%0 Research article
%T Survival-time analysis of white spruce during spruce budworm defoliation
%A Magnussen, Steen
%A Alfaro, René I.
%A Boudewyn, Paul
%D 2005
%J Silva Fennica
%V 39
%N 2
%R doi:10.14214/sf.382
%U https://silvafennica.fi/article/382
%X Mortality and defoliation (DF%) in 987 white spruce (Picea glauca  (Moench) Voss) trees were followed from 1992 to 2003 during an outbreak  of the spruce budworm Choristoneura fumiferana (Clem.) in 15  white-spruce-dominated uneven-aged stands in the Fort Nelson Forest  District near Prince George, British Columbia. 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-times was captured in a Cox proportional hazard  model with a defoliation stress index (DSI), diameter (DBH), 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.