A nonparametric survival model for balsam fir during a spruce budworm outbreak

Abstract

A nonparametric model that relates spruce budworm (Choristoneurafumiferana (Clem.)) and blowdown-caused mortality of balsam fir trees (Abiesbalsamea (L.) Mill.) to individual tree and stand characteristics is presented. Estimation of the annual base-line hazard, λ0(t), also known in epidemiological research as the force of mortality or the instantaneous rate of death, provides an explicit estimate of the rate of change (accelerating or decelerating) of cause-specific mortality during the spruce budworm outbreak. Interpretation of the spruce budworm mortality model indicates that suppressed trees are more likely to survive than the other crown classes. In general, mortality caused by spruce budworm increased with increasing cumulative plot defoliation, increasing spruce (Picea spp.) and balsam fir basal area per hectare, and decreasing hardwood basal area per hectare. Interpretation of the blowdown model indicates that blowdown mortality generally increases with increasing balsam fir, spruce, and percent balsam fir basal areas per hectare and decreasing cumulative plot defoliation.