Area: 54.08 hectares.
Description and Reason for Notification:
Borlasevath and Retallack Moor is of special interest for its rich mix of fen and associated vegetation types which is rare nationally and the strong and characteristic western (or oceanic) nature of its flora. In addition some of the fen plant communities present are nationally rare. It represents an important part of the range of variation in the wet ‘heaths’ of Cornwall being one of the largest purple moor grass Molinia caerulea dominated mire systems.
It is situated in a shallow water logged valley (at around 100-150m altitude) which slopes towards a central stream flowing north-south. The underlying strata comprise grits and slats of Lower Devonian age with an area of alluvium around part of the stream. This geology linked to climate and topography has produced peaty soils with lateral water movement through them which is strongly reflected in the vegetation.
The site is characterised by the dominance of purple moor grass and an abundance of black bog rush Schoenus nigricans. They appear prominently in the fen vegetation types which form complex mosaics and important transitions to wet heath, south western humid heath, marshy grassland, scrub and open water.
On the valley slopes vegetation characterised by purple moor grass and tormentil Potentilla erecta is particularly dominant with black bog rush, rushes and tall herbs such as devil’s bit scabious (Succisa pratensis), wild angelica (Angelica sylvestris) and smaller herbs like saw-wort Serratula tinctoria. Oceanic species like lesser skullcap Scutellaria minor, the uncommon royal fern Osmunda regalis and wavy St John’s Wort Hypericum undulatum, the latter nationally scarce are well represented. Scrub, notably bog myrtle Myrica gale and bramble Rubus fruticosus agg. is scattered amongst the mire which grades imperceptibly to wet heath vegetation on the flatter and less well drained ground with a greater cover of heathers, particularly cross leaved heath Erica tetralix and scattered bog mosses notably Sphagnum compactum and S. tenellum.
In more water logged patches with deeper peat especially towards the valley floor, wet heath gives way to a rare type of valley bog with increased bog moss cover especially Sphagnum papillosum, with more abundant bog asphodel Nathecium ossifragum, round-leaved sundew Drosera rotundifolia and common cotton grass Eriophorum angustifolium.
Further complexity is provided by seepage waters leading to isolated small flushes of a rare type with black bog rush and bog asphodel more prominent along with Sphagnum auriculatum, round-leaved sundew, bog pimpernel Anagallis tenella and the uncommon pale butterwort Pinguicala lusitanica.
Grey willow (Salix cinerea) scrub has developed along part of the main mire axis completing the transition downslope. It becomes more abundant in the southern part of the site both scattered and in dense carr in a mosaic with tall western fen meadow vegetation where either purple moor grass or sharp flowered rush Juncus acutiflorus / soft rush Juncus effusus dominates in ill-drained areas. The rich and colourful suite of associates includes greater birds-foot-trefoil Lotus pedunculatus, fleabane Pulicaria dysenterica, purple loosestrife Lythrum salicaria, hemp agrimony Eupatorium cannabinum ragged robin Lychnis flos-cuculi, common mint Mentha aquatica and lesser spearwort Ranunculus flammula.
These mosaics have much to do with the breeding bird interest of the site which is outstanding in county terms for warblers. Species include grasshopper warbler Locustella naevia, sedge warbler Acrocephalus schoenbaenus, blackcap Sylvia atricapilla, graden warbler Sylvia borin, whitethroat Sylvia communis, willow warbler Phylloscopus trochilus and chiffchaff Phylloscopus collybita along with the locally uncommon willow tit Parus montanus. The valley mire also provides valuable wintering ground for hen harrier Circus cyaneus and short-eared owl Asio flammeus.
As well as natural transitions from mire to standing and running water, a series of ponds (some large) have been constructed on the eastern slope. They are however developing a typical semi-natural emergent and transitional flora including bog pondweed Potamogeton polygonifolius, marsh St John’s-wort Hypericum elodes and floating sweet grass Glyceria fluitans and have augmented available habitat for dragonflies amphibians and waterfowl.
Borlasevath and Retallack Moor supports a limited area of south western humid heath in an upslope transition delineated by increased prominence of western gorse Ulex gallii, bell heather Erica cinerea and bristle bent Agrostis curtisii. This adds to the site’s value illustrating another part of the classic zonation of southern lowland heath mire complexes which is fairly well represented here though much complicated by the nature of the long, gentle, concave slopes blurring the transition between communities.