Postfire carbon balance in boreal bogs of Alberta, Canada

Boreal peatland ecosystems occupy about 3.5 million km 2 of the earth's land surface and store between 250 and 455 Pg of carbon (C) as peat. While northern hemisphere boreal peatlands have functioned as net sinks for atmospheric C since the most recent deglaciation, natural and anthropogenic disturbances, and most importantly wildfire, may compromise peatland C sinks. To examine the effects of fire on local and regional C sink strength, we focused on a 12 000 km 2 region near Wabasca, AB, Canada, where ombrotrophic Sphagnum ‐dominated bogs cover 2280 km 2 that burn with a fire return interval of 123±26 years. We characterized annual C accumulation along a chronosequence of 10 bog sites, spanning 1–102 years‐since‐fire (in 2002). Immediately after fire, bogs represent a net C source of 8.9±8.4 mol m −2  yr −1 . At about 13 years after fire, bogs switch from net C sources to net C sinks, mainly because of recovery of the moss and shrub layers. Subsequently, black spruce biomass accumulation contributes to the net C sink, with fine root biomass accumulation peaking at 34 years after fire and aboveground biomass and coarse root accumulation peaking at 74 years after fire. The overall C sink strength peaks at 18.4 mol C m −2  yr −1 at 75 years after fire. As the tree biomass accumulation rate declines, the net C sink decreases to about 10 mol C m −2  yr −1 at 100 years‐since‐fire. We estimate that across the Wabasca study region, bogs currently represent a C sink of 14.7±5.1 Gmol yr −1 . A decrease in the fire return interval to 61 years with no change in air temperature would convert the region's bogs to a net C source. An increase in nonwinter air temperature of 2 °C would decrease the regional C sink to 6.8±2.3 Gmol yr −1 . Under scenarios of predicted climate change, the current C sink status of Alberta bogs is likely to diminish to the point where these peatlands become net sources of atmospheric CO 2 ‐C.