Postfire carbon balance in boreal bogs of Alberta, Canada

Abstract

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 AE 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 AE 8.4 mol m À2 yr À1 . 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 À2 yr À1 at 75 years after fire. As the tree biomass accumulation rate declines, the net C sink decreases to about 10 mol C m À2 yr À1 at 100 years-sincefire. We estimate that across the Wabasca study region, bogs currently represent a C sink of 14.7 AE 5.1 Gmol yr À1 . 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 1C would decrease the regional C sink to 6.8 AE 2.3 Gmol yr À1 . 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.

Responses of carbon dioxide flux and plant biomass to water table drawdown in a treed peatland in northern Alberta: a climate change perspective

“…Our findings together with others (e.g. Griffis et al, 2000;Bubier et al, 2003;Aurela et al, 2007;Wieder et al, 2009) demonstrate the important interaction between temperature and water availability for GPP ff and R ff response, as either factor alone could not determine the overall growth response of peatland vegetation under changing climatic conditions. Persistently deep water table at the drained site likely limited any response to the short term drying in 2012 as this did little to further lower the water table.…”

“…Depending on time since fire, Wieder et al (2009) report that treed bogs in the same region represent an annual CO 2 sink of 120 to 220 g C m −2 and thus our value is slightly below this range. Within the same region of northern Alberta as the present study, Adkinson et al (2011) report net growing season CO 2 exchange across three study years of −110.1 and −153.5 to −34.5 g C m −2 at poor fen and rich fen sites, respectively.…”

N2-fixation by methanotrophs sustains carbon and nitrogen accumulation in pristine peatlands

“…In our bogs, cryptogamic mosses represent *90 % of the ground cover, contribute up to 65 % of NEP (Wieder et al 2009), and host diazotrophs that contribute 85-96 % of the bog N-input from N 2 -fixation. These exceptionally high rates of Sphagnum-associated N 2 -fixation are the first to be reported for boreal bogs.…”

“…Site differences were largely due to composition of the moss ground-cover, which is related to bog age as defined as time since the most recent fire (cf. Wieder et al 2009). …”

“…This linkage is especially true for ombrotrophic bogs-peatlands isolated from groundwater and surface water inputs, such that external supply of new nutrients is derived solely from the atmosphere (Damman 1988;Vitt 2006;Limpens et al 2006;Wu and Blodau 2013). Bogs of boreal western Canada persist under some of the most extreme conditions known to peatlands, where mean annual temperature averages \2°C, annual precipitation averages \ 500 mm (Vile et al 2011), which is nearly balanced by potential evapotranspiration (Wieder et al 2009), and N inputs from atmospheric deposition are among the lowest found globally, typically \1 kg N ha -1 year -1 (Vitt et al 2003;Wieder et al 2010). Despite these conditions, Sphagnum mosses are surprisingly productive with rates of NPP *2500 kg ha -1 year -1 (Vitt et al 2003;Wieder et al 2010), but can be much higher in considerably wet years (cf.…”