Blysmus compressus

Oxfordshire Flora Group

Species

Blysmus compressus, Flat-sedge

The flowering spike of Flat-sedge
Photo by David Morris

Flat-sedge is a small (to around 35cm) rhizomatous perennial member of the sedge family (Cyperaceae) found in short herbaceous vegetation around base-rich springs and flushes in a variety of habitats, such as wet grassland, fens and dune slacks. Its English name refers to its laterally compressed inflorescence (flowering spike). This dense, flattened inflorescence is distinctive among British sedges.

Oxfordshire site

The site at Marston comprises a complex of habitats along the floodplain of the River Cherwell. Flat-sedge can be found on the bank of the river terrace where there is a spring line above the seasonally flooded meadows along the river. Drier grassland is situated above the river terrace.
The terrace on which Flat-sedge grows supports other plants that are of national concern. The wettest part of the terrace supports vegetation with a fen-like quality, characterised by Devil’s-bit Scabious Succisa pratensis (England Near Threatened), Lesser Pond-sedge Carex acutiformis, Marsh Thistle Cirsium palustre, Marsh Valerian Valeriana dioica (England Near Threatened) and Water Mint Mentha aquatica. These species are characteristic of MG8 Cynosurus cristatus-Carex panicea-Caltha palustris grassland, a variable vegetation type found in floodplains, often supplied by base-rich groundwater (Wallace & Prosser, 2017). The liverwort, Riccia subbifurca, has also been recorded here at its only Oxfordshire site.

The southern end of the bank in its unmanaged state, with conspicuous marsh thistle.
Photo by Judy Webb

Strikingly, Flat-sedge avoids the foregoing zone of vegetation, and is found in a less wet area to the north dominated by Red Fescue Festuca rubra, with frequent Tall Fescue Schedonorus arundinaceus, and abundant Field Horsetail Equisetum arvense and Silverweed Potentilla anserina. Characteristic of MG11 Festuca rubra-Agrostis stolonifera-Potentilla anserina grassland, this rank, species-poor community can be found on heavier, more readily water-logged soils and is usually less ground-water dependent, but can also develop from richer wet grassland following the abandonment of cutting and/or grazing management.

Rank vegetation to the north, supporting Flat-sedge, with abundant Field Horsetail and Silverweed.
Photo by David Morris

Below the terrace is a small area of eutrophic tall-herb vegetation dominated by Reed Sweet-grass Glyceria maxima with a variety of species including Common Valerian Valeriana officinalis (England Near Threatened), Great Willowherb Epilobium hirsutum, Common Nettle Urtica dioica, Yellow Loosestrife Lysimachia vulgaris and Water Mint Mentha aquatica. This is S26d Phragmites australis-Urtica dioica tall-herb fen, Epilobium hirsutum sub-community). This area abuts the river and an old watercourse (possibly an old course of the Cherwell) that runs along the bottom of the terrace. This edge and most of the channel is covered in secondary willow (Salix,) woodland.

Conservation status

Flat-sedge is widespread across Great Britain but is very local and has declined across its range due to changes to grassland management, including drainage and agricultural improvement and the cessation of grazing (Walker and Braithwaite, 2018). The species was studied nationally for the Threatened Plants Project (Walker, et al., 2017) which found that populations were more likely to have persisted in the uplands than in the lowlands, and that grazing management and/or disturbance caused by flooding were key to maintaining the open conditions required.
Due to the national decline of Flat-sedge and its association with habitats of high nature conservation value it is a Species of Principal Importance in England and Wales (former UK Biodiversity Action Plan species) and has been assessed as ‘Vulnerable’ on the vascular plant red lists for both Great Britain and England (Cheffings, et al., 2005; Stroh, et al., 2014).

In the vice county of Oxfordshire (v.c.23) Flat-sedge has always been a rare plant, only ever found at a small number of locations. It is thought to survive at only one of these, a site adjacent to the Victoria Arms pub in Old Marston, just north of Oxford (SP 51994 09046), within the Almonds Farm and Burnt Mill Meadows Local Wildlife Site (LWS). This site, near to the former Marston ferry, was known by G.C. Druce, who also saw Flat-sedge in Wychwood Forest around 1885 (Druce, 1886); it was recorded there again in the 1970s but has not been seen since.
The Marston population covers an area approximately 20m x 3m, with around two thousand inflorescences having been counted in a previous census of the population. It is impossible to estimate the number of individuals for such an extensively rhizomatous plant, however.

Site management and monitoring

The LWS is owned by Oxford City Council and over the years has been tenanted by local farmers for grazing. Until c.2009 the bank was grazed by horses, but is not grazed by the current tenant’s flock of sheep as there is a public right of way popular with dog walkers along the top of the bank, making grazing such a small disturbed area difficult. Following several years of no management, since spring 2017 the bank supporting the Flat-sedge is now mown by volunteers using scythes in the autumn and spring. It is hoped that in future a better arrangement can be reached, enabling grazing.
As the ecology of Flat-sedge is poorly understood, the Marston site offers an opportunity to investigate its hydrological requirements as well as to monitor responses to change in management. Thus, while monitoring to date has not been systematic, we are aiming to start more detailed monitoring in order to explore some of these questions.