Raised bog as a habitat
Moors are rugged, austere and inaccessible landscapes – neither water nor land. Low bogs are rich in nutrients and form on retentive subsoils, through the silting-up of stagnant waters, as well as on flood plains of water courses. Raised bogs, by comparison, form above the groundwater table and are fed merely by nutrient-poor rainwater.
What is a raised bog?
Raised bogs are, by nature, mostly treeless flat plateaus whose terrain comprises small areas of raised hummocks and lower-lying bog hollows. In individual cases they also feature natural small ponds or pools at their centre. At their periphery and near to the natural water areas there is undergrowth and vegetation, such as purple moor grass (Molinia caerulea), indicative of nutrients and periodically wet conditions.
From a geological-pedological point of view, a bog has a peat layer at least 30 cm in thickness and a 30 percent organic substance content by weight in the dry state. The geobotanical definition of a moor, on the other hand, focuses more on the presence of typical marshland vegetation.
Intact raised bogs consist 90 percent of water that is oxygen-deficient and extremely acidic. They depend exclusively on rainwater and the nutrients contained within it and are therefore also referred to as ombrotrophic bogs or mires.
Raised bogs require a continuous water surplus for their growth. The amount of precipitation must exceed the loss of water resulting from runoff and evaporation. Species of peat moss (Sphagnum) without any roots are able to absorb the few available nutrients through their leaves in order to grow. In their lower region they die off due to increasing exclusion of light. Because of the anaerobic conditions, the dead parts of the moss plants are not broken down, thus binding the carbon and forming peat. This peat formation amounts to only about 1 mm on an annual average and in the long term produces the hour-glass shaped curvature that raised bogs typically have.
A raised bog is made up of various layers. The older and more subjacent a layer, the greater its degree of decomposition. The peat layers are differentiated into older, strongly decomposed black or sapric peat and younger and less decomposed white or fibric peat.
Genesis of a raised bog
The formation of a typical raised bog is a very protracted process that goes on for centuries, if not millennia. Raised bogs in the North German Plain began to form about 3,000 years ago, helped by the Ice Ages. The glacial moulding of the earth’s surface led to the formation of shallow lakes and depressions in impermeable soils. There, mud and not entirely decomposed plant remnants (so-called Mudden [mud / peat deposits]) were able to accumulate. This silting-up process during the Neolithic Age helped pine, alder and birch carrs to form on the swampy subsoil. The plant remnants left over in the process, such as cones, seeds, leaves and bits of wood, form carr or brushwood peat and continue to exist at raised bog sites to this day.
The Atlantic climate with its equable temperatures, high atmospheric humidity and substantial precipitation created favourable conditions in the carrs for the growth of peat moss. The peat moss was able to spread increasingly and overgrow the carrs. The bodies of marshland thus created were fed mainly by rainwater. Their growth led to the typical treeless marshland areas made up of small water-filled depressions, referred to as bog hollows, alternating with smaller, drier knolls or so-called hummocks.
Significance and functions
Bogs are remnants of a primeval landscape and, being a near-natural type of landscape, are unique in today’s densely populated cultural landscape. Raised bogs lie like sponges in an elevated position in the landscape and offer an impressive show of nature all year round.
Raised bogs are irreplaceable as wetland habitats and sanctuaries for many species of fauna and flora. Certain groups of fauna species such as snails, mussels or fish, on the other hand, are totally absent. The wet, nutrient-poor and acidic environment has enabled a biocenosis of closely interdependent flora and fauna communities to develop. These highly specialised communities include numerous, particularly endangered species worthy of protection. In some cases, the name of these species already indicates their close association with the marshland habitat, such as bog asphodel (Narthecium ossifragum), moor frog (Rana arvalis) or cranberry blue (Plebejus optilete).
The dietary habits of the larva of the cranberry fritillary (Boloria aquilonaris) underline this dependence, it being able, as a species characteristic of ombrotrophic mires, to develop only on the fenberry (Vaccinium oxycoccus).
The sundew, a representative flora species, has developed a sophisticated technique as a carnivorous plant. With its sticky leaves it traps insects and consumes them to obtain a sufficient amount of nutrients. The subarctic darner (Aeshna subarctica), a dragonfly, lays its eggs in floating peat moss and in other parts of plants living in bog hollows and water holes. The acidic nutrient-poor water contains only a limited food supply (mites, fleas) for the larvae, meaning that it can take two to three years for them to develop into fully fledged dragonflies.
Raised bogs usually have a paucity of mammals. But, unlike the extensively utilised cultural landscape, they do have a substitute function, also offering a sanctuary for species that are not necessarily dependent on a raised bog environment. Ground-breeding birds, such as the highly endangered common snipe (Gallinago gallinago), a wet meadow species, also find good conditions there.
Near-natural bogs stabilise the landscape’s hydrologic balance. In conditions of extreme rainfall they have a regulatory effect on water runoff, thus having an influence, in regard to water protection, on high water and erosion. They also serve as a buffer and filter by retaining substances dissolved in the water.
Intact bogs store carbon, as they absorb carbon dioxide (CO2) during growth and bind it in the peat over the long term. They are natural climate protectors, acting as ‘sinks’ for climate-relevant gas.
Because they began to develop thousands of years ago, and on account of the preservative conditions and the resistance of their layers, Bogs serve as archives of natural and cultural history. Bog vegetation developments (retraceable by means of pollen diagrams) and embedded animal and plant remains make it possible to reconstruct our environment far back into the past.
History of utilisation
Original raised bogs were long deemed by people to be wasteland and hostile and dangerous environments. The marshland areas were drained extensively so as to develop and utilise them for agricultural purposes, for fuel extraction (sapric peat), or as a source of substrates for horticulture (e.g. potting soil comprising fibric peat). Water drainage causes peat moss to stop growing, leaving behind huge peat deposits that would be extracted manually by farmers and also, in some areas, on a large-scale industrial basis.
To cultivate for agricultural use (farming and grassland), drainage and ploughing was carried out and a subjacent layer of sand, manure and lime added.
Nearly 95 percent of all German bogs are considered to be degraded. Bog cultivation was practiced into the 1970s, causing the ones in Lower Saxony to almost disappear entirely. Today, raised bogs and low bogsaccount for approximately 9 percent of the land area there. Almost 70 percent of Germany’s raised bogs are, moreover, located in Lower Saxony.
Raised bogs react sensitively to change. Drainage and nutrient enrichment, in particular, have far-reaching consequences, causing the bogs to lose their above-outlined ecosystem functions where nature conservation, biodiversity, hydrologic balance, water quality and climate protection are concerned.
What has come about over millennia and has, to a large part, been destroyed by man in a relatively short period, needs the right stimuli and a lot of time for regeneration. The aim of such regeneration is, therefore, to create natural conditions, including a hydrologic balance that is as intact as possible, so as to leave the further development to take care of itself again in the long term.
Nds. Landesbetrieb für Wasserwirtschaft, Küsten- und Naturschutz
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