Recognition of the prominence of boreal old-growth stands, here defined as stands driven by gap dynamics, represented a major paradigm shift in boreal forest ecology (Kneeshaw and Gauthier 2003; Kuuluvainen 2009). Prior to this shift, the assumptions held that old-growth forests were almost absent in boreal landscapes because of relatively frequent forest fires and that some boreal forest stands did not remain stable in an old-growth phase, but they rather became open peatlands in the absence of fire (Bergeron and Harper 2009; Wirth and Lichstein 2009). Nonetheless, old-growth forests are abundant in the boreal landscape, even in territories where the fire cycle is relatively short; this pattern reflects the random distribution of fires across all age classes (Bergeron et al. 2001). In addition, the distinctive structural attributes and ecological continuity of old-growth stands imply the presence of specific habitats that are absent from younger stands (Fenton and Bergeron 2008; Boudreault et al. 2018). For these reasons, boreal old-growth forests are now recognized as key components of boreal landscapes.
Since the mid-twentieth century, intensive industrial forest harvesting has expanded markedly across the boreal landscape. In territories that are characterized by severe crown fires, such as the boreal forests in eastern Canada, the harvested surface area can approximate 75% of the burned surface area during the same period (Bouchard and Pothier 2011; Boucher et al. 2017). Furthermore, in landscapes driven by low-severity surface fires and where fire mitigation is very efficient, logging has almost become the only type of severe-intensity disturbance (Östlund et al. 1997). Clearcutting systems are by far the most common harvesting method in the boreal forest biome. This approach is most profitable for forest companies, and the effects of clearcutting are assumed to be equivalent to those from fire (Bergeron et al. 2001). However, fire burns trees of all age classes, while logging primarily affects mature and old forests, thereby rejuvenating the landscape (Östlund et al. 1997; Fall et al. 2004; Bergeron et al. 2006). In addition, the short-term rotation of clearcutting systems accelerates this rejuvenation by inhibiting the establishment of new mature or old stands (Bergeron et al. 2002; Kuuluvainen 2009). The result is a markedly diminished abundance of boreal old-growth forests in managed territories over the last century. The consequential environmental effects include a decline of floral, faunal, and fungal forest species associated with abundant dead wood of diverse decay stages (Siitonen 2001).
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Alternative management models have emerged over the two last decades to counter the loss of old-growth stands in managed landscapes. These new approaches copy the natural disturbance regimes of the different managed regions (Kuuluvainen 2002; De Grandpré et al. 2009) or imitate stand-scale natural processes (Vanha-Majamaa et al. 2007; Kuuluvainen 2009). The efficacy of these models, however, depends of an accurate understanding of disturbance regimes in a given area; however, each boreal region differs in terms of climatic factors, disturbance dynamics, and species’ traits (Kneeshaw et al. 2011; Shorohova et al. 2011). As such, local-scale studies are necessary to assess whether a particular management strategy is or can be adapted to the local characteristics of a landscape.
The main drivers of secondary disturbance in eastern Canadian boreal forests are spruce budworm (SBW—Choristoneura fumiferana (Clem.)) outbreaks and windthrow events (Bouchard et al. 2006; Kerharo 2013; De Grandpré et al. 2018). Other disturbances that drive mortality in these forests include jack pine budworm (Choristoneura pinus pinus) and forest tent caterpillar (Malacosoma disstria) (Jan and Volney 1988; Bergeron and Charron 1994). However, as the main boreal hosts of these two defoliating insects are pioneer species (Nealis and Lomic 1994; Cooke and Lorenzetti 2006), their influence on old-growth dynamics is negligible.
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The disturbance regimes of Fennoscandian boreal forests are relatively well understood and involve a mix of low-, moderate-, and high-severity disturbances (Kuuluvainen and Aakala 2011). In contrast, the perception of dynamics within eastern Canadian boreal forests remains relatively simplistic, dividing the disturbance regime into low-severity secondary disturbances and high-severity primary disturbances (Bergeron and Harper 2009; Shorohova et al. 2011). Nonetheless, previous studies have highlighted that secondary disturbance dynamics in regional boreal old-growth forests vary in their severity, nature, and spatial distribution (Kneeshaw and Bergeron 1998; Pham et al. 2004; Aakala et al. 2007). It is likely that disturbances of moderate severity play an important role in the overall landscape disturbance regime, in particular because of the dynamics of the SBW (Kneeshaw et al. 2009; Shorohova et al. 2011).
Similarly, the structural diversity of eastern Canadian boreal forests suggests that these ecosystems are driven by both low- and moderate-severity disturbances (Martin et al. 2018). However, the transition toward an old-growth stage is a progressive process where late-successional species replace progressively the cohort that appeared following the last primary disturbance (Harper et al. 2005; Lecomte et al. 2006; Gauthier et al. 2010). It supposes that disturbances of moderate severity gain progressively in importance at the later stages of the old-growth succession process. Furthermore, differences in the severity of secondary disturbances also imply differences in the post-disturbance regeneration dynamics (Kneeshaw and Bergeron 1998; Montoro Girona et al. 2018). It is therefore likely that the secondary disturbance regime of eastern Canadian boreal forests is more complex than is currently portrayed.
Our study aims to determine if the secondary disturbance regime of eastern Canadian boreal stands is driven by various types of secondary disturbance. We hypothesized that (1) low- and moderate-severity disturbances present different temporal patterns and that (2) moderate-severity disturbances are more abundant in true old-growth stands and favor different components of the regeneration layer. The innovative nature of our research lies in using dendrochronological data for analyzing disturbances of low and moderate severity over the two last centuries in forests at different stages of the old-growth succession. Thus, our study is not restricted to the effects of the most recent secondary disturbances, but we will observe their long-term patterns of secondary disturbance and their influence on old-growth stand dynamics. Consequently, our research contributes to the increasing body of literature discussing the complexity of secondary disturbance regimes in boreal landscapes by reconstructing the disturbance history of eastern Canadian boreal old-growth forests.
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