Powerstock Common

Area: 161.68 hectares.

Reasons for Notification: 

Powerstock Common and Wytherston Farm is of special scientific interest by reason of its oak Quercus robur, ash Fraxinus excelsior and alder Alnus glutinosa woodland, acid, neutral and calcareous grassland, fen meadow and rush pasture, rare and scarce invertebrates; rare and scarce lower plants, dormouse Muscardinus avellanarius and a population of great crested newts Triturus cristatus at the south-western edge of their range. 

General Description: 

Powerstock Common and Wytherston Farm comprises enclosed and unenclosed woodland, grassland, fen meadow and rush pasture, and a series of ponds. The site lies on soils derived from Fullers Earth Rock, which is overlain on the higher, eastern slopes and plateau by Upper Greensand and Gault Clay. Numerous springs and seepages arise at the junction of the greensand and clay feeding streams which dissect the site; landslips are a characteristic feature in this landscape. The varied geology and soils are reflected in the pattern of land use within the site. Well drained brown earth soils are present on outcrops of free-draining greensand, where bracken Pteridium aquilinum is abundant, but are quickly replaced by waterlogged stagnogleys on the impermeable Gault Clay over which much of the woodland and grassland is situated. 

Woodland

Powerstock Common remained unenclosed until 1866 and many of the multi-stemmed ash and alder date from this time when rights to collect firewood and graze stock were finally extinguished. The railway divided the Common and Wytherston Wood in 1857 and several phases of oak regeneration followed enclosure until post war felling and coniferisation from the 1950’s until 1973. Historically the woods were more open than at present and many of the veteran oak, ash and field maple Acer campestre present originated in wood pasture, which has through natural regeneration developed into high forest. 

The following woodland National Vegetation Classification (NVC) communities have been identified: Quercus robur-Pteridium aquilinum-Rubus fruticosus woodland (W10); Fraxinus excelsior-Acer campestre-Mercurialis woodland (W8); Alnus glutinosa-Fraxinus excelsior-Lysimachia nemorum woodland (W7) and Salix cinerea- Galium palustre woodland (W1). 

Oak high forest is the predominant community on the brown earth soils across the site. The drier stands have an understorey of hawthorn Crataegus monogyna and holly Ilex aquifolium, and bluebell Hyacinthoides non-scripta is a frequent vernal species, with occasional bramble Rubus fruticosus, tway blade Listera ovata and ground ivy Glechoma hederacea. Oceanic species such as broad buckler fern Dryopteris dilatata, scaly male fern D. affinis, soft shield fern Polystichum setiferum and wood sorrel Oxalis acetosella are abundant on damper soils and in stream gullies in stands of high forest and hazel coppice. 

Calcareous clay soils support ash woodland in high forest, coppice or as standards over hazel coppice. Oak, maple and alder are common associates in the canopy and understorey with occasional shrubs such as spindle Euonymous europaeus, privet Ligustrum vulgare and wayfaring tree Viburnum lantana. Dog’s mercury Mercurialis perennis, primrose Primula vulgaris and bugle Ajuga reptans are frequent in the driest soils and are replaced by anemone Anemone nemerosa, celandine Ranunculus ficaria and locally abundant pendulous sedge Carex pendula on the moister, seasonally water-logged soils. Ramsons Allium ursinum are locally frequent, with occasional thin-spiked wood-sedge Carex strigosa, herb paris Paris quadrifolia and broad-leaved helleborine Epipactis helleborine

Stands of willow Salix cinerea and alder replace ash and oak on the wettest soils on seepages, springs and landslips. Alder high forest which has developed from coppice has occasional ash and oak, with willow and rarely hawthorn in the understorey. Saturated, slumping soils with deep runnels support abundant remote sedge Carex remota, opposite-leaved golden-saxifrage Chrysosplenium oppositifolium, yellow pimpernel Lysimachia nemorum and creeping buttercup Ranunculus repens, with wood sorrel and broad buckler fern occasional species on the spreading alder roots. Willow carr is restricted to areas of deep silt and open water with tall herbs, such as hemlock water dropwort Oenanthe crocata, water mint Mentha aquatica and occasional great horsetail Equisetum telmateia. The bog moss Sphagnum palustre is locally frequent in pools. 

The woodland rides and glades contain many species found in the surrounding fen meadows elsewhere on the site and provide important habitat for invertebrates such as butterflies, moths and hoverflies. Purple moor grass Molinia caerulea is frequent with betony Stachys officinalis, saw-wort Serratula tinctoria and devil’s bit scabious Succissa officinalis, whilst common dog violet Viola riviniana is abundant on more free draining soils. 

Grassland, fen meadow and rush pasture 

The following grassland, fen meadow and rush pasture NVC communities have been identified: Festuca ovina-Agrostis capillaris-Galium saxatile grassland (U4); Centaurea nigra-Cynosurus cristatus meadow (MG5); Festuca ovina-Avenula pratensis grassland (CG2), Bromus erectus grassland (CG3), Juncus subnodulosus-Cirsium palustre fen meadow (M22); Molinia caerulea-Cirsium dissectum fen meadow (M24) and Juncus effusus/acutiflorus-Galium palustre rush pasture (M23). 

Fen meadow dominated by tussocky purple moor grass and occasionally tufted hair grass Deschampsia caespitosa forms much of the rough grazing on peaty mineral soils which are seasonally waterlogged. Grasses such as velvet bent Agrostis canina, heath-grass Danthonia decumbens and red fescue Festuca rubra are frequent and sedges are a distinctive component of the community and include glaucous sedge Carex flacca and carnation sedge C. panicea with occasional tawny sedge C. hostiana and flea sedge C. pulicaria. Herbs which are frequent in the shorter, grazed turf include meadow thistle Cirsium dissectum, devil’s-bit scabious and betony with occasional petty whin Genista anglica, dyer’s greenweed G. tinctoria and pepper saxifrage Silaum silaus. The low growing shrubs cross-leaved heath Erica tetralixand ling Calluna vulgaris are locally frequent whilst thickets of taller scrub contain western gorse Ulex gallii, blackthorn Prunus spinosa, downy birch Betula pubescens and eared willow Salix aurita

Outcrops of free draining soils on greensand have stands of bracken over abundant bluebell with greater stitchwort Stellaria holostea and sweet vernal grass Anthoxanthum odoratum. Bracken is suppressed where grazing and trampling have maintained an open sward of fine grasses with frequent sheep’s-fescue Festuca ovina and common bent Agrostis capillaris with herbs such as mouse-ear Pilosella officinarum,common dog violet, tormentil Potentilla erecta and adder’s-tongue Ophioglossum vulgatum. The open, acid grassland is restricted in its distribution to the sandy brown earth soils and is quickly replaced by neutral grassland with rush pasture and fen meadow on the seepages and heavier clay soils with impeded drainage, which cover much of the site. This succession of grassland communities is found in pastures at Wytherston Farm on south and west facing slopes influenced by landslips, which have created a range of conditions in microtopography and soil moisture. The dry neutral pasture has frequent sheep’s fescue, red fescue, sweet vernal grass and common bent. Heath-grass is occasional with frequent glaucous sedge and spring sedge Carex caryophyllea. Devil’s bit scabious and bird’s foot trefoil Lotus corniculatusare abundant with frequent betony and hardhead Centaurea nigra whilst saw-wort is rare. Fen meadow and rush pasture replaces the dry grassland community on seepages and soils with impeded drainage. Rushes and tall herbs are characteristic and frequent and include hard rush Juncus inflexus, sharp-flowered rush, J. acutiflorus, fleabane Pulicaria dysenterica, ragged-robin Lychnis flos-cuculi and locally meadowsweet Filipendula ulmaria. Meadow thistle is locally frequent with square-stalked St John’s wort Hypericum tetrapterum, creeping-Jenny Lysimachia nummularia and southern marsh orchid Dactylorhiza cf. praetemissa

Free draining calcareous soils on the railway cutting support a sward dominated by tor grass Brachypodium pinnatum and scrub such as dogwood and blackthorn. Tor grass is suppressed in areas of short turf maintained by rabbit grazing and here is replaced by meadow oat-grass Helichtotrichon pratense, glaucous sedge, heath grass and quaking grass Briza media. Frequent herbs include bird’s foot trefoil, stemless thistle Cirsium acaule, wild thyme Thymus polytrichous and salad burnet Sanguisorba minor with occasional hairy violet Viola hirta, yellow wort Blackstonia perfoliata and lesser centaury Centaurium pulchellum. The calcicolous mosses Ctenidium molluscum and Homalothecium lutescens are occasional in the sward whilst wood small-reed Calamagrostis epigejos and great horsetail are locally dominant on seepages. 

Invertebrates 

The rare and protected marsh fritillary Eurodryas aurinia is present at Powerstock in fen meadows containing devil’s-bit scabious and the nationally scarce wood white Leptidea sinapis is present in the woodland and scrub fringing the disused railway line. The declining small pearl-bordered fritillary Boloria selene occupies woodland glades and edges and silver-washed fritillary Argynnis paphia and purple hairstreak Quercusia quercus both local butterflies are present in the oak woodland. The diverse moth fauna includes the nationally scarce species small eggar Eriogaster lanestris on blackthorn and Mere wainscot Chortodes fluxa found in damp open woods, the notable waved carpet Hydrelia sylvata and the rare micro moth Cosmoites stabiella. The notable hazel leaf roller Byctiscus betulae is present in the hazel understorey and the hoverfly fauna includes the rare Cheilosia carbonaria and the nationally scarce species, Epistrophe diaphana and Cheilosia soror

Lower plants 

Epiphytic lichens indicative of ecological continuity and associated with old forest conditions are present through the site such as the rare New Forest lichen Parmelia minnarum on alder and Japewia carrollii on willow at one of only two localities in Dorset. Other species are present on ash, hazel, holly, elder and oak including Phaeographis dendritica, Catillaria atropurpurea and Dimerella lutea. Bryophytes include the nationally scarce moss Leptodon smithii, the local moss Zygodon rupestris and the liverwort Frullania tammariisci

Dormouse 

The woodland and scrub supports the rare and protected dormouse. Great crested newt 

On the northern edge of Powerstock Common a series of ponds supports a population of great crested newts. The ponds range from heavily shaded to unshaded and most are sparsely vegetated by floating sweet-grass Glyceria fluitans and common duckweed Lemna minorcommunities. Some ponds have an unusual flora which includes branched bur-reed Sparganium erectum, bog pondweed Potamogeton polygonifolius, bogbean Menyanthes trifoliate, lesser water-plantain Baldellia ranunculoides and submerged floating club-rush Eleogiton fluitans, all of which are rare in west Dorset. The particular combination and juxtaposition of aquatic and terrestrial habitats provide ideal breeding, foraging and hibernation conditions for the great crested newt. 

In addition to the reasons for notification, otters Lutra lutra have been recorded in a stream that lies in the headwaters of the River Hooke. 

Other Information: 

Powerstock Common and Wytherston Farm SSSI lies in the Wessex Vales Natural Area within an area of exceptional biodiversity. Whetley Meadows SSSI, Drakenorth SSSI and Eggardon Hill and Luccas Farm SSSI adjoin the site whilst Toller Porcorum SSSI, Woolcombe SSSI and Mapperton and Poorton Vales SSSI are approximately 350 metres, 700 metres and 1400 metres away respectively.