Drought years in peatland rewetting: rapid vegetation succession can maintain the net CO2 sink function

The rewetting of peatlands is regarded as an important nature-based climate solution and intended to reconcile climate protection with the restoration of self-regulating ecosystems that are resistant to climate impacts. Although the severity and frequency of droughts are 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 vegetation development and the exchange of the two important greenhouse gases CO2 and CH4, especially in rewetted fens. Based on long-term reference records, we investigated anomalies in vegetation dynamics, CH4 emissions, and net CO2 exchange, including the component fluxes of ecosystem respiration (R-eco) and gross ecosystem productivity (GEP), in a rewetted fen during the extreme European summer drought in 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). R-eco 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. CH4 emissions were 20 % below average on an annual basis, though stronger reduction effects occurred from August onwards, when daily fluxes were 60 % lower than in reference years. Our study reveals an important regulatory mechanism of restored fens to maintain their net CO2 sink function even in extremely dry years. It appears that, in times of more frequent climate extremes, fen restoration can create ecosystems resilient to drought. However, in order to comprehensively assess the mitigation prospects of peatland rewetting as a nature-based climate solution, further research needs to focus on the long-term effects of such extreme events beyond the actual drought period.