Canadian Forest Service Publications
Wildfire effects on vadose zone hydrology in forested boreal peatland microforms. 2013. Thompson, D.K.; Waddington, J.M. Journal of Hydrology 486:48-56.
Issued by: Northern Forestry Centre
Catalog ID: 35507
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Peatland vulnerability to wildfire disturbance has been shown to vary as a function of hummock and hollow microforms and vadose zone hydrology, with low-lying hollow microforms most susceptible to deep combustion of peat. To better understand how this microform induced pattern of burning alters vadose water storage, pore-water pressure, and water table relationships, we examined a paired burned and unburned peatland in the boreal plain region of north central Alberta. Water table response to rain events increased significantly after wildfire, resulting in a more variable unsaturated zone thickness that was more responsive to smaller rain events. Water storage losses in the vadose zone occurred primarily at depths greater than 15 cm. Large peat surface water loss occurred in hummock microforms in the early spring due to the presence of unsaturated frozen peat at depth, likely a result of a vapour gradient from the unfrozen peat into the frozen peat underneath. During this period, the loss of water storage in the vadose zone satisfied up to 25% of daily evaporative demand, compared to only 3–5% during ice-free periods. A similar but less severe drying was observed late in summer, with burned hummocks the most vulnerable with high pore-water pressures. The enhanced surface drying observed is a precursor to high pore-water pressure conditions that inhibit Sphagnum regeneration. Our observations point to a paradox where the hummocks, being most resistant to combustion, are themselves most prone to high pore-water pressures following wildfire. The harsher hummock environment may contribute to the observed delay in post-fire Sphagnum regeneration in hummocks compared to hollows.
Plain Language Summary
In this study, we examined two water-related aspects of burned and unburned bog peatlands near Slave Lake, Alberta. The two characteristics were the amount of moisture in the peat just below the surface and an indicator of water stress in mosses called pore-water pressure. These peatlands have raised areas called hummocks, which mostly escape heavy burning during fires. They also have low areas called hollows that are more severely burned during fires. Even though the hummocks do not burn as much as the hollows, we found that the pore-water pressures in hummocks are high enough to keep mosses from regrowing after a fire. The hummocks may be important for preserving carbon stocks in the soil after burning, but they are less important than the hollows in encouraging mosses to regrow after fire. We also discuss how the presence of frozen peat just under the surface of a bog may increase the risk of a fire in springtime.