The unexpected long period of elevated CH4 emissions from an inundated fen meadow ended only with the occurrence of cattail (Typha latifolia) (Part 1 of data collection, table ENVIRONMENTAL_VARIABLES_AND_FLUXES)

Drainage and agricultural use transform natural peatlands from a net carbon (C) sink to a net C source. It is well accepted that rewetting of peatlands, despite of high methane (CH4) emissions, holds the potential to mitigate climate change by greatly reducing CO2 and thus net greenhouse gas (GHG) emissions. However, the time span required for this transition is unknown because measurements of peatland GHG emissions are often limited to a few years. Here, we present 14 consecutive years of CH4 flux measurements following rewetting of a formerly long-term drained peatland in the Peene valley. Measurements were made at two rewetted sites (non-inundated vs. inundated) using an opaque manual chamber system. During the 14-year study period, significant differences in measured CH4 emissions occurred that overlapped with stages of ecosystem transition from a cultivated grassland in a drained peatland to a polytrophic lake dominated by helophytes. The polytrophic shallow lake formed during the first two years of the observation period, immediately following rewetting in autumn 2004. This development was accompanied by a rapid vegetation shift from dying cultivated grasses to floating and submerged hydrophytes and significantly increased CH4 emissions. Since 2008, helophytes have gradually spread from the shoreline into the established shallow lake, especially in drier years. This process was periodically delayed by exceptional inundation and eventually resulted in the measurement site being covered by helophytes in 2016 and 2017. While the period between 2009 and 2015, in particular, showed exceptionally high CH4 emissions, these decreased significantly after cattail (Typha latifolia) and other helophytes appeared at the measurement site. Therefore, CH4 emissions declined only after ten years of transition following rewetting, possibly reaching a new steady state. Thus, this study highlights the importance and need to identify ecosystem transition and conduct long-term measurements to capture responses of these transitional stages. This table contains data about environmental variables and fluxes.

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