CO₂ exchange of a sedge fen in southern Finland—the impact of a drought period

Eddy covariance (EC) measurements of net ecosystem CO₂ exchange (NEE) were conducted on a boreal sedge fen in southern Finland (61°50’N, 24.12’E) during a 1.5-yr period covering two summers in 2004.2005. The EC data were complemented by chamber measurements, which enabled the partition of the daytime NEE into respiration and photosynthesis. A special emphasis was put on the hydrometeorological responses of CO₂ exchange during a drought period in July 2005. A mean CO₂ efflux of 0.009 mg CO₂ m^-2 s^-1 was observed during mid-winter (January.February), while the night-time respiration during the two Julys averaged 0.09 mgCO₂ m^-2 s^-1. During both years the mean midday uptake in late July was about -0.16 mgCO₂ m^-2 s^-1. An annual CO₂ balance of -188 g CO₂ m^-2 was observed in 2005.Aslightly higher net sink of -219 gCO₂ m^-2 was estimated for 2004. The drought period experienced in July 2005 caused a clear depression in the daily NEE values. From the combined analysis of EC and chamber measurements it was concluded that this was mainly due to increased respiration, but evidence was also found of suppressed photosynthesis due to a high VPD.