A comparison of charcoal reflectance between crown and surface fire contexts in dry south-west USA forests

The historical and modern importance of crown fires in ponderosa pine and dry mixed-conifer forests of the south-west USA has been much debated. The microscopic reflectance of charcoal in polished blocks under oil shows promise as a semiquantitative proxy for fire severity using charcoal from post-fire landscapes. We measured the reflectance of 33 modern charcoal samples to evaluate (1) whether charcoal reflectance can distinguish between crown fires and surface fires in these forests; and (2) whether surface fires with masticated fuels burn with severities similar to surface fires in grass, litter and duff fuels. The charcoal analysed was primarily collected after wildland fires under two different conditions: (l) wildfires with moderate to high severity and crown fire behaviour (n = 17), and (2) prescribed fires with low to moderate severity but no crown fire behaviour (n = 16). Statistical analysis indicates that charcoal reflectance produced in crown fires significantly differs from surface fire charcoal, particularly surface fire charcoal formed in grass, duff and litter fuels. However, charcoal produced from surface fires in masticated fuels is indistinguishable from crown fire charcoal, suggesting that fires in areas that have experienced in situ mastication may have soil impacts similar to crown fires.