Cortinarius osloensis is a mycorrhizal species associated 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), 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 is very old, most of them probably 6000-8000 old relic forest stands, and some of their fungi seems to be habitat specific, and of relic nature. 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 treated as threatened on the national redlist as well. Cortinarius osloensis is redlisted as EN on the Norwegian redlist. As the probability to find this species beyond SE Norway is considered very low, the species should likewise be treated as EN (C2a(i)) on the European-global redlist as well.

The species is confined (endemic) to the Oslofjord area, SE Norway.

The species is known from 8 localities in the Oslofjord district. The habitat (calcareous lime forests) is very well investigated for fungi (incl. a specific monitoring program), and the total number of localities is estimated not to exceed 13, corresponding to 260 individuals. The calcareous Tilia forest type is redlisted as VU in Norway, and C. osloensis is nationally redlisted as EN based on decline in habitat a a very small population. Proposal for European/global redlist: EN (C2a(i); B2ab(ii, iii, iv)).

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. It is possible that the species formerly had a wider distribution associated with Tilia forests on calcareous ground, but that the species has been outcompeted 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.

The calcareous Tilia forests of SE Norway have been declining by >30% the last 50 years due to increase of urban areas, including increase in roads, and limestone quarries. 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.

A couple of the Tilia forests with C. osloensis has recently become nature reserves, but more sites needs conservation.

Brandrud, T.E. 1999. Cortinarius subgenus Phlegmacium species associated with Tilia cordata (and Corylus avellana) in SE Norway: A relictual element? J. Journées Européennes Cortinaire 1: 83-88.
Brandrud, T.E., Hanssen, O., Sverdrup-Thygeson, A. & Ødegaard, F. 2011. Kalklindeskog – et hotspot-habitat. Sluttrapport under ARKO-prosjektets periode II. - NINA Rapport 711. 41 s. [in Norwegian w/ English summary]
Frøslev, T.G., Jeppesen, T.S. & Brandrud, T.E. 2006. New species and combinations in Cortinarius subgenus Phlegmacium section Calochroi. Mycotaxon 97: 367-377.
Brandrud, T. E., Frøslev, T. G. & Jeppesen, T. S. 2015.The taxonomy of Cortinarius subgenus Phlegmacium subsection Humolentes in Europe. In prep. for Mycological Progress.