Many wetland ecosystems such as peatlands and wet tundra hold large amounts of organic carbon (C) in their soils, and are thus important in the terrestrial C cycle. We have synthesized eddy covariance data of the carbon dioxide (CO2) exchange from twelve wetland sites across Europe and North America, ranging from ombrotrophic and minerotrophic peatlands to wet tundra ecosystems, in temperate to arctic climates. The average summertime net ecosystem exchange of CO2 (NEE) was highly variable between sites. However, all sites with complete annual datasets, seven in total, acted as annual net sinks for atmospheric CO2. To evaluate the influence of gross primary production (GPP) and ecosystem respiration (Reco) on NEE, we first removed the artificial correlation emanating from the method of partitioning NEE into GPP and Reco, After this correction the level of significance for the previously significant relationship between annual NEE and GPP increased to p=0.110. The most important variables controlling the between-site variation in annual and summer-time Reco were growing season period, air temperature, growing degree days, normalized difference vegetation index and vapour pressure deficit. Summer-time GPP showed weaker correlations with environmental variables as compared to Reco, the exception being leaf area index (LAT), which correlated with both GPP and NEE, but not with Reco, Annual GPP and NEE correlated significantly with LAI and pH, indicating that various peatland and tundra types with different vegetation properties and nutritional status do not respond in the same way to changes in environmental conditions. This means that models of wetland C exchange and its response to climate change should address the issue of ecosystem structure as well as ecosystem function.

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Keywords Carbon, Carbon dioxide, Eddy covariance, Mire, Net ecosystem exchange, Peatland, Photosynthesis, Respiration, Tundra, Wetland
Lund, M. (Magnus), Roulet, N.T. (Nigel T.), Lindroth, A. (Anders), Lafleur, P.M. (Peter M.), Christensen, T.R. (Torben R.), Aurela, M. (Mika), … Nilsson, M.B. (Mats B.). (2009). Exchange of carbon dioxide across twelve northern peatland and tundra sites.