Effect of forest drainage on the peat bulk density of pine mires in Finland

Drainage of peatlands for forestry causes the water level to draw down, which results in subsidence of peat surface and increased peat bulk density (D b ). Later on, an accelerated rate of organic matter decomposition and the pressure of the growing tree stand further compacts the peat. We measured the peat D b (0-80 cm) in 180 undrained and 209 drained (ca. 60-year-old) peatland sites representing three nutrient levels of pine fens and five macroclimatic regions from southern to northern Finland. D b was affected by climatic conditions, nutrient status, and the tree stand volume of the site. D b was significantly higher in drained than in undrained sites in all regions and site types: the mean (±SD) D b in the whole material was 82 ± 23 kg ·m -3 for the undrained sites and 133 ± 22 kg ·m -3 for the drained sites. The increase in D b was significant in all regions down to a depth of 60 cm and in southern Finland, even deeper than 80 cm. When the increase in carbon concentration was taken into consideration, the average postdrainage increase in the D b of the 0-80 cm layer equalled the amount of carbon in the 54-cm layer in average undrained peat. As the reported subsidences of peat surface in forest drainage areas in Finland are usually below this limit, the carbon storage of peat is likely to increase after drainage.