Wildfire, permafrost, and vegetation interactions in a discontinuous permafrost region revealed by dual-frequency airborne radar observations

Abstract:

Tundra and boreal environments underlain by permafrost constitute the largest below-ground reservoirs of carbon globally, with roughly twice the current mass of atmospheric carbon, (Hugelius et al. 2014). Wildfires, a ubiquitous phenomenon across boreal forests and the arctic, can release teragrams of carbon into the atmosphere through i) the rapid burning of vegetation and ii) increased microbial decomposition of thawed permafrost, which persists for years after a burn event (Grosse et al. 2011, Natali et al. 2014). Here we investigate permafrost changes in the Yukon-Kuskokwim (YK) river delta in Southwest Alaska, a tundra environment of discontinuous permafrost that experienced extensive wildfires in the 1970s, 1980s, 2005-2007, and 2015. Interferometric synthetic aperture radar (InSAR) has been previously used to estimate the long-term recovery of wildfire-affected permafrost here (Michaelides et al., 2019), and Landsat imagery in conjunction with field measurements and plant functional type maps has been used to infer long-term vegetation succession in wildfire-affected tundra (Frost et al., 2019). We combine dual-frequency synthetic aperture radar (SAR) observations acquired during the 2017 campaign of NASA’s Arctic Boreal Vulnerability Experiment (ABoVE) with plant functional type maps to investigate the short-term impacts of wildfire on permafrost stability, seasonal thaw depth, vegetation cover and type, and soil moisture content. We use both L-band InSAR data and P-band polarimetric SAR data from the UAVSAR instrument to estimate seasonal thaw depth, active layer thickness, and vertical soil moisture profiles of the active layer at 30 m spatial resolution across a complex of wildfires that burned in 2015. Combined with plant functional type mapping, we compare descriptors of recently burned tundra with those of older burns and unburned tundra. We find that wildfire can double the amount of seasonal thaw, corresponding to up to a doubling of the active layer thickness. In turn this process can result in a corresponding increase in soil microbial decomposition, and changes in vegetation cover and soil moisture content. By better quantifying the short-term post-fire changes to the active layer, we can constrain the long-term behavior of wildfire-affected permafrost.