Gorsllwyn, Onllwyn

Site of Special Scientific Interest · 51.7843, -3.6622

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Powys CountyWest Glamorgan County

Climate exposure

This is about exposure to a nationwide pressure this habitat type carries — not a verdict on this site's own management.

Peatland

Peat habitats store centuries of carbon and depend on a stable, waterlogged water table. Warmer, drier summers dry out peat, making it far more vulnerable to wildfire and to carbon release — a pressure that can undo years of restoration work regardless of how well the site is otherwise managed.

Habitats

Carr, Fast flowing river and stream

Geology & landscape

Landscape processes: Fluvial, Peatland / Mire

Description

Description This site contains a range of peat-depositing vegetation communities which has developed on a col between the Pyrddin and Dulais valleys. These peatlands are surrounded by an area of acidic grassland. Peat deposition has been sufficiently great in part of the Neath Port Talbot portion of the site to form a dome shaped mass of peat above the general water table of the site. Such a feature is known as a raised mire. There are very few other examples of this formation known in mid and south Wales. Unfortunately, regular burning and grazing of the site has grossly modified its surface vegetation. Much of this mire drains north into an area dominated by common reed Phragmites australis. The spectacular development of clumps of greater tussock-sedge Carex paniculata, in many cases over one metre high, amongst the common reed, affords drier sites for the establishment of willow species Salix spp., and willow carr now covers most of the area bordering the main east-flowing drainage stream. A range of woodland species including royal fern Osmunda regalis occurs beneath the tree canopy. Peat has not completely infilled the area. In some parts, the vegetation has grown as floating lawns over water. The more nutrient-rich sites support an extremely diverse flora with up to 28 species recorded in an area of 4 square metres. North-east of the complex of mires an area of acidic pasture is included in the site. Drier ridges divide up a series of wet flushes which support a range of wetland species not encountered elsewhere in the site, e.g. whorled caraway Carum verticillatum, meadow thistle Cirsium dissectum and sharp-flowered rush Juncus acutiflorus. These plants form a clearly defined community of extremely limited distribution in Europe, occurring only along the southern Atlantic seaboard. At the north-west corner of the site, several hay meadows are included on account of their diverse flora. Species now of limited occurrence in mid Wales due to the ploughing and reseeding of grasslands, such as globe flower Trollius europaeus and meadow fescue Schedonorus pratensis, are recorded here.

Species mentioned

English name
search for this in other sites		 Latin name
search with Google		 IUCN / Red List Type Family
royal fern Osmunda regalis LC fern osmundaceae
common reed Phragmites australis LC grass poaceae
meaow fescue Schedonorus pratensis VU grass poaceae
whorled caraway Carum verticillatum Not listed in GB list herb apiaceae
meadow thistle Cirsium dissectum EN herb asteraceae
globe flower Trollius europaeus VU herb ranunculaceae
sharp-flowered rush Juncus acutiflorus LC rush juncaceae
greater tussock sedge Carex paniculata LC sedge cyperaceae

Plant communities

A9 — Fast flowing river and stream, Marginal and inundation, Pond ditch and canal, Sluggish river and stream

Potamogeton natans community

Schoenoplectus lacustris, Juncus bulbosus, Sparganium angustifolium, Sparganium erectum, Sparganium minimum, Alisma plantago-aquatica, Lobelia dortmanna, Equisetum fluviatile, Myriophyllum alterniflorum, Elodea canadensis, Lemna minor, Lemna trisulca, Nymphaea alba, Callitriche platycarpa, Callitriche stagnalis, Littorella uniflora, Potamogeton natans, Ranunculus trichophyllus, Glyceria fluitans, Chara subspinosa

A14 — Fast flowing river and stream

Myriophyllum alterniflorum community

Isolepis fluitans, Juncus bulbosus, Fontinalis antipyretica, Lobelia dortmanna, Equisetum fluviatile, Myriophyllum alterniflorum, Lemna minor, Callitriche stagnalis, Littorella uniflora, Potamogeton filiformis, Potamogeton gramineus, Potamogeton natans, Ranunculus peltatus, Chara spp., Nitella hyalina

A17 — Chalk river and stream, Fast flowing river and stream

Ranunculus penicillatus ssp. pseudofluitans community

Berula erecta, Nasturtium officinale, Elodea canadensis, Lemna minor, Callitriche stagnalis, Veronica beccabunga, Polygonum amphibium, Polygonum hydropiper, Potamogeton crispus, Potamogeton pectinatus, Ranunculus penicillatus ssp. pseudofluitans

A18 — Fast flowing river and stream

Ranunculus fluitans community

Fontinalis antipyretica, Fontinalis squamosa var. curnowii, Myriophyllum alterniflorum, Myriophyllum spicatum, Elodea canadensis, Mentha aquatica, Lemna gibba, Lemna minor, Potamogeton perfoliatus, Ranunculus fluitans, Glyceria fluitans, Phalaris arundinacea