Raised bog
Raised bogs, unlike fens, are exclusively fed by precipitation (ombrotrophy) and from mineral salts introduced from the air.
They thus represent a special type of bog, hydrologically, ecologically and in terms of their development history, in which the growth of peat mosses over centuries or millennia plays a decisive role.
[1] Raised bogs are very threatened by peat cutting and pollution by mineral salts from the surrounding land (due to agriculture and industry).
Raised bogs in Europe have been developing for about 11,000 years, since the beginning of the Holocene and after the retreat of the last ice sheet.
Oxygen deficiencies and high acidity in the constantly moist substrate inhibited the decomposition of dead plant parts and led to peat formation.
By contrast, mire-formed raised bogs are created directly on the mineral substrate of low-salt areas without having been initially formed as fens (see figure on the left in the sequence).
The formation of a typical raised bog is a very slow process, which lasts from centuries to a thousand years even in favourable, undisturbed conditions.
This layer is counted as a geological subsoil due to the small earth-forming processes that are still going on and is known as the peat preservation horizon (Torferhaltungshorizont).
The formation of raised bogs is dependent on the climate, that is to say the amount of precipitation and rate of evaporation, which in turn are decisively determined by the temperature.
In these regions, bogs of all kinds and peat deposits of four million square kilometres have been formed, covering three percent of the Earth's surface.
The distribution of coastal bogs in Europe extends from Ireland to the east via South Norway to Southwest Sweden and north to the Lofoten.
Raised bogs also occur in precipitation-rich upland regions at the montane and, more rarely, alpine levels (i.e. above the tree line).
In northern Russia and western Siberia, kermis frequently occur in giant complexes where the bogs have grown into one another.
On level ground these islands are irregularly distributed; on hillsides they form ridges parallel to the contours and at right angles to line of slope.
The main distribution area for string bogs are the Scandinavian hills, central Finland, Karelia and north Siberia.
Like string bogs, the so-called palsas frequently lie within peatlands fed by mineral soil water.
Polygonal bogs (Polygonmoore) are widespread on the Arctic and sub-Arctic plains of Siberia and North America and cover vast areas.
A scanty layer of peat-forming vegetation can occur in the inner honeycomb-shaped areas of this frost pattern terrain (cryoturbation) and are fed during the short summers with sufficient moisture, because the meltwater is prevented from draining away by the raised polygonal margins.
As in North America there is a succession of raised bog types along the line of descent towards the ocean, from northwest to southeast.
The largest contiguous raised bog in central Europe was the Bourtange Moor, which originally covered an area of about 2,300 km2 including the Dutch portion, but only small sections remain.
The raised bogs of the Central Uplands of the Harz, Solling, Thuringian Forest (Großer Beerberg, Schneekopf - Teufelsbad, Fichtenkopf, Saukopf), Giant Mountains, Ore Mountains, Fichtel and Rhön (Black Moor, Red Moor) are, by contrast, comparatively small.
The Wurzacher Ried (Haidgauer Regenmoorschild) is considered the largest and best preserved raised bog in central Europe.
These habitats are considered to be among the best examples in Ireland due to their relatively large size and the generally low levels of disturbance.
The combination of raised bog, oligotrophic lake and turlough habitats is unique in Ireland and thus the entire system is very important from both a hydrological and ecological perspective.
