CH4 exchange at the forest floor of a forestry-drained fen: low flux rates but high temporal variation
We measured methane (CH<sub>4</sub>) exchange rates with automatic chambers at the forest floor of a nutrient-rich drained peatland in 2011–2013. The fen, located in southern Finland, was drained for forestry in the 1970s and the tree stand is now a mixture of Scots pine, Norway spruce and pubescent birch. Our measurement system consisted of six transparent polycarbonate chambers and stainless steel frames, positioned on a number of different field and moss layer types. Flux rates were calculated with both linear and exponential regression. The use of linear regression systematically underestimated CH<sub>4</sub> flux rates by 20–50 % when compared to exponential regression. However, the use of exponential regression with small fluxes (< 3.5 µg CH<sub>4</sub> m<sup>−2</sup> h<sup>−1</sup>) typically resulted in anomalously large flux rates and high hour-to-hour deviations. We therefore recommend that flux rates are initially calculated with linear regression and that higher flux rates are then recalculated using exponential regression. There was clear seasonal dynamics in the CH<sub>4</sub> flux: the forest floor acted as a CH<sub>4</sub> sink particularly from early summer until the end of the year, while in late winter the flux was very small and fluctuated around zero. However, the magnitude of fluxes was relatively small throughout the year, ranging mainly from −40 to + 50 µg CH<sub>4</sub> m<sup>−2</sup> h<sup>−1</sup>. CH<sub>4</sub> emission peaks were occasionally observed, particularly in spring during the snow melt, and in summer during heavy rainfall events. Diurnal variation was observed in all of the chambers, mainly in the summer and particularly in dry and warm conditions. Annual net CH<sub>4</sub> exchange among the six chambers varied from −31 to −155 mg CH<sub>4</sub> m<sup> 2</sup> yr<sup>−1</sup>; with an average of −67 mg CH<sub>4</sub> m<sup>–2</sup> yr<sup>−1</sup> over the two-year measurement period.