Raised bog

Raised bogs, 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 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. They also differ in character from blanket bogs which are much thinner and occur in wetter, cloudier climatic zones. Raised bogs, 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 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. They also differ in character from blanket bogs which are much thinner and occur in wetter, cloudier climatic zones. 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 Siberia 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, that form a more or less dome shape in the landscape. In Germany, the term Hochmoor ('high bog'), strictly refers only to the classical, lens-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-fed bog'. Thus the latter refers to all bogs, not just those that are arched or only slightly arched, but which nevertheless are characterized by an extreme mineral salt deficiency and other resulting ecological properties. 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 lake mires or '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, fens emerged under the influence of groundwater (minerotrophy). 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 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. 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). 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 bogs and fens. (See bog.)