Area: 30.7 hectares.
A new site.
Includes the Rimsmoor Geological Conservation Review site.
Description and Reasons for Notification:
Oakers Bog is a relatively large and undamaged valley mire near the western extremity of the Poole Basin with its highly acidic sands and clays. There is little hydraulic gradient at Oakers Bog and a depth of peat has accumulated across the valley. At its edges there is gradation to wet and then dry heathland communities, though most of the latter have been lost to the extensive forestry plantations on the original Bryants Puddle Heath. The site has a rich assemblage of heathland plants and animals. There is also considerable geological interest at the solution pits of Rimsmoor and Oakers dolines where an unusually deep accumulation of peat provides a valuable palynological record.
Rimsmoor is a key site both for studies of palynology and doline formation. It is exceptional for the depth of peat it contains; most dolines in the area (apart from Oakers doline) are free-draining and lack significant organic accumulation. Palynologically, Rimsmoor is important as one of few sites providing evidence of vegetational change near the Chalklands since the early Holocene. High temporal resolution is possible by the apparently extremely rapid rate of organic accumulation. There is little precise evidence for the ages of the Dorset dolines and similar hollows elsewhere in England, but radiocarbon dating at Rimsmoor does provide one basal date of approximately 8,000 years before present date (BP). Stratigraphical evidence demonstrates gradual subsidence as the means of formation of Rimsmoor doline. The nearby Oakers doline is also included for its palynological research potential.
The valley mire forms an L shaped feature with central drainage channels. The eastern arm is more deeply incised resulting in a narrower bog and more transitional wet heath. Valley mire is a nationally scarce habitat virtually confined to the New Forest and South East Dorset heathlands. The bog communities at Oakers Bog and the basin mire of Rimsmoor pond are characteristically rich with extensive lawns of bog mosses Sphagnum spp. These are in places dominated by the nationally rare S. pulchrum together with S. papillosum. The local S. magellanicum also occurs. The bog vegetation includes frequent bog asphodel Narthecium ossifragum, round-leaved sundew D. rotundifolia and white beak-sedge Rhynchospora alba. Bogbean Menyanthes trifoliata and bog pondweed Potamogeton polygonifolius are common in the shallow pools as is emergent vegetation including common cotton-grass Eriophorum angustifolium and jointed rush Juncus articulatus. The eastern arm of the mire has a more extensive area dominated by tussocks of purple moor- grass Molinia caerulea. The bog edges grade into wet heath in a narrow fringe, with vegetation dominated by cross-leaved heath Erica tetralix, deergrass Trichophorum cespitosum and stands of bog myrtle Myrica gale. The nationally scarce marsh gentian Gentiana pneumonanthe occurs locally in this community. Where the adjoining drier slopes have not been afforested, small areas of dry heathland vegetation survive, with heather Calluna vulgaris, bell heather Erica cinerea, dwarf gorse Ulex minor and bristle Bbent Agrostis curtisii.
This dry heath with a southern aspect supports breeding colonies of the rare and protected sand lizard Lacerta agilis and smooth snake Coronella austriaca. The wetter parts of the site also support commoner reptiles and amphibians including grass snake Natrix natrix and palmate newt Triturus helveticus. Those invertebrates recorded within the site include the rare large marsh grasshopper Stethophyma grossum, and bog bush cricket Metrioptera brachyptera and swamp spider Dolomedes fimbriatus, both species of restricted distribution. Dragonflies are well represented here and include the local small red damselfly Ceriagrion tenellum.