R-L categories correct, but text here does not match final assessment. Developed and updated version published at IUCN´s Red List update, July 18th 2019.
Rhodotus palmatus, apricot pink eye-caching saprotrophic fungus growing on dead trunks and branches of elm (Ulmus), ash (Fraxinus), sometimes beech (Fagus) or other deciduous tree species in dense, moist broadleaved forests usually close to streams or rivers. The amount of most suitable substrate, dead wood of elm or ash, has been recently increasing due to fungal diseases which killed these trees. Such an increase in the substrate may be temporal because a population decline of elm and ash is probably going to continue in the future. Elm tree has widespread distribution but it is considered as threatened to various degree at the country level and as Vulnerable (VU) throughout Europe due to population decline caused by the Dutch elm disease (Barstow & Rivers 2017); common ash is listed as Near Threatened (NT) approaching the criterion of population decline of at least 20-30% in the future (Khela & Oldfield 2018). R. palmatus is connected to this particular substrate, especially in northeastern and northern Europe. The most abundant population of R. palmatus is known to occur in the Netherlands and the UK but in many other countries it was found in few localities only and is red-listed and/or protected in Estonia (I cat), Hungary (2), Latvia (EN), Norway (EN), Poland (E; strictly protected), Serbia (strictly protected), Slovenia (EN), Sweden (CR). The observed increase in localities in the Netherlands and the UK during the last decades could have been caused by an increased amount of elm dead wood. The size of the current population of R. palmatus evaluated in Europe and subjected to 20-30% decline of habitat due to tree pathogens allow to apply the criterion A3c and to evaluate the threat for this species as Near Threatened (NT) on the European scale. On the world scale, it can be evaluated as Least Concern (LC) because of broad distribution and reported good situation in Russia or Data Deficient (DD) if accepting the delimitation of new Rhodotus species in tropics and subtropics (see Taxonomy).
Rhodotus was regarded as a monotypic genus with R. palmatus species, but recent studies from China revealed that there are two different phylogenetic species: R. palmatus and R. asperior. R. asperior occurs in the tropical and subtropical environment on broadleaved trees of Fagaceae (Castanopsis and Lithocarpus) (Tang et al. 2014).
Rhodotus palmatus is a beautiful fungus occurring on dead wood of deciduous trees (mainly elm and ash) in deciduous temperate forests, usually in the vicinity of streams and rivers, in North America, Europe and Asia. Although it is not rare in a few European countries, especially with mild, Atlantic-type climate (the UK, the Netherlands, northern France) or in Russia, in most of the other areas it is rare or extremely rare and considered threatened. The main threats are the reduction in the amount of available substrate, as both elm and ash are subject to aggressive fungal pathogens reducing their populations even by 20-30%, removing large diameter dead wood, as well as degradation and loss of proper habitats, e.g., floodplain riparian forests. The species is red-listed and/or protected with law in several European countries.
It has circumboreal distribution and is rare elsewhere. It is known to occur in North America (USA and eastern Canada), northern Africa (Algeria), northern Asia (Caucasus, Japan) and Europe. In Europe, it occurs in almost all countries, from Spain, France, Ireland and the UK in the west, Italy, Romania and Ukraine in the south and Norway and Sweden in the north (Krieglsteiner 2001, Bujakiewicz 2002). It is much more common in the Atlantic countries with mild climates, namely England, northern France and the Netherlands (Gulden 1983). Some authors claim that it has expanded its range in recent years with the death of elms caused by the Dutch elm disease (Wattling and Gregory 1989). In Denmark, the species was found only after 2006; in Switzerland, it occurs only on planted trees in the urban environment. The species has been also recorded from China but recent studies show that it could be R. asperior, a species typical of tropical and subtropical regions of China (Tang et al. 2014).
The estimated possible population in Europe is 100000 estimated from possible 1000 localities and 10 individual mature individuals in the locality (5 genets, 2 ramets per genet).
Most records known from the United Kingdom (700 in NBN atlas) are on elm and ash and due to the abundance of dead wood considered rather frequent. In The Netherlands, it was recorded in 170 localities after 1990. In Germany, 45 localities are known; the species is not rare or red-listed. In these countries, it could be found on beech (Fagus) wood. In many other countries, R. palmatus has been recorded from one to several localities and is red-listed in, e.g., Estonia, Hungary, Latvia, Lithuania, Czech Republic, Poland, Slovakia, Romania, Armenia. In Norway, it is known from 13 localities and red-listed as Endangered (EN) C2a(i) on the bases of small population < 2500 and ongoing reduction in habitat. In Sweden, it is known from 10 localities and is Critically Endangered (CR). The species is protected with law in a few European countries, e.g., in Poland, Slovakia, Estonia, Serbia. The European part of the population is considered to be endangered by a host tree decline by up to 20-30% due to the effect of fungal pathogens killing elm and ash trees.
The distribution in North America is restricted to the eastern part of the USA and Canada and the species is considered to be rare (Sundberg et al. 1997).
In Russia, numerous populations of R. palmatus are known and they are considered stable especially in mature forests; the species is red-listed in two regions: Leningrad and Sakhalin.
R. palmatus was recorded in China, however, another species of Rhodotus confined to tropical regions of China has been recognized (Tang et al. 2014) and real distribution of R. palmatus is thus unknown.
Population Trend: Decreasing
Rhodotus palmatus is a saprotrophic fungus growing on dead deciduous wood, especially on fairly recently fallen trunks, stumps and logs of elm (Ulmus). In Europe, it grows on various tree species, mainly on elm (Ulmus) and horse-chestnut (Aesculus), rarer on ash (Fraxinus), beech (Fagus), oak (Quercus) and wild apple (Malus). In the United States, it was recorded on Liriodendron, maple (Acer), linden (Tilia) and elm (Ulmus). It prefers closed canopy broadleaved forests on moist soil. It tends to occur on dead wood lying near streambeds and rivers, in the areas that are periodically flooded, hanging over them (Svensson 2015). It produces basidiomata during the summer till late autumn and sometimes in spring. R. palmatus is easy to identify by large fruit body with apricot or pink salmon colour, distinctly wrinkled gelatinous surface and curved stipe (Phillips 2006).
The species is threatened by the reduction of suitable substrate (elm and ash wood) due to fungal diseases of these tree species and removal of dead wood from forests and parks. Also, the changes of composition, openness and moisture regime in broadleaved forests with elm, especially floodplain forests could influence the survival of this fungus specialized towards particular microenvironmental conditions.
It is important to preserve dead wood of fallen trees not only in forests in nature conservation areas but also in parkland and urban areas where elm or ash trees die. In the countries where few localities of the species are known, establishing the legal species protection is recommended.
Further research on species genetical identity and delimitation is needed. The occurrence limits of R. palmatus and R. asperior should be investigated. The occurrence of the species should be monitored especially against the background of elm and ash dieback.
There is no question of use or trade in case of this species.
Barstow, M., Rivers, M.C. 2017. Ulmus glabra. The IUCN Red List of Threatened Species 2017: e.T61966807A61966819.
Bujakiewicz A. 2002. Rhodotus palmatus (Bull.: Fr.) R. Maire. [In] Wojewoda W. (ed.) Atlas of the geographical distribution of fungi in Poland 2: 95-98.
Khela, S., Oldfield, S. 2018. Fraxinus excelsior. The IUCN Red List of Threatened species 2018: e.T203367A67807718.
Krieglsteiner, G.J., 2001. Die Groβpilze Baden-Württeemberg Band 3. Verlag Eugen Ulmar, Stuttgart.
Phillips, R. 2006. Mushrooms. Macmillan. London. 384 pp.
Sundberg, W.J., Methven, A.S., Monoson, H.L. 1997. Rhodotus palmatus (Basidiomycetes, Agaricales, Tricholomatceae) in Illinois. Mycotaxon 65: 403-410.
Svensson, V. 2015. Ådermusslig (Rhodotus palmatus) – en akut hotade svamp i Söderåsens nationalpark [Rhodotus palmatus found in Söderåsen National Park, southernmost Sweden]. Svensk Mykologisk Tidskrift 36: 42-45.
Tang, L.P., Hao, Y.J., Cai, Q., Tolgor, B. & Yang, Z.L. 2014. Morphological and molecular evidence for a new species of Rhodotus from tropical and subtropical Yunnan, China. Mycological Progress 13: 45–53.