1395 Petalwort Petalophyllum ralfsii
Description and ecological characteristics
Petalwort Petalophyllum ralfsii is a pale green thalloid liverwort with erect lamellae on its upper surface, that grows in open, damp, calcareous dune slacks, often on low hummocks rather than on the very wet ground, on compacted sandy/muddy bryophyte-rich turf. Most localities are referable to Annex I type 2190 Humid dune slacks. It has occasionally been recorded in other coastal grassland where conditions are similar. Closely associated species may include the mosses Barbula convoluta, Bryum spp., Didymodon tophaceus, Ditrichum flexicaule (sensu lato), Hypnum lacunosum, glaucous sedge Carex flacca, the grasses common bent Agrostis capillaris, red fescue Festuca rubra and Yorkshire-fog Holcus lanatus, and buck’s-horn plantain Plantago coronopus. At some sites, it appears to be increasing as a result of trampling and soil compaction. At one site, the area where P. ralfsii grows is used by vehicles and it can be found on the sides of paths. It does not grow in water-filled slacks or in slacks where willow Salix spp. scrub predominates.
European status and distribution
Although it occurs throughout western Europe, Petalophyllum ralfsii has a predominantly Mediterranean distribution. It is infrequent and declining throughout its range, being rather closely confined to dune slacks of a certain kind, which are under threat in many areas, and the species is classed as Vulnerable. The UK and Ireland may now be its stronghold.
UK status and distribution
Petalophyllum ralfsii has always been widely but sparsely distributed in the UK. A high proportion of the known localities are in south-west England and Wales. Many of the sites support large populations but in some parts of the species’ range only small populations occur. All the sites are large dune systems with extensive dune slack habitat. Petalwort is classed as Lower risk (Nationally scarce) in Britain.
Site selection rationale
The site series includes the largest known populations of petalwort, and overall a very high proportion of the total UK population has been selected. Most of the selected sites are within the species’ centre of UK distribution in Wales and south-west England. Outlying localities in Scotland, East Anglia and Northern Ireland have also been selected to ensure full geographical coverage. Most of the sites selected for Petalophyllum ralfsii have also been identified as SACs for a range of Annex I sand dune habitats.
Highlands and Islands
This sand dune site in north-west Scotland hosts the largest population of petalwort Petalophyllum ralfsii in Britain. It is the only known Scottish site and the most northerly location recorded for this species in the world. The characteristic habitat of P. ralfsii at Achnahaird is moist dune slacks and the banks of small burns and drainage lines which run through the site. It also occurs in areas of bare, stable sand. Some of the largest colonies occur on areas of open sand which are kept damp by diffuse flushing from above. A number of other notable bryophyte species have been recorded from the site, often growing in close proximity to P. ralfsii.
A large population (around 3000 thalli) of petalwort Petalophyllum ralfsii is recorded from Braunton Burrows, one of two sites selected for this species in south-west England. There are extensive open dune slack habitats of the types required by this species for survival.
Carmarthen Bay Dunes/ Twyni Bae Caerfyrddin
East Wales, West Wales and The Valleys
Carmarthen Bay Dunes is one of two sites selected for petalwort Petalophyllum ralfsii in south Wales. Whiteford Burrows is included in this site and has a large population of the liverwort. Some of the largest and least-disturbed calcareous dune slack systems in the UK occur in this area, a high proportion of which are very open in character.
Large populations of petalwort Petalophyllum ralfsii occur in two dune slacks at Dawlish Warren. One of the slacks is on a natural, sandy substrate, and here the population appears to be expanding. In the other slack, petalwort grows on sand overlying an artificial masonry/stone substrate, which receives run-off from an adjacent limestone gravel track. Elsewhere in this slack the sand is more acidic and supports populations of the liverwort Fossombronia incurva. Both slacks are closely grazed by rabbits Oryctolagus cuniculus.
Kenfig is one of two sites selected for petalwort Petalophyllum ralfsii in south Wales and supports a large population of the species, numbering thousands of thalli. The calcareous dune system has many dune slacks that include the early successional, open slack vegetation this species requires.
Morfa Harlech a Morfa Dyffryn
West Wales and The Valleys
Petalwort Petalophyllum ralfsii has been recorded in dune slacks in the two dune systems at this site; it is most frequent at Morfa Dyffryn.
North Northumberland Dunes
Northumberland and Tyne and Wear
This site represents petalwort Petalophyllum ralfsii in north-east England. There are extensive dune systems with slacks here, both on the mainland and on Holy Island. Petalwort has been recorded from Holy Island and from two locations on the mainland.
Cornwall and Isles of Scilly
Penhale’s extensive dune system supports a large population of petalwort Petalophyllum ralfsii and is one of three sites selected to represent the species in south-west England. Recorded here since 1919, it occurs in damp, calcareous dune slacks.
A large population of petalwort Petalophyllum ralfsii occurs at Sefton Coast, the only site chosen for this species in north-west England. The plant was first recorded on the Sefton Coast at Ainsdale in 1861 and it is still found within the dune system between Southport and Ainsdale. It seems to prefer damp ground around the edges of dune slacks of fairly recent origin, with the largest populations found in slacks of less than 25 years old. The plant is often found in association with footpaths, where light trampling keeps the ground vegetation sparse; infrequently-used paths or less-trampled edges of pathways seem to be favoured. Although the preferred habitat is short damp turf with plenty of bare patches, populations have been found growing amongst dense marram Ammophila arenaria with few other associated species.
Y Twyni o Abermenai i Aberffraw/ Abermenai to Aberffraw Dunes
West Wales and The Valleys
Abermenai to Aberffraw Dunes is an extensive complex of sand dunes, dune slacks, marsh, shingle and cliffs in south-west Anglesey, north Wales. There is a large population of petalwort Petalophyllum ralfsii here that was first recorded in 1828. This historical continuity indicates that the site is especially favourable for the survival of this species. Although partly afforested, the open dunes have a very rich bryophyte flora, including the mosses Amblyodon dealbatus, Catoscopium nigritum and the liverwort Southbya tophacea, particularly in damp, calcareous slacks and flats.
SACs where this Annex II species is a qualifying feature, but not a primary reason for site selection
- Dee Estuary/ Aber Dyfrdwy Cheshire, East Wales, Extra-Regio, Merseyside, West Wales and The Valleys
- Limestone Coast of South West Wales/ Arfordir Calchfaen de Orllewin Cymru East Wales, West Wales and The Valleys
- Magilligan Northern Ireland
- North Norfolk Coast East Anglia
Many designated sites are on private land: the listing of a site in these pages does not imply any right of public access.