Graduate Project
 

Identification of variables influencing residual stand damage from skyline thinning of young western hemlock-Sitka spruce stand

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https://ir.library.oregonstate.edu/concern/graduate_projects/nc580s365

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  • Western hemlock and Sitka spruce are relatively thin- barked species and susceptible to damage during thinning operations. Damage to the wood allows decay-causing fungi to enter resulting in loss of merchantable volume at the time of final harvest. Cable yarding systems are needed for much of the thinning because most hemlock-spruce stands in the Pacific Northwest are located on steep slopes and fragile soils. It is during these thinning operations that much of the damage occurs. The purpose of this study was to determine the significant harvesting variables affecting residual stand damage due to cable thinning a 30-year-old hemlock -spruce stand. Detailed stand damage measurements were made during logging on nine skyline units in two study areas in western Oregon. Less detailed damage was measured on 18 other units to determine differences in damage levels between three different thinning treatments: conventional low intensity thinning, conventional high intensity thinning, and a herringlone (strip) thinning. Twenty-two variables were measured in two categories: harvesting variables and stand damage variables. Total scar area per turn (ft2/turn) was used as the dependent variable. As a result of regression analysis, the following variables were found to most significantly influence residual stand damage: number of carriage repositions, log. angle, carriage clearance., narrow treatment, rigging slinger, and cutter. An analysis of variance showed mean scar area per acre for the narrow and wide treatments were significantly different from the strip treatment. Only 12 percent of the residual stand (trees/acre) in the strip treatment were damaged. The narrow and wide treatments experienced much higher levels of 47 and 61 percent, respectively. Conventional thinning treatments experienced extensive damage levels (84.78 ft2/acre in the narrow treatment and 91.64 ft2/acre in the wide treatment) compared to the strip treatment (17.57 ft2/acre). Individual scars ranged in size from 0.02 to 14.00 square feet. Of the total scar area in the detailed units, 66.6 oercent was found within 20 feet of the skyline corridor centerline.
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