Hydnellum nemorosum is an apparently rare ectomycorrhizal tooth fungus which lives in symbiosis with several genera of broadleaved trees including members of Fagaceae. It was described as new to science in 2021 in a morphological and molecular study involving the generation of 136 DNA barcode sequences from Hydnellum species of European origin, of which only four sequences were shown to represent this species. These four sequences were derived from basidiomata (mushrooms) sampled in four different northwestern European countries and each was likely to have had an association with four different tree species. Therefore it is currently not possible to base any conservation assessments on criteria involving declining populations or habitats. The species does, however, satisfy Criterion D in having a very small estimated global population. There are four known mycelial patches, which is assumed to represent 40 mature individuals. It is to be expected that further discoveries of this species will be made, but current evidence suggests that it will remain a relatively rare to extremely rare tooth fungus. It is estimated that the global population might be up to 5 x 40 mature individuals, thus conforming to an assessment of EN according to Criterion D.
A batch of 136 European Sarcodon/Hydnellum specimens collected over a period of ca. 20 years was sequenced and analysed in a study by Nitare et al. (2021). This revealed that the new species H. nemorosum was extant in four northwestern European countries and, based on this preliminary evidence, it seemed to be a genuinely rare species. It is a morphologically distinct member of the former Sarcodon section Scabrosi (Maas Geesteranus 1975), but further preserved specimens of H. nemorosum might be found filed in fungaria under the names of other rare European relatives classified within sections Scabrosi or Violacei, for example H. fuligineoviolaceum, H. ioeides or H. lepidum (=S. regalis).
Nitare et al. (2021) showed that H. nemorosum was extant in NW Europe as a single known reproductive patch in each of four countries. Using the IUCN-approved assumptions for terrestrial patch-forming fungi (Dahlberg & Mueller 2011), this equates to 40 mature individuals.
Population Trend: Uncertain
Nitare et al. (2021) concluded that this species is an ectomycorrhizal partner of various broadleaved trees although, as yet, there are no known published molecular root tip analyses to confirm this. Although most related fungi (stipitate hydnoid or tooth fungi) that are found in broadleaved woodland are mycorrhizal with Fagaceae, and this was presumed to be the case with three of the four studied collections (found near Castanea, Fagus and Quercus), the remaining (Swedish) collection was recorded at a site lacking nearby Fagaceae. In common with other tooth fungi, H. nemorosum is found in sparsely vegetated, nutrient-poor, often sandy or gravelly soils and usually associated with sloping terrain (wooded hillsides, banks and ditches).
In common with other ectomycorrhizal tooth fungi, H. nemorosum is sensitive to atmospheric nitrogen deposition. Due to its apparent rarity, it is particularly vulnerable to adverse stochastic events which might disrupt its relationship with its photosynthetic tree partners. Tree-felling, e.g. of Castanea where this tree is not native, storm-induced windthrow of trees or habitat loss caused by invasive plants, e.g. Rhododendron and Pteridium, would constitute some of the major threats at a local scale. Routine site management operations, e.g. targeted to benefit other elements of biodiversity, to improve site infrastructure or to accommodate higher visitor numbers could all inadvertently damage or destroy H. nemorosum mycelia.
There is a need for reductions in the levels of atmospheric nitrogen deposition. At a local scale, perhaps the single most important conservation action would be to ensure that land owners/managers and their relevant subcontractors are made aware of the georeferences of all sites where H. nemorosum is recorded so they can take account of the presence of this species when planning management activities on site. Invasive species should be controlled where they are a problem. Thought should be given to ensuring that there is a supply of suitable ectomycorrhizal partner trees to sustain mycelia of this species at its known sites (ecological continuity)
Molecular analysis of ectomycorrhizal root tips is needed to confirm that this species is ectomycorrhizal and to investigate which tree species are symbionts of H. nemorosum. Further molecular research and fieldwork is required to fully elucidate the distribution of this species.