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 losses3–5. 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. Previous modelling results suggest that increasing fire frequency in boreal regions controls forest composition, increases greenhouse-gas emissions, and serves as a main determinant of boreal carbon (C) balance3–5. However, the net effect of burning on boreal C stocks is determined by both fire frequency and severity, and the consequences of climate-mediated changes in the fire regime for rates of biomass consumption are uncertain6. Although advances in remote sensing have provided more accurate information on annual area burned across the boreal biome7, the vulnerability of boreal biomass to severe burning remains difficult to quantify. A large portion of the boreal C pool is stored in moss, litter and peat layers that are partially or entirely consumed during fires. Combustion of this ground-layer biomass was estimated to represent more than 85% of the total fuels consumed during Canadian forest fires8. In addition to affecting C emissions, the severity of ground-layer biomass burning controls several ecosystem processes, including regulation of soil climate and respiration9, maintenance of permafrost10 and forest succession11. Previous studies investigating the factors that regulate groundlayer burning have concluded that climate change is not likely to cause more severe burning in boreal forests12. However, studies so far have not adequately considered the vulnerability of large C pools in permafrost and peatland regions to burning. The