Soil Disturbance and Hill-Slope Sediment Transport After Logging of a Severely Burned Site in Northeastern Oregon

摘要

Despite considerable public debate in recent years on the practice of postfire logging, few studies have directly evaluated its effects. Soil disturbance and hill-slope sediment transport were measured after a postfire logging operation conducted twoyears after the 1996 Summit Wildfire (Malheur National Forest), in northeastern Oregon. The wildfire was relatively severe, killing an average of 86% of the trees in experimental units, and leaving an average of 34% mineral soil exposed one year after the fire. Soil disturbance was measured both pre- and postharvest in four replicate units in each of three postfire harvest treatments (unlogged control, commercial harvest [most dead merchantable trees removed], fuel reduction harvest [most dead merchantable trees removed plus most dead trees >10-cm diameter]). There was a significant difference among treatments in the percentage of mechanically disturbed soil area, with an average of 19.4% disturbed in fuel reduction units and 15.2% in commercial units.Displacement (13.7% of soil area), apparent compaction (3.1%), and erosion (0.4%) were the most common types of machine-caused soil disturbance. Controls had significantly less change in mean displacement from pre- to posttreatment compared to fuel reduction units, and significantly less change in erosion compared to commercial units. At the experimental unit level, there was a significant correlation between the number of stems removed and the total amount of mechanical soil disturbance observed. Multiple regressions indicated that logging activity, reflected by the number of stems removed, explained more variation in soil disturbance than relative fire severity, reflected by tree mortality, forest floor mass, or the percentage of mineral soil exposed. There was no correspondence between disturbance within units and hill-slope sediment collected in silt fences below units. Visual inspections and sediment collected in silt fences indicated that little sediment exited the experimental units in the short term, and that the existing road system caused most of the observed hill-slope sediment transport. Low observed levels of sediment transport were likely due to a combination of low-to-moderate slopes, low-to-moderate-risk soils, logging over snow or dry ground, hand felling, no new roads, two years recovery of ground cover between the fire and the logging, problems with measuring hill-slope sediment, and the absence of severe weather events in the two years after postfire logging. Given these mitigating factors, hill-slope sediment transport measured in this study should be considered as representative of the low end of the range that would be expected in a postfire tractor logging operation on similar soils and under similar burn severity conditions.