SPRUCE BUDWORM DEFOLIATION AND GROWTH LOSS IN YOUNG BALSAM FIR - PERIOD MODELS OF NEEDLE SURVIVORSHIP FOR SPACED TREES

The age-dependent Weibull model described detailed observations of needle survivorship taken in two protected plots of young, spaced, balsam fir (Abies balsamea [L.] Mill.) on the Cape Breton Highlands, Nova Scotia, Canada. Throughout the midcrown, the rates of age-specific needlefall increased with needle age, indicating a Type I survivorship curve typical of foliage. It was shown that the needle populations experienced distinct stages of needlefall as they aged. Few needles surviving a period of "infant mortality" in their first year fell in their second. There was a low, but steady, increase in the rates up to age 4-5, with a further increase in the rates for older needles. The mean longevity of the needle populations ranged from 4.1 to 6.1 years and generally increased with tree age. Differences between trees of the same age and differences between crown levels explained little more of the variation in needle fall than that already explained by needle age alone.