The return of wetland forests

Wetland forests are an important part of the Swedish forest landscape. These wet environments provide habitats for many species of mosses, lichens and vascular plants, while also playing an important role in the carbon balance. For a long time, many wetland forests have been drained to increase forest production. Land drainage changes both forest vegetation, and the processes that determine how carbon is stored and decomposed in the soil.

Today, extensive efforts are underway to rewet previously drained forest land. By raising the groundwater level, natural water conditions can be restored and carbon dioxide emissions from peatlands reduced. However, rewetting also changes the forest and its biodiversity. How wet should a forest actually be to achieve the greatest possible benefits for both climate and biodiversity?

This is one of the questions we are investigating in the research project The return of wetland forests.

From dry forest land to functioning wetland

In the project, we investigate how forest biodiversity and the carbon balance change along a gradient from relatively dry forest to wet and waterlogged wetland forest. We study both forests that have previously been drained and more naturally wet forest environments.

We survey vascular plants, mosses and lichens and investigate how their species composition is affected by soil moisture. We also study the occurrence of different tree species and dead wood – structures that are important for many forest-dwelling species.

At the same time, we measure how much carbon is taken up by the forest and how much carbon dioxide is released from the soil. By combining these different types of measurements, we can investigate the relationships between water levels, biodiversity and climate benefits.

What is a good target for rewetting?

An important question for the project is whether there is a water level at which several values can benefit at the same time. Very dry conditions may be detrimental both to wetland-forest species and to the ability of the soil to store carbon. At the same time, it is not necessarily the case that the wettest conditions will always result in the greatest biodiversity or highest carbon storage.

By studying the entire soil-moisture gradient, we therefore aim to identify synergies and trade-offs between biodiversity, forest development and climate benefits.

The results can contribute to a better knowledge base for future decisions about which areas are suitable for rewetting and which objectives can realistically be achieved.

Research in collaboration with forest owners

Rewetting affects not only the natural environment but also forest owners and their opportunities to manage their forests. The experiences and perspectives of forest owners are therefore an important part of the project.

In a later part of the project, we will investigate which factors influence forest owners' decisions to participate in rewetting projects and how they view the possibilities of combining climate benefits, biodiversity and other values on their properties.

Through dialogue with forest owners and other stakeholders, we aim to contribute to rewetting measures that are as well-balanced and useful as possible.

Project aims

The aim of The return of wetland forests is to increase knowledge about how rewetting drained forest land can contribute to both biodiversity and climate benefits.

In particular, we aim to answer questions such as:

• How are communities of vascular plants, mosses and lichens affected as forests become wetter?
• How are forest structure and tree growth affected by increased soil moisture
• How does the carbon balance of the soil and forest change along a soil-moisture gradient?
• Are there water levels at which climate benefits and biodiversity can be promoted simultaneously?
• How do forest owners perceive rewetting, and which factors influence their decisions about rewetting?
• Why is this knowledge needed?

Rewetting is a relatively new nature conservation and climate measure being implemented on a large scale in Sweden. To ensure that the right measures are taken in the right places, we need better knowledge of what happens to forests when their water conditions change.

By combining research on biodiversity, forest ecology and the carbon balance, we aim to contribute to a stronger scientific basis for the future rewetting of forest land.

The project is carried out by researchers at the Swedish University of Agricultural Sciences (SLU) and Lund University, in collaboration with forest owners and other stakeholders working with rewetting and forestry

Project leader: Lisa Petersson, SLU

Projektmedlemmar: Patrik Vestin, Lunds universitet; Emma Holmström, SLU; och Järvi Järveoja, SLU

Fältassistenter: Kerstin Fersters, Robin Mang