Peatlands and Changing Climate: Will Prolonged Drought Convert Fen Carbon Sinks to Sources?.

Boreal regions of the northern hemisphere are expected to undergo some of the most dramatic changes in climate.  Boreal peatlands store large quantities of C (455 Pg globally), a testament to their long-term functioning as a sink for atmospheric C.  Peatlands of continental western Canada represent 10% of the global boreal peatland area and contain 17% of the boreal peatland C stock.  Soil surfaces of boreal peatlands typically have close to 100% cover of mosses.  These nonvascular plants, without true stomates and with narrow tolerances to precipitation and temperature, may respond with exceptional sensitivity to changing climate. Boreal, continental, western Canada has been experiencing prolonged drought conditions since 1996, and in 2003 we observed moss decline on southward facing hummocks in our fen sites.  With continued drought conditions predicted, understanding the carbon balance of dying and dead fens is timely and important.  Our goal was to quantify carbon fluxes from peat deposits in fen microenvironments where mosses have recently died. We established 10 replicate plots per site (two stressed fens with dead and dying mosses and two control sites).  Flux measurements were made once or twice monthly throughout the field season in 2004, 2005 and 2006.  While drought conditions were predicted to continue in 2005 and 2006, these years were wetter than the previous year.  Monthly CO₂ fluxes from dying fens represented either a small net source or small net sink for C, while fluxes from control sites always represented net sinks for atmospheric C, consuming ~ 20 times more C than dying fen sites (2.539 vs. 0.141 umol m^-2 sec^-1, respectively). Differences between control and drought-stressed sites were driven mainly by changes in Sphagnum production (i.e, photosynthesis) rather than changes in respiration.  Our observations of dead fens throughout north-central Alberta may indicate fens as harbingers of ecosystem responses to climate change.