Soil compaction and disturbance following a thinning of second-growth Douglas-fir with a cut-to-length and a skyline system in the Oregon Cascades Public Deposited

Graduation date: 1998


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  • Soil bulk density and disturbance were measured before and after a commercial thinning of 3 0-50 year-old Douglas-fir using a cut-to-length (CTL) and a skyline system on the Willamette National Forest in the Oregon Cascades. A dual-probe nuclear densimeter was used to measure bulk density at four and eight inches. Slash depth after thinning and disturbance category were determined at bulk density measurement points in all units. Data were analyzed using paired t-tests, Tukey-Kramer multiple comparison tests, and multiple linear regression. Data were collected in two CTL units pre-thinning but one unit post-harvester and post-thinning (after both harvester and forwarder). During the sampling period thinning was completed only in the northeast corner of one unit (approximately 25 acres) before operations were ended due to concerns about soil moisture conditions and damage to hauling roads. Thus post-harvester and post-thinning data and conclusions apply only to that portion of the CTL units. In this harvested portion, slopes ranged from 0-10 percent and equipment traffic levels were estimated to range from three passes at the end of skid trails, to 30 passes on secondary trails and well over 40 passes on primary trails. These values are considered characteristic of most of Unit 82 except for areas near streams with steeper slopes. Plot centers were located using an incomplete, offset grid system. A sub-sample of bulk densities in old skid trails was also taken. The effects of harvester-only and the combined effect of harvester-forwarder traffic on the bulk density of old skid trails and previously undisturbed soils was assessed. In the three skyline units, bulk density was measured in the center, edge and halfway between skyline roads both before and after thinning. In addition, detailed maps of soil disturbance were created from transects located both directly on top of and halfway between a sub-sample of skyline roads within each unit. From the randomly located plots in the CTL units, it was determined that old skid trails covered approximately 27 and 13% of the two units, respectively. In the portion of the one unit where thinning was completed, it is estimated that an additional 27% of the area was disturbed, for a total of 40% disturbance post-harvester. Further, the harvester was estimated to have left exposed soil on 4% of the area and mixed the mineral and organic horizons on 9%. Post-thinning (both harvester and forwarder), new disturbance was estimated at 25%, for a total combined disturbance of 38%. Exposed or mixed soil was not observed at this time. The discrepancy between post-harvester and post-thinning disturbance estimates is thought to be due to different surveyors, slightly different data sets, redistribution of slash during forwarding or sampling error. In the skyline units, the maximum unit-wide disturbance was 1.8%, though greater than 10% of some units were in skyline roads. Mixing accounted for most of the disturbance, with exposure and rutting never reaching 0.5% on a unit-wide basis. The areas of exposure and rutting were small, discontinuous and usually occurred within 150 feet of the landing. In comparison to undisturbed soil, bulk density was still significantly elevated on the fifty-year-old skid trails sampled in the CTL units. The old skid trail center was 10- 16% more dense than undisturbed soil at the four inch depth in both units, but only ruts in Unit 81 were also greater in density (+8%) at the same depth. At eight inches, old skid trail ruts in both units were consistently more dense (13-15%), as well as the center of trails in Unit 82 (+15%). In neither unit was the bulk density of old skid trail berms statistically different from undisturbed soil. Overall, it is estimated that 4 and 10% of Unit 81 (4 and 8 inch depth) and 6 and 4% of Unit 82 was already in a compacted state prior to this thinning entry. It is important to note, however, that elevated bulk densities on old skid trails may also be explained by measurements being made on naturally more dense subsoils exposed after scalping of the surface soils during the initial harvest entry. The harvester alone was not found to significantly compact either undisturbed soil or old skid trails (p>0. 10). Harvester-forwarder traffic significantly increased the bulk density of previously undisturbed soil by an average of 12 (4 inch depth) and 11% (8 inch depth, p=O.O4 and 0.05), but did not change the bulk density of old skid trails. Compaction levels as a result of this entry were comparable with those of old skid trails. Overall, it is estimated that this new entry contributed an additional 25% (4 and 8 inches) to the compacted area, for an estimated total compacted area of 29-31%. The estimate of total compaction due to this thinning is based on the estimated area in new skid trails and an average of all observed compaction values across the width (both ruts and center) of the skid trail. The bulk densities of the center, edge and zone between skyline roads after thinning were compared to pre-thinning values, but revealedno evidence that a significant difference existing between the four categories (90% level). There was some evidence that mixed soil with ruts greater than six inches deep were approximately 40 and 45% (4 and 8 inch depth) more dense than undisturbed soil (p=0.0006 and 0.0002). Also, points along the skyline roads where over 16000 ft3 were yarded were found to be significantly associated with a reduction in bulk density of approximately 16 and 18% at four and eight inches in depth (p=O.022 and 0.008
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