R-L categories correct, but text here does not match final assessment. Developed and updated version published at IUCN´s Red List update, July 18th 2019.
The European/global population is estimated to 1700 localities/sites. This is equivalent to 34 000 individuals according to IUCN standards.
The decline of the species major habitat, calcareous Quercus-Carpinus forests in the evaluation period (last 50 years) is estimated to be in the magnitude of 15-20% (a similar decline also estimated for other, relevant habitats; Tilia-Corylus forests).
Based on this, the species becomes red-listed as NT according to the A-criterion (A2c + 3c + 4c) (species/habitat decline >15%). According to the C criterion it becomes LC due to population size >20 000 individuals.
The species is characteristic and easy identifiable, due to its initially striking bright violaceous blue cap. However, some older collections/finds can be difficult to interpret due to former, different name usage Many are still labelled C. terpsichores, some old are labelled C. caerulescens).
Cortinarius eucaeruleus (=C. terpsichores var. calosporus) is a mycorrhizal species associated mainly with Carpinus betulus, Corylus avellana, Quercus spp., and Tilia cordata. The striking, deep violaceous blue species occurs in most areas in calcareous Quercus-Carpinus forests, in outpost sites also in calcareous Tilia-Corylus- and Quercus-Corylus forests or woodlands. The species is everywhere rare, but is widespread in Europe, from the Mediterranean area to S Scandinavia, and from Spain-The British Isles east to Caucasus region in Russia.
The habitat (calcareous Carpinus-Quercus forests), and thus also the species is considered to have suffered a decline of 15-20% in the evaluation period, and the species is thus redlisted as NT according to A-criterion.
The species is rare but widely distributed within the European range of Carpinus-Quercus forests; known from France east to the Stavropol region (Causasus N) in SW Russia. It occurs north to the boreonemoral, southern part of Scandinavia and Estonia (in calcareous Tilia or Corylus forests), and south to montane Carpinus-Quercus forests in the mediterranean region.
Due to present knowledge on the species itself, and the distribution of its habitat, this seems to be a more or less strictly European species (possibly including Turkey).
The species is known from approx 20 sites/localities in Scandinavia. The real, total number is here estimated to 200 sites/localities.
Due to former mixing of names in the group (the names C. terpsichores and C. coerulescens often applied formerly), the number of known localities from temperate W and C. Europe is not precise, but is estimated to be approx. 50. The total number in W-C Europe is estimated to 50 x20 = 1000 localities. The number of verified occurrences in E and SE Europe including SW Russia is very little known. The total number here estimated to be somewhat lower than in W and C Europe based on the distributional range of Carpinus-Quercus forests; 500 localities. The European/global population then adding up to 1700 localities/sites. This is equivalent to 34 000 individuals according to IUCN standards.
The decline of the calcareous Quercus-Carpinus forests in the evaluation period (last 50 years) is estimated to be in the magnitude of 15-20%. Based on this, the species becomes red-listed as NT according to the A-criterion (A2c + 3c + 4c) (species/habitat decline >15%).
Cortinarius eucaeruleus is a mycorrhizal associate of Carpinus betulus, Corylus avellana, Quercus spp. and Tilia cordata. The species is found mainly in temperate/nemoral, calcareous Carpinus-Quercus forests, but also in boreonemoral outposts of calcareous Tilia-Corylus and Quercus-Corylus forests/woodland meadows. Sometimes reported from calcareous Fagus forests, but then probably with Quercus or Carpinus present.
Cortinarius eucaeruleus and its habitats (calcareous Carpinus-Quercus-Tilia-Corylus forests) are threatened by (i) areal loss due to urban/settlement expansion, lime quarries, etc, and (ii) lowered habit quality/ecological conditions due to forestry, invasive plants and eutrophication. In some areas also the change of management practices (ceased grazing) has had negative effects. The European area of oak forests has according to statistics decreased by 20% since 1966(?). There are also national evidence for a stronger decline; for instance the area of calcareous Tilia forests in Norway has decreased by >50% since 1970.
To prevent areal decline and further fragmentation of calcareous Carpinus-Quercus-Tilia forests with good habitat quality, it is important to set aside reserves on calcareous hotspots, housing many rare/redlisted, habitat-specific species such as Cortinarius eucaeruleus, C. odoratus, C. alcalinophilus and C. prasinus. It is furthermore important to establish also sites with a less strict conservation regime, such as woodland key biotopes, where some non-destructive human activity can be accepted (such as non-intensive, closed cutting).
More mapping/surveying and monitoring of C. eucaeruleus are needed. First and foremost more data on occurrences in calcareous Carpinus-Quercus forests of E and SE Europe are needed. These are per date only little mycologically surveyed. Secondly, it would also be very advantageous to have established monitoring series in this habitat. Data that can document time trends on C. eucaeruleus and co-occurring habitat-specific fungi are completely lacking. Finally, more documentation on the degree of decline of the habit itself is needed.