to the amount of root zone compacted and the percentage of Increase in soil density (Froehlich 1979; Perry 1964). Therefore, the impact of skid trails on tree growth in a given tract will depend on the amount of area in skid trails or the number of trees affected and on the relative increase in soil density in the skid trails. Some forest managers have turned to skyline or even helicopter logging as a means of reducing site impacts from timber harvesting. These systems spare the soil but are at least twice as expensive as ground skidding with logging vehicles on gently sloping terrain (Aulerich et al. 1974). One method of retaining the economic advantages of ground skidding while reducing the impacts of soil compaction might be an efficiently planned system of skid trails covering only a small portion of the total harvested area. This paper describes a study designed to test the feasibility of such a system.
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