Forest drainage effects on tree growth in Northern Sweden : developing guidelines for ditch network maintenance

Sammanfattning: At present, there are no clear guidelines for when and where ditch network maintenance (DNM) should be performed in Sweden. Recently there have been attempts to create tools that indicate the need for DNM, one of these tools is the Ditch Flow Tracker (DFT). The DFT determines the catchment area (CA) and soil type to predict if a ditch segment does not need DNM. The tool is based on the hypothesis that ditches with small CA’s do not need maintenance because they do not function to drain enough water to improve tree growth. The overall goal of this study was to ground truth the DFT by testing how well the CA, soil type and water flow divided into category can predict ditch function in terms of tree growth. Since the decision of DNM should take into account the ecological impacts the ability of the DFT to predict biodiversity of plants was also tested. A field study was conducted in the Krycklan Catchment Study at 18 drained sites with different CA’s and soil types. At each site, different ditch characteristics were measured as well as forest variables at different distances from the ditch. The analyses indicated that soil type had an effect on the increase in tree growth caused by ditches. Ditch segments in till soils had a higher volume in plots closest to the ditch as well as a higher volume overall compared to peat soils. Contrary to my hypothesis, CA did not affect the effectiveness of a ditch segment. Instead, water flow divided into categories seemed to effect the growth effect from drainage. Ditch segments with a high water flow had the highest standing volume as well as had a gradient of decreasing volume away from the ditch. CA did however indicate the biodiversity of plants within a ditch segment. The analysis showed that ditches with larger CA (1-2 ha) had a significantly higher plant diversity than the ditches with smaller CA’s. Neither soil type nor CA showed any influence on the long term growth effect of ditching when tree rings were examined. However this was calculated from only 10 of the 18 sites due to too few trees having been established when the ditches were first dug, which brings into question how strong this result was. The findings as a whole indicate that soil type and water flow category can be used to predict the effectiveness of a ditch segment in terms of affecting tree volume, while CA can be used to predict the biodiversity of plants of a ditch.