• Proposed
  • Under Assessment
  • Preliminary Assessed
  • VUAssessed
  • Published

Xerula caussei Maire

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Scientific name
Xerula caussei
Common names
Grauhaariger Wurzelrübling
slizečka černá
IUCN Specialist Group
Mushroom, Bracket and Puffball
Assessment status
Proposed by
Irmgard Krisai-Greilhuber
Irmgard Krisai-Greilhuber
Reda Iršėnaitė
A. Martyn Ainsworth, Daniel Dvořák
Anders Dahlberg

Assessment Notes

R-L categories correct, but text here does not match final assessment. Updated version will be published in IUCN´s Red List June or Nov 2019.


Paraxerula caussei is a medium sized thinfleshed gill mushroom with a tomentose blackish grey cap and a long radicant stipe. It is a meso-thermophile saprotrophic species growing on dead Fagus-roots, mainly in climax beech forest over limestone, on alcaline, base-saturated clayey soil, as well as immature “Kalkbraunerde” directly over mussel- or Malmlimestone. It is a European species present from the submeridional to the temperate zone and recorded from South and South-East, West, Central and southern North Europe. It is red-listed in nine European countries. Based on GBIF and databases available (see below) 234 subpopulations are known worldwide. The number of mature individuals has been estimated at 1404. The total area of occupancy is estimated as 470 km2. The habitat types Mullbraunerde-Buchenwald, mesophil limestone beech forest, and thermophile limestone beech forest are endangered biotope types in Austria (Essl & Egger 2010).
It is assessed as Endangered following criterium B2ab(iii), because of an area of occupancy of 470 km2, with criteria A and C it is vulnerable because decline in AOO suspected (A3c) due to endangered and vanishing climax limestone beech forests, i.e. habitat loss, and quality changes and C2ai, small population size (number of mature individuals only 1404, number of mature individuals in each subpopulation < 250). The assessment is based on quite small population size, fragmentation, loss of habitat and a diminishing habitat quality.

Taxonomic notes

Current Name in Index Fungorum:
Paraxerula caussei (Maire) R.H. Petersen

Why suggested for a Global Red List Assessment?

It is a very rare European species of climax Fagus forests, only known from few scattered subpopulations with few mature individuals. It is endangered by habitat loss, alteration and eutrophication.

Geographic range

It is a European species distributed in the submeridional to temperate zones, recorded from South and South-East, West-, Central and southern North Europe.

Population and Trends

Based on GBIF and databases available (see below) 234 known subpopulations worldwide. The number of mature individuals has been estimated at 1404 following these lines: (a) likely number of current localities (estimated to be ca. 234) (b) translation of the estimated total number of localities to an estimate of the total number of mature individuals in 2 steps (i calculate the number of functional individuals (ie. conspecific sporomata inhabiting an individual tree, template = 2, = 702): estimation of total number of localities × estimation of the average number of functional individuals/locality; ii convert the number of functional individuals into mature individuals following Dahlberg & Mueller 2010, template = 2 because few aggregates sporomata on a trunk should be counted as two mature individuals; = 1404). The number of known localities is 117 and the number of yet unknown localities was estimated to be twice as high. Mature individuals are very rare.
The AOO was estimated as being 470 km2, 2 x 2 m = 4 km2 for each locality, because almost all of the basidiospores of a mature fruit body are deposited within this area (Norros et al. 2012). Functional individuals of lignicolous fungi have to be calculated per trunk or log because of the mycelia (ramets) inhabiting the log, stump or root. For the AOO one has also to consider spore dispersal distance. Most spores of a mature individual are dispersed within 10s of metres, only a minor proportion is dispersed over longer distances. The average daily deposition rate, e.g. of Phlebia centrifuga (Norros et al. 2012) shows that dispersal from a single fruit body decreases from a few thousands to ca. 150 spores m–2day–1 in the first 60–80 m. Due to competition with other spores and other environmental factors (e.g. wood degradation stage, succession stage), colonization ability leading to a new generation via spore germination and establishment is only possible with a high number of spores deposited. This high number is only reached within the above mentioned 60-80 m, meaning that the AOO of a single ramet and its offspring is no more than 80 m. So calculating with the suggested 2 x 2 km2 is more than sufficient for AOO. For taxa that have a cryptic life form (such as fungi) occurrences may be estimated by tallying the area of 2 x 2 km2 (= 4 km2) grid cell in which observation records are located using equation 4.1. (IUCN Guidelines 2017): AOO = no. occupied cells × area of an individual cell. As it is still very difficult to have accurate distribution data, in case of macromycetes the no. of occupied cells is equated with no. of sites known.

Population Trend: Decreasing

Habitat and Ecology

It is a meso-thermophile saprobiont on dead Fagus-roots, occuring in climax beech forest over limestone, over light, shallow, skeletal, alcaline, base-saturated clayey soil, as well as immature “Kalkbraunerde” directly over mussel- or Malmlimestone.

Temperate Forest


It is endanged by fertilizer input, surface acidification, biotope destruction (logging) and biotope alteration (conversion of beech forests into coniferous and other forests, infrastructure measures).

Agro-industry plantationsUnintentional effects: subsistence/small scale (species being assessed is not the target) [harvest]Recreational activitiesOther ecosystem modifications

Conservation Actions

Reduction of fertilizer inflow via water and air from adjacent agriculture and habitat protection are needed.

Site/area protectionResource & habitat protectionSite/area management

Research needed

Investigation of population size, population trends and habitat needs would be highly welcome.

Population size, distribution & trends

Use and Trade

There is no use and trade known.



Dahlberg, A., Mueller, G., 2011: Applying IUCN red-listing criteria for assessing and reporting on the conservation status of fungal species. Fungal Ecology 4: 147-162.

Essl, F., Egger, G., 2010: Lebensraumvielfalt in Österreich – Gefährdung und Handlungsbedarf. Zusammenschau der Roten Liste gefährdeter Biotoptypen Österreichs. – Naturwissenschaftlicher Verein für Kärnten und Umweltbundesamt GmbH.

IUCN Standards and Petitions Subcommittee. 2017. Guidelines for using the IUCN Red List Categories and Criteria. Version 13. Prepared by the Standards and Petitions Subcommittee. http://nc.iucnredlist.org/redlist/content/attachment_files/RedListGuidelines.pdf
Krieglsteiner, G.J.,2001: Die Großpilze Baden-Württembergs Band 3. Ulmer.
Norros, V. et al. 2012. Dispersal may limit the occurrence of specialist wood decay fungi already at small spatial scales. 
– Oikos 121: 961–974.

Petersen, R.H.; Hughes, K.W. 2010. The Xerula/Oudemansiella complex (Agaricales). Beihefte zur Nova Hedwigia. 137:1-625.

Known distribution - countries

Regional Population and Trends

Country Trend Redlisted