Charcoal reflectance suggests heating duration and fuel moisture affected burn severity in four Alaskan tundra wildfires

Wildfires are anticipated to increase in frequency and extent in the Arctic tundra. In the unprecedented 2010 fire season, 37 tundra fires burned 435 km2 of the Noatak National Preserve, Alaska. We sampled sixteen soil monoliths from four of these burned areas, which based on microsite burn severity assessments ranged from scorched to moderate- high. Surface charcoals were later studied using reflectance microscopy, as charcoal reflectance may semiquantitatively indicate the duration of heating experienced by a given fuel. Here, the combination of high fuel moisture contents and rapid consumption of fine tussock fuels likely resulted in short fire residence times across the four burned areas, giving an overall low median charcoal reflectance for the entire assemblage (0.82%Romedian). The low charcoal reflectances of the ground fuels provide further evidence for limited heat transference to the organic soil (bryophytes, 0.57 ± 0.17%Romedian; duff and litter, 0.83 ± 0.33%Romedian). The range of observed microsite burn severities is therefore likely attributable to localised variations in above- and ground fuel moisture contents resulting in heterogeneously burned fuels. Consequently, charcoal reflectance is able to provide additional information about current fire behaviour that may improve our understanding of tussock-shrub tundra fires in the future.