Forest Character and Vulnerability of Balsam Fir to Spruce Budworm in Minnesota

Abstract Vulnerability of balsam fir to damage by the spruce budworm was studied from 1956 to 1962 in northeastern Minnesota. Data on tree injury were collected annually on permanent plots in 23 balsam fir stands representing a variety of stand conditions. Multiple regression analyses of preoutbreak attributes of individual trees revealed none that explained more than 27 percent of the variation in damage. Using nine tree characteristics in a discriminant analysis resulted in misclassifying about one-fourth of the trees expected to survive. More promise in predicting mortality was shown by three stand characteristics: (a) percent basal area in spruce, (b) percent basal area in nonhost species, and (c) balsam fir basal area. These components in an equation explained 56 percent of the variation in balsam fir mortality. Though not high, this is reasonably good in view of the other sources of variation in the data. The study helps quantify the relation between forest character and spruce budworm vulnerability.