Empirical modeling of windthrow occurrence in streamside buffer strips Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/w37638790

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  • Streamside buffer strips provide numerous benefits to stream ecosystems. The buffer strips create shade, provide shelter for wildlife, act as barriers to logging debris during and after timber harvest, and serve as a continued source of large woody debris. Quantifying woody inputs resulting from windthrow provides managers with estimates for long-term planning and habitat development strategies. A two step modeling process was used to model stand and landscape level attributes to predict the presence or absence of windthrow and quantify its occurrence. A survey of windthrow was conducted on 22 stream reaches in the Coast Range and western foothills of the Cascade Range of Oregon. Stream reaches were located on Bureau of Land Management lands in Density Management Study sites. Using logistic regression, elevation was found to be the most significant predictor of the presence or absence of windthrow. Linear regression revealed that the mean of ratios of height to diameter of trees in a stream reach was the most significant predictor of the number of stems affected by windthrow. Across all stream reaches windthrow was found to be inversely related to the distance from the stream channel. Approximately 79% of recorded windthrow events occurred within the first 20 meters upslope from the stream, while 21% occurred between 21 and 40 meters upslope distance from the stream. Nearly 40% of the windthrow events fell in a north to northeast direction indicating that the prevailing winds played a large role in dictating the direction of fall. Local topography was also observed to have an impact on the direction of fall. Trees fell toward the stream channel approximately 25% of the time. Windthrow did not appear to be a significant source of mortality. In the majority of stream reaches, windthrow amounted to 1% or less of standing live basal area over the 4 yr. period. The greatest observed loss was 5.2%. In terms of the number of stems available to blow down, the greatest observed loss was 4%.
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