Topkill and stem defects initiated during an uncontrolled spruce budworm outbreak on Cape Breton Island, Nova Scotia

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Spruce budworm (SBW; Choristoneura fumiferana [Clem.]) defoliation begins at the top of the crown, and often results in leader death, loss of annual height growth or gradual top dieback, known as topkill. We used regression tree analysis to examine topkill incidences during a severe SBW outbreak in Nova Scotia, Canada in the 1970s–1980s that resulted in 60–100 per cent mortality of merchantable balsam fir (Abies balsamea (L.) Mill.) volume, and re-examined surviving trees for defects 25 years later. At the study outset in 1979, 4–6 years into the SBW outbreak, stand level topkill ranged from 40 to 88 per cent of trees and mean individual-tree topkill probability was 7, 52, 79 and 87 per cent on trees with ≤25, 26–75, 76–90 and 91–100 per cent total defoliation (all age classes of foliage). By 1987, the SBW outbreak had ended and mortality probability of trees was 12, 59, 85 and 98 per cent for classes of peak total defoliation of ≤25, 26–75, 76–90 or 91–100 per cent. Most mortality occurred following 6–8 years of defoliation, but topkill peaked after 4–6 years, indicating that topkill required less cumulative defoliation than death. When the plots were remeasured in 2012, 25 years later, 90 per cent of the surviving trees with documented topkill during the SBW outbreak had stem defects, of which 26 per cent were crooks, 53 per cent forks and 21 per cent sweeps. From 32 to 88 per cent of stem length was located above the point of the defect, resulting in the long-term downgrading of topkill-damaged logs.