Wildfire combustion and carbon stocks in the southern Canadian boreal forest: Implications for a warming world

Abstract

Boreal wildfires are increasing in intensity, extent, and frequency, potentially intensifying carbon emissions and transitioning the region from a globally significant carbon sink to a source. The productive southern boreal forests of central Canada already experience relatively high frequencies of fire, and as such may serve as an analog of future carbon dynamics for more northern forests. Fire-carbon dynamics in southern boreal systems are relatively understudied, with limited investigation into the drivers of pre-fire carbon stocks or subsequent combustion. As part of NASA's Arctic-Boreal Vulnerability Experiment, we sampled 79 stands (47 burned, 32 unburned) throughout central Saskatchewan to characterize above-and belowground carbon stocks and combustion rates in relation to historical land use, vegetation characteristics, and geophysical attributes. We found southern boreal forests emitted an average of 3.3 ± 1.1 kg C/m 2 from field sites. The emissions from southern boreal stands varied as a function of stand age, fire weather conditions, ecozone, and soil moisture class. Sites affected by historical timber harvesting had greater combustion rates due to faster carbon stock recovery rates than sites recovering from wildfire events, indicating that different boreal forest land use practices can generate divergent carbon legacy effects. We estimate the 2015 fire season in Saskatchewan emitted a total of 36.3 ± 15.0 Tg C, emphasizing the importance of southern boreal fires for regional carbon budgets. Using the southern boreal as an analog, the northern boreal may undergo fundamental shifts in forest structure and carbon dynamics, becoming dominated by stands <70 years old that hold 2-7 kg C/m 2 less than current mature northern boreal stands. Our latitudinal approach reinforces previous studies showing that northern boreal stands are at a high risk of holding less carbon under changing disturbance conditions.

North American Boreal Forests Are a Large Carbon Source Due to Wildfires From 1986 to 2016

“…A study in the southern Canadian fire in 2015 comparing modeled and measured combustion emission also suggested that the model estimation is lower than the measurement by 0.8 kg C m -2 . This difference exists partly because the regional average carbon stock per unit area is lower than that of the field sites 52 .…”

Using a natural experiment to foresee the fate of boreal carbon stores

“…Dieleman et al. (2020) estimated that the 2015 fire season in Saskatchewan emitted 36.3 ± 15.0 Tg C, which the authors suggest is around three‐quarters of the mean annual wildfire carbon emissions from Canada, and around half of the carbon emissions from Alaska and Northwest territories during similar megafire years. Such large emissions unambiguously demonstrate the importance of southern boreal fires to continental and global carbon budgets.…”

“…Dieleman et al. (2020) found that unburnt forests had carbon stocks of around 8 kg C/m 2 , of which half was in the belowground pool. However, they revealed considerable variation amongst ecoregions, forest type, topo‐edaphic conditions, logging and fire disturbance history.…”

“…By framing questions, refining hypotheses and generating empirical data, natural experiments, such as the impressive natural experiment studied by Dieleman et al. (2020), are a critical step in a large program foreseeing the fate of global carbon stores.…”

“…Natural experiments, such as that undertaken by Dieleman et al. (2020), are constrained by numerous assumptions and contain statistical uncertainties blunting their capacity to accurately disclose the trajectory of complex ecological systems such as boreal carbon dynamics. Nonetheless, natural experiments are a critical element in Earth System science because they are important for framing questions, refining hypotheses and generating empirical data that can inform and ground in reality other approaches, such as mechanistic biogeochemical models, essential in predicting the fate of global carbon stores like those in the boreal forest.…”