Peat Bog

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This is the peat bog we were allowed to walk on

What is Peat?       (Information from Irish Peatland Conservation Council)

Peat is a soil that is made up of the partially decomposed remains of dead plants which have accumulated on top of each other in waterlogged places for thousands of years. Areas where peat accumulates are called peatlands. Peat is brownish-black in color and in its natural state is composed of 90% water and 10% solid material. It consists of Sphagnum moss along with the roots, leaves, flowers and seeds of heathers, grasses and sedges. Occasionally the trunks and roots of trees such as Scots pine, oak, birch and yew are also present in the peat.

Peatland Types

There are two major peatland types found in Ireland; fens and bogs. The similarities and differences between these two types are summarized in the five points below.

  • Fens are alkaline with a pH of 7 to 8. Bogs are acid with a pH of 3.2 to 4.2.
  • Fens and bogs are waterlogged habitats.
  • Fens are minerotrophic which means that their water supply is from the mineral-rich ground water. Bogs are ombrotrophic which means that their water supply is from the mineral-poor rainwater.
  • Fen peat has a relatively high ash content, circa 10% or more. Bog peat has a low ash content, circa 3% or more.
  • The average peat depth in a fen is up to 2m. Peat depth in a bog varies from 2 to 12m.

Raised and Blanket Bogs

Bogs can be further divided into two different types; raised bogs and blanket bogs. Note that depending on the altitude, scientists can distinguish between Atlantic blanket bog (below 200m) and mountain blanket bog (above 200m).

Where are the Bogs?

Raised bogs occur in the midlands of Ireland and in the Bann River Valley where rainfall is between 800 and 900mm per year. Blanket bogs are found along the west coast of Ireland and in mountainous areas around the country where rainfall is 1,200mm per year or more.

Peat Formation

Peatlands are composed of deep layers of waterlogged peat ad a surface layer of living vegetation. Peat consists of the dead remains of plants (and to a lesser extent of animals) that have accumulated over thousands of years. Peat accumulates in areas where the rate of plant production exceeds the rate of plant decomposition. Complete plant decomposition is prevented in areas where waterlogging occurs. In Ireland, high rainfall and low temperatures result in low evaporation which means that waterlogged soils are a common feature, for example in shallow basins. These waterlogged soils are anaerobic or poor in oxygen and oxygen is essential for the growth of the soil micro-organisms (bacteria and fungi) that bring about the complete breakdown of plant material. As a result of the poor microbial activity dead plants accumulate in waterlogged areas as peat.

Another factor which contributes to the accumulation of peat by preventing the growth of soil micro-organisms is the acidity of the ground water. In bogs, the acid nature of the ground water is produced y plants known as bog mosses, or Sphagnum species. These plants absorb the cations (positive ions) in rainwater (for example calcium and magnesium) and release hydrogen ions into the water. The more acidic the soil water, the less suitable it becomes for micro-organisms to grow and the plant remains therefore accumulate at a faster rate.


Fen and Raised Bog History

Raised bog formation started at the end of the last glaciation - some 10,000 years ago - when the glaciers had retreated northward. At this time much of central Ireland was covered by shallow lakes left behind by the melting ice. Lakes also formed where glacial ridges, such as eskers, impeded free drainage and trapped the water.

At the base of these shallow lakes there were deposits of lake marl overlying clay and glacial drift. These lakes were fed by mineral-rich groundwater and springs and supported floating plant communities, which sometimes produced a thin peat layer just above the lake marl. The lake edges were dominated by tall reed and sedge beds. As these plants died, their remains fell into the water and were only partly decomposed. They collected as peat on the lake bed. With time this process formed a thick layer of reed peat that rose towards the water surface. As the peat surface approached the upper water level, sedges invaded, and their remains added to the accumulating fen peat.

In time the fen peat layer in these shallow lakes became so thick (up to 2m) that the roots of plants growing on the surface were no longer in contact with the calcium-rich groundwater. When this happened the only source of minerals for the plants came from rainwater, a very poor source of the essential minerals needed for plant growth. As a result plants invaded that were able to grow in the mineral-poor habitats on the surface of the peatland. The best indicator of the changing conditions was the invasion of the bog moss or Sphagnum. This moss became common in such transitional fen/bog habitats, and made the ground even more acid, by its ion exchange activity. This intermediate stage between fen and bog can be seen today at Scragh Bog, County Westmeath, where a mixture of lime-loving fen plants and acid Sphagnum cushions grow together. Plants typical of raised bogs, such as Heathers, Sundews and Deer Sedge invade the tops of the Sphagnum hummocks, completing the invasion of bog species.

The Bog Moss is important as it acts like a sponge or candle wick, drawing up water and keeping the surface of the bog wet and waterlogged, in all but the driest periods. So, even though the bog continued to grow upwards, away from the water table, the Bog Moss ensured that the water table rose in tandem with the rising peat level.

During the long history of bog growth, there have been occasional changes in the overall climate in Ireland. About 4,500 years ago the annual rainfall decreased. This caused bog surfaces to dry, and allows the invasion and establishment of a Pine woodland on the surface of the bog. This woodland persisted for some 500 years, until the climate changed again and became wetter. Rapid bog growth recommenced as the surface became waterlogged, and the trees died. Tree stumps and whole tree trunks were buried and preserved in the rapidly accumulating Sphagnum peat. The layers of fen and Sphagnum peat and the buried Pine stumps are often seen exposed by turf cutters at the margins of raised bogs.

Blanket Bog History

Blanket bog formation in the mountains and west of Ireland also started at the end of the last glaciation, 10,000 years ago. Initially peat formation was confined to shallow lakes and wet hollows and an infilling sequence from open water to fen and acid peat is recorded in these areas. Later, acid peat spread out to form a blanket covering huge areas. While some spread may have taken place as early as 7,000 years ago, many areas were not engulfed until 4,000 years ago when the climate became wetter. Heavy rainfall caused minerals such as iron to be washed out or leached from the surface layers of the soil. These were deposited lower down where they formed an impermeable layer known as an iron pan. Water cannot move down through such a layer and the soil surface became waterlogged as a result. Under these conditions the accumulation and spread of peat was made possible.

Today in the west of Ireland the blanket bog rests directly on the stumps of Pine trees what were once part of extensive woodlands in the area and also covers large areas of farmland that were cultivated by Neolithic (Stone Age) farmers. At the Céide Fields in North Mayo an extensive system of walls and fields has been unearthed dating to the Stone Age period of 5,000 years ago.

Bog Structure

Walking on a bog involves walking on a soft living carpet which floats on a material which is nearly all water. In fact bogs have less solids in them than milk. By weight, a raised bog may be up to 98% water and only 2% solid peat. Blanket bogs are rather more solid, with up to 85% water. This great volume of water is held within the dead Sphagnum fragments. This ability to retain water is one of the properties which makes Sphagnum peat such a prized horticultural material.

A bog consists of two layers: the upper, very thin layer, known as the acrotelm, is only some 30cm deep, and consists of upright stems of the Sphagnum mosses, largely still alive and colorful with their red, yellows and ochre. Water can move rapidly through this layer.

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-colored mass of Sphagnum fragments. Water movement through this amorphous peat is very slow indeed- typically less than a meter a day. This is where most of the rainwater is stored. From here the water slowly seeps down through the bog over several weeks or even months.

The catotelm resembles the lower layers of a tropical rainforest, or the abyssal depths of the oceans, environments which enjoy constant, unvarying conditions because they are protected from the turbulent environment above by a relatively thin canopy or surface layer.

Under normal circumstances, the water table within the bog never drops down into the catotelm. Even drainage merely empties the acrotelm of its water more quickly. Generally the water table is very stable, remaining within a few centimeters of the bog's surface about 95% of the time.

Because the surface of a bog typically consists of low hummocks and ridges, scattered hollows or pools, this stable water table produces intense competition for living space between species. And so, several zones of characteristic vegetation have evolved each depending on their proximity to the water table.

In increasingly wet climates, bogs have adapted interesting surface patterns of pools to hold the surplus water which cannot seep away before the next rainstorm. The pools are formed in the peat and do not go down to the underlying soil. They are unique to bogs and totally devoid of fish. However they are home to a wide variety of insects.

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