Spatial and temporal patterns of windthrow in the Bull Run Watershed, Oregon Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/ww72bf18j

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  • Throughout the 20th century, windthrow has affected forests in the Bull Run watershed, a 26,500 ha basin that is the principal water source for the city of Portland, Oregon. Windthrow from storms in 1973 and 1983 was mapped into a geographic information system (GIS) and compared to a 1931 windthrow pattern that had been determined through field work and photo interpretation. By relating the windthrow patterns to vegetation, topography, soils, and edges created by forest canopy openings such as lakes and clearcuts, the degree to which these factors influence the patterns was tested with a logistic regression. Distanced sampling was used to compensate for spatial autocorrelation among the data. The results showed that topographic exposure most strongly affected the 1931 windthrow patterns, while a greater number of 1973 and 1983 windthrow patches were associated with clearcut edges than other edges, and those edges were more likely to be affected when located on shallow soils with a high hazard for windthrow. All three storms were characterized by northeasterly winds, a common occurrence during winter months in the vicinity of the Columbia River Gorge. However, the particular combination of at least two-consecutive days of severe east winds, and sub-freezing temperatures, had only occurred four times between 1948 and 1994, including both the 1973 and 1983 windthrow events. Multiple approaches were taken to estimate future windthrow risk in the Bull Run that incorporated both spatial and temporal variables. Probabilistic windthrow maps showing zones of low, medium, and high risk were generated for current forest conditions, as well as conditions projected-to the years 2010 and 2075. Because variables such as vegetation height and clearcut edges are ephemeral, the prediction of future risk incorporates assumptions of vegetation succession. However, a mean return interval calculated for predicted windthrow-generating storms suggests that such events could occur as frequently as once every three years. While no model can predict the precise location of a disturbance as variable as windthrow, an understanding of the relative spatial and temporal probability of windthrow provides useful information for forest planners and managers.
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