Coupled hydrological and geochemical impacts of wildfire in peatland-dominated regions of discontinuous permafrost

Caren Ackley; Suzanne E Tank; Kristine M Haynes; Fereidoun Rezanezhad; Colin McCarter; William L Quinton

doi:10.1016/j.scitotenv.2021.146841

Coupled hydrological and geochemical impacts of wildfire in peatland-dominated regions of discontinuous permafrost

Sci Total Environ. 2021 Aug 15:782:146841. doi: 10.1016/j.scitotenv.2021.146841. Epub 2021 Mar 30.

Authors

Caren Ackley¹, Suzanne E Tank², Kristine M Haynes³, Fereidoun Rezanezhad⁴, Colin McCarter⁵, William L Quinton¹

Affiliations

¹ Cold Regions Research Centre, Wilfrid Laurier University, Waterloo, ON, Canada.
² Department of Biological Sciences, University of Alberta, Edmonton, AB, Canada.
³ Cold Regions Research Centre, Wilfrid Laurier University, Waterloo, ON, Canada. Electronic address: khaynes@wlu.ca.
⁴ Ecohydrology Research Group, Department of Earth and Environmental Sciences and Water Institute, University of Waterloo, Waterloo, ON, Canada.
⁵ Department of Physical and Environmental Sciences, University of Toronto Scarborough, Toronto, ON, Canada.

PMID: 33848861
DOI: 10.1016/j.scitotenv.2021.146841

Abstract

Naturally-ignited wildfires are increasing in frequency and severity in northern regions, contributing to rapid permafrost thaw-induced landscape change driven by climate warming. Low-severity wildfires typically result in minor organic matter loss. The impacts of such fires on the hydrological and geochemical dynamics of peat plateau-wetland complexes have not been examined. In 2014, a low-severity wildfire, with minimal ground surface damage, burned approximately one-half of a 5 ha permafrost plateau in the wetland-dominated landscape of the Scotty Creek watershed, Northwest Territories, Canada, in the discontinuous permafrost zone. In March 2016, hydrometeorological and permafrost conditions on the burned and unaffected plateaus were monitored including snowpack characteristics and surface energy dynamics. Pore water samples were collected from the saturated layer as thaw progressed throughout the growing season on the burned and unaffected plateaus. Repeated probing of the frost table depth was coupled with laboratory analyses of peat physical and hydraulic characteristics performed on peat cores collected from the top 20 cm of the ground surface in the burned and unaffected plots. The higher transmissivity of the burned forest canopy accelerated snowmelt promoting earlier onset of the thawing season and increased the ground heat flux to melt ground ice. Wildfire increased the thickness of the supra-permafrost layer, including the active layer and talik, resulting in a more uniform subsurface with limited depressional storage capacity and reduced preferential runoff flowpaths across the burned plateau. The incorporation of ash and char into the peat matrix reduced pore diameters, promoting greater subsurface soil moisture retention and longer pore water residence times ultimately providing greater opportunity for soil-water interaction and biogeochemical reactions. Consequently, pore water showed elevated dissolved solutes, dissolved organic matter and mercury concentrations in the burned site. Low-severity wildfires have the potential to trigger a series of complex, inter-related hydrological, thermal and biogeochemical processes and feedbacks.

Keywords: Ash translocation; Ground thaw; Peat hydraulics; Residence time; Snowmelt; Subarctic.