Fire disturbances are increasing under global climate change and ecological transformations of forests are occurring. Specifically, shifts from productive closed-canopy feather moss forests to low-productivity open-canopy lichen (Cladonia spp.) woodlands have been observed in boreal forests of eastern Canada. It has been hypothesized that high severity of fires would be the cause of this change, but this is difficult to validate a posteriori on mature forest stands. Because charcoal properties are affected by fire severity, we have put forward the hypothesis that the amount and physicochemical properties of charcoal (C, N, H, O, ash, surface area) would be different and indicative of a greater fire severity for open-canopy forests compared to closed canopy ones. Our hypothesis was partly validated in that the amount of charcoal found on the ground of closed-canopy forests was greater than that of open-canopy forests. However, the physicochemical properties were not different, albeit a greater variability of charcoal properties for open canopy stands. These results do not allow us to fully validate or reject our hypothesis on the role of fire severity in the shift between open and closed canopy stands. However, they suggest that the variability in fire conditions as well as the amounts of charcoal produced are different between the two ecosystem types. Furthermore, considering the role that biochar may play in improving soil conditions and promoting vegetation restoration, our results suggest that charcoal may play a role in maintaining these two stable alternative ecosystem states.

ABSTRACT Following deglaciation, lowland sites in eastern Canada that were covered by proglacial Lake Ojibway recorded direct afforestation with boreal mixedwood taxa, without an initial tundra phase, contrary to sites that were never covered by this lake. Because former islands of proglacial Lake Ojibway were colonized by vegetation before lake drainage around 8200?cal a bp, we hypothesized that these paleo-islands went through a non-arboreal phase which eventually transitioned to boreal mixedwoods, hence providing a source of propagules for rapid and later colonization of the lowlands. We carried out a multi-proxy analysis combining pollen, macro-remains, sedimentary ancient DNA and charcoal to document vegetation composition and dynamics on two paleo-islands. Both study sites recorded progressive vegetation establishment starting with a tundra-like phase with only herbs and shrubs, followed by open forests dominated by either larch or pine, culminating with the establishment of the boreal mixedwoods about 300 years before the final drainage of proglacial Lake Ojibway. Fire regimes seem to have partially driven vegetation dynamics and diversity on the paleo-islands. Paleo-islands were sources of propagules, which helps explain how the former Lake Ojibway lowlands were directly colonized by boreal mixedwoods, without an initial tundra phase.