Water balance of a burned and unburned forested boreal peatland

Abstract

We examined the water balance of a forested ombrotrophic peatland and adjacent burned peatland in the boreal plain of western Canada over a 3-year period. Complete combustion of foliage and fine branches dramatically increased shortwave radiation inputs to the peat surface while halting all tree transpiration at the burned site. End-of-winter snowpack was 7-25% higher at the burned site likely due to decreased ablation from the tree canopy at the unburned site. Shrub regrowth at the burned site was rapid post-fire, and shading by the shrub canopy in the burned site approached that of the unburned site within 3 years after fire. Site-averaged surface resistance to evaporation was not different between sites, though surface resistance in hollows was lower in the burned site. Water loss at both burned and unburned sites is largely driven by surface evaporative losses. Evaporation at the burned site marginally exceeded the sum of pre-fire transpiration and interception at the unburned site, suggesting that evapotranspiration during the growing season was 20-40 mm greater at the burned peatland. Although the net change in water storage during the growing season was largely unchanged by fire, the lack of low-density surface peat in the burned site appears to have decreased specific yield, leading to greater water table decline at the burned site despite similar net change in storage.

Low Evapotranspiration Enhances the Resilience of Peatland Carbon Stocks to Fire

“…Postfire ET can exceed prefire losses in Sphagnum ‐dominated boreal peatlands (Figure ) (Thompson et al, ). Prefire, feather moss peatland ET is similar to Sphagnum ‐dominated ecosystems (Kettridge et al, ).…”

“…Evapotranspiration (ET) from burned and unburned Sphagnum and feather moss‐dominated peatlands and their associated components relative to ET from a Sphagnum ‐dominated peatland. Sphagnum evapotranspiration fluxes were derived from Thompson et al (). Unburned feather moss ET assumed equal to unburned Sphagnum peatland ET (cf.…”

Seasonal variation in albedo and radiation exchange between a burned and unburned forested peatland: implications for peatland evaporation

“…Even in the densest tree patches, canopy transmittance at the unburned site is still much larger than closed‐canopy conifer forests where τ a is typically 0.3 or lower (Bisbee et al ., ; Hardy et al ., ). In contrast to the transient shading regime of the unburned site, the burned peatland appears to have a shortwave‐dominated radiation regime similar to that of only the most open and treeless patches of an unburned peatland, which has been shown in a water balance study at this same peatland to increase surface evaporation rates (Thompson et al ., ). However, burned hollows patches with a lower albedo and higher surface temperature that contribute towards enhanced longwave losses have been shown to evaporate at very low rates (Kettridge et al ., ), owing to an evaporative barrier composed of hydrophobic burned peat (Kettridge et al ., ).…”

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

“…Thus, the impacts of fire on base flow may be stronger in regions of discontinuous permafrost with more dynamic changes in hydrologic connectivity (Connon et al, , ; Walvoord et al, ). Furthermore, at our study site fire was associated with an increase in evapotranspiration and concomitant reduction in groundwater recharge (Rocha & Shaver, ); work elsewhere has documented both increases (Thompson et al, ) and decreases (Liu et al, ) in evapotranspiration following fire in cold regions, indicating that advances to our understanding of the land surface water and energy balance are necessary to improve boundary representation in subsurface models.…”

“…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, ).…”