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Ditch network maintenance in drained peatland forests increases sediment load and causes harmful effects on downstream watercourses. The objective of this study was to identify the immediate erosion processes in the ditch network and quantify the impact of the ditch network maintenance on suspended solids (SS) load at the catchment outlet. Ditch level processes were measured by conducting field campaigns and catchment-level responses were obtained by using the paired catchment approach. Measurements were conducted at two nested catchments in Koivupuro, Eastern Finland, where runoff and turbidity were monitored continuously at the catchment outlets and erosion and deposition were measured in selected ditches with a pin meter, erosion pins and sediment collectors. According to the results, the sediment load was at its highest during the first year after the ditch network maintenance and especially high during the first spring snow-melt period. Pin-meter measurements, sediment-collector data and results of ditch-bed erosion pins showed that the erosion was more severe in the ditch cut into mineral subsoil than in the ditch cut into thick peat. The SS load from the whole Koivupuro catchment during the first year after the ditch network maintenance was, depending on the estimation method, 185 or 250 kg treated-ha −1 a −1 (0.56 or 0.75 kg ditch-m −1 ), while the net erosion within the ditch network was much higher, 6300 kg treated-ha −1 a −1 (19 kg ditch-m −1 ), indicating that suspended solids were deposited before reaching the outlet of the catchment. Compared to the earlier studies which have focused on the catchment-level effects of ditch network maintenance on the SS loads, the current study succeeded in showing erosion processes and their variability within the ditch network and provided information for improving water protection during ditch network maintenance. [ABSTRACT FROM AUTHOR]

In boreal-drained peatland forests, tree growth is retarded by the gradual deterioration of drainage ditch networks. In order to avoid the development of suboptimal growth conditions, ditch network maintenance (DNM) operations (ditch cleaning and/or complementary ditching) are annually conducted on an area of about 70,000 ha in Finland. The previous studies indicate that the depth of the water table prior to DNM may influence the magnitude of the growth response to DNM. Tree growth does not necessarily increase after DNM at sites with large stand volume and subsequent low water levels due to tree stand evapotranspiration. We investigated how the pre-treatment water table depth (pre-WTD) in late summer relates to the growth of Scots pine (Pinus sylvestris L.) stands after the DNM operation in 12 field experiments. The increase in mean annual volume growth caused by DNM was negatively related to the pre-WTD, with the highest growth response in stands where the pre-WTD was less than 25–30 cm. DNM did not clearly increase stand growth in sites where the pre-WTD was more than 35–40 cm below the soil surface. There was a high variation in growth response to DNM between the water levels from 20 to 35 cm below the soil surface, indicating that factors other than water table depth are also needed as decision criteria for assessing the appropriate timing of ditch network maintenance. [ABSTRACT FROM PUBLISHER]

In drained, forested peatlands, ditch network maintenance (DNM) is often considered necessary for tree growth, but it also constitutes additional management costs. Commercial thinnings, in turn, in addition to their silvicultural benefits, are generally applied to enhance the financial performance of stand management but results from peatland stands are scarce. In this study, our aim was to find financially feasible management practices for Scots pine-dominated stands on drained peatland sites in Finland. Using mainly inventory data sets, we compiled altogether 29 typical model-stands for four climatic areas, four site types, and two stand conditions according to need for silvicultural care. We used MOTTI stand simulator to predict the development of the model-stands according to different management regimes consisting of various combinations of 0-2 DNM and 0-2 thinnings with different timings and thinning intensities. We then calculated and compared the financial feasibility of the regimes using net present value (NPV; discount rate 3%) analysis. The separate effect of DNM on the profitability was marginal, but the positive effect of thinnings was clear. The harvesting removals varied within a wide range, depending on the timing and intensity of thinnings, but on average, the NPV doubled due to the thinnings. In the stands of initially good silvicultural condition, regimes including only one thinning and a DNM operation generally displayed a good financial result. In the stands of initially poor silvicultural condition due to neglected early care, regimes with two thinnings produced the best NPV regardless of the often low-yielding first thinning. [ABSTRACT FROM AUTHOR]