Tree-ring evidence for a combined influence of defoliators and extreme climatic events in the dynamics of a highaltitude balsam fir forest, Mount Megantic, southern Quebec

Most mature balsam fir trees (Abies balsamea (L.) Mill.) growing at the top of Mount Mégantic (Quebec) exhibit multiple leaders in the upper crown section, an indication of stressful conditions for tree development in this high-altitude stand. We report here on a detailed dendroecological analysis of windthrown trees showing that multiple stem development was linked to spruce budworm activity during the 1970s. In spite of severe defoliation, damaged trees temporarily restored their vegetative structure by forming reiterated axes. About 9% of all leaders sampled died during a 3-year period (1996-1998), which represents 60% of dead leaders sampled, with tree morbidity and mortality also peaking in 1996. Tree-ring dating of growth anomalies indicated the impact of major snow and (or) ice storms during the 1996 and 1998 winters, two events validated by field observations from the Mount Mégantic conservation area authorities. Biotic (defoliators) and extreme climatic events have a combined influence in the dynamics of high-altitude balsam fir forests.