Case studies from Germany and Scotland indicate climate change mitigation potential of riverine and coastal wetlands

A review of case studies from Germany and Scotland finds that riverine floodplain forests and coastal saltmarshes can store and sequester substantial organic carbon, supporting their potential contribution to climate change mitigation. The synthesis combines previously published measurements of vegetation, soils, sediment stocks and carbon sequestration rates, while emphasizing uncertainty around methane emissions, soil sequestration and changes in habitat extent.

The German case study examines remnant and planted hardwood floodplain forests along the Middle Elbe River, a lowland river with a mean discharge of about 680 m³ s⁻¹. Dikes and river engineering have reduced the active floodplain by 80% since the Middle Ages, and only about 1% of the former forest area remains. The study covered forests in the active floodplain, the seepage-water zone behind the dike and floodplains along tributaries, with additional comparisons among young plantations, old forests and dense or sparse canopies.

Researchers measured tree diameter at breast height and height in the field, then used species-specific equations to estimate tree volume and carbon stocks. Root biomass, deadwood, litter and shrubs were also included, while dendrochronological measurements from dominant pedunculate oak, Quercus robur, were used to estimate tree age and growth rates. Soil organic carbon was assessed to a depth of 1 metre using samples from 18 soil pits and 93 horizons, with pedotransfer functions supplying bulk-density estimates for another 755 horizons.

Average vegetation carbon stocks ranged from 50 Mg ha⁻¹ in young plantations to 180 Mg ha⁻¹ in old forests, while forest soil organic carbon stocks ranged from 100 to 150 Mg ha⁻¹. Former floodplain sites that had been disconnected from river hydrology stored 33% less soil organic carbon than active sites, and low sites within the active floodplain stored 50% more than high sites. These comparisons associate hydrological setting with observed carbon stocks but do not by themselves establish causation.

Average vegetation sequestration rates were estimated at 217 ± 86 g organic carbon m⁻² yr⁻¹ for young forests on low active floodplains, compared with 103 ± 20 g m⁻² yr⁻¹ for old dense forests in the same setting, 118 ± 22 g m⁻² yr⁻¹ for old sparse forests, 130 ± 39 g m⁻² yr⁻¹ for old dense forests on high active floodplains, 121 ± 36 g m⁻² yr⁻¹ in the seepage-water zone and 116 ± 35 g m⁻² yr⁻¹ at tributaries. The difference between young and old forests was significant at p ≤ 0.05, while differences among old dense forests under different hydrological conditions were not significant at p ≥ 0.05.

Carbon sequestration by dominant pedunculate oaks was higher during flood years in every hydrological setting. Drought was associated with a significant reduction in oak carbon sequestration on the active floodplain only at comparatively high elevation; the reported outcome concerns sequestration rather than growth. Average annual growth rates were 2.2 mm yr⁻¹ in the low active floodplain, seepage-water zone and tributary floodplains, and 2.8 mm yr⁻¹ in the high active floodplain.

The Scottish case study summarizes estimates for approximately 240 saltmarshes covering 58 km² around the country. Scottish saltmarshes were estimated to hold 0.37 ± 0.09 million tonnes of organic carbon, standardized to a sediment depth of 0.1 metre, and to receive approximately 4,385 ± 481 tonnes of organic carbon annually. About 85% of the annual organic carbon was estimated to originate from terrestrial sources. The average organic carbon stock in surficial sediments was 6.31 ± 1.56 kg m⁻², approximately 23% above the area-normalized average for saltmarshes across Great Britain. Despite its high measured coastal stock, the Scottish sequestration rate was below the global average, which the review associates with Scotland’s temperate climate and greater proportion of sandy soils.

Saltmarsh carbon stocks and sequestration vary among and within sites. Higher marsh zones generally contain more carbon in biomass and soils than lower zones, muddy sediments contain more organic carbon than sandy sediments, more frequently inundated lower marsh zones have higher estimated sequestration rates, and accreting systems generally capture more sediment. National averages can therefore obscure substantial local variation. The review also notes unresolved questions concerning methane fluxes, carbonate burial, grazing, drainage and the effects of changes in precipitation and storm frequency.

The review concludes that floodplains and saltmarshes are efficient carbon-sequestration ecosystems per unit area, but their limited spatial extent means they cannot serve as a stand-alone solution to climate change. Protecting existing carbon stores is important because disturbance can cause decomposition and emissions, while restoration or reduced anthropogenic pressure may support additional sequestration. Important limitations remain, including incomplete estimates of soil carbon sequestration in the German floodplains, uncertain methane emissions and outdated information on the extent and condition of Scottish saltmarshes.

Subject of Research: Organic-carbon storage and sequestration in German floodplain forests and Scottish saltmarshes

Article Title: Case studies from Germany and Scotland indicate climate change mitigation potential of riverine and coastal wetlands

Article References: Ludewig, K., Shupe, H. A., Vásconez Navas, L. K., Mueller, P., Nolte, S., & Hunt, C. A. (2026). Case studies from Germany and Scotland indicate climate change mitigation potential of riverine and coastal wetlands. Nature Conservation, 62, 149-173. https://doi.org/10.3897/natureconservation.62.153910

Image Credits: AI Generated

DOI: 10.3897/natureconservation.62.153910

Keywords: wetland carbon storage, floodplain forests, saltmarsh carbon, organic carbon sequestration, Middle Elbe River, Scottish saltmarshes, blue carbon, teal carbon, nature-based solutions, climate mitigation, Case, studies