Effects of Post-Fire Salvage Logging on Erosion Rates at Multiple Scales

Abstract:

Forest managers sometimes harvest burned trees after wildfires to realize economic value, reduce fuel loads, or achieve other operational goals. This logging can be controversial because some ecosystem effects are negative, yet the potential impacts on erosion rates have not been clearly identified. Our objectives were to quantify hillslope-scale erosion rates and compare the hillslope erosion rates to rates from larger (swale) and smaller (rill) scales. Soil characteristics, vegetative regrowth, and erosion rates were measured in logged areas and unlogged controls at seven severely burned sites in the western US. One site had replicated measurements at all three scales, five sites had only hillslope or swale scale measurements, and one site had only rill measurements. Erosion rates from hillslopes (70-170 m²) and swales (0.1-2.6 ha) were measured with sediment fences. Rill erosion rates were measured with rill experiments, where water was applied to a hillslope at five flow rates for 12 min each; water samples were collected at a point 9 m downslope.

At the hillslope scale the passage of heavy logging equipment reduced soil water repellency, compacted the soil, reduced vegetative regrowth rates, and generally increased erosion rates by one or two orders of magnitude relative to the controls. The rill experiments also showed greater rates of rill incision and erosion from the areas disturbed by heavy logging equipment relative to the controls. At the swale scale erosion rates were higher in the logged areas than the controls when measurements were replicated and simultaneous but there was no detectable change in the other study areas. Overall, the absolute erosion rates from both logged and unlogged areas tended to decline over time while the relative difference in erosion tended to increase due to the slower vegetative recovery in the more heavily disturbed areas. The potential adverse effects of salvage logging can be minimized by reducing compaction and increasing cover on the most heavily disturbed areas, and minimizing the convergence and connectivity between these areas and the stream network.