Legacy of insect defoliators: Increased wind-related mortality two decades after a spruce budworm outbreak

Abstract

Effects of spruce budworm (Choristoneura fumiferana Clem.) outbreaks on growth and survival of balsam fir (Abies balsamea [L.] Mill.) and spruce (Picea spp.) are well documented, but few studies extend beyond 10 years after defoliation ceased. We used inventory data from 106 permanent sample plots in >50-year-old balsam fir stands in northern New Brunswick, Canada, to determine legacy effects of the 1969-1993 budworm outbreak on stand development up to 29 years after defoliation ceased. Defoliation data were from annual aerial surveys from 1945 to 1993 and plot ground sampling from 1985 to 1993. Plots were stratified into net stand volume development categories (decreasing, stable, and increasing 1985-2005 stemwood volume) and related to outbreak phases (outbreak, direct 1-10 years after defoliation ceased, and legacy >10 years), outbreak severity (1-4 [low], 5-8 [medium], and 9-12 [high] years of defoliation), and stand age (mature and overmature). Stand age was an important factor influencing outbreak severity (e.g., r2 0.383, P < 0.01). Trend and rate of volume development over time were related to past outbreak severity and increased rate of postoutbreak wind-related mortality, which peaked at 11 m3/ha/yr 11-15 years after defoliation ceased. Results indicate that aging postoutbreak stands are more vulnerable to wind disturbance events, effecting rapid stand decline. Copyright © 2009 by the Society of American Foresters.