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Habitats - Culm grassland

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  • Rhôs pasture in the Devon Blackdown Hills

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Culm grassland

Culm grassland is a unique habitat that developed above a geological formation known as the Culm Measures laid down in the Carboniferous period (about 300 million years ago) in northwest Devon and northeast Cornwall. The name comes from the occasional presence of a soft, sooty coal, known in Devon as culm. The majority of the formation consists of shales and thin sandstones although there are also areas of slate, limestone and chert.

The geology of the Culm Measures has given rise to acidic, clay soils that are poorly drained in most places. In addition, the relatively high rainfall experienced in southwest England makes the soil very damp, conditions that persist even through the driest of summers. The only similar areas elsewhere in the UK are in south Wales, where the term rhôs pasture is used — Welsh for wet pasture — and in southwest Scotland. Outside the UK, there is a closely related area in north-west France. There may be others in western Ireland, northwest Spain and Portugal.

Some 92% of culm grassland has been lost in the past 100 years, with 48% disappearing between 1984 and 1991 alone. The chief causes have been agricultural improvement of land by drainage, ploughing, reseeding and fertiliser application; afforestation; abandonment and neglect; management inappropriate for conservation purposes (e.g. overgrazing); and habitat fragmentation. Several initiatives have been implemented to halt the decline including those by DWT (Culm Natural Networks and currently the Working Wetlands project), Butterfly Conservation (Reconnecting The Culm), and Natural England (through the Higher Level Stewardship Scheme).

Find out more about DWT's Working Wetlands project

NB. There are some areas of land beyond the Culm Measures that largely or partly comprise rhôs pasture: e.g. four SSSIs on the Devon Blackdown Hills, two of which are DWT nature reserves. DWT’s Biodiversity & Geodiversity Action Plan prescribes targets for all Devon rhôs pastures wherever they are located.
Species associated with culm grassland

Culm grassland comprises a variety of different plant communities, including purple moor-grass dominated mires, rush-pastures, wet heaths and tall herb-fens. These habitats are noted for their biodiversity, and the unimproved nature of the land allows several rare plant species to persist including the heath-spotted orchid, lesser butterfly orchid and southern marsh orchid, in addition to a number of other plants of conservation interest such as whorled caraway, wavy St John’s-wort and marsh cinquefoil. Wetter areas typically support sharp-flowered rush, ragged robin, marsh bedstraw, meadowsweet and wild angelica, while the drier areas hold such species as meadow thistle, tormentil, devil’s-bit scabious, saw-wort and heathers (ling and cross-leaved heath). Where the soil is waterlogged, bog vegetation comprising Sphagnum mosses, bog pondweed and sedges may develop.

The diverse flora supports several butterflies including marsh fritillary, small pearl-bordered fritillary, wood white, marbled white, dingy skipper and small heath. Other insects found on the Culm include the nationally scarce narrow-bordered bee hawkmoth, double line moth and keeled skimmer dragonfly. There is a wide range of bird species including barn owl, curlew, snipe, woodcock, willow tit, reed bunting and grasshopper warbler, and reptiles and amphibians are well represented with common frog, toad, viviparous lizard, adder and grass snake all found across the region. Mammals that occur include dormouse, harvest mouse, fox, roe and red deer, otter and several species of bat.

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