Bogs and Drainage

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Bogs and Drainage

Peat bogs in Ireland may be divided broadly into raised and blanket bogs. Raised bogs, characteristic of central Ireland (Midlands), are so described because of their domed shape and because they hold water above the water table of their surroundings. Many are formed over basins, often in underlying glacial clays, in which water accumulated. Gradually reeds and other fenland plants colonized, and as they died, their remains did not decay fully in the waterlogged anaerobic conditions; fen peat began to form. As layers of fen peat built up, the surface gradually grew above the level of the surrounding land and of the surface runoff. Plants became reliant on rainfall for water and nutrients, and because the nutrient concentration of rainfall is low, there was a change to species tolerant of low-nutrient conditions. Fenland plants gave way to those of acid bog conditions, particularly the bog mosses (Sphagnum species). Continued upward growth of the bog led to the characteristic convex or domed profile.

A pristine raised bog has several distinctive parts. The central area or dome is flat or very gently sloping; it is often extremely wet and may have a micro-topography of pools and hummocks. Pools are sometimes occupied by aquatic Sphagnum species but may be open water. The hummocks have Sphagnum species requiring drier conditions, deer sedge (Trichophorum cespitosum), cotton sedge (Eriophorum vaginatum), and heathers (Calluna vulgaris and Erica tetralix). On the more steeply sloping bog edge (rand), the water table is slightly deeper, there is some water flow through the upper peat, and there is a better supply of nutrients; bog myrtle (Myrica gale) and common heather (Calluna vulgaris) are frequent. The bog may be surrounded by a lagg, an area of mobile water, sometimes with a small stream, and with large tussocks of purple moor grass (Molinia caerulea). Few raised bogs in Ireland are pristine; most have been subject to domestic peat cutting by spade for fuel, and because this extended inward from the bog edges, laggs and rands are rare. Climatic conditions varied throughout the development of raised bogs, some of which began to form 7,000 or even 9,000 years ago, and many display distinct horizons in the peat. For example, wetter, cooler conditions after 500 b.c.e. increased Sphagnum growth, and many bogs have an upper layer of poorly humified, reddish peat, whereas below, the peat is more humified and blacker.

The origin of raised bogs is generally natural, but blanket bog development, although regionally complex, came about probably through a combination of deteriorating climate and clearance of woodland by early farmers around 3,000 to 4,000 years ago. With increased percolation, plant nutrients and finer soil particles were washed down the soil profile, leaving acidic upper horizons. Plants adapted to these conditions colonized (heathers, sedges, mosses), and their remains began to accumulate in the cool, wet environment where biological breakdown was slow. As bog mosses invaded, the organic soils became waterlogged, any remaining trees died, and peat formed. Tree stumps and even Neolithic field systems, as at Céide fields, Co. Mayo, in the west of Ireland, may be seen beneath blanket peat—often exposed by cutting.

Hand cutting of fuel peat has little effect in any one year because the peat face extends into the bog by about one-half to one meter per year, but over centuries, especially since the seventeenth century when population increase was rapid, the impact has been extensive. By the late twentieth century hand cutting had declined considerably; electricity reached almost everyone and oil was readily available. With rural to urban movements, the people required to dig, stack, turn, and transport turf to homesteads were no longer present. Where peat (turf) is still used for fuel, since the 1980s it has often been obtained by compact harvesters attached to a farm tractor; a year's supply can be cut in a few hours. Particularly after Bord na Móna was established in 1946 as a statutory body to develop Ireland's peat resources, the large raised bogs, especially those in the Midlands, became the focus for extensive peat extraction. Bogs were drained, surface vegetation was removed, and the peat was milled. Once dried, the milled peat was collected and burnt in peat-fired power stations. Some peat is still used in this way, but many bogs are reaching exhaustion and public attitudes have changed; there has been increasing recognition of the international value of bogs. Ireland's bogs are examples of ecosystems relatively rare in Europe, and through their plant, pollen, and other microfossil remains they enable the vegetational history to be explored. They are also important carbon sinks. Governments in the Republic and Northern Ireland have designated conservation sites, but outside of those, human impact continues. Forestry expansion has been largely on western peat-lands, and the increased sheep population, following European agricultural policies, has resulted in overgrazing and erosion.