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Recent acceleration of biomass burning and carbon losses in Alaskan forests and peatlands


Climate change has increased the area affected by forest fires each year in boreal North America1,2. Increases in burned area and fire frequency are expected to stimulate boreal carbon losses3,4,5. However, the impact of wildfires on carbon emissions is also affected by the severity of burning. How climate change influences the severity of biomass burning has proved difficult to assess. Here, we examined the depth of ground-layer combustion in 178 sites dominated by black spruce in Alaska, using data collected from 31 fire events between 1983 and 2005. We show that the depth of burning increased as the fire season progressed when the annual area burned was small. However, deep burning occurred throughout the fire season when the annual area burned was large. Depth of burning increased late in the fire season in upland forests, but not in peatland and permafrost sites. Simulations of wildfire-induced carbon losses from Alaskan black spruce stands over the past 60 years suggest that ground-layer combustion has accelerated regional carbon losses over the past decade, owing to increases in burn area and late-season burning. As a result, soils in these black spruce stands have become a net source of carbon to the atmosphere, with carbon emissions far exceeding decadal uptake.

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Figure 1: Fire-regime effects on the severity of biomass burning.
Figure 2: The effect of fire size on biomass combustion.
Figure 3: Increases in both burned area and fire severity accelerate regional C losses.

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We thank L. Ness for remote sensing analyses, A. McAdam for R programming, E. Ellicott, G. Shetler, C. Treat and N. French for laboratory and field assistance and M. Flannigan, B. DeGroot, D. McGuire and S. Liu for helpful comments. This study was supported by NASA (grant NNG04GD25G), the Bonanza Creek Long-Term Ecological Research Program and US Forest Service (grant PNW01-JV11261952-231 and NSF DEB-0080609), the Joint Fire Science Program (project 03-1-3-08) and the USGS Mendenhall Postdoctoral and Earth Surface Dynamics Programs.

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All authors collected data, and commented on the manuscript at all stages. E. S. Kasichke led the geospatial analyses with help from E.H. E. S. Kane led the compilation of soil carbon data with help from J.W.H., K.L.M. and M.R.T. M.R.T. analysed statistical data, led the overall synthesis and wrote the paper.

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Correspondence to Merritt R. Turetsky.

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Turetsky, M., Kane, E., Harden, J. et al. Recent acceleration of biomass burning and carbon losses in Alaskan forests and peatlands. Nature Geosci 4, 27–31 (2011).

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