Interannual variability in net ecosystem CO2 exchange at the arctic treeline

Net ecosystem exchange (NEE) of carbon dioxide was measured at treeline for est and fen tundra sites near Churchill, Canada during three consecutive gr owing seasons (1997-1999). Although both sites demonstrated a net uptake of CO2 in each of the years, there was considerable variation in the flux bet ween sites and between years. Mean daily NEE at the forest varied from -1.5 (+/-0.25 SD) g CO2 m(-2) d(-1) in 1997 to -7.3 (+/-0.39) g CO2 m(-2) d(-1) in 1999. The fen mean daily NEE varied from -1.1 (+/-0.36) g CO2 m(-2) d(- 1) in 1997 to -3.1 (+/-0.50) g CO2 m(-2) d(-1) in 1998. Integrated over the 65-d measurement period the forest was a net sink of -100, -313, and -478 g CO2 m(-2) in 1997, 1998, and 1999, respectively, and the fen net sink was -73, -202, and -38 g CO2 m(-2), respectively. Interannual variations in th e fen NEE were closely related to water table draw-down (soil wetness), the forest NEE was unaffected by water table change. The forest seasonally cum ulative NEE was closely associated with timing of snowmelt and accumulated heat content prior to leaf-out. Earlier snowmelt and greater heat accumulat ion produced a larger growing season sink. It is likely these events also i nfluenced the fen NEE, but the relationship is complicated by water table e ffects. These results suggest that carbon dioxide exchange varies considera bly across the northern treeline, and that there may be an important biosph eric feedback between climate warming, treeline advance, and carbon cycling .