Wood rotting fungi

There are many fungi, both saprotrophic and parasitic, that decay wood and you’ll find a great deal of information in the reference given in the button.

Examples of saprotrophs found on dead wood lying on the ground are Chlorociboria aeruginascens (which often stains the wood the same colour, as shown here ), Plectania campylospora , Polyporus arcularius and Stereum ostrea . The densely clustered caps of Coprinus disseminatus are often found around the bases of old stumps that have been colonised by the mycelium of this species.

When you look at a majestic forest eucalypt remember that the bulk of the tree is the dead heartwood. The living parts are the roots, leaves and relatively thin layer under the bark. Thus, live trees also provide scope for many saprotrophs and Laetiporus portentosus is an example of a saprotrophic fungus, feeding on the dead heartwood in living trees.

On the other hand, Armillaria luteobubalina is an aggressive parasite, capable of attacking and killing many plant species after which it feeds on the dead wood. It often produces dense clusters of mushrooms on dead wood or at the bases of live trees of various species. The fungus is an endemic Australian species and is widespread through south-eastern Australia. Armillaria is a problem in the Australian National Botanic Gardens in Canberra, where it has killed mature trees in several garden beds . In the botanic gardens it spreads mostly by root-to-root contact and one way of controlling its spread is to put a physical barrier between the roots of infected and healthy trees. This photo shows a bitumen road between the Gardens' Rainforest Gully (out of the picture, to the left) and (on the right) a garden bed with an Armillaria infestation. In order to keep Armillaria out of the Rainforest Gully, a trench was dug along the edge of the road, a physical barrier put in and the trench then re-filled and re-bitumened. You can see the later strip of bitumen on the right hand side of the road (just to the left of the white, concrete gutter).

However, mushrooms growing from tree bases don’t always mean trouble. Gymnopilus pampeanus also often produces dense clusters of mushrooms on dead wood or at the bases of various live trees. Some people mistakenly believe they have an Armillaria problem when they see Gymnopilus, for the two have similar growth habits. However, the Gymnopilus mycelium feeds largely on dead wood and is, at most, a mild parasite that may inflict damage to a tree that is already otherwise stressed.

White rot and brown rot

The major constituents of wood are the polymers cellulose, hemicellulose and lignin. A polymer is a bit like a long chain. While a chain can be quite long and heavy, it's just a series of simple links joined together. In a similar way, each of the above polymers is built up by joining together a string of much simpler chemical compounds. In the case of cellulose and hemicellulose the basic building blocks are various simple sugars. For example, the basic building block in cellulose is glucose - a simple sugar that turns up in many foods, as you can see from the ingredients labels on various jars, cans or packets that you buy during your grocery shopping. The building blocks of lignin are more complex and lignin helps protect cellulose against microbial attack.

There are two broad classes of wood-rotting macrofungi. Fungi which decompose cellulose, hemicellulose and lignin leave the wood very pale in colour and often with a fibrous to stringy consistency. These are called white rot fungi and this photo shows an example of wood with white rot. Fungi which can decompose only cellulose and hemicellulose leave the wood coloured brown and, rather than fibrous, cracked into cubical fragments, which break off readily. These are the brown rot fungi. The next photo shows what you get when you crumble, or rub, white and brown rot woods in your hands. There are several types of white rot and you can find out more here: [http://www.forestpathology.org/decay.html]

Examples of white-rot fungi are Armillaria luteobubalina , Schizophyllum commune and Daldinia concentrica .

Examples of fungi causing brown rots are Fomitopsis lilacino-gilva Laetiporus portentosus and Serpula lacrymans .

There are more species of white rot fungi than there are of brown rot fungi. In the northern hemisphere many of the brown rot fungi are found on conifers in the boreal areas but in Australia there has been no systematic study into the geographic or host preferences of brown rot species. However, there is some evidence of a preference for cool-temperate areas.

Fairly often, when you cut open a piece of dead wood that's still fairly firm, you'll see black lines marking out zones of varying size. These black zone lines mark out the extremities of the separate mycelia. Such wood, called spalted wood, is often sought after by craftspeople working with wood, for the irregular lines add to the attractiveness of the finished item. The following photos show, first, numerous fruiting bodies of the polypore Trametes versicolor growing out of a tree-stump and, second, the wood inside the stump.

Ornament using spalted wood.

You can see various zone lines formed by the mycelia in the wood. The next photo is a close-up view of the black zone lines in another piece of wood. Here are an items made from spalted wood.

Some white rot fungi, such as Trametes versicolor, decay lignin and cellulose simultaneously whereas others decay lignin first, leaving a residue of cellulose and hemicellulose for later decay. In Chile, Nothofagus wood that has been selectively de-lignified by Ganoderma australe is fed to cattle, for the lignin-free residue is easily digested with the help of the micro-organisms found in cattle guts. Such wood is called palo podrido and you can find out a bit more about it here. [http://www.forestpathology.org/decay.html] There is also evidence of moose in Alaska feeding on logs decayed by Ganoderma tsugae.

Wood rotting fungi, nematodes and bacteria

Nitrogen is an essential element for all living organisms, but wood has a low nitrogen content. Many wood-rotting fungi can grow at very low nitrogen levels and are very efficient at both scavenging and re-cycling nitrogen, but some have additional strategies. The mycelia of some wood-rotting fungi also trap and feed on nematodes that can often be found in decaying wood. By trapping nematodes (using hyphae with sticky knobs, for example), these fungi supplement their nitrogen intake. Species in the genus Hohenbuehelia and Pleurotus trap nematodes. These fungi produce stemless-mushrooms as fruiting bodies, the caps being attached directly to the wood – either at the top or side of the cap.

Pleurotus ostreatus on a toilet roll.

Bacteria are also common in rotting wood and laboratory experiments with the mycelium of Pleurotus ostreatus have shown that this fungus will destroy and feed on bacterial colonies. Pleurotus ostreatus is widely cultivated (in many colour variations) and sold in supermarkets as the Oyster Mushroom. This fungus can grow on a variety of cellulose-containing materials - even toilet rolls, as shown at the right.

An excellent source of authoritative information about all types of predatory, wood-rotting fungi is in this site, written by George Barron, who has spent many years studying these fungi [http://www.uoguelph.ca/~gbarron/MISC2003/feb03.htm].

Forest litter and woodchips

Many fungi exploit the leaf and twig litter that can be found on forest floors, with the mycelia confined just to the litter layer – extending neither to the soil below nor to the larger pieces of fallen wood. This is especially the case in the tropics, but also in sheltered temperate areas (where the litter layer is protected against the drying effects of sun and wind). Many of the fungi that exploit the litter layers produce tiny fruiting bodies, but at times in great abundance. For example, you may find hundreds of Marasmius haematocephalus mushrooms popping up from the forest litter. The individual caps are under a centimetre in diameter and there are many litter decomposers that produce mushrooms with caps only a couple of millimetres in diameter. You can also find various polypores, ascomycetes and corticioid fungi on leaf and twig litter. Here is a close-up view of the toothed fruiting body of a species of the corticioid genus Sistotrema that was growing on some long-dead, eucalypt leaves lying on the ground. The spines are one to two millimetres long. The fruiting body grew on the underside of the leaf, so the spines were pointing down.

Turning now to a suburban setting, think of all the wood that is spread around parks and home gardens in the form of woodchip mulch. Hypholoma aurantiaca is a common mushroom in woodchip-mulched gardens in Canberra. The species is well-known in Europe and may have been introduced to Australia. Some may think nothing much of the fungi on woodchip mulch but Peter Shaw, of the University of Surrey in England, writes: "It seems that the combination of deep woody litter and regular disturbance provides a habitat with no natural equivalent, so that the fungal community which appears is unusual." You can find more about his investigations in this link. [http://www.chezshaw.demon.co.uk/work/research/woodchips/woodchips.html]

LINKS TO OTHER SECTIONS

• Wood rotting fungi
• Dung fungi
• Fungi and invertebrates
• Fungi and vertibrates
• Fungi and fire
• Fungi versus fungi
• Mycorrhizas