Graduate Thesis Or Dissertation

Soil and understory vegetation characteristics of a tractor-logged forest in eastern Oregon

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  • A 1.5 ha mixed conifer stand in the Blue Mountains of Oregon was intensively examined to assess the impact of logging disturbance on soils and herbaceous vegetation. Sampling was conducted six years after much of the timber overstory was removed in a shelterwood cut and yarded by crawler tractor. A year after harvest slash was machine-piled and burned. Skid trails and other areas of disturbance were seeded with a mixture of perennial grasses. Logging impact was divided into five "disturbance classes'* as treatments: general, berm, slash fire ring, skid trail, and undisturbed. Analysis of variance was used to test treatment differences. Characteristics of skid trails were analyzed separately from those of non-skid trail areas. Soil compaction was evident on skid trails from substantially lower rates of infiltration and saturated hydraulic conductivity (SHC), and a higher average soil strength, relative to undisturbed areas. Skid trail compaction probably resulted from mixing of denser subsoils with the low-density volcanic ash soils at the surface, and from compression by tractor activity. Bulk density of skid trails was not statistically different from that of undisturbed areas. Soil densities of all classes of disturbance were generally lower than 0.9 g/cm³. Non-skid trail areas were not compacted. Soil water conductance rates markedly differed among disturbance classes, but were considerably higher than storm intensities projected for the region. High sample variability of both infiltration and SHC rates made interpreation of statistical comparisons among treatments difficult. Some watershed protection values were diminished on the disturbed areas. Litter cover of skid trails, general disturbance areas, and berms was 27, 45, and 68 percent lower, respectively, than that of undisturbed areas. Depth of litter decreased correspondingly among the disturbed treatments. Litter was nearly absent on the fire rings. Another agent in soil stabilization, belowground biomass, was two-thirds less on skid trails, in comparision with undisturbed areas. Amounts of soil organic matter in the surface 3 cm were significantly lower for the four types of ground disturbance, relative to undisturbed areas; however, organic matter was higher at some subsurface levels of berms, fire rings, and skid trails. Bulk density of non-skid trail soils was inversely correlated with organic matter content. There were no statistically significant differences among treatments for soil water retained at four potentials, for coarse mineral fragment content, or for fine earth particle size distribution. Aboveground vegetation production of the seeded skid trails was 66 percent greater than that of undisturbed areas. Understory plant cover of skid trails and undisturbed areas was equivalent. Correlation between aboveground herbaceous production and soil strength at the 5-10 cm depth of skid trails was not statistically significant.
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