Filippa Fredriksson

Summary: Wetlands are crucial for biodiversity and climate change mitigation. Vast wetland areas have been lost through land‐use intensification. Thus, restoration efforts have gained attention globally. We assessed relationships between environmental conditions, aboveground and belowground plant traits and carbon cycling in restored wetlands in Denmark, both grazed and ungrazed, compared to a baseline in near‐natural wetlands. Wetland plant species strategy varied from acquisitive to conservative. More conservative species tended to have larger root diameters and higher percentages of root aerenchyma. However, at the community level, we found no evidence for differences in the suite of traits expressed in restored and near‐natural wetlands. Environmental and carbon cycling variables were not consistently different between restoration treatments. Average summer carbon fluxes were highly variable, but net ecosystem exchange was negative, indicating sequestration of CO ₂ . Groundwater level was the main driver of CO ₂ exchange in summer, with relatively dry sites tending to fix more carbon. Relationships between plant traits and carbon cycling were weak. Our results suggest that, for carbon cycling, plant community trait composition is less important than the abiotic environment, in particular, the water table depth. This shows that ecosystem functions can be restored, even in cases where plant biodiversity has not recovered.

Introduction: Riparian wetlands provide several important ecosystem services, including carbon sequestration. Drained and degraded wetlands act as carbon sources, but rewetting might result in elevated methane (CH ₄ ) emissions and does not always reestablish a carbon sink. Objectives: This study examines the impacts of riparian wetland restoration and disturbance regime (grazing vs. no grazing) on CH ₄ and CO ₂ exchanges in wetlands 12–17 years post‐restoration. Methods: Greenhouse gas fluxes were measured using a static chamber at monthly intervals over a year to assess the effects of disturbance regime, groundwater level, and soil temperature on CH ₄ and CO ₂ exchange. Results: Our results show no significant differences in CH ₄ emissions between restored and near‐natural sites. We demonstrate that grazed sites, both near‐natural and restored, are net carbon sinks, while ungrazed sites are carbon sources. However, we did not capture the production from shrubs and trees which were abundant at near‐natural ungrazed sites and would be expected to act as carbon sinks. Conclusion: Our results suggest that ecosystem functioning with respect to carbon cycling was reestablished post‐restoration, but that ecosystem function of the restored areas was contingent on the prevalent disturbance regime.

Restoration and rewetting of wetlands previously drained for agriculture, is currently used to decrease net greenhouse gas (GHG) emissions, while improving biodiversity. Wetland hydro(geo)logy is known to exert a key control on GHG-retention and on conditions facilitating improved biodiversity. Yet knowledge of the major hydrochemical processes that occur in wetlands prior to drainage and after restoration is limited, although links between wetland hydrochemistry, GHG-retention and biodiversity might well exist. To reduce the knowledge gap, we sample surface waters, precipitation, and shallow (<1 m) groundwater from 61 wells. The sampling sites are either near-natural or restored wetlands of the riparian zone, and are distributed along three separate stream valleys, with subsurface geologies consisting of carbonate rock, glacial till or sand from glacial outwash. Furthermore, the wetlands are categorized based on management (grazed or unmanaged). Surface and groundwater samples are analyzed for dissolved major ions, methane (CH4), organic carbon (DOC), fluorescence, and all samples are analyzed for stable water isotopes (δ18O, δD) and electrical conductivity (EC). EC, pH, dissolved oxygen (O2) and temperature are measured in the field using a flow cell. Initial results from the groundwater wells in the wetlands indicate EC values between 101-5300 μS/cm (the high end due to marine influence), O2 between 0.1-6.7 mg/L, and that the pH varies from acidic (min. 5.0) to alkaline (max. 7.7). The groundwater’s Fe(II) concentration appears to be significantly elevated in restored stream valley sites versus the near-natural sites. The results suggest differences in redox conditions that in turn may control production of GHGs, such as CH4. In addition, the hydrochemistry and subsurface geology seem to be a key factor in the development of the present vegetation in the various field sites. With shifting climate, terrestrial wetness will change too, under influence of hydrogeochemical-vegetation interactions. To understand the associated climate feedbacks, a detailed understanding of wetland hydrology and ecology is needed. Through a method-independent approach, this study helps clarify the response related to hydrochemistry, geology, and time. The increased understanding could also contribute to fine-tuning of current and future restoration programs, thus increasing their success.