The relation of balsam fir volume increment to cumulative spruce budworm defoliation

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Abstract
The effect of cumulative defoliation caused by spruce budworm (Choristoneura fumiferana [Clem.]) from 1981 to 1987 on tree growth was assessed in a 35-year-old balsam fir (Abies balsamea [L.] Mill.) stand. After six years of moderate to severe defoliation, specific volume increment was reduced by an average of 11, 55, 83, 70, and 83% for trees with visual cumulative defoliation ratings, in 1987, of 1–25%, 26–50%, 51–75%, 76–90%, and 91–100%, respectively. Following defoliation, mean specific volume increment ranged from 0.02 cm3 cm−2 year−1 for trees with >90% cumulative defoliation to 0.16 cm3 cm−2 year for trees with <25% defoliation. Specific volume increment and percentage growth reduction were significantly related to the cumulative defoliation rating, with regression equations explaining 72 and 64%, respectively, of the variability among trees. It was concluded that onetime visual ratings of cumulative defoliation caused by spruce budworm can be used in assessing balsam fir growth rates and growth reduction. Key words: growth reduction, specific volume increment, stem analysis, Choristoneura fumiferana, Abies balsamea