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 Spatially explicit reconstructions of fire activity in northwestern boreal Canada are rare, despite their importance for modeling current and future disturbance regimes and forest dynamics. We provide a dendrochronological reconstruction of historical fire activity along Highway 3 in the Northwest Territories (NWT), Canada, within the boreal subarctic zone. We dated 129 fires that occurred between 1202 and 2015 CE, using samples from 479 fire-scarred living and dead jack pine trees (Pinus banksiana Lamb.). Three distinct periods can be distinguished in terms of historical fire cycle (FC) and fire occurrence. Initially (1340?1440 CE), fire activity was low (FC = 572 years; 1 fire/decade), before increasing notably between 1460 and 1840 (FC = 171 years; 4.45 fires/decade), and even more in recent times (1860?2015 CE; FC = 95 years; 7.63 fires/decade). Climate has been an important factor controlling changes in fire frequency and FC in the NWT since the 1300s. The 1440s and 1850s correspond with periods of increased fire activity synchronized with shifts from negative to positive Pacific Decadal Oscillation (PDO) phases. Since the mid-1800s, human activities may have contributed to the increase in fire activity, but climate remained the leading factor. During the 20th century, years with increased area burned corresponded to periods with drier-than-average conditions associated with positive PDO, suggesting fire activity in the study region is still influenced by climate. Spatial teleconnection patterns among PDO, drought, and large fire years (LFYs) in the NWT reveal persistent relationships between ocean-atmosphere circulation patterns and fire activity. PDO dynamics hold strong potential for predicting regional fire hazards across northwestern North America.