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Spruce Budworm Defoliation and Growth Loss in Young Balsam Fir: Cohort Models of Needlefall Schedules for Spaced Trees

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The relationship between the age-specific rates of needlefall, q(x), and needle age, x, for cohorts of needles from young, spaced balsam fir (Abies balsamea [L.] Mill.) was described by the discrete hazard function of the Weibull model, q(x) = 1 - exp{(xs - [x + 1]s)/us}. Here u = 6.32 yr (SE = 0.122 yr), s = 3.03 (SE = 0.109), and R² = 0.982. These age specific rates increased with needle age, demonstrating a type I pattern of needlefall. Three phases of needlefall were distinguished. An "early life" phase in which the mean rates of needlefall remained under 10% per year occurred during the first 2 years of existence. Later, up to about needle age 4, the rates climbed steadily upwards with age (at 5.5% per year per year). After age 4, the rate of increase in needlefall with age increased to over 8% per year per year. This may be due to the onset of old-age or wear-out effects in the progressive deterioration of the needles with age. Use of crown level and year of observation as independent variables in addition to needle age did little to extend the explanatory power of our models. These results are consistent with the period analysis performed by Fleming and Piene (1992) on the same stands. Because of the large year-to-year variability in age-specific rates of needlefall, fits of the Weibull's discrete hazard function to individual cohorts usually left over half the variation in the rates unexplained. This contrasted with the period analysis in which the Weibull model explained over 80% of the variation each year. Hence, despite the success of the period analysis, the large year-to-year variability drastically restricts our ability to predict age-specific rates of needlefall. Although the Weibull model may describe future needle age distributions within a given year accurately, large fluctuations in parameter values, possibly reflecting weather-caused variations or long-term cycles and trends in needlefall, can be expected. For. Sci. 38(3):678-604.
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Keywords: Life-table; Weibull; age-specific mortality; foliar biomass; mortality schedules

Document Type: Journal Article

Affiliations: Maritimes Region, Forestry Canada, P.O. Box 4000, Fredericton, N.B., Canada E3B 5P7

Publication date: 01 August 1992

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