Drought years in peatland rewetting: Rapid vegetation succession can maintain the net CO 2 sink function

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Abstract. Rewetting is a necessary measure to stop CO2 emissions of degraded peatlands and to restore their natural habitat and C accumulation function. Although the severity and frequency of droughts is predicted to increase as a consequence of climate change, it is not well understood whether such extreme events can jeopardize rewetting measures. The goal of this study was to better understand drought effects on peatland restoration measures. Based on long-term reference records, we investigated anomalies in vegetation dynamics and CO2 exchange, including ecosystem respiration (Reco) and gross ecosystem productivity (GEP), in a rewetted fen during the extreme European summer drought 2018. Drought-induced vegetation dynamics were derived from remotely sensed data. Since flooding in 2010, the fen was characterized by a patchy mosaic of open water surfaces and vegetated areas. After years of stagnant vegetation development, drought acted as a trigger event for pioneer species such as Tephroseris palustris and Ranunculus sceleratus to rapidly close persistent vegetation gaps. The massive spread of vegetation assimilated substantial amounts of CO2. In 2018, the annual GEP budget increased by 20 % in comparison to average years (2010–2017). Reco increased even by 40 %, but enhanced photosynthetic CO2 sequestration could compensate for half of the drought-induced increase in respiratory CO2 release. Altogether, the restored fen remained a net CO2 sink in the year of drought, though net CO2 sequestration was lower than in other years. Our study reveals an important regulatory mechanism of restored fens to maintain their net CO2 sink function even in extremely dry years. Even in times of more frequent climate extremes, fen restoration can create ecosystems resilient to drought. However, further research needs to focus on the long-term effects of such extreme events beyond the actual drought period.

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