THE EFFECT OF THE WINTER/SPRING TRANSITION ON CO2 EXCHANGE AT A TEMPERATE FEN

Northern peatlands have a significant reservoir of carbon stored within meters of the Earth's surface. Most of this carbon is potentially available for exchange with the atmosphere as carbon dioxide (CO2) making the exchange of carbon from wetlands highly sensitive to forcing by both weather and climate. Specifically, the exchange of CO2 in a wetland during the winter/spring transition is extremely responsive to climatic variation. The CO2 exchange during this period is traditionally thought of as a combination of an initial abiological pulse of CO2 due to build up under the snow/ice cover during the winter months and the onset of biological processes controlling CO2 exchange. Manual sampling of CO2 exchange during this period could underestimate this exchange due to the episodic nature of the thawing process. However, automated chamber systems allow us to quantify the effect of warming because of their high sampling frequency throughout the thaw period. Sallie's Fen is an NSF Long-Term Research in Environmental Biology (LTREB) site where CO2 fluxes and a suite of environmental data have been collected since 1989. Opaque, static manual chamber CO2 fluxes have been measured at this site since 1989. Measurements of net ecosystem exchange (NEE) with clear manual chambers began in 1994 and at higher frequency with autochambers in 2000. The high frequency flux measurements for six thaw periods reveal different NEE due to the timing of the thaw in that depended on weather, particularly the latitude of winter frontal boundaries and warm front passage. These high frequency measurements put into context of the longer dataset of manual chamber measurement of dark CO2 flux and NEE indicate that the timing and rate of thaw have considerable impact on the annual carbon exchange in this wetland. Predicted increases in temperature and a lengthened growing season in Northern latitudes will affect the rate of carbon exchange and the carbon balance of wetland environments.