Mires are areas of permanently wet peat caused either by a water table very near the surface (‘siligenous’ mires) or high rainfall which saturates the peat even though it is above ground water level (‘ombrogenous’ mires).
Mires are interesting because the peat preserves its own history, which often extends as far back as the end of the last Ice Age. Corings usually reveal different stages, showing a transition from neutral or alkaline to increasingly acid conditions. Pollen in the peat shows the changing composition of wind-pollinated trees, shrubs and herbaceous plants which surround the mire; these in turn reflect changing climatic conditions over tens of thousands of years.
Basin mires develop from small, deep ponds which lack an outlet. Marginal vegetation is unable to trap sediments or enlarge the fringe of swamp and marsh. Rather, it forms a mat or skin over the water’s surface which eventually becomes so thick that plants are raised on a layer of soggy peat above the water, which becomes anaerobic and acid. Cut off from minerals and leached by the rain, acid-intolerant plants give way to bogmoss (Sphagnum spp) and associates. Where the moss forms tussocks, scrub and trees seed in but as they grow they either depress the mat and drown or topple into the depths, creating temporary open pools.
Britain’s best-developed basin mires are found in Cheshire and Staffordshire. The German term ‘Schwingmoor’ aptly describes the sensation one feels walking on their surface. It is possible, by setting up a rocking motion, to get trees all over a Schwingmoor swaying and dancing in response. It is also possible to fall through the mat. Schwingmoors are best left unvisited unless under expert guidance.
Valley mires develop where water flows through shallow valleys but where the drainage is somewhat impeded. The ground remains waterlogged and peat forms, but whether the vegetation becomes that of bog or fen depends on the mineral content of the ground water and, ultimately, the nature of the surrounding rock. Frequently, hummocks and other leached areas have bog plants while the banks of the drainage channels cutting the bog and the stream which usually flows through its centre have fen species.
Valley bogs develop even in areas of fairly low rainfall since they depend only on ground water. Some of the most interesting occur in southern England, notably the New Forest, where many plants and animals not found in northern areas survive due to the mild climate.
Raised mires develop from valley mires where the rainfall is over 1010mm (40”) per year. Here peat builds up until it is above the ground water table. Rain leaches out remaining minerals from the raised patches, which tend to spread and coalesce in a continuous circular or oval dome of Sphagnum peat up to 1.6km (one mile) across and 6-9m (20-30’) higher in the centre than at the edge, which is usually marked by a rather steep drop into a peripheral drainage channel or ‘lagg’. The dome’s spread halts only at the drier valley sides or at wide or swift streams. The only nutrients available are those in airborne dust, sea spray blown inland, rainwater and leachings from dead plants. Very few species tolerate these extremely impoverished conditions. Tregaron Bog in central Wales and the red-bogs of Ireland are the best British examples of raised mires.
Flood plain mires form where the upper soil is kept permanently wet due to frequent flooding or to the percolation of water through porous alluvial soils. Here swamp and marsh give way to bog or fen as peat accumulates, but the mire vegetation stays in contact with the ground water so its make-up is determined by local water chemistry.
Small flood plain mires occur along many streams and rivers but in the lowlands most have been destroyed by human interference.
Blanket mires develop only where the rainfall is at least 1395mm (55”) per year and the humidity is consistently high. Here, all flat ground and gentle slopes remain permanently waterlogged, peat builds up and large areas become uniformly blanketed, irrespective of local variations in ground water chemistry, although slopes steep enough for continued drainage remain free. The diagram shows four topographical varieties of blanket mire, all of which may occur within the same continuous tract.
Many examples of blanket mires occur in Ireland, the Scottish Highlands, the Welsh uplands, Dartmoor and the Pennines. Botanically, they can be divided into two provinces, north and south of the Scottish Highland line. These are subdivided by analysis of local topography, the wetness of the surface, the extent and degree of flushing, and the altitude. So, while blanket mires present some of the bleakest, most monotonous scenery imaginable to the untrained eye, the botanist finds a great deal of variety even here.
Mires are of two types, ‘bog’ and ‘fen’, depending on whether their vegetation is adapted to acid or alkaline conditions.
Bog vegetation
Acid mires or bogs develop initially due to one or more of these conditions: a water supply deficient in basic minerals, high rainfall leading to extreme leaching of soil minerals, or the raising of the mire above the water table, cutting off the supply of water-borne nutrients.
Very few plants tolerate these conditions, and those that can, notably Sphagnum spp, tend to decompose slowly and to release acids during decomposition. This causes even more effective leaching of minerals which, combined with the other conditions conducive to acidity, results in the extreme depletion of soil nutrients from the mire.
The typical Sphagnum bog is covered almost completely with bog-moss of several species, through which projects a stunted growth of various grassy and sedgy plants and heather (Calluna vulgaris), cross-leaved heath (Erica tetralix), and sweet gale. Bog asphodel (Narthecium ossifragum) is usually found. The Sphagnum carpet supports a specialised community of small plants, including many liverworts and insectivores such as the sundews (Drosera spp). The species composition and balance varies throughout Britain, depending primarily on rainfall and altitude. Certain very wet bogs form pools and hummocks in a continuing local cycle but, in general, bog communities are remarkably stable if undisturbed since little change can occur once the peat becomes leached to the limit possible in any particular climate.
When Sphagnum bog becomes degraded due to climatic effects or human interference, mixed moor results. This occurs at high altitudes or on slopes where the peat tends to break down and become dissected by drainage channels. Here the sheathed cottongrass (Eriophorum vaginatum) is almost always prominent, usually accompanied by heather. Although most bog species can be found locally, the Sphagnum carpet is absent and the balance of species is entirely different from that in the continuous-cover Sphagnum bog.
Elsewhere, man’s management is thought to have caused the changeover from Sphagnum bog to mixed moor. Drainage, burning and grazing dramatically affect bog vegetation, particularly when applied in combination. First, they disrupt the Sphagnum carpet, quickly eliminating many widespread Sphagnum species as well as the sundews, some of the sedges and liverworts and many rarer plants. After this, the smooth, mossy bog surface with its scattered projecting shoots is replaced by an irregular tussocky surface, commonly consisting of sheathed cottongrass and heather, although in western bogs deer-sedge and purple moorgrass may be locally abundant. Bog-mosses and other small species become increasingly rare. Finally, certain new mosses and liverworts appear which take advantage of decreased competition or which are associated with the rather higher salt content in the peat due to oxidation.
Fen vegetation
Fens are mires which lack extreme acidity due to the presence of alkaline ground water. Because of this, fen vegetation is typically more like that of marshes than of acid bogs, often containing a great many diverse communities wherever there are small variations in relative soil acidity or the height of the water table. Here rare plants or plants with restricted ranges flourish. Woodwalton Fen, in Cambridgeshire, supports over 400 species including the nationally rare fen violet (Viola stagnina), the woodrush (Luzula pallescens), and Deptford pink (Dianthus armeria).
Mixed fen is dominated by tussock-forming grasses such as purple quailreed (Calamagrostis canescens), reed canary- grass, purple moorgrass, reed sweet grass (Glyceria maxima), blunt-flowered rush (Juncus subnodulosus) and black bog rush (Schoenus nigricans). There is a considerable proportion of meadowsweet (Filipendula),yellow iris (Iris pseudacorus), water mint (Mentha aquatica) and reedmace (Typha spp). These plants are rather lower growing than reedswamp species, allowing the survival of a great many smaller herbaceous species. Mixed fen is favoured by grazing and mowing but, if left unmanaged, gradually succeeds to ‘mixed carr ’ which is relatively dry and contains a variety of shrubs and trees.
Reed fen develops from swamp which is managed to the advantage of common or Norfolk reed (Phragmites communis). Reedbeds may be created artificially by flooding, with subsequent control of water levels for reed, or they may be recovered from old beds which have become derelict. If the reeds are not harvested, they die and break down each year, forming a loose litter and eventually a thick organic ooze. Peat then builds up, unless silt is continually added through tidal action or currents. Normally, derelict reed fen succeeds to one or another type of sedge fen, where the reeds are not dense, or otherwise to mixed carr.
Great saw sedge (Cladium mariscus) develops where the water is stagnant but rich in calcium and poor in nitrogen and phosphorus. This is often the case in the interior of alkaline fens where nutrient-rich drainage water cannot penetrate from outside. Saw sedge fen has few associated species since it forms a dense surface mat of dead litter with evergreen leaves reaching through and above it. This type of fen resists tree invasion for a long time even if unmanaged. Eventually a thick, firm mat builds up and sweet gale may invade, followed by creeping willow (Salix repens) and other trees.
Fens were once widespread in the English lowlands, notably in the Fenland proper of Cambridgeshire and the surrounding counties. In the wet north and west, fens form a relatively short-lived transition stage between swamp or marsh and acid bog. Here fens develop when peat accumulates in high concentration but succeed to bog when the peat builds up above the water table.
Even in East Anglia, mires may shift from fen to bog as they become subject to leaching by rain. In this part of the country, however, rainfall alone is not usually sufficient to keep the peat permanently waterlogged, and shrub and tree species gradually seed in, leading to the development of fen scrub or ‘carr ’. Carr may be of several types. ‘Mixed carr ’ is relatively dry and contains a variety of scrub and tree species. ‘Swamp carr ’, where trees colonise tussocks over a layer of water, is comparatively unstable since the trees tend to fall over or drown as they grow larger, forming open pools, but it may persist for a long time until the floor begins to firm through the buildup of brushwood and other detritus. In ‘semi-swamp carr ’, trees colonise a strong, interlaced rhizome mat which can support their weight. Another important distinction is between ‘open carr ’, in which trees are spaced far enough apart to allow sunlight to penetrate to ground level, and ‘closed carr ’, in which tree growth is dense, shading out most flowering ground plants. A mixture of open and closed carr in association with areas of open fen often supports an exceptionally varied plant, insect and bird life.
