Undrained peatland areas disturbed by surrounding drainage: a large scale GIS analysis in Finland with a special focus on aapa mires

Half of the Finnish peatland area is drained for forestry. The remaining undrained peatlands are not all pristine, because surrounding drainage may disturb their hydrology. This concerns especially aapa mires, which are dependent on hydrological connections to their upper catchments. We investigated the amount and sizes of Finnish undrained peatlands, the drainage state of their margins, and the naturalness of aapa mires in large (> 50 ha) undrained peatland areas, using a GIS analysis based on digital map data, aerial images and an elevation model. The results show that a majority (66.7 % of count, 84.7 % of area) of undrained peatland areas have at least partly drained margins. Drainage activities commonly disturb minerotrophic water discharge to aapa mires. In the middle boreal zone, on average 41.6 % (median 42.8 %) of the catchment area of aapa mires is such that hydrological connection with the mire is disturbed by intervening drainage. In the southern boreal zone, the figure is 25.1 % (median 16.1 %), and in the southern part of the northern boreal zone 24.2 % (median 9.9 %). Possible implications of the disturbances include tree encroachment, hummock formation and fen–bog transition, which is likely to cause a loss of biodiversity but could potentially increase peat growth and carbon sequestration.