Forestry in drained boreal peatlands: managing nutrient and sediment loads to watercourses

Ditch network maintenance has been common practice in peatland forestry. It increases forest growth but at the same time, it significantly weakens water quality due to increased nutrient and sediment exports to watercourses. Society needs to balance the increased growth and harvest revenue against water protection costs and water quality damages. In a given sensitive headwater catchment and using a large set of parameter values, we find that implementing ditch network maintenance is not always socially optimal.

Boreal peatland forests have been extensively drained for forestry purposes. In practice, Finland no longer drains pristine peatlands. However, in drained peatlands, ditch network maintenance is a common practice to maintain the forest growth achieved by first-time ditching. As a negative externality, ditch network maintenance increases nutrient and sediment loads, which are detrimental to water quality. The focus of this study is on water quality externalities caused by forest management in drained boreal peatland forests.

The main question is how does the socially optimal even-aged forest management in drained peatland forests look like when negative impacts on water quality and the measures to reduce them are taken into account. The forest rotation model provides framework, where ditch network maintenance, carried out after the first commercial thinning, and final harvesting are analyzed as management practices, which cause negative water quality impacts. The damage caused by nutrient and sediment loads can be reduced by using water protection measures, such as establishing overland flow fields, and abstaining from ditch network maintenance. In addition, stream restoration measures can re-establish damaged watercourses but similarly as water protection measures, there are costs associated with these measures.

First, we characterize qualitatively the socially optimal choice of the rotation age, ditch network maintenance and water protection. Second, we examine numerically the key factors affecting the optimal forest management and water protection measures. We use field data collected from 33 forest streams in northeastern Finland to describe the sediment load damage in ecologically vulnerable boreal headwaters. On the chosen drained forest site using a large set of parameter values, we find that implementing ditch network maintenance is not always socially optimal. Furthermore, the optimal size of the overland flow field depends highly on estimates of sediment load damage and overland flow field costs. To conclude, the decision when to implement or abstain from ditch network maintenance in drained boreal peatlands is a site-specific question depending on various economic parameter values.

The key lesson of the study is that ditch network maintenance should be carefully considered and when implemented, it should be complemented with water protection measures. In many cases, ditch network maintenance is not the optimal choice from society´s point of view if negative water quality externalities are taken into account.


Reference: Miettinen, J., Ollikainen, M., Aroviita, J., Haikarainen, S., Nieminen, M., 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 (2020)