Damage Characteristics in Young Douglas-Fir Stands from Commercial Thinning with Four Timber Harvesting Systems

Damage to residual trees from commercial thinning was characterized and compared among four common harvesting systems in western Oregon: tractor, cut-to-length, skyline, and helicopter. Six young (30to 50-year-old) Douglas-fir (Pseudotsuga menziesii) stands with various residual densities were studied. Scarring was the most typical damage to crop trees, accounting for 90% of the total damage in most cases. Damage levels greatly decreased as the minimum scar size that defines damage was increased. Scarring by ground-based systems was more severe: scars were larger, and gouge and root damage were more prevalent than in skyline and helicopter systems. Damaged trees were concentrated within 15 ft of skid trails or skyline corridor centerlines. In the cut-to-length system, the harvester caused more wounding (70%) to crop trees than did the forwarder (30%), but forwarder scars were larger and sustained severe gouging. Recommendations for minimizing stand damage are included. NOTE: The authors would like to acknowledge Steve Pilkerton and Mark Miller for assistance in making key local contacts as well as obtaining materials and supplies. The logging contractors involved in the stand damage study deserve our thanks for their cooperation during the project. Thanks are extended to USDA Forest Service sale administrators on the Blue River, McKenzie, Oakridge, Yachats, and Hebo Ranger Districts for their help. This research was supported by Oregon State UniversityOs Strachan Forestry Research Fund and the USDA Center for Wood Utilization. Present address: H.-S. Han, Faculty of Natural Resources and Environmental Studies, University of Northern British Columbia, 3333 University Way, Prince George, BC, V2N 4Z9, Canada. Thinning prescriptions are increasingly necessary for intensive management of private and public forestlands in western Oregon. Sessions et al. (1991) reported that within 25 years, thinning will be required on about two-thirds of industrial lands. In the same survey, the U.S. Forest Service and the Bureau of Land Management (BLM) planned to implement intensive management on virtually all of their forested acres allocated to timber production in western Oregon. Unfortunately, thinning activities produce residual stand damage that may adversely affect timber growth and value. Each harvesting system can cause distinctive damage to remaining crop trees during thinning operations. Most scars from cut-to-length thinning are relatively small (Bettinger and Kellogg 1993). Damage to leave trees was less severe with skyline thinning than with conventional skidding or tractor-based operations (Aulerich et al. 1976, Fairweather 1991). Flatten (1991) also found that helicopter thinning damage was far less than that typically found with skyline systems. Government agencies and private industry have inconsistent and often ambiguous policies for defining a maximum acceptable damage level and what constitutes a damaged tree. The minimum scar size that qualifies as damage ranges from 1 in. to 144 in. (British Columbia Ministry of Forests 1983; Oregon Department of Forestry 1995; Weyerhaeuser Co. 1996; Washington State Department of Natural Resources 1997). The main concern is how a critical size affects tree health and value loss at final harvesting. The maximum acceptable damage level for scarring, crown, and root damage is not consistent either, varying from 3-5% of total residual trees or at an inspectorOs discretion. Species, logging systems, thinning treatments, timber value, rotation age, and snags for wildlife habitat are factors to consider when determining the maximum acceptable damage level. Logging injury can be reduced by promoting damage-free efforts, such as suspending thinning during sap flow (Wallis and Morrison 1975, Aho et al. 1983), clearly marking leave trees (Kelley 1983), and matching the logging system to topography and timber size (Aho et al. 1983). Shorter rotations to minimize decay loss after thinning (Lamson et al. 1985), limiting use of drive-tobunch vehicles to deep-rooted species (Ostrofsky et al. 1986), directional felling (Kellogg et al. 1986), reduced road and trail densities (Hoffman 1990), use of straight skid trails (Hoffman 1990), and temporary rub trees (McLaughlin and Pulkki 1992) also protect residual trees. Potential problems can be identified and resolved, but, no matter how well-planned and designed the thinning operation is, success depends on loggersO skills and experience in their efforts to lessen stand damage (Kellogg et al. 1986, Ostrofsky et al. 1986, Hoffman 1990). The winter in western Oregon is the rainy season; most logging activities occur during summer and fall. Logging during winter often causes soil disturbances from ground skidding and hauling on unpaved spur roads. In some higher elevation areas, however, snow covers the ground and soils are frozen, allowing winter logging. Root damage from equipment travelling and logs being dragged over the snow is lower, compared with summer logging. Also, because sap flow is less during winter, the stems may be less susceptible to scarring damage then.