Assessment Notes

Justification

Rhodotus palmatus is an apricot pink eye-caching saprotrophic fungus growing on dead trunks and branches of dead deciduous wood, especially on fairly recently fallen trunks, stumps and logs.  In Europe, it grows on various tree species, mainly on elm (Ulmus)  horse-chestnut (Aesculus), and ash (Fraxinus), rarer on other deciduous trees. In the United States, it was recorded on tulip tree (Liriodendron), maple (Acer), linden (Tilia) and elm (Ulmus). It prefers closed canopy broadleaved forests on moist soil.

The European part of the population is considered to be VU due to a decline in host trees by up to 20-40% caused by fungal pathogens killing its primary host, elm and ash. The population decline in other parts of its distribution (i.e., North America and temperate Asia) has not been quantified, but it is in decline. 

Globally the species is assessed at NT A3c+4c

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Taxonomic notes

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 regions on broadleaved trees of Fagaceae (Castanopsis and Lithocarpus) (Tang et al. 2014).

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Geographic range

This species has a circumboreal distribution and is rare elsewhere. It is known from North America (eastern and north central USA and southeastern 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), in Russia it is known from St. Petersburg and Sakhalin regions.  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).

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Population and Trends

In Europe, most records from the United Kingdom (700 in NBN atlas) are on elm and ash and the species is considered relatively common. In the Netherlands, it was recorded from 170 localities since 1990. In Germany, 45 localities are known while Norway and Sweden have 15 and10 localities, respectively.  In many other countries, R. palmatus has been recorded from one to several localities. It is included in Red Lists of 11 countries mostly as EN or CR, e.g.,  Estonia, Hungary, Latvia, Lithuania, Czechia, Poland, Slovakia, Romania, Armenia, Austria, Norway, and Sweden.  The species is protected by law in several European countries, e.g., Poland, Slovakia, Estonia, Serbia. The European part of the population is considered to be vulnerable due to a decline in host trees by up to 30-40% due to the effect of fungal pathogens killing its primary host, elm and ash trees. 

The distribution in North America is restricted to the eastern USA and south eastern Canada and the species is considered to be rare (Sundberg et al. 1997). 

In Russia, numerous localities of R. palmatus are known and the species is considered stable in mature forests.  It has been listed as threatened in other habitats, and is included in the Red Data Books of five regions.

As an additional species of Rhodotus confined to tropical regions of China has been recognized (Tang et al. 2014), and the distribution of R. palmatus in this country is thus unknown.

Population Trend: decreasing

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Habitat and Ecology

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), horse-chestnut (Aesculus), and ash (Fraxinus), rarer on beech (Fagus), oak (Quercus) and wild apple (Malus). In the United States, it was recorded on tulip tree (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 stream beds and rivers, in the areas that are periodically flooded or on wood hanging over the water (Svensson 2015). It produces basidiomata during from summer until late autumn and sometimes in spring. R. palmatus is easy to identify by its large fruit body with apricot or pink salmon colour, distinctly wrinkled gelatinous surface and curved stipe. 

The two primary hosts, elm and ash, are under serious threat and decline due to pathogens. Dutch elm disease is one of the most serious tree diseases in the world. It has killed over 60 million British elms in two epidemics and continues to spread today (Forest Research Newsletter 2019). Dutch elm disease has also decimated elms in North America. Ash dieback and Emerald Ash Boer have caused major declines of ash in Europe and eastern North America, respectively.

In Europe, there appears to be a trend that the species in western Europe, where the climate is more temperate and humid, has a broader habitat range including urban areas whereas towards the eastern part of its range it is restricted to mature, moist, natural to semi-natural forests.

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Threats

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 and ash, especially floodplain forests, could influence the survival of this fungus specialized towards particular micro-environmental conditions.

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Conservation Actions

It is important to preserve dead wood of fallen trees, not only in forests 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 legal species and habitat protection is recommended.

Further research on species identity and delimitation is needed. The distirbution 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.

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Use and Trade

There is no use or trade of this species.

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Source and Citation

Iršėnaitė, R., Kałucka, I.L. & Olariaga Ibarguren, I. 2019. Rhodotus palmatus. The IUCN Red List of Threatened Species 2019: e.T70402359A70402387. https://dx.doi.org/10.2305/IUCN.UK.2019-2.RLTS.T70402359A70402387.en .Accessed on 31 January 2025

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• Country occurrence