THE EFFECTS OF WATER-TABLE MANIPULATION AND ELEVATED-TEMPERATURE ON THE NET CO2 FLUX OF WET SEDGE TUNDRA ECOSYSTEMS

lit situ manipulations were conducted in a naturally drained lake on t he arctic coastal plain near Prudhoe Bay, Alaska (70 degrees 21.98' N, 148 degrees 33.72' W) to assess the potential shortterm effects of de creased water table and elevated temperature on net ecosystem CO2 flux . The experiments were conducted over a 2-year period, and during that time, water table depth of drained plots was maintained on average 7 cm lower than the ambient water table, and surface temperatures of plo ts exposed to elevated temperature were increased on average 0.5 degre es C. Water table drainage, and to a lesser extent elevated temperatur e, resulted in significant increases in ecosystem respiration (ER) rat es, and only small and variable changes in gross ecosystem productivit y (GEP). As a result, drained plots were net sources of approximate to 40 gC m(-2) season(-1) over both years of manipulation, while control plots were net sinks of atmospheric CO2 of about 10 gC m(-2) season(- 1) (growing season length was an estimated 125 days). Control plots ex posed to elevated temperatures accumulated slightly more carbon than c ontrol plots exposed to ambient temperatures. The direct effects of el evated temperature on net CO2 flux, ER, and GEP were small, however, e levated temperature appeared to interact with drainage to exacerbate t he amount of net carbon loss. These data suggest that many currently s aturated or nearly saturated wet sedge ecosystems of the north slope o f Alaska may become significant sources of CO2 to the atmosphere if cl imate change predictions of increased evapotranspiration and reduced s oil water status are realized. There is ample evidence that this may b e already occurring in arctic Alaska, as a change in net carbon balanc e has been observed for both tussock and wet-sedge tundra ecosystems o ver the last 2-3 decades, which coincides with a recent increase in su rface temperature and an associated decrease in soil water content. In contrast, if precipitation increases relatively more than evapotransp iration, then increases in soil moisture content will likely result in greater carbon accumulation.