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Since 2006, the area defoliated by the spruce budworm (SBW) doubles every year, reaching in 2015 more than 6 million hectares (MFFP 2015). The North Shore of Quebec is the most affected region, with almost 60% of defoliated areas in 2015. Since 2011, mortality is observed in trees located in areas heavily defoliated over several years. Although advance regeneration is a very important factor with regards to the future of our forests, very few studies have focused on how it is affected by the SBW, much less directly (and not through reconstruction by dendrochronology). Also, knowing that salvage logging in stands displaying high mortality rates started in some places on the North Shore, it is important to question the effects that these could have on defoliation sustained by advance regeneration. In years to come, post-SBW salvage logging could become a major economic issue in boreal forests and reducing its impact involves an understanding of the ecological processes affected by the outbreak in progress. Until now, however, our knowledge of the effects of harvesting in disturbed forest have focused on those affected by wildfires, and some affected by the mountain pine beetle in western Canada. This study aims to fill the gaps in the literature on the impact of an outbreak of SBW and of salvage logging in forests disturbed by the SBW on the defoliation sustained by advanced regeneration of mixed stands of balsam fir and black spruce.
A permanent design installed since 2006 on the North Shore by Natural Resources Canada has helped us better understand "live" changes undergone by the advance growth of the boreal forest in Quebec in a context of a SBW outbreak. Moreover, with the salvage operations that happened and those planned in this sector, it was possible to compare the defoliation sustained by advance regeneration in harvested stands to that of control stands which have also been attacked by the SBW.
The specific objectives were, first, to assess the effect of regenerating stem’s height and species on their defoliation rate. Second, to assess the effect of stand composition on defoliation experienced by advance regeneration. Third, to compare the impact of salvage logging on the defoliation sustained by advance regeneration in mixed stands of balsam fir and black, by comparing harvested stands to unharvested stands with similar composition.
Our results showed that larger and wider individuals experienced more defoliation, probably because there is a higher chance of a larvae falling onto a bigger individual than on a smaller one. Balsam fir regenerating stems were usually more defoliated than black spruce stems. As for stand composition, advance regeneration in fir dominated stands was usually more defoliated than advance regeneration in stands dominated by the black spruce. We thought that salvage logging could potentially reduce defoliation sustained by the regenerating stems, since by removing mature trees of an attacked stand you remove the female SBW favorite egg-laying sites, thus reducing the larvae abundance in those stands. However, although we saw such a trend in our results, this was not significant in our models.