Spruce budworm defoliation and growth loss in young balsam fir: patterns of shoot, needle and foliage weight production over a nine-year outbreak cycle

Abstract

Patterns of shoot numbers, shoot length, needle length, and foliage weight were examined throughout a spruce budworm ( Choristoneura fumiferana (Clem.)) outbreak cycle, for young balsam fir ( Abies balsamea (L.) Mill.), representing four spacing and defoliation conditions. Treatments were defoliated spaced and unspaced, and protected (control, by annual insecticide spraying to prevent defoliation) spaced and unspaced. Foliage parameters were measured on one branch from every second whorl of 20–38 trees per treatment, each year from 1976 to 1984. We found that the number of shoots for protected spaced trees was about three times higher (1140 vs. 400 shoots/tree) than for unspaced trees. There was, in general, no significant difference between protected spaced and defoliated spaced trees, and between protected unspaced and defoliated unspaced trees. For protected trees, the number of shoots increased from the top toward mid-crown, and then decreased or stabilized toward the base, whereas defoliated trees had the greatest number of shoots in the lower crown. Shoots were longer on protected than on defoliated trees (means of 3.9–4.0 vs. 2.2 cm/shoot), but there was no effect of spacing. Shoot length was greatest in the uppermost whorls of protected trees and decreased toward the base, while defoliated trees had similar shoot lengths throughout the crown, but were converging to the non-defoliated pattern by 1984. Defoliation resulted in longer needles in one year and shorter needles in three years; there were no consistent relationships with crown position while spacing affected needle lengths in three and two years for protected and defoliated trees, respectively. Foliage weight per tree was 2.7–3.3 times higher for the protected spaced than unspaced trees, while by 1981, foliage weight of defoliated trees was only 7–12% that of the protected trees. However, by 1984, the rate of foliage weight increase was similar between protected and defoliated trees. Regardless of treatment, the maximum foliage weight was in the lower crown. For protected, unspaced suppressed trees, there was a shift in the foliage weight distribution over time, with increased weight in the upper crown.