Geography and distribution
Hericium erinaceus fruitbody emerging from trunk fissure on live beech (Photo: Dr A. Martyn Ainsworth)
Bearded tooth is found in Europe, Asia and North America and generally throughout the North temperate latitudes. In Britain, most of the fruitbody records are on beech in southern England, and the UK stronghold is undoubtedly the New Forest in Hampshire. Three unpublished New Forest Hericium surveys in 1998, 2007 & 2009 each documented around 12-15 occupied trees with fruitbodies present on the day of survey. Judging from available records, this stronghold has 40-45 different trees that have supported fruiting at some time between 1960-2007. This represents around 45% of the estimated 105 trees in Britain that are known to have supported fruiting of H. erinaceus during the same period.
H. erinaceus ccupies central deadwood of trunks and large diameter branches of living and dead, standing and fallen, broadleaved trees. Typically these are old pollards or mature trees with spreading crowns, but it can also fruit occasionally on wounded or damaged younger trees growing in plantations.
The fruitbody of bearded tooth is a large, irregularly bulbous, compact, white to creamy mass of 5-40 cm in diameter. It sometimes comprises a few stout basal branches and is always dominated by crowded, rubbery, hanging, spore-producing spines, 10-40 mm long. The fruitbody becomes yellowish-brown with age.
The hyphal system is monomitic (contains only generative hyphae – undifferentiated filamentous structures), amyloid, comprising thin- to thick-walled hyphae, around 3-15 μm wide, with clamped septa and numerous gloeoplerous elements (filled with oily droplets) which can protrude into the hymenium (spore-bearing layer) as gloeocystidia. The basidia (spore-producing organs) are 25-40 x 5-7 μm, four-spored, and have a basal clamp. The spores are white in mass, finely roughened or warty, amyloid, short ellipsoid, and around 5-7 x 4-5.5 μm.
Escaping to the surface - how fruiting occurs
Since the fungus feeds in the central dead parts of the tree, it is to some extent imprisoned by the outer cylinder of healthy living wood, at least while the tree is alive. The challenge is to find an escape route to reach the surface, produce a fruitbody and disperse its spores in the open air. Observations show that fruiting occurs where this outer living cylinder is locally absent or damaged and dysfunctional. For example, the inner wood might be directly exposed to the air around trunk fissures, fractures, hollows and other breaches of the outer wood, including saw-cuts. On the other hand, there might be a deadwood bridge, such as occurs within branch stubs and below large wounds, which provides a medium for the fungus to grow through to reach the surface. The lower frequency of such features, often those conferring 'veteran tree' status, on younger trees probably helps to explain the lower frequency of associated Hericium erinaceus fruitbodies.
Hericium erinaceus fruitbody at the earliest stage of spine development (Photo: Dr A. Martyn Ainsworth)
Fruiting is usually seen from September to December and only rarely are there two 'flushes' of fruiting from a single location on a tree within a year. The fungus is characterised by intermittent fruiting on occupied trees over several decades and this often continues for many years after the branch or trunk falls to the ground. Given the importance of branch stubs for fruiting, the apparent preference for trees in relatively open woodland might reflect the relatively greater frequency of large, long-lasting lower branch stubs in such habitats.
Young trees growing in denser woodland have lower branches that are relatively quickly shaded out and soon fall. Under these conditions, the branch stubs are smaller and therefore take less time to become sealed off by successive annual rings of living wood. Consequently, their mature standing trunks present relatively few escape opportunities for any bearded tooth that might be feeding within.
Other similar species
There are two other Hericium species in Britain, also found fruiting on broadleaved trees, especially beech. Of the three species, H. cirrhatum, sometimes called Creolophus cirrhatus, has the most recorded sites, but tends to fruit for one or, at most, just a few years on the same piece of wood and then is not seen again. Its fruitbody differs from that of H. erinaceus in taking the form of tiered layers of white brackets whose lower surfaces are covered in white, downward-projecting, but shorter, spines. Confusion is most likely when the fruitbodies are very young, old, or high on the tree and not clearly visible. Microscopically, there are slight differences in fruitbody hyphae and spores. The third species, H. coralloides, is probably the rarest and usually fruits on beech or ash with less of a tendency to fruit high up on the standing tree. It has its own unique fruiting behaviour in that once it has commenced fruiting, it usually produces fruitbodies annually for up to five years or so and then reproduction ceases. The fruitbody form is much more delicate than that of its two relatives and the white spines hang, comb-like, from beneath a mass of coralloid branches which dominate the structure. Only in its very youngest stage, before the branches have developed, could it be confused with its relatives.
Hericium erinaceus young fruitbody emerging from branch stub on living beech (Photo: Dr A. Martyn Ainsworth)
- Hericium cirrhatum
- Hericium coralloides
Threats and conservation
Hericium erinaceus is of conservation concern across its European range. It is threatened at several scales ranging from changes in land use down to the removal and use of individual pieces of wood supporting its mycelia (vegetative parts). Of particular concern in this respect is the removal of older wood, particularly trunks and main branches of beech in areas with known fruiting populations of the fungus, because of the extended period required for their natural regeneration and replacement. Storm-damaged branches and branches cut from trees overhanging paths and car parks for safety reasons should be relocated wherever possible to less formal woodland settings nearby. Judging from the low density of fruitbody production in its New Forest stronghold, sustainable breeding populations of the fungus favour open-grown trees, especially beech, patchily distributed throughout a wide landscape with a full range of age-classes present and good ecological continuity of beech presence. Further research on the establishment phase of the fungus is required to help inform decision-making regarding which and how many trees are best left 'for the fungus' where woodland is being managed for the dual aims of forestry and biodiversity.
Bearded tooth is listed as one of only four non-lichenised fungi on Schedule 8 of the Wildlife and Countryside Act 1981, and is thereby accorded the highest level of protection for a fungus in the UK. It was red-listed as Vulnerable in Great Britain in 1992, but removed from the 2006 GB red list (both listings were unofficial). It is a UK BAP (Biodiversity Action Plan) priority species and a species of 'principal importance for the conservation of biological diversity' in England and Wales, listed in accordance with Sections 41 & 42 of the Natural Environment and Rural Communities Act 2006.
In 2003 around half (220) of its post-1980 European localities were thought to be located in Britain or France and it was red-listed in 13 of the 23 European countries in which it had been recorded. It was one of 33 fungi proposed for inclusion in the Appendices of the Bern Convention to try to secure fungal representation within the EU Habitats Directive.
Why is bearded tooth so rare?
Hericium erinaceus fruitbody inside fallen hollow beech (Photo: Dr A. Martyn Ainsworth)
Firstly, it should be noted that the term 'rare fungus' simply refers to a fungus whose fruitbodies are rarely recorded. Due to the challenging issues of sampling and detection, the true number of fungal individuals in a geographic area is unknown. This number would have to take into account all the cases where bearded tooth was present in the wood, but was not in fruiting condition at the time of survey. Hence bearded tooth might be genuinely rare or perhaps it is relatively common in our woodlands but only rarely found producing fruitbodies. Even so, it is remarkable that a fungus that is so rarely seen fruiting in the wild, even in its stronghold areas, fruits so rapidly when in cultivation or even grown in Petri dishes in the laboratory. Typically, fruiting can occur in as little as 2-3 weeks and, in 9 cm diameter Petri dishes, sometimes before the fungus has grown to the edge of the dish. This begs the question: 'Why is it so rarely seen in the wild?'
An experiment in tree re-erection
A section of upper trunk of an ancient hollow beech in Windsor Forest, known to have intermittently supported fruiting of Hericium erinaceus from the 1960s until 1998, collapsed and the largest (4 m) length was re-erected in 2001. It was dragged to a nearby beech of appropriate diameter and strapped to the live tree to simulate the former environment in which the fungus had successfully fruited. No H. erinaceus fruitbodies have been recorded since, despite searches of the standing hollow tree, collapsed trunk sections and re-erected portion, although a variety of other rare fungi have been recorded on the dampest fragments of fallen trunk left in ground contact.
Detail of Hericium erinaceus upper surface of fruit body (Photo: Dr A. Martyn Ainsworth)
Bearded tooth is an edible species when young. According to some, its succulent texture and mild flavour make it one of the very best edible mushrooms, although unauthorised collection of wild fruitbodies in the UK is prohibited. Despite the rarity of wild fruitbodies, it is surprisingly easily cultivated on woodchips, sawdust or agricultural waste materials rich in cellulose, such as cotton, sugar cane, rice and corn waste, and is commercially produced as a speciality foodstuff.
Various medicinal properties have been associated with Hericium erinaceus, such as enhancing the immune system, and in China it is traditionally used for gastric ailments.
Another, quite different, use is in ranking the conservation importance of dead beech habitat in Britain and across Europe. To this end, it is one component of a suite of wood-inhabiting fungi whose fruiting presence is used to rank beech-dominated sites in much the same way as lists of wild plants are used as ancient woodland indicators. The number of indicator species recorded at a site is then compared to that recorded within the relatively unmanaged stronghold regions of the tree’s natural range and the site ranked accordingly. Although much survey work remains to be carried out, well-known English beech sites such as Burnham Beeches, Epping and the New Forest are already of demonstrable international importance for their dead beech fungal habitat.
Bearded tooth at Kew
Hericium erinaceus has not been recorded in the Gardens. Collections of H. erinaceus and related species from throughout their worldwide range are preserved in the Kew Mycology Herbarium, and made available to bona fide researchers by appointment.