Assessment Notes

Justification

Tricholoma acerbum is associated with rich/calcareous Quercus(-Carpinus) forests, in Europe with a southern European, maybe a Southwestern European core area, but also known from northern Europe and east to Japan. The species is regarded as threatened (Endangered to Critically Endangered) in the Nordic countries, as well as in some countries in western and central Europe (Germany, The Netherlands, Switzerland, formerly also Great Britain). The species has been included in a preliminary proposal for a European red-list in 1993. The major habitat of the species; temperate, deciduous oak forests, is a strongly decimated/depauperate and fragmented habitat on a European base, where less than 2% of the original, old, thermophilous, deciduous forests are left at present (Hannah et al. 1995). In the evaluation period (50 years), the richer (mesotrophic-calcareous) oak forests have had a habitat loss estimated to be >30% in Europe (including a number of Natura 2000 habitats).

Tricholoma acerbum is estimated to have decline >30% during the evaluation period of 50 years (three generations) due to a long-term and persistent decline in oak forest habitats, both in area (due to altered land-use/urbanization, Phytophtora deseases on oak, etc.) and in habitat quality (lack of old-growth forests due to forestry, and also lack of “old-fashioned” grazed forests/woodland meadows). Hence, this species qualifies for listing as Vulnerable under criteria A2c+3c+4c.

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Geographic range

Tricholoma acerbum has a wide distribution in southern, western and eastern Europe, with outpost areas to the north in southwestern Fennoscandia (Brandrud 2013, Christensen and Heilmann-Clausen 2013). The species is also reported from southeast Asia (the conspecificity of the Asian populations should be verified genetically).

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Population and Trends

The distribution and population size in various regions of Europe are treated in Brandrud (2013) and Christensen and Heilmann-Clausen (2013). Tricholoma acerbum probably has its core areas in S-SW Europe, where it occurs both in Mediterranean areas as well in montane districts, e.g. reported to be rather frequent in the Apennines in Italy, and less frequent in Catalonia in Spain. The species is widely distributed but rather infrequent in W and C Europe. The species is locally not so rare in Great Britain, Switzerland and Austria (approx. 100 localities known from Great Britain and Switzerland+Austria), rare in Germany (70 locations known), scattered/not rare in Hungary, and reported from  southeast to montane areas of Greech and Turkey. On the northern flank, the species has one, larger outpost area along the southernmost coast of Norway (with 22 known localities). The species is extremely rare in Sweden (two known locations) and Denmark (five locations).

The fungus is also reported from North Africa (Morocco; Canary Islands), and from temperate regions of E Asia, both Japan, China and Korea.

In Europe as a whole, the number of known localities is probably around 400-500, with an estimated number of approx. 4,000-5,000 total localities in this area, maybe a total of 7,000-10,000 localities in the entire Eurasian (-North African) distributional area. It is very doubtful that the reports of Tricholoma acerbum s. lat. (including T. manzanitae) from Canada and USA represent the same species (they have deviating morphology and should be phylogenetically studied), so we do not consider the American populations in this assessment.

The species is regarded as threatened (Endangered to Critically Endangered) in the Nordic countries, as well as in some countries in W/C Europe (Germany, The Netherlands, Switzerland, formerly also Great Britain). The species was included in a preliminary proposal for a European Red- List in 1993 (Ing 1993).

An ongoing decline is due to decrease and decline in health of European oak forests. Habitat loss is estimated as >30% in Europe in the evaluation period (50 years), for these richer (mesotrophic-calcareous) oak forests, including a number of Natura 2000 habitats. Oak forests decreased by approximately 20% 1960-98 in Russia, and in richer/calcareous, old-growth forests probably more. The situation in E Asia is not known, but it is likely that the species (assuming this is the same taxon) is also decreasing in this region, due to deforestation of oak forests.

Population Trend: decreasing

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Habitat and Ecology

The species is mainly associated with Quercus spp. (mycorrhiza), but apparently also with Castanea, Tilia and Corylus, possibly also with Fagus. The species occurs in montane forests with deciduous Quercus species (such as Quercus robur, maybe also Q. pubescens) and Castaneus in southern Europe, but in Mediterranean areas also with the sclerophyllous Quercus ilex. In southern Europe, the species is reported mainly from mesotrophic to somewhat base-richer, but not calcareous forests (mainly on loamy-clayey soils). In central-western Europe, the species occurs in richer forests of the Quercus-Carpinus type. In the Norwegian outpost area, the species occurs in richer to calcareous Quercus-Tilia forests, in Russia reported from calcareous oak forests In southern Sweden, the species is reported from Fagus forests, possibly with isolated Quercus trees.

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Threats

The species is threatened by loss and fragmentation of intact Quercus forest habitat due to altered land-use, including deforestation, shift in silviculture from oak to conifer species, as well as a decline in health of oak forests due to  intensive forestry, loss of forest traditional grazing, which kept the forests semi-open, and assured oak recruitment, oak diseases, etc.

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Conservation Actions

To prevent decline and fragmentation of oak forests with good habitat quality, it is important to set aside Quercus forest reserves and woodland key biotopes, and in many cases (re-)introduce a moderate disturbance regime with cattle grazing, assuring semi-open conditions and oak recruitment.

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Use and Trade

The species is considered edible.

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Source and Citation

Brandrud, T.-E. 2015. Tricholoma acerbum. The IUCN Red List of Threatened Species 2015: e.T76265852A76266227. https://dx.doi.org/10.2305/IUCN.UK.2015-4.RLTS.T76265852A76266227.en .Accessed on 6 February 2023

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• Country occurrence