MI1 Drainage status

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Development

Altogether 5.7 million hectares of Finland's mires have been drained as a result of forestry operations that aim at increasing tree growth in peatland forests. This equals 55% of the original mire area. Due to draining, nearly one million hectares of the original mire area has been so strongly affected that it is no longer considered mire in the latest National Forest Inventory (NFI). Mires with a thin peat layer have dried up and turned into mineral soils according NFI definitions.

According to NFI11 (2009–2013) there is 4.1 million hectares of pristine mires left in the whole country. There are notable regional differences in the extent of mire drainage. In Southern Finland drainage has been much more common than in the north. In the southern half of the country the present proportion of drained mires is approximately 80% whereas the same figure for the northern half is slightly over 40%

Impact on biodiversity

The draining of mires has had a very significant impact on mire habitats both quantitatively and qualitatively. Draining alters the hydrology of mires which, at its strongest, leads to the disappearance of mire vegetation and the cessation of peat accumulation. After drainage the original flora begins to be supplanted by forest species. The species number of drained areas is often at its peak a few years after draining when original mire plants still grow next to invasive forest species. In time the invasive forest species become more common at the expense of original mire species. The succession is fastest on wet and nutrient rich mires. Drainage increases the growth of woody plants which decreases the area of open habitats and increases shading in turn.

Most susceptible to the effects of draining are plants growing on wet surfaces such as the Bogbean (Menyanthes trifoliata) and some larger sedges (including Carex rostrata, C. lasiocarpa and C. chordorrhiza). Mire shrubs and other plants growing on tussocks often benefit from drainage first, but begin to decline as the changes in the hydrology advance. Almost all Shpagnum mosses (peat mosses) decline as a result of drainage, most notably the species growing in wet conditions. Most tussock species decline too, one of the few exception being Sphagnum magellanicum which seems to tolerate draining quite well. Several forest mosses such as Pleurozium schreberi become more common on drained areas.

As a result of the disappearance of open mire habitats the populations of bird species preferring these habitats decline. Examples include Golden Plover (Pluvialis apricaria), Whimbrell (Numenius phaeopus) and Ruff (Philomachus pugnax). The fall in the water level in mire ponds after drainage has also caused a decline in the number of breeding waterfowl, including the Red-throated Diver (Gavia stellata). Drainage and the resulting disappearance of open mire habitats also has a negative impact on the number and abundance of mire butterflies.

Species suffering from draining include also some mound ant species that specialize in mire habitats (e.g. Formica uralensis, F. fennica, F. forsslundi). In studies conducted in conjunction with NFI10 the occurence of the mire specialist species was found to correlate negatively with time passed since drainage. Mire species were found most often in undrained mires and much less often in recently drained and transforming habitats. Only one mire mound ant species was found on dry peat heaths. On the other hand, the Red Wood Ant - a species very common in heathland forests - became abundant in drained mires.