Assessment Notes

Justification

Cortinarius osloensis is a fungus forming  mycorrhiza with Tilia cordata in calcareous Tilia forests of the Oslofjord area. The species belongs to a group (subsect./clade Humolentes) that has been studied in detail in entire Europe (incl. sequencing; Brandrud et al. 2015), and based on available data, it seems probable that this is an endemic species for the Tilia forest region of SE Norway. The calcareous Tilia forests of the Oslofjord region are very old, most of them probably 6,000-8,000 old relic forest stands, and some of their fungi seems to be habitat specific, and of relict nature  (Brandrud 1999, Brandrud et al. 2011). The calcareous Tilia forests are treated as a threatened nature type in Norway (due to severe habitat-loss during 50 years), and many of its habitat-specialists are assessed as threatened on the Norwegian national Red List. Cortinarius osloensis is Red Listed as Endangered (EN) in the Norwegian national Red List 2015. As the probability to find this species beyond SE Norway is considered very low, the species should likewise be assessed as EN (C2a(i)) on the global level based on the assessment that the total population consist of less than 2,500 but more than 250,  mature individuals and that the habitat of C osloensis is subjected to a continuing decline. The  species is known from eight localities and the potential total number of localities is estimated not to exceed 13. The number of genotypes (genetically unique mycelia, corresponding to "fairy rings" of sporocarps) is between 2-7 at the known localities corresponding to an estimate of at least 260 mature individuals (cf. Dahlberg and Mueller 2011).

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Geographic range

Cortinarius osloensis  is confined (endemic) to the Oslofjord area in southeastern Norway.

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Population and Trends

The species is known from eight localities in the Oslofjord district. The habitat (calcareous lime forests) is very well investigated for fungi (incl. a specific monitoring program; Brandrud et al. 2014), and the total number of localities is estimated not to exceed 13. The number of genotypes (genetically unique mycelia, corresponding to "fairy rings" of sporocarps) is between 2-7 at the known localities corresponding to an estimate of at least 260 mature individuals (Dahlberg and Mueller 2011). The calcareous Tilia forest type is Red-listed as Vulnerable in Norway, and C. osloensis is nationally Red-listed as Endangered (EN) based on decline in habitat and a very small population.

Population Trend: decreasing

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Habitat and Ecology

The species is associated (mycorrhiza) with Tilia cordata in calcareous lime forests, on very dry, shallow-soil limestone outcrops or small scree areas with limestone/shale gravel (Brandrud et al. 2015). It is possible that the species formerly had a wider distribution associated with Tilia forests on calcareous ground, but that the species has been out-competed by Fagus-Quercus-Carpinus species in forests nowadays dominated by these tree species and that the species has survived in some Tilia forests of relic nature, outside the distributional area of Fagus and Carpinus.

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Threats

The calcareous Tilia forests of south east Norway have been declining by >30% the last 50 years due to increase of urban areas, including increase in roads, and limestone quarries (Brandrud et al. 2011). Also a “densification” due to increase in thickets e.g. of Fraxinus and Acer and invasion of Picea abies is seen in many stands (due to loss of traditional management and increased seed pressure from adjacent Picea plantations), increasing the humification/acidification of the calcareous topsoil, and threatening the old, relic biodiversity of the Tilia forests.

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Conservation Actions

A couple of the Tilia forests with C. osloensis have recently become nature reserves, but more sites need conservation. The calcareous lime forest is a so-called Selected habitat type under the Norwegian Nature Diversity Act, with a certain level of regulation (mild protection, actions plans, etc.).

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Use and Trade

The species is not known to be used.

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Source and Citation

Brandrud, T.-E. 2015. Cortinarius osloensis. The IUCN Red List of Threatened Species 2015: e.T76196620A76196690. https://dx.doi.org/10.2305/IUCN.UK.2015-4.RLTS.T76196620A76196690.en .Accessed on 1 February 2022

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• Country occurrence