Mires and Peat (Apr 2013)
Domination of growing-season evapotranspiration over runoff makes ditch network maintenance in mature peatland forests questionable
Abstract
In Finland, ditch network maintenance (DNM) is carried out annually on 60–70,000 ha of drained peatland to promote tree growth for forestry purposes. However, it is important to avoid ditching that contributes little to the stand growth and productivity, both to improve the economical profitability of forestry and to mitigate DNM-induced nutrient release to watercourses. We hypothesised that mature forest stands with substantial evapotranspiration potential do not necessarily need DNM, even if the ditch networks are in poor condition.We estimated evapotranspiration (EVT) of forest vegetation during the growing seasons (June–September) of 2007–2011 in four forested artificial peatland catchments dominated by Scots pine stands (Pinus sylvestris L.) (stand volume 93–151 m3 ha-1) located in southern, western, central and northern Finland. Precipitation (P), runoff (R) and water table level (WTL) were monitored continuously in the field. The water storage change (ΔS) was estimated on the basis of WTL measurements and peat pF curves determined from in situ peat samples. In addition, tree stand transpiration (T) was estimated in two of the catchments using the sap flow method. EVT was estimated as the residual term of the water balance equation.During the growing season, EVT (153–295 mm) was 49–161 % of the total accumulated P (155–368 mm), and decreased from south to north. Within each growing season, EVT was always largest in July or August. Tree transpiration was about 50 % of the total forest EVT in the two sites where it was measured directly. The mean WTL was at depth 36–63 cm during the growing seasons. Clear-cutting of a 100m3 ha-1 stand on one site resulted in an average rise of WTL by 18 cm.The results suggested that, in the southernmost site in particular, no drainage network management would be necessary to sustain satisfactory drainage conditions for tree growth because growing-season precipitation is transferred back to the atmosphere by forest EVT. In the northernmost site, ditch networks were considered important in controlling drainage conditions because of the low EVT potential of <100 m3 ha-1 tree stands. Further research is needed to fully understand the mechanisms behind forest EVT potential, as one site with high tree stand volume had surprisingly low EVT potential.
