As boreal and arctic ecosystems respond to warming climate, a key question is: will Alaskan landscapes become wetter or drier after fire? Previous studies have shown that, at least for a few years post-fire, organic soil duff has higher moisture content compared to controls, both in boreal forest (Hinzman et al. 2000, Potter & Hugny, 2020) and tundra (Liljedahl, et al. 2007).  Liljedahl noted that denser deep organic duff layers remaining after a Seward Peninsula fire had a relatively larger surface area per volume and different pore shape, maybe making them better at retaining water than the pre-burn fluffier upper layers. Yet, over the long term, burn scars tend to become better drained and drier. Hinzman et al. (2000) sampled old burns up to 50 years old and noted “severely burned sites have progressively become drier and warmer resulting in major changes to the vegetation type and soil structure.” What are the drivers of these changes?

In a recent study, Thunberg et al. 2021 studied factors that drive wetting (precipitation) vs. those that drive drying (evaporation and plant transpiration). Solar radiation speeds evapotranspiration (ET) and relative humidity (RH) slows it down. Thunberg used flux tower data from stations between Fairbanks and Utqiagvik (Barrow) including burnscars from 2004 and 2007. The towers measure greenhouse gas and water fluxes between the land and air. Her study showed ET increased after fire, which could (independent of other factors) indicate a trend toward surface drying. Increased ET was pronounced at a North Slope tundra fire in early years and remained detectable for about 8 years post-fire. ET at Poker Flats forest burn site also exceeded that of an adjacent unburned site during 5/6 years studied.  Interestingly, Rocha et al. (2011) linked the early increases in ET at the Anaktuvuk River tundra fire to an increase in evaporation from loss of the spongy moss and organic duff, resulting in surface water pooling and increased soil temperatures. Overall, the ET at forest sites seems to be strongly influenced by relative humidity while ET at tundra sites depends more strongly on air temperature.

Conclusion: Evaporation and transpiration are both important influences on soil moisture budget which tend to increase in young burns, but may be moderated by influences of soil structure, microtopography, and subsurface ice-melt.

Citations: Sarah M. Thunberg, JE Walsh, ES Euskirchen, K Redilla, and AV Rocha. 2021. Surface moisture budget of tundra and boreal ecosystems in Alaska: Variations and drivers. Polar Science 29: 100685.

Hinzman, L, K Yoshikawa, W Bolton, K Petrone, and M Fukuda. 2000. Impacts of wildfire on the soil moisture content and thermal regime in the boreal forests of interior Alaska. Proc. Fire Conference 2000: First National Congress on Fire Ecology, Prevention and Management, 27 Nov -Dec 1, San Diego, CA.

Potter, C and C Hugny. 2020. Wildfire effects on permafrost and soil moisture in spruce forests of Interior Alaska. J. For. Res. 31(2):553–563.

Liljedahl A, Hinzman L, Busey R and Yoshikawa K. 2007. Physical short-term changes after a tussock tundra fire, Seward Peninsula, Alaska J. Geophys. Res. Earth Surf. 112 F02S07 

Rocha, A and G Shaver. 2011. Postfire energy exchange in arctic tundra: the importance and climatic implications of burn severity. Global Change Biol. 17, 2831–2841.