Beavers can turn streams into carbon stores – we measured how much

WildMedia/Shutterstock Across Europe, beaver numbers are increasing after a long period of decline. As these aquatic mammals recolonise rivers, they are gradually rebuilding wetlands that once existed across many river valleys. As geographers, we have been investigating how these changes could also affect the movement of carbon through river systems. To find out, we measured the full carbon balance of a wetland created by beaver damming. Our new study shows that a wetland created by beaver damming can store carbon at rates up to ten times higher than an equivalent stretch of river and floodplain without beavers. Over just 13 years, the wetland we studied in northern Switzerland locked away more than 1,100 tonnes of carbon. That’s comparable to two Olympic swimming pools filled with charcoal. So when beavers dam rivers, they can also fundamentally change how carbon is stored in river landscapes. Our team studied a wetland where beavers have been active for more than a decade. We monitored the site intensively for a full year to measure the flow of water, the amount of carbon dissolved in the water, released greenhouse gases (such as carbon dioxide and methane) and plant growth across the wetland. We also sampled and analysed sediments and dead wood that had accumulated since the beavers arrived. By combining these measurements, we have built one of the most complete carbon budgets for a beaver landscape in Europe. The results surprised us. Despite some seasonal carbon emissions during summer, the wetland acted as a strong carbon sink. Each year it stored around 98 tonnes of carbon that would otherwise have flowed downstream or returned into the atmosphere. But this annual carbon balance is strongly linked to water flow and the extent of flooding, which can vary year-to-year. What really determines long-term benefits is how much carbon ultimately becomes buried and stored in the landscape for decades.

When a dam slows the water, sediments begin to settle. These sediments carry organic material such as leaves, soil and plant fragments that contain carbon. Instead of washing away downstream, the material becomes buried in wetland soils. Beaver dams also raise water levels and can flood existing vegetation. Some trees die and fall into the water, adding large amounts of dead wood that slowly stores carbon over long periods. Meanwhile, a new succession of wetland plants and algae growing in the wetland absorb carbon from the atmosphere. Over time, the wetland becomes a natural storage system. Sediment, wood and vegetation build up layer by layer. This locks carbon into the landscape and eventually fills the wetland. In the wetland we studied, sediments contained up to eight times more organic carbon than nearby forest soils.

Wetlands usually produce methane, a powerful greenhouse gas. This has raised concerns that beaver ponds might actually worsen climate warming. But in our study, methane emissions were extremely small – less than 0.1% of the total carbon balance. Most greenhouse gas emissions came from carbon dioxide released from sediments exposed during the drier summer periods. Even then, these emissions were smaller than the amount of carbon being stored in sediments and wood. Over the course of one year, the wetland stored more carbon than it released. With and without beavers To understand the role of the animals themselves, we compared the beaver wetland with a scenario where the same river remained a normal flowing stream with a forested floodplain. Forests are already important carbon stores. Trees capture carbon as they grow, and some of that carbon remains locked in soils and dead wood. Without beaver dams, the river would stay largely confined to its channel. Water would move quickly downstream, carrying sediments and carbon away rather than trapping them across the floodplain. Our calculations show that this forested river corridor would store only a small fraction of the carbon held in the beaver wetland. So the presence of beavers increased carbon storage by about an order of magnitude over the course of a decade. As beaver populations expand across Europe, they could improve carbon storage in river landscapes. When we scaled our results up to the area of floodplains in Switzerland suitable for beaver recolonisation, we estimated that the potential carbon storage could offset roughly 1–2% of the country’s annual emissions. That might sound small. But it would happen without any expensive technology, infrastructure or active intervention. It would simply come from allowing a native species to rebuild the wetlands that once existed along many of these rivers. Beavers are not going to solve climate change, but our research shows these natural engineers can quietly help river landscapes store more carbon for decades to come.

Joshua Larsen receives funding from UK Research and Innovation (UKRI) Annegret Larsen receives funding from the Swiss Federal Office for the Environment. Lukas Hallberg does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.