Published on: 21.3.2022

Information produced by Finnish Environment Institute

Mitigating the impacts of forest drainage on waters

In the late 20th century, peatlands were drained extensively in Finland to promote silviculture. While these ditches still discharge nutrients and other substances into water bodies, there are ways of mitigating their impact.

Forest drainage was a national project in Finland that helped to speed up the rise of the forest industry. Drainage projects peaked in the early 1970s when around one per cent of Finland’s surface area was drained every year. Later this area started decreasing and it was recently thought that new drainage projects had ceased altogether. Nevertheless, the results of the latest National Forest Inventory showed that they have continued to this day. Old ditch networks are also actively maintained.

Forestry accounts for 14% of the phosphorus loading and 12% of the nitrogen loading of Finnish water bodies, peatland drainage being the main cause. Nutrients may continue to leach for decades after a drainage project has been completed. Drainage may also release and carry organic matter into water bodies which results in the darkening of the water.

There are several ways of reducing this loading, such as avoiding clearcutting and introducing continuous cover forestry instead. This reduces the need to maintain ditches, as the transpiration of trees prevents the groundwater level from rising excessively.

The ditches should not be made unnecessarily deep, and different water protection structures can be built in them. Additionally, sufficiently wide tree-covered buffer zones should be left along water bodies where the trees and soil retain nutrients and solids. Some areas can be restored and the peatland ecosystem revitalised.

The optimal solutions can be found when peatland forests are incorporated into catchment-level river basin management where different water issues of the catchment or sub-catchment are examined in an integrated manner.

Image: Yrjö Huusko, Vastavalo