Sensitivity of CO2 exchange of fen ecosystem components to water level variation

Climate change is predicted to bring about a water level (WL) draw-down in boreal peatlands. This study aimed to assess the effect of WL on the carbon dioxide (CO2) dynamics of a boreal oligotrophic fen ecosystem and its components; Sphagnum mosses, sedges, dwarf shrubs and the underlying peat. We measured CO2 exchange with closed chambers during four growing seasons in a study site that comprised different vegetation treatments. WL gradient in the site was broadened by surrounding half of the site with a shallow ditch that lowered the WL by 10-25 cm. We modeled gross photosynthesis (P-G) and ecosystem respiration (R-ECO) and simulated the CO2 exchange in three WL conditions: prevailing and WL draw-down scenarios of 14 and 22 cm. WL draw-down both reduced the P-G and increased the R-ECO, thus leading to a smaller net CO2 uptake in the ecosystem. Of the different components, Sphagnum mosses were most sensitive to WL draw-down and their physiological activities almost ceased. Vascular plant CO2 exchange, en bloc, hardly changed but whereas sedges contributed twice as much to the CO2 exchange as shrubs in the prevailing conditions, the situation was reversed in the WL draw-down scenarios. Peat respiration was the biggest component in R-ECO in all WL conditions and the increase in R-ECO following the WL draw-down was due to the increase in peat respiration. The results imply that functional diversity buffers the ecosystem against environmental variability and that in the long term, WL draw-down changes the vegetation composition of boreal fens.