Long-term effects of spruce budworm outbreak and insecticide protection on forest recovery and structure in Eastern Quebec

Insect outbreaks can significantly alter forest structure through tree mortality. While surviving trees often exhibit growth release, overall productivity may still decline. Aerial insecticide applications have been widely used in Canada to mitigate spruce budworm ( Choristoneura fumiferana) impacts, though their long-term effects remain poorly understood. This study assessed the long-term impacts of the 1967–1992 outbreak, 12 years after its collapse, in protected and unprotected mixed forests in Eastern Quebec. We evaluated growth release at tree- and stand-levels, stand recovery duration, structural changes, and the influence of spraying. We remeasured 78 permanent 500 m 2 plots, recording DBH for all commercial species ≥10 cm and classifying stems as alive or dead. Growth losses were assessed via stem analysis. Balsam fir ( Abies balsamea) mortality was significantly higher in poorly protected plots (45.33%–57.11%) compared to well-protected ones (<25%). Surviving trees showed growth release across treatments, but only average- or well-protected plots had improved productivity and faster recovery. Poorly protected plots showed higher mortality, limited recovery, and more pronounced thinning effects compared to well-protected plots, with smaller trees most affected. The outbreak caused growth losses equal to 16 years for balsam fir, highlighting the need to prioritize this species in protection programs.