Spruce budworm defoliation and growth loss in young balsam fir: defoliation in spaced and unspaced stands and individual tree survival

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Abstract
A severe outbreak of spruce budworm (Choristoneurafumiferana (Clem.)) began in 1976 in young, spaced and unspaced stands of balsam fir (Abiesbalsamea (L.) Mill.) located on the Cape Breton Highlands, Nova Scotia, Canada. A study was initiated that year to relate decreases in foliar biomass, caused by defoliation, to reductions in annual volume increment. Detailed defoliation assessments on individual trees from 1976 to 1984 showed severe defoliation of the current foliage in 1976. In 1977 and 1978, current foliage was destroyed in the early part of shoot elongation and severe back-feeding occurred on older age-class needles. After 1978, defoliation decreased as a result of a decline in budworm populations and, in general, only the current foliage was defoliated until the populations decreased to low levels in 1983. Defoliation was significantly higher in spaced than in unspaced stands in 1977, 1978, and 1980, and as a result, by 1984, average tree mortality caused by spruce budworm was 43.8% in spaced and 18.9%.in unspaced stands. The survival of some balsam fir trees and not others following a budworm outbreak is not related to differential defoliation, but to the ability of some balsam fir trees to rapidly increase foliar biomass through prolific epicormic shoot growth.