Assessment Notes

Justification

This species is a European endemic resembling small oyster mushrooms restricted to coastal sand dunes and fruiting at the base of dead Ammophila. It was only distinguished from Hohenbuehelia culmicola at species rank in 2016 (but recognised as a distinct taxon at variety rank since 1970) and currently only known from three countries (and formerly, in 1949, in the Republic of Ireland). The relatively inconspicuous mushrooms are likely to have been overlooked due to the restricted microhabitat and late/early reproductive season. Although current evidence suggests this is much rarer than H. culmicola s.str., this assessment acknowledges that the two species were historically confused and not recognised as distinct entities. A few historical collections or records with associated photographs currently filed as H. culmicola might therefore be redetermined as H. bonii.

Criterion C seems most readily appropriate for this species, using an estimate of 2,000 mature individuals in total, but fewer than 1,000 mature individuals in each subpopulation, and accepting a continuing decline due to continued trampling of dunes, scrub invasion at some sites and conversion of coastal dunes to other uses (golf courses, caravan sites, more permanent housing). This yields an assessment of Vulnerable under criterion C2a(i).

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Taxonomic notes

Historically, Hohenbuehelia culmicola has been confused with this species now recognised as Hohenbuehelia bonii (Ainsworth et al. 2016). The occurrence of this latter species is now recognised in France, UK and the Netherlands (this last country added on basis of Flickr report of a redetermination of a “culmicola” collection). When all existing collections labelled as H. culmicola are checked, further redeterminations as H. bonii may be possible.

Earlier synonyms are Acanthocystis petaloides var. macrospora Bon and H. petaloides var. macrospora (Bon) Courtec. The two taxa were synonymised in Fraiture and Otto (2015).

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Geographic range

This species is restricted to coastal Ammophila stands, particularly around Atlantic and North Sea coasts. It has not been recorded in the Republic of Ireland since 1949. GBIF holds one New Zealand record of this species made in 2008 and originating from the redetermined collection PDD87504. This collection was found associated with Ammophila arenaria which is an introduced plant in New Zealand. Until there is evidence to the contrary, this is regarded as lying outside the natural range of this species and not considered in the assessment. There is also a 2019 Danish (Hirtshals øststrand) record of Hohenbuehelia culmicola accompanied by a photograph which is possibly of H. bonii. This is, as yet, an unconfirmed record of H. bonii and so it was not considered in the assessment, but it was included on the distribution map as Denmark is very likely to be within the distribution range of this species.

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Population and Trends

There are six sites known from the UK (one site known to have produced fruitbodies for two decades), but only seen on a few (fewer than five) culms of Ammophila at each site. Four sites are known from the Netherlands but it is described as a “characteristic species on the Northern French dune habitat" (Guinberteau 2011). It is possibly extinct in Ireland, with no known records in the last 70 years. 

It is estimated that there are two genets inside each occupied Ammophila clump and two ramets per genet. Surveyed sites in the UK had only one to a few occupied Ammophila clumps so using an estimate of five occupied clumps per site and apply a further x5 to estimate the true number of sites (and this seems a high estimate) we obtain 100 x 20 (10 confirmed sites plus an estimated 10 sites along the dunes of northern France) which gives 2,000 mature individuals in total. The largest subpopulation along the north coast of France is estimated to comprise ca. 1,000 mature individuals, but this should be reassessed when further georeferenced surveying has been carried out

Population Trend: decreasing

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Habitat and Ecology

It occurs in coastal sand dunes, rarely inland dunes, growing saprotrophically on Ammophila (also rarely on other Poaceae in same habitat such as Leymus arenarius). It has narrow niche requirements, i.e. dynamic sand dune ecosystems with sparse vegetation of herbs and grasses.

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Threats

Threats are mainly from conversion of coastal dunes to caravan/camping sites, golf courses or other development and, on a slower timescale, to visitor pressure and recreational disturbance (trampling, vehicles). Some UK sites have diminishing Ammophila stands due to invasion by Hippophae rhamnoides and management of this shrub can cause further damage to the habitat of Hohenbuehelia bonii. Changes in large-scale offshore dredging activities are also suspected to alter coastal dynamics unfavourably for this species, for example by shifting the prevailing deposition from sand to pebbles and shingle accretion (evidence noted at type locality, Kent, UK). Storm surges and long-term sea level rise, coupled with the associated coastal management responses, present a clear threat to dynamic dune systems upon which this species depends.

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Conservation Actions

Further research is needed to inform practical conservation actions. Management (cutting) of invasive Hippophae might be feasible for retention of partially invaded Ammophila habitat when the shrub is at early stage of encroachment. Control of mature Hippophae stands, on the other hand, is more likely to recreate a highly disturbed habitat which is then in a condition for new Ammophilla establishment and so may benefit Hohenbuehelia bonii in the longer term. Further study is needed to study the effects of increased sand dredging on the relationship between accretion of coastal sand and shingle (pebbles) and to what extent this threatens H. bonii's habitat and requires conservation action.

The main focus of research should be on further systematic survey of suitable habitat and checking of historic collections labelled "H. culmicola" to determine whether redetermination as H. bonii is required. Surveillance is also important to reveal how its fruiting presence (size and location of occupied grass culms/roots) changes from year to year to assess how ruderal this species is. Population genetics is required to more accurately estimate the number of genets per plant and to locate the whereabouts of the fungus within the plant (root systems or separate stems). Further ecological research is also required to determine whether particular Ammophila-dominated habitats are favoured or whether the age/vigour of the plants can provide clues to help direct field survey work to dune areas which might yield further records of reproductive individuals of H. bonii.

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Use and Trade

No trade is currently known, although some culinary experimentation by wild food foragers is to be expected given the overall similarity to edible oyster mushrooms.

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Source and Citation

Ainsworth, A.M. 2024. Hohenbuehelia bonii. The IUCN Red List of Threatened Species 2024: e.T126002961A247853241. .Accessed on 29 June 2024

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• Country occurrence