Increasing wildfires threaten historic carbon sink of boreal forest soils

Abstract

Boreal forest fires emit large amounts of carbon into the atmosphere primarily through the combustion of soil organic matter1,2,3. During each fire, a portion of this soil beneath the burned layer can escape combustion, leading to a net accumulation of carbon in forests over multiple fire events4. Climate warming and drying has led to more severe and frequent forest fires5,6,7, which threaten to shift the carbon balance of the boreal ecosystem from net accumulation to net loss1, resulting in a positive climate feedback8. This feedback will occur if organic-soil carbon that escaped burning in previous fires, termed ‘legacy carbon’, combusts. Here we use soil radiocarbon dating to quantitatively assess legacy carbon loss in the 2014 wildfires in the Northwest Territories of Canada2. We found no evidence for the combustion of legacy carbon in forests that were older than the historic fire-return interval of northwestern boreal forests9. In forests that were in dry landscapes and less than 60 years old at the time of the fire, legacy carbon that had escaped burning in the previous fire cycle was combusted. We estimate that 0.34 million hectares of young forests (<60 years) that burned in the 2014 fires could have experienced legacy carbon combustion. This implies a shift to a domain of carbon cycling in which these forests become a net source—instead of a sink—of carbon to the atmosphere over consecutive fires. As boreal wildfires continue to increase in size, frequency and intensity7, the area of young forests that experience legacy carbon combustion will probably increase and have a key role in shifting the boreal carbon balance.

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Fig. 1: Hypothesized patterns of legacy C presence and combustion as a proportion of total organic-soil depth across the soil moisture gradient of a boreal forest.
Fig. 2: Radiocarbon concentration Δ14C in the atmosphere over time, Δ14C values of soil-depth increments and atmospheric concentration of 14C during the year in which the stand established.
Fig. 3: Predicted presence and combustion of legacy carbon.

Data availability

The raw data are included in Fig. 2 and are available from the Oak Ridge National Laboratory Distributed Active Archive Center (ORNL DAAC)48. Data for emissions are archived at ORNL DAAC41,48.

Code availability

The R code for emissions is archived with the original publication2. The R code used for all analyses in this study is available from the corresponding author upon request.

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Acknowledgements

This project was supported by funding awarded to M.C.M. from NSF DEB RAPID grant number 1542150, and from the NASA Arctic Boreal and Vulnerability Experiment (ABoVE) Legacy Carbon grant NNX15AT71A; by NSERC Discovery Grants to J.F.J. and M.R.T.; by Government of the Northwest Territories Cumulative Impacts Monitoring Program Funding project number 170 to J.L.B.; by an NSERC-PDF to N.J.D.; and by Polar Knowledge Canada’s Northern Science Training Program funding awarded to Canadian field assistants. Logistical and financial support was provided through the Government of the Northwest Territories–Wilfrid Laurier University Partnership Agreement.

Author information

M.C.M. conceived the study with help from S.G., J.F.J., E.A.G.S. and X.J.W. M.C.M., X.J.W., J.F.J., M.R.T., J.L.B., S.G.C. and N.J.D. designed the field sampling. X.J.W. and N.J.D. collected the field data. X.J.W. and C.E. completed the laboratory work with technical support from M.C.M. and E.A.G.S. X.J.W. analysed the data with help from M.C.M. and E.A.G.S. B.M.R. and S.P. provided the data and wrote the Methods section ‘Estimation of young burned area’. X.J.W. wrote the manuscript and all co-authors edited the manuscript.

Correspondence to Xanthe J. Walker.

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The authors declare no competing interests.

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Peer review information Nature thanks Mary Edwards and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Extended data figures and tables

Extended Data Fig. 1 Area burned in the 2014 wildfires in the Northwest Territories of Canada that was considered as young burned (younger than 60 years at the time of fire).

a, b, Percentage (a) and cumulative percentage (b) of area burned in the 2014 wildfires, expressed in years since the fire and year of burn. In a, the dashed line represents the best fit between the observed values in the fire databases and the solid line represents the linear extrapolation to 1954. Grey shading indicates the standard error of the observed values and red shading indicates the predicted standard error (y = 0.01430x + 0.01097; R2 = 0.09; P < 0.05).

Extended Data Table 1 Firth’s bias-reduced logistic regression results
Extended Data Table 2 t-test results
Extended Data Table 3 SOL depth and carbon in pre-fire and burned pools
Extended Data Table 4 Comparison between Δ14C values of two soil profiles in five sites
Extended Data Table 5 Results of Firth’s bias-reduced, mixed-effect model and simple logistic regression

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