Interannual variability in net ecosystem CO₂ exchange at the arctic treeline

Net ecosystem exchange (NEE) of carbon dioxide was measured at treeline forest and fen tundra sites near Churchill, Canada during three consecutive growing seasons (1997-1999). Although both sites demonstrated a net uptake of CO₂ in each of the years, there was considerable variation in the flux between 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 the fen NEE were closely related to water table draw-down (soil wetness), the forest NEE was unaffected by water table change. The forest seasonally cumulative NEE was closely associated with timing of snowmelt and accumulated heat content prior to leaf-out. Earlier snowmelt and greater heat accumulation produced a larger growing season sink. It is likely these events also influenced the fen NEE, but the relationship is complicated by water table effects. These results suggest that carbon dioxide exchange varies considerably across the northern treeline, and that there may be an important biospheric feedback between climate warming, treeline advance, and carbon cycling.