Patterns of balsam fir mortality caused by an uncontrolled spruce budworm outbreak

Abstract

Tree mortality caused by spruce budworm (Choristoneurafumiferana (Clem.)) defoliation was assessed annually from 1976 to 1985 in 20 mature balsam fir (Abiesbalsamea (L.) Mill.) stands on Cape Breton Island, Nova Scotia, and was related to defoliation and to tree, stand, and site characteristics. Ten to 12 years after the start of the budworm outbreak, fir mortality averaged 87% of the merchantable volume (range 60–100%) among the stands. Timing of mortality was similar to that found in studies of previous outbreaks. In the first 4 years of the outbreak, virtually all the trees that died had more than 90% cumulative defoliation but, overall, 64, 21, and 14% of the dead trees had cumulative defoliation >90, 76 to 90, and 51 to 75%, respectively. Early in the outbreak, fir mortality was generally negatively correlated with tree vigor, relative crown position, or diameter at breast height, but in later years, trees were killed irrespective of these factors. Fir mortality was evenly distributed among different sized trees, and 73 to 86% of the trees in each 5 cm diameter at breast height class died. A linear regression equation between dead fir volume and total fir volume explained 89% of the variability in mortality among stands. Percent fir mortality was correlated (r = 0.84) with visual estimates of cumulative defoliation (including all age-classes of foliage) in 1981, but mortality was not correlated with cumulative current annual defoliation or with site characteristics. Using regression equations, fir mortality during this budworm outbreak was predicted to within ±6 m2/ha in 14 of 18 (78%) of the stands, with a relative accuracy of 17.7%.