Spruce budworm populations, defoliation, and changes in stand condition during an uncontrolled spruce budworm outbreak on Cape Breton Island, Nova Scotia

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Abstract
Effects of an uncontrolled spruce budworm (Choristoneura fumiferana (Clem.)) outbreak in 20 mature balsam fir (Abies balsamea (L.) Mill.) stands on Cape Breton Island, Nova Scotia, were studied from 1976 to 1985. Spruce budworm populations were extremely high, higher than peak levels recorded for other outbreaks, and averaged over 380 third-instar larvae per square metre of foliage from 1976 to 1980. As many as 1570 third-instar larvae per square metre occurred 5 to 7 years after the start of the outbreak, resulting in complete current defoliation and back-feeding on older foliage. Stands were divided into three groups, based on the pattern of cumulative current defoliation; the sum of current annual defoliation during the outbreak was 343, 445, and 543% for these groups, equivalent to the removal of 3.5 to 5.5 age-classes of foliage. Budworm defoliation caused the death of 78, 80, and 89% of the merchantable balsam fir volume in the three groups of stands, respectively, as well as 27% of the spruce (Picea sp.) volume; another 39% of the spruce volume died as a result of spruce beetle (Dendroctonus rufipennis Kby.) activity. Fir mortality commenced 3 years after the start of the outbreak and spruce mortality 2 to 3 years later. About one-half of the total mortality occurred in the 4 years after budworm populations and defoliation returned to low levels. Tree mortality and loss of foliage opened the stands, and 4 years after the collapse of the outbreak, 4% of the surviving trees and 17% of the dead trees had blown down, whereas 60% of all trees had broken tops.