Surface and Subsurface Soil Response to Tethered Harvest Systems on Steep, Ash-Cap Forest Soils: A Case Study in Eastern Oregon

Restoring resilient forest structure in the fire-prone, densely stocked forests of the Inland Pacific Northwest depends on mechanical thinning, much of it too steep for conventional ground-based equipment. Winch-assisted (tethered) harvesting has expanded mechanized access to such terrain, but its soil impacts on the ash-cap soils of the Blue Mountains remain poorly characterized, with research focused on near-surface response. This study characterized soil response following two operationally distinct tethered mechanized systems, cut-to-length (CTL) and whole-tree (WTL), within the same 15-ha unit on a 40% ash-cap slope in the Wallowa-Whitman National Forest, Oregon. Bulk density and moisture content were measured at the surface and at 30 cm across pre-harvest, immediate post-harvest, and one-year post-harvest periods, and cross-slope profiles were measured to quantify surface soil displacement. CTL corridors showed minimal immediate surface disturbance due to slash-mat protection but densified significantly by one year, likely driven by environmental exposure rather than machine traffic; sub-surface compaction was transient, relaxing within a year. WTL corridors densified immediately at the surface and remained elevated at one year, while sub-surface bulk density declined significantly below pre-harvest levels by one year, though this decrease may partly reflect sampling variability rather than a purely soil-driven process. Neither system showed sustained compaction at 30 cm after one year. These results indicate that, at this study site and under the operating conditions evaluated, no persistent increase in bulk density was detected at 30 cm depth for either system within the one-year monitoring period, though CTL and WTL produced distinct disturbance timelines that land managers should weigh when selecting equipment for similar terrain.