Interannual variability of net ecosystem CO2 exchange at a subarctic fen

Landscape-scale net ecosystem CO2 exchange (NEE) and the energy balance of a subarctic fen were studied during five growing seasons near Churchill, Ma nitoba. Interannual variability in NEE was large and ranged from a net sink of -235 g CO2 m(-2) in 1996 to a net source of +76 g CO2 m(-2) in 1994, An nual estimates of CO2 exchange indicate that during the present period the fen is losing carbon nearly 3 times faster than its long-term historical ga in of about -11 g CO2 m(-2) yr(-1). Our estimates suggest that gross ecosys tem photosynthesis may be more variable than ecosystem respiration on diurn al, seasonal, and interannual timescales. Our data strongly indicate that a n early snowmelt combined with wet and warm conditions during the spring pe riod lead to large carbon acquisition even when drier conditions were exper ienced over the majority of the growing season. The phenological stage of t he vegetation relative to the climatic conditions experienced is an importa nt cause of the interannual variability in NEE. An accurate representation of phenology in climate models is, therefore, critical to the success of fo recasting the carbon budgets of northern wetlands.