Abstract
Climate warming is increasing the prevalence of overwintering ‘zombie’ fires, which are expected to occur primarily in peatlands, undermining carbon storage through deep burning of organic soils. We visited overwintering fires in Northwest Territories, Canada, and Interior Alaska, United States, and present field measurements of where overwintering fires are burning in the landscape and their impact on combustion severity and forest regeneration. Combustion severity hotspots did not generate overwintering, but peat and woody biomass smouldering both supported overwintering, leading to wintertime smouldering in both treed peatlands and upland forests. These findings create challenges for fire managers and uncertainty about carbon emissions, but forest regeneration was not compromised.
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Acknowledgements
The NT sampling campaign was supported by the Government of the Northwest Territories (GNWT) Environment and Climate Change funding to J.L.B. and in-kind helicopter support. Other logistical support was provided through the GNWT–Wilfrid Laurier University Partnership Agreement. J.L.B. was supported by the Canada Research Chairs Program and Natural Science and Engineering Research Council Discovery Grant funding (RGPIN-2019-05889). T.D.H. was supported under the umbrella of the Netherlands Earth System Science Centre, which received funding from the European Union Horizon 2020 research and innovation program under Marie Skłodowska-Curie grant agreement no. 847504. S.V. was funded through a Consolidator grant under the European Union Horizon 2020 research and innovation program (European Research Council; grant agreement no. 101000987). X.J.W. and M.C.M. were supported through the Bonanza Creek Long-Term Ecological Research (LTER) which is funded by the National Science Foundation (DEB-2224776 and DEB-1636476) and by the USDA Forest Service, Pacific Northwest Research Station (RJVA-PNW-20-JV-11261932-018). X.J.W. and M.C.M. were also supported by National Aeronautics and Space Administration (NASA), Terrestrial Ecology award no. 80NSSC22K1244 and National Science Foundation, Office of Polar Programs, Arctic Natural Sciences award nos. 2019515 and 2116862. We thank O. Melnik, H. Kleiner, W. Konwent and J. Paul for help in the field and with data processing.
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J.L.B., M.R.T., S.V., R.O., X.J.W. and M.C.M. conceived the study. J.L.B., X.J.W., S.V., M.C.M., T.D.H. and M.R.T. designed the field sampling. J.L.B., X.J.W., S.V., T.D.H., R.A.-S., M.J.v.G., E.L.O. R.C.S. and M.R.T. collected the field data. J.L.B. analysed the data. J.L.B. wrote the manuscript and all co-authors edited the manuscript.
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Extended data
Extended Data Fig. 1 Sampling locations within single season and overwintering fires.
Locations of sampling in the Interior Boreal Alaska ecoregion of Alaska (A) and the Taiga Plains ecoregion of Northwest Territories (B). Fire perimeters for 2009 (AK)28 and 2014 (NT)29 within the sampling region are shown in brown. Delimitations of ecoregions are provided in panel C and follow the US EPA classification (US EPA: Ecoregions of North America [data], https://www.epa. gov/eco-research/ecoregions-north-america (last access: 15 May 2024), 2015). Green shading in A and B represent Landsat-based tree cover product from 2008 for AK and 2013 for NT30.
Extended Data Fig. 2 Ground conditions in overwintering fires.
Indication of low severity burning as evidenced by intact fine fuels (A) and light combustion on the underside of downed trees (D). Evidence of woody biomass smouldering as exemplified by complete combustion of large roots (B), boles (C) and frequent occurrences of complete stem fall, which was not evident in single season pairs (E, F). Photo credits: J. Baltzer (A,B, D-F), M. Turetsky (C).
Extended Data Fig. 3 Comparison of combustion variables between overwintering (O) and single season (S) fires in the Northwest Territories (NT) and Alaska (AK).
Boxplots showing distribution of combustion variables; plots include median, 1st and 3rd data quartiles and outliers. Canopy combustion is an ordinal score where each tree was ranked from 0 to 3; 0 = none, alive and no biomass combusted; 1 = low, only needles/leaves consumed; 2 = moderate, all foliage and majority of fine branches combusted; 3 = high, most of the aboveground canopy including foliage, branches, and bark combusted. Model results are presented in Supplementary Table 2. Samples sizes for burn depth and proportional combustion were as follows: NT, n = 75 (9 overwintering and 6 single season fire sites with 5 nested plots per site] and AK, n = 66 (7 overwintering and 4 single season fires sites with 6 nested plots per site). Canopy combustion was measured at the stand level meaning n = 15 for NT and n = 11 for AK.
Extended Data Fig. 4 Comparison of material legacy variables between overwintering (O) and single season (S) fires in the Northwest Territories (NT) and Alaska (AK).
Boxplots showing distribution of material legacy variables; plots include median, 1st and 3rd data quartiles and outliers. Model results are presented in Supplementary Table 2. Samples sizes for residual soil organic layer thickness (rSOL) were as follows: NT, n = 75 (9 overwintering and 6 single season fire sites with 5 nested plots per site] and AK, n = 66 (7 overwintering and 4 single season fires sites with 6 nested plots per site). Deadstanding and deadfallen basal area (BA) were measured at the stand level meaning n = 15 for NT and n = 11 for AK.
Extended Data Fig. 5 Aerial image of an overwintering fire that ignited in 2023 and continues to smoulder without a flare-up as of July 2024.
This overwintering fire is at the perimeter of SS022 near Fort Smith, NT. Note the trail of downed stems leading to the current smouldering hotspot. Photo credit: Duane Sinclair, Government of the Northwest Territories Environment and Climate Change.
Extended Data Fig. 6 Comparison of regeneration variables between overwintering (O) and single season (S) fires in the Northwest Territories (NT) and Alaska (AK).
Boxplots showing distribution of regeneration variables including seedling density (seedlings m−2), basal diameter (m2 ha−1), and relative abundance of conifers (proportion conifer; unitless). Samples sizes for all of these variables were as follows: NT, n = 75 (9 overwintering and 6 single season fire sites with 5 nested plots per site] and AK, n = 66 (7 overwintering and 4 single season fires sites with 6 nested plots per site). In the fourth panel, Time indicates whether the proportion conifer value is for pre-fire (before) or post-fire (after). In this plot site-level means were used as the pre-fire composition is at the site level meaning n = 15 for NT and n = 11 for AK. Plots include median, 1st and 3rd data quartiles and outliers. Model results are presented in Supplementary Table 2.
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Supplementary Tables 1 and 2.
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Baltzer, J.L., Walker, X.J., Veraverbeke, S. et al. Overwintering fires can occur in both peatlands and upland forests with varying ecological impacts. Nat Ecol Evol 9, 559–564 (2025). https://doi.org/10.1038/s41559-024-02630-2
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DOI: https://doi.org/10.1038/s41559-024-02630-2
