Resistance of the boreal forest to high burn rates

Abstract

Boreal ecosystems and their large carbon stocks are strongly shaped by extensive wildfires. Coupling climate projections with records of area burned during the last 3 decades across the North American boreal zone suggests that area burned will increase by 30-500% by the end of the 21st century, with a cascading effect on ecosystem dynamics and on the boreal carbon balance. Fire size and the frequency of large-fire years are both expected to increase. However, how fire size and time since previous fire will influence future burn rates is poorly understood, mostly because of incomplete records of past fire overlaps. Here, we reconstruct the length of overlapping fires along a 190-km-long transect during the last 200 y in one of the most fire-prone boreal regions of North America to document how fire size and time since previous fire will influence future fire recurrence. We provide direct field evidence that extreme burn rates can be sustained by a few occasional droughts triggering immense fires. However, we also show that the most fire-prone areas of the North American boreal forest are resistant to high burn rates because of overabundant young forest stands, thereby creating a fuel-mediated negative feedback on fire activity. These findings will help refine projections of fire effect on boreal ecosystems and their large carbon stocks.climate change | fire-free intervals | fuel feedback | probability of burning | tree ring dating

Accounting for spatial autocorrelation improves the estimation of climate, physical environment and vegetation’s effects on boreal forest’s burn rates

“…As this probability was lower for LDs dominated by open forests than for those dominated by the denser spruce-moss forests, this suggests that fires need a continuous forest cover for spreading (Murray et al 1998;Senici et al 2015). This also confirms previous findings suggesting that boreal forests present a resistance to high BRs, as when stands are open, fires cannot spread because of the lack of fuel, thus inducing a negative feedback between forest cover continuity and fire spread (Héon et al 2014). Wetlands have an important water retention potential, and often reduce or stop fire spread (Senici et al 2015;Erni et al 2016).…”

“…As a result, open forests were suggested to lead to the highest probabilities of belonging to any non-null BR class, which is a misinterpretation of the current vegetation being a cause instead of a consequence of the BRs. This also contradicted the results obtained with potential vegetation which suggested that potential open forests could limit BRs because of their lack of fuel (Héon et al 2014). Similarly, fir-dominated and spruce-moss forests are combined into a single coniferous-moss forest type in the current vegetation classification, a consequence of the impossibility of distinguishing spruce and fir from photointerpretation.…”

The pyrogeography of eastern boreal Canada from 1901 to 2012 simulated with the LPJ-LMfire model

“…First, annual burn rates for 1980-2012 were compiled from the Natural Resources Canada fire database (M. A. Parisien, personal communication, 2016) using Canada's national fire polygons with the hexagonal cells approach from Héon et al (2014), but extended to our study area. We used 365 hexagonal cells to cover our study area and to compute the 1980-2012 simulated mean annual burn rates with 95 % CI for each hexagonal cell.…”

“…Here, historical annual proportions of burned areas were obtained for 26 locations (Fig. S3) using post-fire stand initiation reconstructions based upon field and archival data that were digitized and included in GIS databases (Girardin et al, 2013b;Héon et al, 2014;Portier et al, 2016). Using a 100 km radius around each location centroid, we calculated the simulated mean annual burn rates between 1911 and 2012, together with the 95 % CI.…”

Differences in Human versus Lightning Fires between Urban and Rural Areas of the Boreal Forest in Interior Alaska

“…Models predict continued increases in area burned and carbon emissions in boreal North America in the future [11][12][13][14] in response to a strengthening of arctic amplification of global climate change [15]. However, there are indications of an ecosystem shift from highly flammable mature spruce to less flammable early successional deciduous vegetation [16][17][18][19].…”