Burn severity alters peatland moss water availability: implications for post‐fire recovery

Abstract

Wildfire is the largest disturbance affecting northern peatlands; however, little is known about how burn severity (organic soil depth of burn) alters post-fire hydrological conditions that control the recovery of keystone peatland mosses (i.e. Sphagnum). For this reason, we assessed the impact of burn severity on moss water availability by measuring soil tension (Ψ ) and surface volumetric moisture content (θ) in burned and unburned portions of a peatland complex 2 years after fire. We found that both high and low burn severity decreased post-fire water availability by altering peat hydrophysical properties (moisture retention and water repellency). Locations covered by Sphagnum fuscum prior to fire exhibited a decreasing post-fire water availability with an increasing burn severity. In contrast, the lowest water availability (Ψ > 400 cm, θ < 0·02) was observed in feather mosses that underwent low burn severity (residual branches identifiable). Deep burning (>0·20 m) in peatland margins and burn depths >0·05 m in the middle of the peatland exhibited the highest water availability (Ψ < 60 cm). Locations with low surface θ and high Ψ , notably feather mosses undergoing low burn severity, exhibited minimal moss recolonization. Such areas dominate postfire surface cover (~40%) within late successional (mature) peatlands or peatlands located in dry hydrological settings. We argue that such environments are under-represented in conceptual models of post-fire recovery. A new conceptual model is proposed in which (1) deep burning is counterbalanced by rapid recolonization and (2) pre-fire species interact with burn severity to produce substantial lags in post-fire moss recovery.

Ecosystem scale evapotranspiration and CO2 exchange in burned and unburned peatlands: Implications for the ecohydrological resilience of carbon stocks to wildfire

“…The conditions of burn severity (complete above‐ground mortality) and depth of burn inherent in this work are also likely an important control on short‐term recovery dynamics (Lukenbach, Devito, Kettridge, Petrone, & Waddington, ). Kettridge et al () show the importance of burn severity on post‐fire ET rates at the plot scale, where high burn severity totally burns through a protective capping layer in feather moss environments.…”

Low Evapotranspiration Enhances the Resilience of Peatland Carbon Stocks to Fire

“…The dominance of peatland communities varies widely among peatlands driven by differences in climate, hydrogeology (Devito et al, ), age (Benscoter & Vitt, ), disturbance regime (Turetsky et al, ), and recovery period (Benscoter & Vitt, ; Lukenbach et al, ). Sphagnum ‐dominated systems, where increased evaporation may exceed small reductions in tree transpiration postfire (Figure ) (Thompson et al, ), tend to be wetter and deeper peatlands with larger water and carbon stocks available to endure discrete disturbances.…”

“…Even under high postfire evaporative demand, Sphagnum profiles maintain connectivity with subsurface water stores. In comparison, within the study site, a severe disconnect occurs between the burned feather moss surface and saturated water stores just 0.33 m below (Lukenbach et al, ). This results from (i) the nature of the peat moss structure which, unlike Sphagnum , does not have an effective external wicking system along the moss surfaces (Callaghan et al, ) and (ii) the low moisture content observed at the study site within the near‐surface of the peat (Lukenbach et al, ).…”

“…In comparison, within the study site, a severe disconnect occurs between the burned feather moss surface and saturated water stores just 0.33 m below (Lukenbach et al, ). This results from (i) the nature of the peat moss structure which, unlike Sphagnum , does not have an effective external wicking system along the moss surfaces (Callaghan et al, ) and (ii) the low moisture content observed at the study site within the near‐surface of the peat (Lukenbach et al, ). Lower water contents reduce the unsaturated hydraulic conductivity, which limits the supply of water to the peat surface, leading to further drying of the near surface (McCarter & Price, ; Waddington et al, ).…”

Groundwater Controls on Postfire Permafrost Thaw: Water and Energy Balance Effects

“…Based on our results, we suggest that the degree to which fire effects can be transported laterally via groundwater flow are strongly dependent on postfire hydraulic gradients and soil properties. Given that both the vertical water balance and soil hydraulic properties may be modified by fire (Kettridge et al, , ; Lukenbach et al, ; Semenova et al, ; Sherwood et al, ; Thompson et al, ; Thompson & Waddington, ), this represents a potential postfire feedback which merits further investigation. For example, since deeper organic soils are often less conductive than near‐surface organic soils, burning off the near‐surface soil would lead to a decrease in the average hydraulic conductivity of the organic soil layer (Hinzman et al, ; Neilson et al, ; Quinton et al, ).…”