Grifola frondosa is widespread but rare European wood-inhabiting fungus with large annual fruitbodies forming at the trunk base of old and large oak veteran trees. Its distinctive and wellknown multipileate fruitbodies may also occasionally develop on other old-growth deciduous hardwoods, such as hornbeam, chestnut, beech and elm. Such large-diameter trees are are declining as cutting exceeds new addition of such trees and and inappropriate management in cultural landscapes.

The species is assessed as to be NT due past and ongoing population decline with 20-25% during the last 50 years (which is an evaluation period equal to three generations, each generation about 17 years long).The ongoing substrate loss and decrease of population is expected to continue over the next 50 years. Due to population decline referring to habitat quality, the species meets the threshold for Near Threatened A2c+3c+4c.

Grifola frondosa was long considered a temperate circumpolar species with Northern Hemisphere distribution (Ryvarden, Gilbertson, 1993). However, molecular data revealed different phylogenetic species with continental biogeographic patterns. According to Shen et al. (2002), G. frondosa clade was partitioned in two phylogenetic species represented by one eastern North American and Asian line and one distinct European lineage, although in that research only a single European isolate was analysed. Following recent phylogenetic analysis (Gargano et al., 2020), the Italian sample of white G. frondosa clustered together with other isolates from Europe, separately from the rest of sequences deriving from East Asia and the USA, and thus a distinct taxonomic entity was also suggested for the terminal G. frondosa clade.
A recent molecular study of G. frondosa complex based on 45 samples from East Asia, Europe and North America (Xie et al., 2024) demonstrated highly supported clades corresponding to each continent; East Asian and European isolates were shown to belong to two different species: G. albicans, described from Japan and representing the East Asian clade, and G. frondosa, remaining for the European clade. For this assessment, we consider Grifola frondosa to be restricted to the European taxon.

Grifola frondosa is a lignicolous fungus confined to old and large trees, mainly oak but also other deciduous hardwoods or occasionally conifers. This conspicuous and rarely occurring fungus is declining globally due to deforestation, particularly large-scale logging of old-growth trees.

Grifola frondosa is native to temperate hardwood regions of Europe. It is occur within the natural range of oak in almost all European countries, but is unevenly distributed throughout the continent. The fungus is more common in areas with milder climate, influenced by the Gulf Stream. According to GBIF, its highest occurrences are reported from three countries – the Netherlands, southern Sweden and UK. In other countries, in many localities within its range it is considered a rarely occurring species.

Grifola frondosa is a well-known species forming large and conspicuous fruit bodies atthe base of old trees. Its is much searched and can hardly can be overlooked. If present, the fungus typically occur at one or a few trees per site, on average estimated as three trees with one genetic individual each.
Based on population decline, the species is nationally red-listed as EN in Bulgaria and Lithuania, as VU in Austria, Czech Republic, Denmark, Germany, Latvia, Norway and Ukraine. It is assessed as NT in Finland, Romania and Sweden. The species is protected by law in Estonia (category 1 species), Latvia (currently included in the list of specially protected species), Poland (listed among the species under partial protection), Lithuania and Ukraine. In Slovenia, Russia and Belarus it is considered a rare species. In some countries the species is regarded uncommon but not rare (Slovak Republic), common but not frequent (Italy), common and not threatened (Netherlands).

Overall in Europe, G. frondosa is considered to be Near Threatened due to continuous population decline by up to 20-25% over three generations (50 years), which is estimated to be ongoing process, largely because of limited reproductive capacity of the fungus and persistent loss of its host/substrate (giant oak trees) throughout the continent.

Grifola frondosa is a wood-inhabiting saprotroph forming large fruit bodies at the base of living or dead hardwoods as well as on decayed tree trunks and stumps. It is almost exclusively found fruiting on large old-growth trees where it decay heartwood and submerged roots. Its mycelium may live for many decades in wood. Although mycelium persists in the substrate for a long time, large annual sporocarps may not form every year.

In Europe, G. frondosa is mostly associated with oaks (Quercus spp.) but has been also recorded on other deciduous trees, such as hornbeam (Carpinus), beech (Fagus), elm (Ulmus), maple (Acer), birch (Betula), chestnut (Castanea), eucalypt (Eucalyptus), walnut (Juglans), poplar (Populus) (Ryvarden, Melo, 2014).

The fungus occurs in forests as well as in pasturelands, parklands and gardens. Habitat preferences of the species vary across its range. In the milder climates of Western Europe, it is frequently found in urban plantations and agricultural landscapes (wood-pastures); under the more continental climate in Eastern Europe, it is mostly limited to natural or semi-natural forests and forest parks.

The main threat is felling of old trees in forests, parks and gardens (Quercus spp., other Fagaceae and Betulaceae). Old growth deciduous forests are highly vulnerable. Forests dominated by slow-growing oak trees are under increasing threat of deforestation, uncontrolled logging for timber in mature forests, replacement of old growth forests by fast-growing trees. Large size oak trees are also declining in pasturelands and urban plantations. In addition to habitat destruction, in areas where picking wild mushrooms is traditional, collection of large edible fruit bodies can be also a threat.

Conservation actions should focus on preserving currently existing and potential host trees, particularly multi-aged stands of mature oak in forests, open pastures, parks and gardens. These habitats must be managed to ensure regeneration, to maintain stands of oak trees of all ages and to support the long-term viability of host trees so that they can reach significant age and size.

Further research on phylogenetic and taxonomic diversity of Grifola within the Northern Hemisphere is required, following the recent studies on the Southern Hemisphere group of species (Rugolo et al., 2023), new taxa in Grifola proposed from Asia/China based on morphology and molecular data (Tang et al., 2024; Xie et al., 2024), etc. Surveys are also needed to study population trends of G. frondosa and to monitor its habitat trends.

In Europe, G. frondosa is not in use. In Asia, a close phylogenetic species, known as edible maitake mushroom, is commercially cultivated and widely used in traditional medicine (Xie et al., 2024).

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