Raised bog

Ewiges Meer Nature Reserve, raised bog element of the remains of a bog in East Frisia

Raised bogs (German: Regenmoore or Hochmoore) also called ombrotrophic bogs (ombrotrophe Moore) are acidic, wet habitats that are poor in mineral salts and are home to flora and fauna that can cope with such extreme conditions. Raised bogs, unlike lowland bogs are exclusively fed by precipitation (ombrotrophy) and from minerals 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.

Raised bogs are very threatened by peat cutting and pollution by mineral salts from the surrounding land (due to agriculture and industry). There are hardly any raised bogs today that are still living and growing. The last great raised bog regions are found in western Sibiria and Canada.


The term raised bog derives from the fact that this type of bog rises in height over time as a result of peat formation. They are like sponges of peat moss, full of water, than form a more or less dome shape in the landscape. In Germany, the term Hochmoor ("high bog"), strictly refers only to the classical clock-face-shaped bogs of northwest Germany. The bogs are not influenced by mineral-rich groundwater or surface water, but are fed exclusively by precipitation - mainly rainwater, hence their alternative German designation of Regenmoor or "rain bog". Thus the latter refers to all bogs that are not arched or only slightly arched, but which nevertheless are characterized by an extreme mineral salt deficiency and other resulting ecological properties.

Formation and development

Layering of a raised bog: plant remains, white peat and black peat (from the top)

A living raised bog needs a moist, balanced climate in which to grow. The quantity of precipitation has to be greater than the water losses through discharge and evaporation. In addition, the precipitation must be evenly spread through the year.

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. As far as their origins are concerned, a distinction is made between 'siltation-formed raised bogs' (Verlandungshochmoore) and 'mire-formed raised bogs' (wurzelechte Hochmoore). The former emerged in a secondary process after the silting up of lakes or oxbows (see illustration on the right in the sequence). At first, lowland bogs or fens emerged under the influence of groundwater (mineral soil water). Oxygen deficiencies and high acidity in the constantly moist substrate inhibited the decomposition of dead plant parts and led to [peat] formation.

Thus the raised bog rises very slowly above the level of the groundwater level, hence its name. As the resulting peat slowly rises above the influence of mineral salts in the groundwater, it reaches a point where the development of the raised bog begins to change in nature; that is, the bog now becomes fed solely by rainwater, which is low in salt water. By contrast, mire-formed raised bogs, are created directly on the mineral substrate of in low-salt area without having been initially formed as lowland bogs (see figure on the left in the sequence). They are formed either as a primary bog due to the erosion of previously dry mineral soils, for example due to clearing, climate change or infiltration, or as a secondary process as a result of the growth of a raised bog on neighbouring mineral soil. 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. Furthermore, there are a number of transitional and intermediate bogs, which in different ways combine characteristics of both raised and lowland bogs. (see bog).

The main constituent of the peat are the rootless peat mosses, that only slowly grow in height, whilst at the same time the lower layer becomes peat as the air is excluded. Depending on the geographical location, various species of peat moss are involved in making the raised bogs. The growth rate of the peat layer is only about a millimetre per year.

Structure of a growing bog

Growing bogs can be divided into two layers. The 'acrotelm' (Greek: akros = highest; telma = bog) is the upper part and includes the vegetation layer and the bog 'floor'. Here fresh organic substances (peat formation horizon) are created by the growth and dying of plant elements. The "catotelm" (Greek: kato = below) is the underlying water-saturated part with less biological activity. 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). In raised bogs, the upper peat layer is called white peat, since it consists of largely undecomposed light brown peat mosses. The lower layer is black peat, which is already well humified and has a black-brown colour with still recognizable plant remains.

Raised bog types and distribution

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 addition, the relief of the terrain has an influence on the water discharge behaviour and thus the shape of a raised bog. This results in geographical limitations to the formation of raised bogs. Favourable conditions for the development of raised bogs are found mainly in North America (Canada and Alaska), Northern Europe and Western Siberia, South America, Southeast Asia and in the Amazon Basin. 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. In the southern hemisphere low-mineral-rich bogs are rarely formed from peat mosses. Only in the Tierra del Fuego do peat moss raised bogs exist. The most peaty countries in the tropics are found in Southeast Asia. In many cases it is not yet clear how these bogs have emerged as mosses are entirely absent here.

Blanket bogs

Blanket bog in Connemara, Irland
Main article: Blanket bog

In areas with a strong maritime character, so-called blanket bogs are formed in conditions of high, very evenly-distributed rainfall (on more than 235 days a year). In Europe, these mostly very thin peat layers without significant surface structures are distributed over the hills and valleys of Ireland, Scotland, England and Norway. In North America, blanket bogs occur predominantly in Canada east of Hudson Bay. These bogs are often still under the influence of mineral water (groundwater). Blanket bogs do not occur north of the 65th latitude.

Planregen bogs

Due to their proximity to the sea, Planregen bogs (Planregenmoore) are also referred to as coastal bogs or Atlantic bogs. In the regions covered by blanket bogs there are also lightly convex planregen bogs with low energy surface relief in level locations. The distribution of planregen bogs in Europe extends from Ireland to the east via South Norway to Southwest Sweden and north to the Lofoten. In North America there are planregen bogs in the area of the Great Lakes (especially in Minnesota and Ontario). Planregen bogs are also fed exclusively by the rain.

See also



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