Graduate Project

The effects of stand thinning on soil erosion rates at Jim's Creek in the Willamette National Forest, Oregon

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  • Surface sediment transport is an important geomorphic process which can be significantly altered by management activities in forested ecosystems. Disturbance of the soil surface may result in increased sediment delivery to fish bearing streams and degradation of soil structure. Selective thinning and low impact yarding techniques were utilized in an effort to restore oak savanna ecosystem structure to the Jim’s Creek Savanna Restoration Project Area southeast of Oakridge, Oregon. This study monitored sediment transport rates following the thinning of nearly 90% of the existing 120 year old Douglas-fir stand within the project boundary. The study followed a before-after control-impact (BACI) design, in which the 12 sediment traps were monitored for 12 months prior to thinning, were removed during the logging activities, and then returned to the same locations following the thinning and monitored for an additional 2 year period. Estimated soil erosion rates derived from traps were then compared to baseline (pre-restoration) transport rates collected at the same locations in a previous study to assess change over time. Two control traps were included in the analysis and were placed in an area that did not undergo thinning activities during the study. The rest of the traps were place in areas in which thinning via skyline yarding techniques occurred (near the existing road network) or in areas where helicopter yarding was utilized to remove felled trees. Average estimated soil erosion rates from traps located in thinned areas declined from 983.5 g/m/yr to 379.8 g/m/yr following logging activities at the site; and rates declined from 156.5 to 84.1 at the two untreated reference plots. Previous work at the site suggests that rain- and throughfall-drop splash is the dominant surface soil erosion mechanism and that an alteration of canopy structure may influence this process. Reduced canopy structure can lead to a reduction in throughfall splash erosion. The hypothetical reduction in throughfall splash combined with possible loss of drop splash by fog drip as a result of canopy thinning may explain the observed decrease in surface soil erosion following stand thinning. There is some indication of a difference in the seasonality of peak surface sediment transport following the site treatments. However, this comparison may not be valid due to the duration of the pre-treatment study not including the later summer months which were included in the post-treatment monitoring and consistently showed the highest transport rates. Surface soil transport was simulated using a non-linear diffusion model as well as a version of the web-based GIS WEPP erosion model. Simulated rates generally poorly represented observed rates, and the WEPP model consistently over-predicted surface sediment transport rates. Continuation of existing erosion boxes and addition of sites to sample a range of canopy cover conditions would increase understanding of effects of canopy influences and long-term effects of site disturbance. Acquiring a finer scale digital elevation model may facilitate more accurate modeling of surface erosion.
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