SPRUCE BUDWORM DEFOLIATION AND GROWTH LOSS IN YOUNG BALSAM FIR - SPACING EFFECTS ON NEEDLEFALL IN PROTECTED TREES

Age-dependent Weibull models were used to describe precise measurement s of needlefall from a pair of protected plots of young, unspaced, bal sam fir (Abies balsamea [L.] Mill.), These models showed that the age- specific rates of needlefall tended to increase with needle age throug hout the observed crown, Where crown level effects were evident, needl e lifespans were shorter in the lower crown than in the upper crown, T he influence of crown level in the unspaced plots, however, was too in consistent to be useful as a general predictor of needlefall rates. Co m paring the results with previous work on spaced sample plots showed that spacing affected many aspects of needlefall, The mean needle life span of 5.3 yr (SE = 0.17) on the spaced sample plots was significantl y greater than the 4.7 yr (SE = 0.22) estimated for the unspaced plots , These differences were most extreme in the lower crown. Spacing also influenced the earliest phase of needlefall, For needles under 2 yr, the mean age-specific rates of fall were negligible in the spaced plot s, but were about 6%/yr in the unspaced plots, Thereafter, annual incr eases in the age-specific rates of needlefall were similar in the spac ed and unspaced plots, Spacing also affected the pattern of year-to-ye ar variation in needlefall,There was evidence of statistically signifi cant trends in mean needle lifespans overtime in the spaced plots, but not in the unspaced plots.