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H4: Valley Mire and Bog

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H4: Valley Mire and Bog

Habitat Class: 

Although valley mire and bogs are technically wetland habitats I have included them with heath because they form in low lying valleys and depressions in heathland areas where water percolates through the sand and gravel of the neighbouring higher ground into the low lying area from where it cannot readily escape, possibly because of the topography of the ground (bog) but also, frequently, where the vegetation slows down the run off into a stream or river (mire). This seepage through vegetation into a river can be seen quite clearly at Morden Bog or Hartland Moor in Purbeck. 

The water in these areas is usually acidic and poor in nutrients and so favours a unique variety of plant species but, notably, sphagnum mosses are a key feature of such sites. Valley mire is wet at all times of the year although the water table level will vary depending on the amount of rain and so the fringes may not always under water. Plant species near the edge of a valley mire may well differ from those who will have their foundations in water all year. Certain specialised insects also thrive in these conditions and, being able to move, can relocate to appropriate degrees of 'wetness' to suit their needs.

Valley mire can show a variation in surface patterns with open ponds, or even lakes, where the surface is clear to what is known as blanket bog where the water surface is covered in dense vegetation and the water is hidden. These can be particularly dangerous places for some animals, including humans! Both plants and animals will vary depending on the degree of open or covered water. Looking down on a valley mire it is possible to see changes in colouration of the vegetation as the mix changes with the depth of the water.  

Of 120 known valley mire sites in northern Europe 90 are found in the New Forest. I do not know exactly how many of the remaining 30 are in Dorset but it is certainly in double figures and the Dorset heath around Poole Harbour is an important area for this unique and rare habitat. Most mires have existed for thousands of years almost unchanged although there is evidence of peat digging is some areas where the depth of the peat is significant. In some cases valley mire has formed in places where clay has been extracted at some point in the past. 

Valley mire is a unique and interesting habitat. The variations between very wet and occasionally wet areas and between open and covered wet areas leads to diversity of species, many of which are quite unique to such conditions.

Under the Phase 1 habitat survey classification system valley mire is coded as E3.1 and is described as follow:

"A valley mire develops along the lower slopes and floor of a small valley and receives water from springs and seepages on the valley sides, feeding a central water course. Such a fen can be distinguished from a flush because the former is a complex, whereas a flush is a discrete single feature, usually of limited extent.Valley mires are often dominated by acidophilous vegetation containing sphagnum species, carex species and ericoids. However, vegetation typical of base rich conditions can also occur, for instance, Schoenus nigricans (bllack bog-rush) and Juncus subnodulosus (blunt-flowered rush). Floating mats of mosses and sedges may be present. Acid watercourses often contain Hypericum elodes (marsh St John's-wort) and Potamogeton polygonifolius (bog pondweed)."

Under the National Vegetation Classification system valley mire is classified primarily as M21 and M29


The information about this Site of Special Scientific Interest has been organised into reports, charts, maps and photos. Click a pic below to see the detail:

Distribution Map Linked Species Linked Sites Some Photographs Guidance Notes