& Other Habitats
Peatlands make up 21% of Co. Offaly, 14% of north Tipperary and 12% of Co. Laois.
Bogs are particular types of wetlands which are waterlogged only by direct rainfall. Hence the water entering a bog contains only those nutrients found in rainfall, which is slightly acidic and almost without any nutrients. In fens however, it is groundwater, enriched by the chemistry of mineral soils, which causes waterlogging.
It is this waterlogging that creates the oxygen deficient conditions which make peat by preventing the complete decomposition of dead plant material, and this partially decomposed plant material that steadily accumulates as a thickness of peat. For bogs, this takes place under conditions of high acidity and nutrient deficiency. It is the presence of peat which is the defining feature of a peatland.
Active growing bogs have two layers:
- the acrotelm – a thin top layer of peat-forming Sphagnum mosses and other plants about 10 to 40 cm deep which form the living surface of the bog and
- the catotelm – a relatively inert, permanently waterlogged and compacted peat store beneath.
An important distinction therefore exists between a primary bog surface, where the surface and peat beneath have been created by natural peat accumulation, and a secondary bog surface, where peat has been removed by human action to create an artificial morphology.
Primary bogs include all peatlands where the vegetation has been left intact, however degraded it may be by drainage or fire.
Where a primary surface is retained, the overall shape of the bog together with its entire peat archive remains largely intact. This makes restoration of theses areas easier to achieve.
In creating a secondary bog surface the shape of the bog becomes markedly artificial and part of the archive is removed. Such secondary surfaces are generally created by agricultural land reclamation, peat cutting or open-cast peat mining.
Perhaps surprisingly, drainage and even forestry may still retain a primary surface even though desiccation and subsidence may result in significant changes to the morphology of the bog.
Consequently restoration of a stable bog hydrology after drainage or forestry may be easier and (ultimately) more complete than is the case for the complex morphologies and truncated peat archives of secondary surfaces.
Bogs & Fens
Peatlands, or to use the international term of mires, can be divided into two categories, namely:
- Bogs which are ombrotrophic or which receive their water and nutrition from rainwater, and
- Fens which are minerotrophic which are under the influence of nutrient-rich groundwater.
Nutrient-rich fen communities are typically dominated by lusher vegetation than that found on bogs, typically reeds, cattails and larger sedges.
Bogs (after bogach, the Irish word for a mire) can be further divided into raised bogs or blanket bogs, which can be further subdivided into lowland or the mountain blanket bogs which typify the Slieve Bloom Mountains.
The accumulated peat laid down in a healthy peatland is a particularly unusual and important feature and allows us to examine the entire history of the ecosystem’s development in the form of the plant remains laid down at each stage. Most significantly, the peat archive holds enormous quantities of carbon gathered from the atmosphere by living plants in the surface layer (acrotelm) as they photosynthesise and grow. When these plants die their semi-decayed remains are locked away in the peat under anaerobic waterlogged conditions, limiting further decay and loss of carbon.
As a result, once stored in the waterlogged catotelm zone as peat, the carbon is locked up for millennia. This equates to about 1000 tonnes per hectare for every 2-2.5 metres of peat sequestered.
Peatlands: An Archive
The peat also stores a historically record of the surrounding landscape in the form of pollen grains blown onto and preserved in the peat. Using a combination of plant remains (macrofossils) and pollen (microfossils) it is possible to reconstruct pictures of past landscapes and climatic periods which in the case of Irish peatlands stretches as far back as 10,000 years ago.