Tricholoma colossus is an ectomycorrhizal fungus associating with Pinus in open old-growth pine forests, possibly favoured by fire. It is a characteristic and easily reckognizble species forming large, conspicous and robust sporocarps. It is widespread, but generally very rare in Europe with only few known localities in most countries. It has it main occurence in Fennoscandia and Russia. It is also rarely encountered in eastern USA.

It mainly occurs in oldgrowth forests, and usually with only a few mature individuals at each location. Individuals are considered to potentially be very old. It has been and is being negatively impacted by clear cutting and reducing areas of old growth pines outside protected areas. It is estimated to have declined and to be continuously declining at the global scale due to decreasing area of old growth pine forests and an apparent poor ability to spread and re-establish in managed forest after clear-cutting.

Large scale modern rotation forestry in Fennoscandia, Russia have significantly reduced and will continuously significantly reduce the potential amount of appropriate habitat. In Fennoscandia, the past, ongoing and future population decline inferred from habitat change, i.e. clear cutting, have been estimated from forest statistics of potential habitat decline to be between 30-50%  since 1970.

The species is globally assessed as VU (A2c+3c+4c) on account of the decline of its population size and habitat as indicated above.

very rare in old pine forests in the northern alpine hemisphere

It is widespread in Europe, but generally very rare and with only few known localities in most countries. An exception is its main distribution in the boreal parts, Sweden Finland and Russia (Karelia, St. Petersburg,  Kirov, Penza regions, Kabardino-Balkarian Republic). It is also roprted from North East USA (cf Gbif 2023).

The majority of the European population is located in Sweden and Finland and possibly also in Russia. In Sweden the total no of localities are estimated to be 400 (SLU Artdatabanken 2020), the number is probably in the same range in Finland and likely to be higher in Russia. In the remaining Europe, it is widespread but very rare and with only few localities in most countries.

In Fennoscandia, old-growth pine forest is estimated to have declined with more than 50% since 1960 (cf. Svensson et al. 2019 from Sweden, Kotiaho 2017 from Finland). Clear-cutting forestry was introduced in large scale in these countries around 1950 and on average 1% of the productive forest land is clear-cut annually. The habitat loss and reduced habitat quality is estimated to be simlar in Russia. The area of boral oldgrowth forests is estimated to correlate well with the population of T. colossus, hence the forest decline to be an approximate proxy of its population trend (cf. Brandrud and Bendiksen 2014). The species has a poor ability to re-establish after clear-cutting due to increasingly low proportion of old-growth-pine-forest where it occurs and hence very small likeliness to disperse and re-establish from spores. The species is estimated to be declining at a rate of >30% in 3 generations (50 years) worldwide (cf. Dahlberg & Mueler, 2011),

Tricholoma colossus forms ectomycorrhiza with Pinus on mainly slightly mineral rich, shallow soil with hin humus layer and in Cladonia-rich dry forest on nutrient-poor sandy soil (Christensen & Heilmann-Clausen, 2013). It is generally very rare typically with only a few mycelia per stand in old-growth pine forests on sediment/sandy soils.It is listed as ahigh-rank indicator species for dry sandy pine forest of large conservtion interest in the Nordic countires (Nitare, 2023). The mycelium of the fungus is considered to be long-lived, potentially several decades to centuries, comparable to its host pine trees (cf. Dahlberg and Mueller 2011). The mycelia of the fungus survives forest fires if their associated old tree survives.

Tricholoma colossus is primarily threatened by clear-cutting of old-growth pine forests (for decline of old-growth (pine) forests, see e.g. Svensson et al. 2019 from Sweden, Kotiaho 2017 from Finland). It is rarely observed in forest regenerated after clear-cutting. Hence conversion of old growth forests to managed forest, i.e. reduced amount of habitat, is the main cause of the decline. It may also to some extent be negatively affected by areal loss due to expansion of urban settlements, military areas, roads etc. of ) of sandy pine forests (see Brandrud and Bendiksen, 2014).

The major part of the potential pine habitat is located in northern Europe and some parts of Russia. Around 1% of the forest is cut annually, so a conservative estimate of the potential decline of appropriate habitat is approximately 30% over 50 year period (three generations, Dahlberg & Mueller, 2011) taking into account uncertainties of numbers and size of subpopulations and rates of forest cutting in Russia.

It is nationally redlisted in multiple countries throughout Europe. Set aside Scots pine forest reserves where the species have good subpopulations. At these forests, natural or prescribed burning should be considered to maintain desired forest dynamics.

A better understanding of the specie’s population dynamic would facilitate a better management, e.g.at what conditions it may establish, the demographic structure within populations and the pattern (and causes) of individual mycelia. It is possible that T. coloccus may survive selective cutting if a sufficient number and density of retention trees are preserved in a managed forest. In this case it will be critical to ensure that as many retention trees as possible are located where the mycelia of the fungus are growing, e.g. where the sporocarps have been observed.

There is no use and trade known.

Brandrud, T. E. & Bendiksen, E. 2014. Fungi of sandy pine forests in Norway, and a comparison of this threatened element elsewhere in Europe(-Asia). Agarica 35: 67-87.

Dahlberg A & Mueller G. 2011. Applying IUCN red-listing criteria for assessing and reporting on the conservation status of fungal species. Fungal Ecology 4: 1-16

GBIF (2023). Tricholoma colossus (Fr.) Quél. in GBIF Backbone Taxonomy. Checklist dataset https://doi.org/10.15468/39omei accessed via GBIF.org on 2024-12-09.
Hyvärinen, E., Juslén, A., Kemppainen, E., Uddström, A. & Liukko, U.-M. (eds.) 2019. The 2019
Red List of Finnish Species. Ympäristöministeriö & Suomen ympäristökeskus. Helsinki. 704 p.

Kotiaho jS. 2017. On effective biodiversity conservation, sustainability of bioeconomy, and honesty of the Finnish forest policy. Ann. Zool. Fennici 54: 13-25.

Nitare J. 2023. Skyddsvärd skog: naturvårdsarter och andra kriterier för naturvärdesbedömning (In Swedish: Protected forest: nature conservation species and other criteria for nature value assessment). 3rd ed. Skogsstyrelsen. Jönköping. ISBN 987-91-986297-0-5

SLU Artdatabanken (2020). Rödlistade arter i Sverige 2020. (Red-listed species in Sweden 2020)
SLU, Uppsala ISBN 978-91-87853-54-8

Svensson J, Andersson J, Sandström P, Mikusinski G and Jonsson B G. 2019. Landscape trajectory of natural boreal forest loss as an impediment to green infrastructure. Conservation Biology 33(1): 152-163.