Fungi and the
Fungi serve as one of the principal decomposers in ecosystems. As decomposers fungi return various important elements such as carbon and nitrogen back to the environment thus preventing them from becoming tied up in organic matter. Fungi are often found growing on woody substrates, soils, leaf litter, dead animals, and animal exudates (Tree of Life, http://tolweb.org/tree?group=fungi).
The best wood rotters in the natural world are fungi (Barron, 26). The major wood rotters belong to the phyla Basidiomycota and Ascomycota. These fungi have the distinctive capacity to infiltrate and enzymatically digest the lignin, cellulose and hemicellulose components of the hardest wood. Roughly 80 billion tons of carbon are returned to the atmosphere each year as carbon dioxide through the biodegradation of woody material. Consequently, fungi play an important role in the carbon cycle (Barron, 27).
Fungi are highly adapted for the task of breaking down plant material. Invasive hyphae penetrate tissues and cells, secreting enzymes, which break down plant cell wall components. (Campbell, 629). The major components of wood are cellulose and lignin. Certain fungi only attack and digest the cellulose component of wood, which in turn leaves the lignin component behind. The lignin that remains gives the wood a brown color. This type of decay is called “Brown Rot”. As brown-rot fungi attack wood they follow the lines of least resistance, vascular rays and the lines between annual rings of growth. When the wood dries out it fractures into angular pieces along these lines attack producing a type of decay referred to as “Cubic Rot”. Other types of fungi attack both the lignin and cellulose components resulting in what is called “White Rot”. When these fungi destroy both the cellulose and lignin a white or bleached residue is left behind (Barron, 27).
Depending on their relationship to the substrate in which fungi grow, they can be separated into the following three categories: (1) parasitic fungi, (2) saprophytic fungi, and (3) Mycorrhizal fungi (Arora, 6).
The majority of the serious fungus pests are parasitic fungi. These fungi survive by feeding on living organisms (Arora, 6). Included in this group are some of the most dangerous plant pathogens. For example, Cryphonectria parasitica, is the agent of destruction for an estimated 4 billion chestnut trees in the eastern United States. Likewise, parasitic fungi infect humans causing diseases like Pneumocystis and Valley Fever (Tree of Life, http://tolweb.org/tree?group=fungi).
On the other end of the spectrum are saprophytic fungi that live off dead or decaying matter such as wood, humus, soil, grass, and dung (Arora, 6). A common and easily recognizable fungus in this group is Fairy Ring Mushroom (Marasmius oreades) which grows on lawns. Aptly named, Fairy Ring Mushroom periodically produces circles of mushrooms on it outer fringes. The reason behind this phenomenon is that the mycelium of this terrestrial fungus if supplied with an even distribution of nutrients will grow outward at the same rate in all directions thereby producing a “fairy ring.” As each year passes the fairy ring grows in size as the mycelium grows outward. Eventually an obstruction or lack of food inhibits further mycelia growth. At this point portions of the fairy ring’s mycelium dies or breaks up into arcs. It has been estimated that various fairy rings found in the prairies of the Midwest are six hundred years old (Arora, 7).
The third and final category is mycorrhizal fungi. Fungi in this group form symbiotic relationships with plants, typically trees, in which the mycelium of the fungus forms a sheath of hyphae around the rootlets of its host. The plant provides the fungus with water and organic compounds, while the fungus assists the plant in the absorption minerals such as phosphorous. Additionally, studies have revealed that mycorrhizal fungi do not grow without their host and the host plant itself is out competed by other plants that do possess their normal mycorrhizal partner. Many mycorrhizal fungi are host specific, meaning they only grow with one kind of tree. On the other hand host trees can have several mycorrhizal partners. It has been reported that a single Douglas fir had over 50 species of mycorrhizal mushrooms associated with it (Arora, 7).
Arora, David. Mushrooms Demystified. 2nd ed. Berkley, Ten Speed Press, 1986.
Barron, George. Mushrooms of Northeast North America.
Tree of Life Web Project.
2006. The University of Arizona College of Agriculture
and Life Sciences and The University of Arizona Library. 21 April 2006.