Ecohydrological Controls on Post‐Fire Sphagnum Moss Recovery in Boreal Shield Peatlands

ABSTRACT Following wildfire, the return of boreal peatlands to a net carbon sink depends greatly on the ecohydrological recovery of Sphagnum mosses. We examined post‐fire Sphagnum moss accumulation and moss moisture stress (soil water tension, soil moisture) in triplicate burned and unburned peatlands (shallow peatlands, middle of deep peatlands and margins of deep peatlands). Shallow (< 70 cm maximum peat depth) peatlands had significantly less post‐fire moss growth than deeper peatlands, and near‐surface soil tension exceeded 100 hPa (an established ecohydrological threshold for Sphagnum moss) when the entire peat profile became desaturated, which only occurred in the shallowest peatlands. We found no significant difference in moss moisture stress between the burned and unburned landscapes 5 years following wildfire. Rather, current peat depth best explains moss moisture stress in burned and unburned landscapes, suggesting a peat depth threshold, above which Sphagnum drought resilience increases. To further examine this threshold and the ecohydrological controls on near‐surface soil tension in unburned, burned, and post‐fire moss‐recovered peatlands, we modelled the length of time until near‐surface soil tension exceeded 100 hPa under a drying scenario using HYDRUS‐1D. Burned profiles reached a near‐surface soil tension of 100 hPa faster than unburned or recovered profiles, and the greatest control on time to threshold soil tension was wildfire burn severity (depth of burn). In moss‐recovered peatlands, when the depth of recovered Sphagnum moss was greater than 4 cm, near‐surface soil tension took significantly longer to reach the ecohydrological threshold. We suggest this moss recovery depth represents an important post‐fire metric and can be used to target peatlands requiring ecohydrological adaptation strategies to enhance Sphagnum moss recovery.