Evaluating the potential impacts of steep slope logging on hillslope hydrology and erosion

Abstract

Bark beetle outbreaks in high-elevation mountain forests in the Western United States have increased in extent and severity in recent decades, motivating land managers to log beetle- killed forests to reduce canopy fuel loads and mitigate safety hazards from dead standing trees. Current slope policies restrict logging on slopes greater than 40%. Cut-to-length tethered logging provides an option for treating forests with slopes greater than the current limit because it is cable-assisted, exerting less force on the ground surface compared to untethered ground-based methods. To evaluate erosion potential of cut-to-length tethered logging, and slash application as an erosion mitigation strategy, we investigated runoff and soil erosion after a cut-to-length tethered logging project on a steep hillslope (10 - 80%) on Monarch Pass, Colorado. The site had severe infestation levels and tree mortality from spruce beetle (Dendroctonus rufipennis) and Western balsam bark beetle (Dryocoetes confuses) outbreaks, providing potentially inadequate slash supply consisting of dead timber. For three summer field seasons, we measured soil erosion that accumulated in 39 sediment fences. We additionally measured runoff and erosion during rainfall simulations of 10 and 100-year local rainstorm intensities on 20, 40, and 60% slopes. We evaluated erosion on tether roads (pathways equipment creates up and down the hillslope) with bare soil patches, tether roads covered with slash, and cut hillslope areas in between tether roads. Runoff per unit rainfall was 2x higher on bare tether roads compared to tether roads with slash. There was strong evidence that erosion was greater on bare tether roads than slashed tether roads. Sediment yield was 91% less on slashed tether roads relative to bare tether roads when we measured sediment accumulation in sediment fences after natural rainfall and 81% less on slashed tether roads relative to bare tether roads when we simulated high-intensity rainfall. The low to moderate erosion rates we observed (0 - 2 t/ha/yr) in a steep-sloped system with dead- timber slash indicate this logging method may have a reasonable erosion risk for logging on slopes greater than 40%.