Regulation of peatland evaporation following wildfire; the complex control of soil tension under dynamic evaporation demand

The capability of peatland ecosystems to regulate evapotranspiration (ET) following wildfire is a key control on the resilience of their globally important carbon stocks under future climatic conditions. Evaporation dominates post‐fire ET, with canopy and sub‐canopy removal restricting transpiration and increasing evaporation potential. Therefore, in order to project the hydrology and associated stability of peatlands to a diverse range of post‐fire weather conditions and future climates the regulation of evaporation must be accurately parameterised in peatland ecohydrological models. To achieve this, we measure the surface resistance (rₛ) to evaporation over the growing season one year post‐fire within four zones of a boreal peatland that burned to differing depths, relating rₛ to near surface soil tensions. We show that the magnitude and temporal variability in rₛ varies with burn severity. At the peatland scale, rₛ and near‐surface tension correlates non‐linearly. However, at the point scale no relationship was evident between temporal variations in rₛ and near‐surface tension across all burn severities; in part due to the limited fluctuation in near‐surface tensions and the precision of rₛ measurements. Where automated measurements enabled averaging of errors, the relationship between near‐surface tension and rₛ switched between periods of strong and weak correlation within a burned peat hummock. This relationship, when strong, deviated from that obtained under steady state laboratory conditions; increases in rₛ were more sensitive to fluctuations in near‐surface tension under dynamic field conditions. Calculating soil vapour densities directly from near‐surface tensions is shown to require calibration between peat types and provides little if any benefit beyond the derivation of empirical relationships between rₛ and measured soil tension. Thus, we demonstrate important spatiotemporal fluctuations in post‐fire rₛ that will be key to regulating post‐fire peatland hydrology, but highlight the complex challenges in effectively parameterising this important underlying control of near‐surface tensions within hydrological simulations.