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 recognizable species forming large, conspicuous and robust sporocarps. It is widespread, but generally very rare in Europe with only few known localities in most countries. It has its main centre of occurrence in Fennoscandia and Russia. It is also rarely encountered in eastern USA.

It mainly occurs in old growth forests, and usually with only a few mature individuals at each site. Individuals are considered to potentially be very old. It has been, and is being, negatively impacted by clear-cutting and the reduction of areas of old growth pines outside of protected areas. Is suspected to have declined, and to be continuously declining, at the global scale due to the 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 and Russia have significantly reduced, and will continuously significantly reduce, the potential amount of appropriate habitat. In Fennoscandia, the past, ongoing and future population decline suspected from habitat change, i.e. clear-cutting, has been estimated from forest statistics of potential habitat declines to be between 30-49% over 50 years (three generations).

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

It is widespread in Europe, but generally very rare and with only few known localities in most countries (and so the map is incomplete because of uncertainties). An exception is its main distribution in the boreal parts of Sweden, Finland and Russia (Karelia, St. Petersburg, Kirov, Penza regions, Kabardino-Balkarian Republic). It is also reported from eastearn USA (see 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 remainder of 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 (see 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 similar in Russia. The area of boreal old growth forests is estimated to correlate well with the population of Tricholoma colossus, hence the forest decline is appropriate to be an approximate proxy of its population trend (see Brandrud and Bendiksen 2014). The species has a poor ability to re-establish after clear-cutting due to the increasingly low proportion of old-growth-pine-forest where it occurs and hence very small likeliness of dispersal and re-establishment from spores. The species is suspected to be declining at a rate of 30-49% in three generations (50 years; Dahlberg and Mueller 2011) worldwide.

Tricholoma colossus forms ectomycorrhiza with Pinus on mainly slightly mineral rich, shallow soil with a humus layer and in Cladonia-rich dry forest on nutrient-poor sandy soil (Christensen and 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 a high-rank indicator species for dry sandy pine forest of large conservation interest in the Nordic countries (Nitare 2023). The mycelium of the fungus is considered to be long-lived, potentially several decades to centuries, comparable to its host pine trees (see 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 the decline in 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 this species' decline. It may also to some extent be negatively affected by expansion of urban settlements, military areas, roads etc. (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 a 50 year period (three generations, Dahlberg and Mueller 2011) taking into account uncertainties over numbers and the size of subpopulations and rates of forest cutting in Russia.

There is no use and trade known.

It is nationally Red Listed in multiple countries throughout Europe. It will be important to set aside Scots Pine (Pinus sylvestris) forest reserves where the species has good subpopulations. In these forests, natural or prescribed burning should be considered to maintain desired forest dynamics. A better understanding of the species' population dynamics would facilitate better management, e.g. at what conditions it may establish, the demographic structure within subpopulations and the pattern (and causes) of individual mycelia growth. It is possible that Tricholoma colossus may survive selective cutting if a sufficient number and density of trees are preserved in a managed forest. In this case it will be critical to ensure that as many retained trees as possible are located where the mycelia of the fungus are growing, e.g. where the sporocarps have been observed.