When do we ditch the ditch? – A field test of the “DitchFlowTracker” to prioritize forest drainage ditch maintenance for sustainable forest management

The goal of our study is to provide an easily applied method for prioritizing the need for ditch maintenance. To meet forest production goals while maintaining Sweden’s environmental objectives, more research is needed to provide guidelines for prioritization of ditch maintenance.

Almost 25% of the forests in Fennoscandia have been artificially drained by ditches over the past century to increase timber production, with a recent estimate of 1 million km of ditches occurring in all of Sweden, twice the length of all natural streams. Ditching can improve forest production but is often ineffective and may cause nutrient losses as well as increased transport of mercury and sediment. As ditches age, ditch maintenance may be required and the Swedish Forest Agency and County Administrative Boards from around Sweden are in urgent need of guidance on how to prioritize ditch maintenance. This is a research topic with a truly bottom-up approach that was developed by listening to stakeholders’ needs. Our goal is to test a new model, the DitchFlow-Tracker, as support tool for prioritizing ditch maintenance in an effort to balance the positive effects on tree productivity against negative environmental impacts.

The DitchFlowTracker uses new digital elevation models generated from recent aerial laser scans of Sweden combined with flow accumulation models in GIS to estimate how much water drains in particular ditches. In a recent case study, the DitchFlowTracker estimated that ~25-50% of ditches within the Krycklan Catchment Study in northern Sweden are dry year-round and never have flowing water; these ditches are obviously top candidates for not being maintained. However, we need to validate the DitchFlowTracker with field testing before recommendations can be made. Particularly because hydrology, and thus the effects of ditches on tree growth, can depend on forest age and soil type.

We propose to do field tests of the DitchFlowTracker by measuring water flow in ditches and tree growth next to ditches at sites with different forest ages and soil types. By doing this, we will be able to provide a tool to semi-automatically identify a large number of ditches that are not in need of ditch maintenance, thus contributing significantly towards saving forest production costs and increasing the environmental sustainability of forest operations.