A practical demonstration of a method for rating forest stands as to their probability of defoliation by the Douglas-fir tussock moth using aerial photographs and available cruise data was done in the Palouse Ranger District, Clearwater National Forest, Idaho. A nonlinear computer program, RISK, was used to identify significant independent variables for the prediction of probability of defoliation (dependent variable). A hazard rating model developed in the Blue Mountains of Oregon and Washington was localized for the Palouse District using data collected from previously established plots. Data for the Palouse RD hazard rating were obtained from Stand Examination and Management Status Records, aerial photos, topographic maps, and Orthophoto Quadrangles. Four final hazard rating models were developed using various combinations of the following independent variables: elevation, slope, cosine of the aspect times the tangent of the slope, sine of the aspect times the tangent of the slope, topographic position, stand level, stand density, presence of volcanic ash in the soil, stand age, and percent host species. Hazard ratings which indicate the probabilities of a stand being defoliated during a tussock moth outbreak ware computed for 496 forest stands, and stands were color-coded on a map by probability of defoliation classes.
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