Articles by Mikko T. Niemi

According to the 13th Finnish National Forest Inventory, 0.8 Mha of drained peatland forests require ditch network maintenance (DNM). The annual DNM area has decreased radically during the past ten years, leading to gradual shallowing of ditches and rise of water table (WT) in peatland forests. To study the impacts of ditch shallowing on ecosystem services, we applied Peatland Simulator SUSI for 20 average peatland forests representing four different geographical regions in Finland. The simulation period was 20 years and the initial ditch depths were set to 0.3 m, 0.6 m and 0.9 m. The study included drained peatland forest site types from nutrient rich to nutrient poor, with main species as Scots pine (Pinus sylvestris L.) or Norway spruce (Picea abies (L.) Karst.). We studied how ditch shallowing affected stand volume growth, ecosystem and soil carbon (C) balances, and nitrogen (N) and phosphorus (P) export loads to water courses in different peatland sites. The results showed that due to ditch shallowing, the ecosystem C sinks increased in most sites when the initial ditch depth was 0.6 m or 0.9 m. Ditch shallowing generally increased stand volume growth in Southern Finland when the initial ditch depth was 0.6 m or 0.9 m. Regardless of the location and initial ditch depth, ditch shallowing decreased N and P exports, and soil C emissions. The study calls for new water management guidelines for drained forested peatlands in Finland.

• Saari, Department of Forest Sciences, P.O. Box 27, FI-00014 University of Helsinki, Finland https://orcid.org/0009-0001-5931-4210 E-mail: annastina.saari@helsinki.fi
• Palviainen, Department of Forest Sciences, P.O. Box 27, FI-00014 University of Helsinki, Finland https://orcid.org/0000-0001-9963-4748 E-mail: marjo.palviainen@helsinki.fi
• Niemi, Department of Forest Sciences, P.O. Box 27, FI-00014 University of Helsinki, Finland https://orcid.org/0000-0003-0461-3667 E-mail: mikko.t.niemi@helsinki.fi
• Laurén, Department of Forest Sciences, P.O. Box 27, FI-00014 University of Helsinki, Finland; Faculty of Science and Forestry, University of Eastern Finland, P.O. Box 111, FI-80101 Joensuu, Finland https://orcid.org/0000-0002-6835-9568 E-mail: annamari.lauren@helsinki.fi

The pulse density of airborne Light Detection and Ranging (LiDAR) is increasing due to technical developments. The trade-offs between pulse density, inventory costs, and forest attribute measurement accuracy are extensively studied, but the possibilities of high-density airborne LiDAR in stream extraction and soil wetness mapping are unknown. This study aimed to refine the best practices for generating a hydrologically conditioned digital elevation model (DEM) from an airborne LiDAR -derived 3D point cloud. Depressionless DEMs were processed using a stepwise breaching-filling method, and the performance of overland flow routing was studied in relation to a pulse density, an interpolation method, and a raster cell size. The study area was situated on a densely ditched forestry site in Parkano municipality, for which LiDAR data with a pulse density of 5 m^–2 were available. Stream networks and a topographic wetness index (TWI) were derived from altogether 12 DEM versions. The topological database of Finland was used as a ground reference in comparison, in addition to 40 selected main flow routes within the catchment. The results show improved performance of overland flow modeling due to increased data density. In addition, commonly used triangulated irregular networks were clearly outperformed by universal kriging and inverse-distance weighting in DEM interpolation. However, the TWI proved to be more sensitive to pulse density than an interpolation method. Improved overland flow routing contributes to enhanced forest resource planning at detailed spatial scales.

• Niemi, Department of Forest Sciences, University of Helsinki, FI-00014 University of Helsinki, Finland E-mail: mikko.t.niemi@helsinki.fi