Carbon dioxide fluxes in a northern fen during a hot, dry summer

Atmospheric gradient techniques were used to measure the net ecosystem exch ange of CO2 for a subarctic sedge fen near Churchill, Manitoba, during the summer of 1994. This was the second driest and warmest summer since 1943. T he mean daily temperature was 2 degrees C above average, and the rainfall w as 55% below normal. More than half of the rain fell after the main growth period. The fen landscape comprises hummocks and hollows. Equilibrium reten tion storage occurs at an average standing water depth of 80 mm above the h ollow bottoms (water table reference is 0). During the summer of 1994 the a verage water table position at -117 mm resided well below the zero equilibr ium retention depth. Periodically this decreased to -265 mm, well below a 3 0-year average depth of -70 mm. During the full summer period, measurements indicate that the fen was a source of CO2. Only during a relatively short period of most active photosynthesis in midseason was there a small net CO2 uptake. A deep and warm soil aerobic layer promoted a large respiration fl ux, and this exceeded the photosynthetic CO2 uptake of the stressed sedge c ommunity. Diurnally changes in surface temperatures and incident solar radi ation can explain most of the changes in the net CO2 exchange. It is hypoth esized that in 1994 photosynthesis was significantly decreased and the resp iration loss enhanced by the hot, dry conditions. If this hypothesis is cor rect, by analogy climate warming would need to be accompanied by a substant ial rainfall increase to maintain a condition of net CO2 gain to this peatl and.