Restored wetlands need help rebuilding vegetation, study finds

A four-year study found that restoring water levels alone did not lead to significant vegetation growth at two long-drained wetlands. Active support for plant recovery may also be needed, ERR reports, citing a study co-authored by University of Tartu researcher Ilona Tamm.

Wetland restoration projects are common, but less is known about whether restored areas begin functioning as wetlands again. Monitoring is often short, and projects may lack clear goals for assessing success, Tamm writes.

When a peatland is drained, oxygen enters the peat and speeds the breakdown of stored organic matter, releasing carbon dioxide. Raising the water level can slow this process and prevent further peat loss, but a wet area is not necessarily a restored wetland. Conditions must also allow wetland vegetation to grow again.

Tamm and colleagues from the University of Tartu and the University of California, Berkeley, tracked water levels, vegetation and carbon dioxide and methane flows for four years after restoration at two wetlands with long histories of drainage. One was Ess-soo, a former peat extraction area in southern Estonia; the other, Hill Slough, is in California’s Sacramento-San Joaquin River Delta. Both were restored in 2021 after decades of drainage and land use.

At Ess-soo, peat extraction had substantially changed what was originally a natural raised bog. Restoration began in 2021 on a 58-hectare area that had been unused for nearly 30 years. The study focused on an 8.3-hectare section where ditches were blocked and shallow pools created to raise the water level and encourage bog vegetation to return naturally.

The restoration produced both overly wet and overly dry conditions in different parts of the study area. Lower sections remained under water, where vegetation receded, while water levels in higher places fell too far. During a dry summer, water levels across much of the study area remained too deep for more than a month. Four years after restoration, vegetation still covered only about 30 percent of the drier ground.

The findings suggest that restoring a wetland’s water regime may not be enough, particularly on heavily damaged peat extraction sites where little original vegetation or plant material remains. Bare peat can be acidic, nutrient-poor and dry, making it difficult for plants to recolonise.