We compared the CO2 exchange and its controls in the plant communities of a strongly patterned aapa mire, the Kaamanen fen in northern Finland. Based on a systematic vegetation inventory and an ordination analysis, four plant community types were chosen for the study: Ericales–Pleurozium string tops, Betula–Sphagnum string margins, Trichophorum tussock flarks and Carex–Scorpidium wet flarks. We measured plant community CO2 exchange with a closed chamber technique during the growing season of 2007 and early summer of 2008. Nonlinear regression models were used for simulating the CO2 exchange over the measurement period for different mire components and for the whole mire. The CO2 exchange dynamics distinguished two functional components in the mire: an ombrotrophic component (Ericales–Pleurozium string tops) and a minerotrophic component (other plant community types). Minerotrophic plant communities responded similarly to environmental controls, the most important of these being variation in leaf area and aerobic peat volume (water level). The ombrotrophic component dynamics were more obscure; frost and possibly peat moisture played a role. The minerotrophic communities functioned as effective CO2 sinks in the simulation, while the net CO2 exchange of the ombrotrophic community was close to zero. The smaller NEE of the ombrotrophic community was due to less efficient photosynthesis per unit leaf area in combination with high ecosystem respiration resulting from a large aerobic peat volume. Our study shows that a fen/bog functional dichotomy can also exist within one mire. Wet minerotrophic communities within northern mires can act as effective CO2 sinks.
