Survival-time analysis of white spruce during spruce budworm defoliation.

摘要

Mortality and defoliation (DF%) in 987 white spruce (Picea glauca) trees were followed from 1992 to 2003 during an outbreak of the spruce budworm (Choristoneura fumiferana) in 15 white spruce-dominated uneven-aged stands in the Fort Nelson Forest District near Prince George, British Columbia, Canada. Four stands were aerially sprayed with Bacillus thuringiensis (Bt). Defoliation and mortality levels were elevated in non-sprayed stands. The relationship between defoliation and survival was captured in a Cox proportional hazard model with a defoliation stress index (DSI), diameter, crown class (CCL), a random stand effect, Bt-treatment, and number of years of exposure to stand-level defoliation (DYEAR) as predictors. The DSI, optimized for discrimination between survivors and non-survivors, is the discounted sum of five lagged DF% values. Survival probabilities were predicted with a maximum error of 0.02. Hazard rates increased by 0.06 for every one point increase in DSI. CCL and random stand effects were highly significant. Bt-treatment effects were fully captured by DSI, CCL, and DYEAR..