Fungi and Aspens: Promoting Biodiversity
Aspen friends and foes

The Biodiversity and Management of Aspen woodlands: Proceedings of a one-day conference held in Kingussie, Scotland, on 25th May 2001

Ernest and Valerie Emmett
Drumlins, Newtonmore Road, Kingussie, Inverness-shire, PH21 1HD. Email: E-cubed@netcomuk.co.uk

It has been estimated that about 80% of all the organic energy on the Earth is locked up in wood of various kinds. This enormous store of energy is under constant attack, both when the trees are alive and more so when they are dead: from fungi, bacteria, insects and smaller animals. (Ryvarden 2001).

Foremost in the relationships with trees are the fungi and these play several different roles, not all of them detrimental to the health of the tree and some also provide food - for insects and other invertebrates, as well as mammals.

The fungi can be classified under three main headings:

The Mycorrhizal species are essential for the healthy growth of the tree. These fungi are of two sorts: endo-mycorrhizal and ecto-mycorrhizal. The hyphae of the latter sheath the tree's roots and, by breaking down material in the forest soil, they provide the tree with nitrogenous and other nutrients, including mineral trace elements. In exchange, the fungus receives carbohydrates manufactured by the tree during photosynthesis. Endomycorrhizal species are the most widespread, but their presence is not revealed by the formation of fruit bodies on the soil surface. These fungi enter the plant root cells forming specialised inclusion bodies, where exchange of nutrients occurs.

With Aspens, the ectomycorrhizal fungi are more important, and fairly specific associates in this category include familiar toadstool shaped fungi such as Leccinum aurantiacum, Leccinum duriusculum and Lactarius controversus. These are the friends of the Aspen, helping it to grow.

The second group of fungi are the Saprophytes and these include both host specific and cosmopolitan species. They are the litter decomposers, reducing fallen leaves, twigs and other already dead woody material to humus, a principal part of forest soils. These fungi include some that look like familiar toadstools with lamellae ("gills"). Others are poroid fungi (the Polypores), releasing their spores from pores instead of lamellae.

Some others that help to decay the woody material look rather like paint splashes and sheets of fungal tissue adhering to the surface but loose at the edges; and yet others form hard warty growths on twigs and branches. These are the Corticioid fungi. They do not have lamellae or pores; instead, they form amorphous sheets of spore bearing tissue covering the surface of logs and twigs.

Examples of detrivores specific to Aspen are the Polypores: Ceriporiopsis anaerina, Antrodia malicola and mellita, and the Corticioid: Peniophora polygonia. The two Antrodias mentioned have not yet been found in Britain, but it is hoped that they will be found in Scotland.

The cosmopolitan species are legion; for example, many Mycena species. One that was found in an Aspen grove on The Royal Society for the Protection of Birds' (RSPB) Insh Marshes Reserve last November may be a new species for science requiring description.

The Poroid and Corticioid fungi are among the dominating species in the decay of Aspens. In two Norwegian studies of the species occurring on cut and fallen logs, it was found that most of them were Polypores and Corticioids (Table 1).

Note that in one of these studies, the Aspen logs yielded 155 species of fungi, an indication of the value of lying timber for biodiversity.

The diversity of the fungal species increases as the wood decay proceeds: in the early stages of decay relatively few fungi colonise the wood, but as defensive substances are removed by early colonisers, a succession of species become involved. By the last stage, when the trunk is losing its shape completely, a great number of species have inhabited the former tree, during its decay cycle.

Some of the rarer fungi fruit only sporadically, with long gaps of many years between appearances of the sporocarps, although they are presumably present throughout in the vegetative state. In several long-term studies, while some species fruited regularly, others were only recorded once. It has also been found that some fungi can only invade decaying wood after a pioneer species has overcome the wood's armoury of defensive chemicals and started the partial decay. (Niemel‰ et al. 1995).

So far, nearly 100 species of fungi have been recorded on or with Aspen in Britain (Table 2), by members of the British Mycological Society - mostly in England, reflecting where most mycologists live or collect and where Aspen is not considered a common tree. The authors anticipate increasing the number of species recorded from Aspen in the coming years.

The third group, the Pathogenic fungi, are especially interesting, and include several species that are specific to Aspen. They are not friends of the Aspen but do great things for biodiversity. They include the group known as Rusts, as well as larger poroid fungi, the Polypores, often referred to as Bracket fungi.

Aspen is the host for several species of Rust (Melampsora spp.), which cause decay spots on the leaves and some will blacken and kill the growing tips of new shoots. Heavy infestation can result in defoliation of the Aspen. These fungi thus reduce the growth rate of the tree and obviously are detrimental to the life cycle of insects that feed on the young leaves and growing shoot tips, such as the Dark-bordered beauty moth (Epione vespertaria).

Taphrina johansonii © Ernest and Valerie Emmett Taphrina johansonii, a spectacular flower parasite of Aspen.

The Aspen bracket fungi, Phellinus tremulae © Ernest and Valerie Emmett Previously not recorded in Great Britain until 2000, when it was discovered in Strathspey on the RSPB's Insh Marshes Reserve. Phellinus tremulae is the most serious pathogen of Aspen and may help create the ideal habitat for a range of saproxylic invertebrates. Fig A. above illustrates the commonest form of bracket on the main stem of the dying/dead Aspen. Fig B below illustrates the less common 'branch creeper' form on a dead Aspen branch.

A spectacular flower parasite is Taphrina johansonii, and this obviously interferes with seed production where it occurs. It is probably of limited significance in Badenoch and Strathspey, where flowering is rare - although in 2001 the trees were flowering and the Taphrina was found locally in Strathspey.

Another small parasite is the Ascomycete, Encoelia fascicularis, which can be found erupting like small brownish black cups from the bark of living trees and on fallen branches on the ground. This fungus causes carbonising rots.

The wood decaying parasites include a significant Polypore, Phellinus tremulae, which is responsible for the death of most Aspens. It is a "white rot" type fungus, decomposing both the lignin and the cellulose. The wood from decayed trees has little economic value. Sporocarps can be found erupting as wedge shaped brackets from the trunks of Aspens, or else as a coating on the underside of branches at the point where they emerge from the main trunk - the "branch creepers".

This fungus, which is the most serious pathogen of Aspen, was previously not recorded from Britain until last year, when it was found for the first time on the RSPB Insh Marshes Reserve (Emmett and Emmett 2001) - and so it is not even on the Red Data List for fungi. It was thought that Britain did not have any Aspen trees large enough to support it. In Fennoscandia, where Aspen is much more common, it usually occurs on large and old trees. In Badenoch, however, it has been found on comparatively young trees - the diameter at breast height of the smallest of the infected trees measured so far, is under 20cm and the largest is about 50cm.

Since the first recording of Phellinus on the Insh reserve, it has been found at many sites in the Badenoch and Strathspey area: at three places in Kingussie, at Kincraig, at Loch an Eilein on the Rothiemurchus estate, Granish near Aviemore, at two sites near Grantown on Spey and westwards towards Laggan, and it has also been found on the RSPB Abernethy Forest reserve and across the Cairngorm massif near to Balmoral. It is likely to be found at other Aspen sites and this has been confirmed from other areas; for example, recent records from Glen Affric (Watson Featherstone this volume).

The Aspens that grow on the poorer soils, for example on stony moraines, seem to produce sporocarps more readily than those that grow on the richer, damper sites closer to water bodies. These findings confirm similar ones made by mycologists in Finland and Norway.

The problem in recording fungi is that most of them are ephemeral and there may not be a friendly mycologist on hand when a fungus fruits! Fortunately, Phellinus tremulae is perennial, the fruit bodies are persistent and one can see the annual growth phases on the fruit bodies. They are not easy to spot in the early stages of their growth though, often looking like a thumbnail on the trunk. The fungus is typically a parasite of living trees, but fruit bodies remain alive for a few years after the death of the host tree. It is said not to form new fruit bodies on dead trunks (Balaban and Kotlaba 1970). The current authors, however, have observed fruit bodies that have apparently formed after trees have fallen.

Entomologists hunting rare saproxylic insects in decaying Aspens, record a sweet smell in the soft decay material that the larvae feed on. Cultures of the Phellinus are unusually interesting in that they emit a sweet smell like Oil of Wintergreen, due to the presence of methyl benzoate, methyl salicylate, benzyl alcohol, linalool and ethyl benzoate (Collins and Halim 1972). It is likely that mycelium of the Phellinus is present in the decomposing sapwood which is home to the larvae of these invertebrates, and contributes these compounds to the mixture of smells.

References

Ballaban, K. and Kotlaba, F. 1970. Atlas drevokaznych hub. - 136 pp Praha.

Collins, R. and Hallim, A. 1972. An Analysis of the odorous constituents produced by various species of Phellinus. Canadian. J. Microbiology. 18: 65-66.

Emmett, E.E. and Emmett, V.E. 2001. Phellinus tremulae, a new British Record on Aspens in Scotland, Mycologist 15:3 105-106.

Niemel‰, T. 1974. On Fennoscandian Polypores. III. Phellinus tremulae (Bond.) Bond. and Borisov, Ann. Bot. Fennici 11: 202-215. 1974

Niemel‰, T., Renvall, P. and Penttil‰, P. 1955. Interactions of fungi at late stage of wood decomposition. Ann. Bot. Fenn. 32: 141-152.

Ryvarden, L. 2001. An Introduction to Wood-rotting Fungi, Biological Institute, University of Oslo, Norway.

(For work by Andersen and Hermansen, see Ryvarden 2001)

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For further information about aspen, please go to the Aspen Information Resource

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