Stand breakdown and surface fuel accumulation due to spruce budworm (Choristoneura fumiferana) defoliation in the boreal mixedwood forest of central Canada

Spruce budworm (Choristoneura fumiferana (Clemens)) defoliation has been shown to increase the likelihood of large forest fires in central Canada. However, the time frame of heightened risk based on the duration of spruce budworm defoliation has not yet been quantified. In this article, we document the extent of stand breakdown and surface fuel accumulation after a period of spruce budworm defoliation that occurred between 1972 and 1976. Data on stand characteristics were derived from previous studies at three different locations in the boreal mixedwood forests of central Canada: Aubinadong (B.J. Stocks. 1987. For. Chron. 63: 8-14), Gogama, and Gowganda in Ontario. Stand breakdown was measured using a series of transects set in plots 7 years following aerially mapped defoliation (1977-1983). Results show that during the 4 years following 5 years of defoliation, crown breakage, a typical symptom of defoliation, increased by nearly 200%, and surface fuel increased by 145% from predisturbance levels. The high correlation between crown breakage and surface fuels linked defoliation to fuel buildup. We begin to solve the challenge of measuring fuel structure over the expansive scale of spruce budworm outbreaks by quantifying the relationship among stand breakdown, time since the end of defoliation, and the duration of defoliation so that the expected fuel structure can be modelled from annual defoliation surveys. Key words: spruce budworm defoliation, crown fire, forest fire management, forest structure, natural disturbance. Il a ete demontre que la defoliation causee par la tordeuse des bourgeons de l'epinette (Choristoneura fumiferana (Clemens)) augmente la probabilite que de vastes feux de foret surviennent dans la partie centrale du Canada. Cependant, la periode de risque accru fondee sur la duree des defoliations causees par la tordeuse des bourgeons de l'epinette n'a pas encore ete quantifiee. Dans cet article, nous documentons l'ampleur de la deterioration des peuplements et de l'accumulation des combustibles de surface a la suite d'une periode de defoliation survenue entre 1972 et 1976. Les donnees sur les caracteristiques des peuplements ont ete extraites d'etudes precedentes realisees a trois endroits differents dans les forets boreales mixtes du centre du Canada : Aubinadong (B.J. Stocks. 1987. For Chron. 63: 8-14), Gogama et Gowganda, en Ontario. La deterioration des peuplements a ete mesuree a l'aide d'une serie de transects etablis dans des parcelles 7 ans apres une defoliation cartographiee par la voie des airs (1977-1983). Les resultats montrent que durant les 4 annees qui ont suivi 5 annees de defoliation le bris des cimes, un symptome typique de defoliation, a augmente de pres de 200% et les combustibles de surface de 145%, relativement aux niveaux existants avant la perturbation. La correlation etroite entre le bris des cimes et les combustibles de surface relie la defoliation a l'accumulation de combustibles. Nous commencons a relever le defi que represente la mesure de la structure des combustibles considerant l'ampleur croissante des epidemies en quantifiant la relation entre la deterioration des peuplements, le temps ecoule depuis que la defoliation a cesse et la duree de la defoliation, de telle sorte que la structure anticipee des combustibles puisse etre modelisee a partir des releves annuels de defoliation. [Traduit par la Redaction] Mots-cles: defoliation causee par la tordeuse des bourgeons de l'epinette, feu de cime, gestion des feux de foret, structure de la foret, perturbation naturelle.
Introduction Spruce budworm (Choristoneura fumiferana (Clemens)) outbreaks and forest fire are two important natural disturbances in Canada's boreal forests; however, the relationship between these two ecological drivers has not been [...]