Effect of fire and insect outbreaks on long-term forest succession in boreal forests of eastern Quebec

Background/Question/Methods Knowledge on natural disturbances is important to understand the origin and dynamics of boreal forest ecosystems. We quantified the effect of forest fires and insect outbreaks, particularly from the eastern spruce budworm, in a 65000 km² section of the eastern North-American boreal forest. Fire return intervals were studied based on information coming from a fire map for the 1800-2000 period, and post-fire succession was estimated from more than 3000 forest inventory plots. The historical reconstruction of spruce budworm outbreaks was based on dendrochronology and the interpretation of old aerial photographs.

Results/Conclusions Fire return intervals varied between 275 and 500 years in this region. Despite these long intervals, fires of relatively large sizes were common (including 14 fires of more than 500 km² during the last 200 years), which generated a forest mosaic characterized by large post-fire forest patches dominated by black spruce accompanied by pioneer species such as trembling aspen and jack pine, imbedded within a matrix of late-successional forests co-dominated by balsam fir and black spruce. As for past spruce budworm outbreaks, they had little impact in the study area during the 1800-2000 period, with the notable exception of the most recent outbreak (~1972-1984), whose impact was restricted to the south. This recent outbreak resulted in the replacement of old stands co-dominated by balsam fir and black spruce with young stands co-dominated by balsam fir and white birch. Overall, when considered over large spatio-temporal scales, the occurrence of the two disturbance types appeared to be influenced separately and strongly by climate and weather patterns, and to a lesser extent by forest characteristics such as composition and structure.