Conservation status of the rare endemic Centaurium tenuiflorum subsp. anglicum, English Centaury (Gentianaceae)

The status of the rare English endemic Centaurium tenuiflorum subsp. anglicum, English Centaury, has been assessed from field surveys in 2020 and compared against previous population counts. In Dorset, 16 populations with c.25,815 plants occurred and there was no evidence of overall decline. It was not refound in one site in the Isle of Wight. The IUCN threat status is ‘Least Concern’.

The JNCC (2020) assessment of its IUCN (2001) threat status as 'Least Concern' was based on 1996-2002 data in Edwards & Pearman (2004). In 2020, all accessible populations were revisited to obtain up-to-date population counts. The aim of this short paper is to present the data and update the threat status.

Methods
In Dorset the entire coastline between St Gabriels and West Bay was checked from the base of the cliffs by E. Downey; no populations occur on the cliff tops. Each population was counted as far as practical on the steep slopes, mapped with GPS and photographed. The larger populations tend to be somewhat fragmented and the counts are summations of the sub-populations. The populations were correlated against the sites in Edwards & Pearman (2004), some of whose 1996 grid references were estimated from maps rather than with a GPS. Full details are held by the authors and are available on request for bona fide researchers. On the Isle of Wight, the Medina Estuary, the only publicly-accessible site, was visited by T.C.G. Rich; no access was available to the King's Quay site.
Estimates of extent of occupancy were calculated using MAGIC maps (DEFRA, 2013).

Results
The counts from 2020 are summarised below with the equivalent 1996 or 2002 counts from Edwards & Pearman (2004).

Broom Cliff, St Gabriels SY392923
On 9 August 2020, five small plants were found growing amongst sparse vegetation.

Ridge Cliff, Seatown SY427912
In 1996, 20 plants occurred. On 29 July 2020, 100 plants were found on unstable ground on the edge of a cliff slump.

Dog House Hill (Thorncombe Beacon A) SY431913
On 30 July 2020, 2550 plants were found amongst sparse vegetation on the edge of a cliff slump.
Thorncombe Beacon SY432913 In 1996, 289 plants occurred. On 30 July 2020, 110 plants were found growing among Phragmites at the base of the cliff.

Eype (west A) SY442912
In 1996, 250 plants occurred. On 28 July 2020, one plant was found growing among vegetation on an unstable slump.

Eype (west B) SY44311
On 28 July 2020, five plants were found growing amongst vegetation on the edge of a slump.

Eype (west C) SY446910
In 1996, 108 plants occurred. On 2 August 2020, 90 plants were found growing among Phragmites and on bare clay.

Eype car park SY447910
On 2 August 2020, 5670 plants were found growing on cliff slopes below the car park, some of which were trampled.

Eype (east A) SY448909
On 3 August 2020, 1876 plants of varied height were found growing over c.20 m length at the edge of a cliff slump.

Eype (east B) SY449909
On 3 August 2020, 768 plants of varied height were found growing over c.10 m length at the edge of a cliff slump.
Eype (east C) SY450909 On 3 August 2020, 890 mostly small plants were found over c.15 m length at the edge of a slump.
West Cliff SY451908 In 2002, c.1000 plants occurred. On 3 August 2020, 3440 plants, mostly small, were found on dry, bare ground with some among Phragmites on a fairly steep slope.

Medina Estuary SZ502939
No plants were found on the Medina estuary on 16 July 2020; the site has changed since it was last recorded in 2006.

Discussion
The Dorset populations were estimated to contain c.14,455 plants in 15 sites in 1996/2002(Edwards & Pearman, 2004. In 2020, c.25,815 plants were found in 16 sites. There is no evidence of decline overall though individual colonies may have increased or decreased, the metapopulation dynamics related to the mosaic of cliff slumps where, following a slip, the plants colonise after 3-4 years and persist until the habitat closes over again (Pearman, 1999