Traditional hand-felling on steep slopes in the Pacific Northwest (USA) minimizes soil disturbance but is labor intensive and hazardous. Tether-assist machine harvest offers a safer alternative, yet its soil impacts are less understood. This study compared soil impacts (bulk density (Db), sediment production, penetration resistance) and soil resilience (volumetric water content (VWC), seedling growth) between hand-felled and tethered machine-felled areas on > 30% slopes with Ultisol soils (silty clay loams) in Southwest Oregon. Pre- and post-yarding soil measurements were taken in both harvest areas, with tracked and untracked zones assessed for resilience in the tethered system. Both methods showed a general decrease in near-surface Db that persisted through to the post-yarding phase. A general increase in subsurface penetration resistance occurred across treatments, with varying significance. No sediment production was observed over two years. Tracked soils exhibited higher VWC, especially during dry months with high vapor pressure deficit, and greater Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) seedling biomass than untracked soils. These results suggest, for the two-year observation period, that tether-assist harvest on these soils does not more negatively impact soil compared to the current steep slope harvest standard of hand-felling. The higher plant biomass and VWC in tracked zones suggest machine traffic collapses macropores, increasing water retention, which may allow for more plant available water during dry seasons. This study contributes to the growing understanding of tethered versus traditional harvesting on steep terrain and suggests that tethered machine-felling could be a safer and viable alternative to hand-felling on steep (> 30% slopes) terrain with Ultisol soils in dry conditions.
Chung et al. (Tue,) studied this question.