Ditch maintenance in drained peatland forestry – good or bad?

Cleaning of ditches to maintain an adequate water table is a common practice in peatland forestry. Ditch maintenance promotes forest growth by keeping the water table at favourable level but as a negative side effect it increases nutrient and sediment loads to waterways.

In Finland, scientists have examined the trade-off between increased harvest revenues and declined water quality due to ditch network maintenance. According to the main author, postdoctoral researcher Jenni Miettinen at the University of Helsinki, there are only few economic analyses focusing on this trade-off. This is surprising, as it is generally known that drained peatland forests contribute considerably to nutrient and sediment loads. The authors assess this trade-off from the society’s point of view and determine the social net benefits from maintaining or not maintaining ditches, and compare it to the management of a private forestland owner who is here assumed to ignore the negative water quality externalities.

Finnish researchers have analysed how ditch maintenance – or the lack of this – can affects forest owners and the society in different ways. Photo: Eva Skarbøvik

The scientists chose a drained forest site for which ditch network maintenance was optimal management for the private landowner. This outcome was contrasted to the socially optimal solution that also considered the negative water quality impacts. The social optimum relied on a large set of parameters relating, for example, to forest growth, nutrient and sediment load damages, and water protection costs. Ditch network maintenance was in many cases not the optimal choice for the society because of the negative effects caused by nutrient and sediment load damage in headwater streams.

The researchers also focused on mitigation measures and found that negative water quality impacts can be cost-efficiently reduced by establishing overland flow fields. However, in certain cases it would be even more efficient to not carry out the ditch maintenance at all. Thus, both society and private landowners should scrutinize very carefully when to conduct ditch network maintenance, since abstaining from it is in many cases the better option.

These results are important for future management of drained peatland forests in the Nordic countries, both under a forest bioeconomy where more biomass is needed, and also under a changing climate, when more intense rainstorms may give higher erosion and runoff. The results are also relevant for the respective legislation and subsidies in the Nordic countries related to drainage maintenance of peatland forestry.

Miettinen J, Ollikainen M, Aroviita J, Haikarainen S, Nieminen N, Turunen J, Valsta L 2020. Boreal peatland forests: ditch network maintenance effort and water protection in a forest rotation framework. Canadian Journal of Forest Research 50: 1025–1038. https://doi.org/10.1139/cjfr-2019-0339

Feature photo: Jarno Turunen.