The Blackdown Hills. Part 1: Ecology

Special Character

The Blackdown Hills were designated as an Area of Outstanding Natural Beauty (AONB) in 1991, much later than the East Devon AONB in 1963. They cover 360 km² and straddle the Devon-Somerset county border south of the Taunton Vale. Other AONBs have been designated primarily in open, upland, coastal or river landscapes, but the Blackdowns are different: their landscape is intimate and enclosed, as a direct result of low intensity livestock farming. The area is also essentially lowland, between 150 and 310m above sea level. The Hills have a sense of remoteness and tranquility in a landscape punctuated by small villages, hamlets and isolated farmsteads of distinctive architecture and often a rich history. Prehistoric sites, from the Mesolithic onwards, are widespread. The diverse pattern of landscapes and the long history of land use reflect the unique underlying geology and the drainage pattern superimposed upon it.

Landscape and Geology

From the steep, north-facing scarp, the Blackdowns dip gently southward to the A35 as an extensive flat-topped but dissected plateau consisting of an acidic Upper Greensand, mantled by a thin layer of clay-with-flints. Greensand was laid down under a shallow sea that covered southern Britain between 65 and 135 million years ago during the Cretaceous, and varies in colour from yellow to dark green, with beds of flint-like chert much used in local building. It is rich in iron and is one of the main aquifers in the area. At the base of the greensand is an unconformity, so that the rock sequence normally lying beneath it is incomplete. In some areas, greensand is underlain by Jurassic Blue Lias limestone and, in parts of Bishopswood and Membury for instance, such outcrops give rise to calcareous soils. Elsewhere, the greensand is underlain by Keuper Marls or White Lias limestones, both of Triassic age. The latter produces small patches of calcareous soils on the northern scarp in Adcombe Wood, and near Thurlbear.

Three major rivers arise and flow south to the sea from the Blackdowns: the Yarty (the major tributary of the Axe), the Otter, and the Culm, that joins the Exe. As a result of this diverse geology, the pattern of river erosion, and the predominance of livestock farming, the Blackdown Hills boast a very diverse range of habitats.

Habitats and Plant Communities

The woodland that develops on heavier, more acidic soils is generally dominated by Pedunculate Oak, often with a shrub layer of Hazel and Holly amid Bracken and Brambles. Ash woodlands, typical of the small areas of calcareous soils where Field Maple is also common, are the places to look for scarce trees like Small-leaved Lime and the Wild Service; herb communities typically include Dog’s Mercury, Ramsons, Hart’s-tongue Fern, Gladdon, Pendulous Sedge, Sweet Woodruff and Yellow Archangel. Rare species include Herb Paris, Goldilocks Buttercup and both Greater Butterfly and Bird’s-nest Orchids. In wet places, often at the spring-line, there may be wet woodlands with Downy Birch, Alder, Grey Sallow and Alder Buckthorn underlain by Opposite-leaved Golden Saxifrage, Marsh Violet, Kingcup, Tussock Sedge, Purple Moor-grass and, locally, Royal Fern and Marsh Valerian.

Hedgebanks are characteristic of the AONB. Perhaps because of the preponderance of dairy farming, fewer hedges have been grubbed out than in many areas: hedgerow loss is estimated at about 6% in the Blackdowns. Recently established hedges are often of Beech and Hawthorn, typically where land has been enclosed since the Enclosure Acts, as on Stockland Hill where hedgerows are straight and regular. Ancient hedgebanks are irregular, enclosing a mosaic of small fields, some dating from medieval times. They are also species-rich, with 8 or even 12 species of woody plant per 30m. Field Maple, Elm, Hazel, Ash, Oak, Dogwood, Spindle, Hawthorn, Blackthorn and Dog Rose are typical, usually to the exclusion of Beech and Sycamore. Herb communities include typical spring flowers such as Primrose, Lesser Celandine and Wood anemone. Sweet Violet (often white-flowered here), Early Purple Orchid and, more rarely Green Hellebore and Orpine are also hedgebank plants in the Blackdowns; Black Bryony, Honeysuckle, Tufted Vetch and Lesser Periwinkle are common climbers and scramblers.

Very little of the grassland is now ‘unimproved’. The old hay meadows, cut in July, have largely been replaced by temporary Rye-grass leys which are often first cut for silage in May. Remaining unimproved mesotrophic grasslands typically include various combinations of Black Knapweed, Cat’s-ear, Corky-fruited Water-dropwort, Sweet Vernal-grass, Crested Dog’s-tail and Yellow Rattle, among many others. Some sites support strong populations of Green-winged Orchids, while others have an abundance of Southern Marsh Orchids, or waxcap fungi. Species-rich, unimproved hay meadows, as at Goren Farm, may support about 16 species per m²; improved grassland usually has about 4 species per m². Calcareous grassland is confined to a few sites to the east and northern scarp of the AONB; typical species include Common Spotted, Bee and Pyramidal Orchids, Rockrose, Quaking Grass, Autumn Gentian, Hoary Ragwort, Hawkweed Oxtongue and Purging Flax.

Heathland developed about 4000 years ago as a result of forest clearance followed by grazing on land poor in nutrients. The greensand plateaux of the Blackdowns were once dominated by heathland, but now only a few blocks remain. Well-drained sites on greensand support species-poor dry heath with Ling, Bell Heather, Bristle Bent and Western Gorse the main species. In wetter areas at the spring-line, much more species-rich mires are found and it is this community that is perhaps the most important for conservation in the AONB. Cross-leaved Heath, Purple Moor-grass, Heath Spotted Orchid and Bog Asphodel are typical components; specialities include all three British species of sundew, Pale Butterwort, Bog Myrtle, Lesser Butterfly and Early Marsh Orchids, Dorset Heath, Petty Whin, Wood Horsetail and Broad-leaved Cotton-grass. Blackdown and Sampford Commons include the largest areas of dry and wet heath in the AONB, and Hense Moor and Deadman are rich sites for both wet heath and bog communities.

Strongholds and threatened species

The Blackdown Hills are a stronghold for the Dormouse which is declining nationally. Among invertebrates, the Raft Spider is apparently frequent in spring-line mires throughout the AONB. ‘County Rarity’ higher plant species with strong populations here include Small Teasel and Wood Horsetail, as well as Early Marsh Orchid, Bog Myrtle and Corky-fruited Water-dropwort, which are ‘Devon Notables’.

Species that are local or rare in the Hills because they are at the edge of their ranges within Britain include: Dartford Warbler, Pied Flycatcher and Redstart (both the latter bred regularly in the Yarty catchment until about 1998); Duke of Burgundy and perhaps the Large Marsh Grasshopper; and Dorset Heath and White Helleborine (the latter in its most southwesterly site in the UK).

Rapidly declining species include the Curlew (as a breeding bird), Marsh Fritillary, Petty Whin, Broad-leaved Cotton-grass, Lesser Butterfly Orchid and Great Sundew. The Silver-studded Blue, Pearl-bordered Fritillary and Southern Damselfly seem now extinct in the Blackdowns. However, Fir Clubmoss, last seen on Hense Moor in 1997, was refound in 2008, raising hopes that other ‘extinct’ species may turn up anew.