Seasonally frozen soil modifies patterns of boreal peatland wildfire vulnerability

Abstract. Peatlands play a vital role in the global carbon cycle, acting as one of the most important global carbon sinks. However, an understanding of their environmental processes, particularly in relation to a changing climate, remains inchoate. In particular, the role seasonal ice or frost layers play in altering spring water balance, and thus vulnerability to deep smoldering combustion during wildfire is not fully understood. Continental boreal peatlands are characterized by periodic wildfire disturbance, which releases carbon, but can also inhibit short-term peat productivity and carbon sequestration as the peatland recovers, with recovery timescales linked to the severity or depth of burning. The presence of seasonal frost layers coincides with drier spring conditions and an enhanced risk of wildfire. Two-dimensional numerical modelling was conducted using HYDRUS-2D, a variably saturated flow model, to simulate water balance in the vadose zone and assess vulnerability to fire during prolonged rain free periods in the presence of continuous and discontinuous frost. Our results show there is a lack of horizontal water transfer which increases spatial variability in water balance and leads to pronounced heterogeneity in the risk of smoldering combustion and the potential for deep combustion at hummock-hollow interfaces. Peatlands are broadly divided into areas which are characterized by a dry near-surface and high water contents at depth (water conserving), and those with a wetter near-surface, but comparatively lower water contents at depth (productive). Those areas with dry near-surfaces will be more vulnerable to wildfire and characterize around 50 % of hummocks and 25 % of hollows. In the presence of a seasonal frost layer productive peat layers in hollows will show substantial drying out due to the frost layer disconnecting the surface from the water table; this approximately doubles the proportion of hollows vulnerable to wildfire. Breaks in the frost layer allows areas to maintain hydrological connectivity to a falling water table, but this connectivity is limited in lateral extent and can drive further spatial heterogeneity in vulnerability to wildfire ignition in the weeks when the frost layer begins to thaw.