Ecohydrological Thresholds to High Peat Burn Severity: Implications for Peatland Wildfire Management

Abstract

Northern peatlands represent a globally significant carbon stock, equating to almost one-third of the world’s organic soil carbon. The largest areal disturbance to northern peatlands is wildfire where carbon loss, through peat smouldering combustion, is highly variable. The tightly-coupled ecohydrological nature of peatlands results in autogenic feedbacks and the occurrence of threshold behaviour. High depth of burn has been evidenced in black spruce dominated peatlands in the sub-humid Boreal Plains ecozone of Alberta, Canada so this was chosen as the area of study. A landscape-scale assessment of peat hydrophysical properties found that peat smouldering combustion vulnerability was greatest at stand-age > 80 years, in coarse/heterogeneous hydrogeological settings, and in peatland margins compared to peatland middles. In combination, and when exposed to a climatic water deficit, we found that these drivers of cross-scale variability could lead to high peat burn severity. Assessment of a partially-drained and burned peatland enabled the identification of a black spruce basal diameter threshold that corresponded to the occurrence of high peat burn severity. We suggest that the above-ground fuel load threshold could occur due to the initiation of a self-reinforcing feedback by anthropogenic disturbance or climate change. Moreover, surpassing a peat burn severity threshold can cause the breakdown of an important feedback that limits evaporation losses post-fire, likely leading to further carbon losses through increased decomposition rates and/or ecosystem regime shift. We found that although peat moisture content was increased by fuel modification treatment, combustion carbon losses were greater in fuel-treated areas compared to the control because of the addition of mulch (wood) to the surface. Hence, peatland wildfire management that integrates the modification of above- and below-ground fuels, considers ecohydrological thresholds, and drivers of cross scale variability, is required to effectively reduce the risk of high peat burn severity in black spruce dominated peatlands.