Raised Bogs

Raised bogs are discreet, raised, dome-shaped masses of peat occupying former lakes or shallow depressions in the landscape. They occur throughout the midlands of Ireland. Their principal supply of water and nutrients is from rainfall and the substrate is acid peat soil, which can be up to 12m deep. Raised bogs are characterised by low-growing, open vegetation dominated by mosses, sedges and heathers, all of which are adapted to waterlogged, acidic and exposed conditions.

Distribution of Raised Bogs in Ireland

Raised bogs are a distinctive and characteristic feature of the landscape of the midlands of Ireland in which they are concentrated. They also occur in the Bann River Valley in Northern Ireland, in the vicinity of Omagh, Co. Tyrone, and in East Clare and North Limerick on either side of the mouth of the River Shannon. They occur on land below 130m and in that climatic zone where rainfall is between 800 and 900mm per year.

Formation of Raised Bogs

Raised bogs began to develop 10,000 years ago in depressions occupied by shallow lakes in which anaerobic conditions occurred. Complete decomposition of plant material is prevented. In time this un-decomposed plant material forms a thick layer of peat that rises towards the surface of the lake. Eventually the surface peat is invaded by sedges to form a fen. The fen peat layer thickens so that the roots of plants growing on the surface are no longer in contact with the calcium-rich groundwater. The only source of minerals for plants is now rainwater, which is very poor in minerals. Raised bog species, such as Sphagnum mosses begin to invade and eventually the fen becomes a raised bog

Raised Bog Structure and Hydrology

The surface of a raised bog consists of a soft living carpet of vegetation, which floats on a material, which is nearly all water. By weight, a raised bog may be up to 98% water and only 2% solid matter. This volume of water is held within dead Sphagnum moss fragments. Raised bogs consist of two hydrological layers (see figure 2.3); the upper, very thin layer, known as the acrotelm, is usually less than 50cm deep, and consists of living stems of Sphagnum mosses, recently dead plant material and water. The acrotelm has high permeability to water near to the surface, but becomes more impermeable with depth as the peat becomes more consolidated and decomposed (humified). Water movement and fluctuations mean that conditions in the acrotelm remain largely aerobic and it is here that microbial activity is strongest. These properties mean that the acrotelm is critical to the normal development and functioning of a raised bog. Below this is a very much thicker bulk of peat, known as the catotelm, where individual plant stems have collapsed under the weight of mosses above them to produce an amorphous, chocolate-coloured mass of Sphagnum fragments. By contrast with the acrotelm, catotelm peat is typically well consolidated and often strongly humified. It is permanently saturated with water. Water movement through this amorphous peat is very slow, typically less than a meter a day. This is where most of the rain water is stored. In a raised bog, which is ombrotrophic, the only source of water to the surface is from precipitation. Under normal circumstances the water table is very stable, remaining within a few centimetres of the bog’s surface about 95% of the time. Because the surface of a bog typically consists of low hummocks, hollows and pools, this stable water table produces intense competition for living space between species.

Importance of Raised Bogs

Raised bogs are beautiful landscapes with a unique biodiversity. They provide mankind with services that are worth billions of euro that may be easily jeopardised by inappropriate or short-sighted exploitation. Raised bogs are:

  • the finest example of their type in Europe, and probably the world
  • a unique repository of information of past climates, vegetation and human activity
  • a valuable genetic resource of potential use to humanity
  • valuable outdoor laboratories in which plants, animals and natural processes in an extremely inhospitable environment can be studied
  • of national and international importance as part of the biosphere in which they are inextricably linked to to other ecosystems
  • a unique feature of the Irish landscape of considerable tourist value
  • a priority habitat under the EU Habitats Directive because of their scarcity in Europe
  • an important store of carbon, helping to control greenhouse gases
  • an important store of water within river catchments

Conservation Status of Irish Raised Bogs

Under the EU Habitats Directive the National Parks and Wildlife Service (NPWS) are obliged to complete a report every 7 years on the condition of designated habitats. The assessments carried out in 2006 found that 10 years after the introduction of the habitats directive, no peatland type of priority importance in Ireland is in good conservation status. Raised bogs have been given a BAD status in this report because of the significant decrease in their range, habitat structure, habitat function and area.

Extent and Utilisation of Raised Bog

The original extent of raised bog in the Republic of Ireland was 308,742ha according to the Peatland Map of Ireland drawn by Hammond in 1979. IPCC monitors the status of the resource on an on going basis. Our latest assessment was carried out in 2009 and published in Ireland’s Peatland Conservation Action Plan 2020. Developmental pressures on raised bogs are intense, particularly extraction for fuel and horticulture mainly due to the development of new markets for these products and the establishment of numerous peat producing businesses. The most serious impact of mechanised peat extraction in Ireland has been on the Midland raised bogs accounting for a loss of 24% of the resource in less than 50 years. Hand peat cutting accounts for a staggering 64% loss and afforestation accounts for 2% of the loss of habitat in the Republic of Ireland. This leaves 10% of the raised bogs remaining which are deemed suitable for conservation.

Text, Photographs and Images © Irish Peatland Conservation Council, Bog of Allen Nature Centre, Lullymore, Rathangan, Co. Kildare.  Email: bogs@ipcc.ie; Tel: +353-45-860133.

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