- Timber harvesting in the Pacific Northwest (PNW) region of the United States of America (USA) has a long history of explosive growth, environmental litigation and regulation, economic downturn, and workforce dynamics that have created a current environment where cable-assisted harvesting is quickly gaining popularity among private and public land managers alike. While logging continues to be one of the deadliest occupations in the United States, an increasing primary concern for many privately- and federally-owned lands in the western United States is soil disturbance. In addition, the large long-term capital investment required of these systems places additional pressures on prospective adopters. The economic and environmental impact from cable-assisted harvesting machines is largely undocumented from a scientific perspective, and lack of information can in some cases be grounds for exclusion of cable-assistance as a possible harvesting system.
Cable-assistance could provide a host of benefits to the timber industry, such as a reduction in logging accidents and the ability of an aging workforce to continue working. However, with such a significant capital investment required and undocumented environmental (soil) impact, adopters of cable-assisted systems are left to face these decisions with an inadequate supply of information.
The purpose of this research, through a case study approach, is to quantify the productivity and cost (P&C) and soil impacts of a harvester and forwarder operating with and without the use of cable-assistance on dry clay soils in western Oregon. Detailed time study data was collected in order to generate regression models for both machines under each system, while bulk density and penetration resistance data was collected before harvesting, after harvesting, and after forwarding across the machine corridor and down to a soil depth of 50 cm.
Untethered and tethered harvesting showed estimated productivity values of 115.45 and 79.01 m3 per productive machine hour (PMH), respectively, while forwarding productivity was estimated at 35.94 and 42.54 m3/PMH for untethered and tethered, respectively. Depending on the coupling of the harvest operation, machine productivity ranged from 28.75 m3/SMH to 92.36 m3/SMH, with resulting unit costs for untethered and tethered systems ranging from $13.19/m3 to $17.78/m3. Differences in harvester productivity were possibly attributable to different operators and the narrower operating window when operating using cable-assistance.
Soil compaction results showed a smaller geographically impacted area, smaller increases in soil density, and in some cases a decrease in soil density using cable-assistance. The reduced coverage effect, primarily through the reduction of track wander, creates a tradeoff between coverage area and coverage intensity. The smaller increases, and decreases, in soil density is a product of reduced shear displacement of soil. This shear reduction is due to a better weight distribution provided by cable-assistance. As a result, this soil can likely be subjected to more passes before the onset of densification occurs if using cable-assistance.