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

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 cm(3) cm(-2) year(-1) for trees with >90% cumulative defoliation to 0.16 cm(3) 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.