Spruce budworm defoliation and growth loss in young balsam fir: relationships between volume growth and foliage weight in spaced and unspaced, defoliated and protected stands

Abstract

Foliage weight (FW) and tree volume, volume increment (VI), and specific volume increment (SVI) were examined throughout a spruce budworm ( Choristoneura fumiferana (Clem.)) outbreak cycle from 1976 to 1984, for young balsam fir ( Abies balsamea (L.) Mill.). Treatments were spaced and unspaced, defoliated and protected (annual insecticide spraying to prevent defoliation). Six years into the outbreak, FW in defoliated plots reached its minimum levels, representing a reduction of 83.3 and 84.3% for spaced and unspaced stands, respectively, compared with the protected stand. For surviving trees, FW recovery was rapid and substantial (threefold to fivefold over 3 years), due to the release of suppressed buds. Still, at the end of the outbreak, FW was reduced by 37.5 and 68.5% for spaced and unspaced stands, respectively. VI was reduced by 92.5 and 91.1% and SVI by 92.0 and 89.3% for spaced and unspaced stands, respectively, compared with the protected stand. This severe budworm outbreak reduced growth rates by about the same level, regardless of initial differences in growth rates and density. By the end of the outbreak, volume was reduced by 32.6 and 54.1 m 3/ha for spaced and unspaced stands, respectively, or a decrease of 51.7 and 35.7% compared with the protected stand. Relationships between VI and FW were, in general, linear for spaced trees and a mixture of linear and non-linear for unspaced trees. On an individual-tree basis, VI: FW relationships were strong at the beginning and end of the spruce budworm outbreak, but weaker in the middle. Combining all years, on a per hectare basis, the relationships between VI and FW were strong ( r 2=0.78 for spaced and 0.80 for unspaced stands), and still held despite defoliation.