Fuel availability not fire weather controls boreal wildfire severity and carbon emissions

Abstract

Carbon (C) emissions from wildfires are a key terrestrial-atmosphere interaction that influences global atmospheric composition and climate. Positive feedbacks between climate warming and boreal wildfires are predicted based on top-down controls of fire weather and climate, but C emissions from boreal fires may also depend on bottom-up controls of fuel availability related to edaphic controls and overstory tree composition. Here we synthesized data from 417 field sites spanning six ecoregions in the northwestern North American boreal forest and assessed the network of interactions among potential bottom-up and top-down drivers of C emissions. Our results indicate that C emissions are more strongly driven by fuel availability than fire weather, highlighting the importance of fine-scale drainage conditions, overstory tree species composition, and fuel accumulation rates for predicting total C emissions. By implication, climate change-induced modification of fuels needs to be considered for accurately predicting future C emissions from boreal wildfires. Main TextClimate warming and drying in parts of the boreal forest have led to heightened wildfire activity 1,2 , with large increases in the annual area burned over recent decades 3,4 (Figure 1).Climate influences the amount and type of fuel available to burn over long timescales. At shorter timescales, weather patterns dictate the flammability of fuels and weather parameters are expressed as percentiles relative to longer-term climate patterns. Consequently, carbon (C) emissions from boreal wildfires have been considered to be dominated by top-down controls of fire-conducive weather [5][6][7] . The Canadian Forest Fire Weather Index (FWI) System 8 is broadly used to predict fire activity and C emissions throughout the boreal forest and even globally [9][10][11]

Improving prediction and assessment of global fires using multilayer neural networks

“…While other studies predict fuel and climate to be the drivers in Equatorial Asia 33 , we found that precipitation alone explained the variability in fires in this region. In Boreal areas, we find an important effect of fuel load, which is not predicted by previous models, but consistent with observations 59 . In Southeast Asia, Abatzoglou et al 33 find aridity alone as a driver, whereas we find climate, fuel, as well as population density to be important drivers.…”

Peat depth as a control onSphagnummoisture stress during seasonal drought

“…Alternately, the loss of accumulated peat due to wildfire may be affected by interacting processes related to peat depth, peat properties, and WT draw down. Organic soil depth has been linked to burn severity (e.g., depth of burn or carbon loss) for shallow organic soils (Shetler et al, 2008; Walker et al, 2020). Moreover, higher average depth of burn has been observed at peatland margins (Hokanson et al, 2016), where small, perched peatlands tend to have deeper WT at their edges (Hokanson et al, 2020) and the WT is more likely to drop below the peat profile.…”

Boreal Forest Wildfire and Climate Linked Drivers of Carbon and Nitrogen Loss

“…Particularly, the strongest driver of per area emissions of C in boreal wildfires appears to be total fuel (i.e. potentially combustible organic material) which is strongly controlled by long term forest moisture (Walker et al, 2018(Walker et al, , 2020. However, in order for this fuel to be available to ignite and propagate fire it must be sufficiently dried and spatially arranged to be susceptible to high heat and oxygen exposure during active fire.…”

“…This field sampling has been regionally limited and biased towards a few high intensity burn complexes in North America which may in turn bias global emission estimates (van Leeuwen et al, 2014). For example, the Eurasian boreal region is dominated by relatively fire resistant vegetation that promotes lower intensity fire (Rogers et al, 2015;de Groot et al, 2013a) and C loss (0.88 kgC/m 2 ) (Ivanova et al, 2011) than that in typical (Walker et al, 2020) North American wildfire (3.3 kgC/m 2 ) (Boby et al, 2010). Though Eurasia contains over 70% of the boreal global land area (de Groot et al, 2013a), and about 50% (20 Mha/yr) of its yearly burnt area (Rogers et al, 2015), wildfire emissions from this region are severely under sampled in the field (van Leeuwen et al, 2014).…”

“…Lastly, boreal wildfire research has often focused on individual or small groups of fires located relatively near to each other, with little information about the representativeness of the observations or context of the results within the broader spectrum of fire impacts across the wider region, especially those relating to variation in climate. This knowledge gap has thus far been addressed with conglomerated studies spanning different fire seasons, ecosystem types and research methodologies (Walker et al, 2020;Gaboriau et al, 2020). Therefore, widely replicated, simultaneous and systematic field measurements of fire processes in under sampled regions with particular attention to climate are needed to derive more robust, generalizable conclusions about boreal forest responses to wildfire.…”