peatlands and bogs Peatlands are biological landforms and can cover large areas, completely changing the pre-existing ecology and hydrology. They are the only plant community to lay down
in situ a detailed record of their own history in the form of the partially decomposed remains of the plants themselves. Included within these plant remains are stratified records of pollen of their own and surrounding communities, of some invertebrates, of windblown dust, charcoal, and magnetic particles, of volcanic ash, and of past hydrological conditions on the bog surface. These records considerably enhance the scientific and conservation value of peatlands, for it is only by understanding the recent past that we can evaluate fully the present environment.
Of the types of peatlands that form this ‘living history book’, the
ombrotrophic or ‘rain-fed’
raised bogs are most distinctive. By the accumulation of peat over the millennia they become raised above the mineral groundwater limit and assume the form of a giant upturned soup plate with a flat top and quite steeply sloping sides. They are distinguished from the other main type of peatland, the
minerotrophic or ‘rock-fed’, which occur in valleys and basins or on river flood plains and are nourished by inflowing streams as well as by precipitation.
Raised bogs are dominated, in their natural state, by species of the genus Sphagnum, the bog moss. These small plants grow from their tips at a rate of about 2–3 cm in a season. The lower parts of the plants (which, being mosses, have no roots) are shaded because they grow close together, and are soon submerged in the bog water-table, which is usually only a few centimetres below the surface. Here the lower part of what is potentially a long-lived individual dies, but the plant remains are preserved because, in contrast to virtually all other ecosystems, decomposition is extremely slow in the waterlogged, acid, and anoxic conditions. Peat accumulation consequently takes place at a rate of about 100 cm in 1000–2000 years.
By this simple process, involving a small and insignificant plant when viewed in isolation, but a powerful one when growing packed together on a bog surface with sedges and heathers, whole peat landscapes developed during the Holocene, the past 10 000 years. By the infilling of ponds and lakes, and the colonization of ill-drained surfaces such as estuarine flats, raised bogs developed by about 7500 years ago. During succeeding millennia they grew larger through the process of
paludification, involving waterlogging of the surrounding area. In low-lying maritime areas, such as the German–Dutch border zone or the central plain of Ireland, vast bog complexes some tens of kilometres across were at their zenith by 2000 BP. Individual raised bogs, such as the three bogs known as Cors Caron, near Tregaron in Wales, which expanded to almost fill the valley of the River Teifl, are impressive landscape features and of international importance to nature conservation.
Blanket bogs also cover huge areas in cold, wet temperate zones such as northern Scotland, coastal areas of Norway, and the tip of South America. They are a mixture of ombrotrophic and minerotrophic bogs, and while they may contain much bog moss they also may be dominated by sedges. Minerotrophic peatlands, such as the valley bogs of the New Forest in southern England, are generally much smaller, being a few tens of metres across but some hundreds of metres long. They commonly have wet-loving trees such as alder and willow growing on the peat surface.
Although peatlands have long been valued for nature conservation on account of their specialized plant and animal life and the birds that are attracted to areas such as the Flow Country of northern Scotland, their value as a scientific archive is immense. In studies of environmental change and palaeoecology the advantages of raised bogs as proxy-data sources are manifold. Since raised bogs derive all their moisture from the atmosphere, there is a relatively direct relationship between the mean water-table of the bog and precipitation. There is similarly a relatively direct relationship between the occurrence and zonation of species on the hummocks and hollows of the bog surface and the mean water-table. By reconstructing the peat-forming vegetation from the plant remains we can therefore reconstruct the changing bog water-table through time, and this is a proxy for past climate. A number of records, particularly from northern Europe, have shown consistent trends with deterioration to wetter, cooler climates during the past 5000 years compared to the warmer climate of the previous 5000 years.
Other results from the proxy-records contained in the peat include the use of pollen analysis, both for the investigation of long-term natural changes, such as the migration of European forest vegetation after the end of the last glacial stage, and for tracing human impact on the wider landscape through forest clearance and the growth of agriculture in prehistory and more recent times. The increase in air pollution consequent upon industrialization has been traced using sensitive techniques of low-level measurement of the concentrations of magnetic minerals which have been trapped during the past few centuries of peat accumulation. The detection and geochemical identification of microscopic shards of volcanic ash (tephra) in peat is a recent development. Tephra from Icelandic eruptions of known historical dates, and from prehistory, have been found in Irish and Scottish bogs, and can be used as synchronous time markers in the peat, linking the other proxy-records precisely in time. Data such as these, and the remarkable preservation of prehistoric human bodies in peat, show what potential there is for unravelling the past in the peat.
Keith E. Barber
Bibliography
Biodiversity and conservation, Vol. 2, No. 5, 1993; special issue, Peatlands and people. Chapman and Hall, London.
Chambers, F. M. (ed.) (1993) Climate change and human impact on the landscape. Chapman and Hall, London.
Gore, A. J. P. (ed.) (1983) Ecosystems of the world 4A. Mires: swamp, bog, fen and moor. Elsevier, Amsterdam.
