An introduction to poroide fungi

5. Decay characteristics

Polypores are mostly wood inhabiting fungi able to utilize components of wood cell walls as their main source of energy for growth and reproduction. 

Wood consists mainly of lignin, cellulose and hemicellulose. Cellulose is a long chain polymer of glucose anhydride units joined by ß 1‑4 linkages. Hemicellulose consists of similar polymers of glucose joined by other linkages or polymers of monosaccharides other than glucose. Lignin is a complex polymer of phenolic units. Conifer wood in general has a higher lignin content (27‑35 %) than wood of dicotyledonous angiosperms or "hardwoods" (19‑24 %). 

Wood rotting fungi can be grouped into two categories according to which enzyme systems they produce to decay the wood. The two groups are referred to as white rot and brown rot fungi. 

White rot fungi produce cellulase and lignase enzyme systems enabling them to degrade all components of wood cell walls. Most of them apparently remove lignin and polysaccharides at about the same rate, and wood in intermediate or advanced stages of decay contains about the same proportions of these components as sound wood (Kirk & Highley, 1973). 

Wood decayed by white rot fungi tends to lose its strength properties gradually and retains its fibrous structure into the advanced stages. The decayed wood becomes spongy, stringy, or laminated and is usually bleached and paler in colour than sound wood. White rot fungi eventually decay wood completely and the residues are not stable components of forest soils. 

Brown rot fungi selectively remove cellulose and hemicellulose from wood. Wood decayed by brown rot fungi loses its strength properties rapidly and undergoes drastic shrinkage and cracking across the grain. In advanced stages wood is reduced to a residue of amorphous, crumbly, brown cubical chunks composed largely of slightly modified lignin. Brown rot residues are extremely stable and important organic components in forest soils. 

Only a fraction of the known species of wood rotting basidiomycetes are brown rot fungi. The number of wood rotting basidiomycetes in Europe has never been calculated, but it is our guess that brown rot fungi constitute less than 10% of the total number. Apparently this is a case of parallel evolution, and the brown rot type of enzyme system has developed independently in a number of evolutionary lines and does not in itself necessarily indicate close relationships between species. 

However, within each family, genera containing brown rot fungi tend to have close relationships on the basis of morphological characters. This is certainly true for the polypores. The general impression is that the brown rot type of wood degrading system is correlated with morphology at the genus level and is a character of taxonomic significance.

The total number of polypores recorded in Europe and included in this book is 393 species  out of which 99 or about 25% has brown rot. 

 Of  these 99 brown rot species, about 60 or about 60%  occur exclusively or primarily in conifers. The general data on geographical distribution of brown rot fungi throughout the world supports the belief that brown rot polypores are found primarily in coniferous forest ecosystems. On a geographic basis, brown rot fungi are primarily distributed north of the Tropic of Cancer at 23 1/2 degrees north latitude. The heaviest concentration is actually north of 35 degrees north latitude and thus, brown rot fungi in general may be said to be boreal in distribution. South of 35 degrees north latitude concentrations of brown rot fungi occur at high elevations in coniferous forest regions like the southern Rocky Mountains or Himalayas or in pine forest regions such as the Gulf Coast of the United States or in Vietnam. In the tropics and in the southern temperate zone brown rot fungi are few. 

 Reputable fossil records indicate that polypores were present about 300 million years ago during the carboniferous age when the development of woody gymnosperms began. 

There is a possibility that brown rot polypores evolved several times from white rot fungi as a late specialization with conifers. Later, when coniferous forests ecosystems were forced towards the temperate and boreal zones, brown rot polypores had to evolve in parallel or perish. 

Today these fungi play an important ecological role in these ecosystems. Brown rot residues are extremely stable and remain essentially unaltered in the soil for hundreds of years. In coniferous forest ecosystems they may comprise up to 30 per cent of soil volume in the upper layers (McFee & Stone 1966). 

Soils with high brown rot residue content have greatly increased water holding capacity. They are also major sites of ectomycorrhizal development and nonsymbiotic nitrogen fixation. In addition such soils have a high cation exchange capacity and a favourable acid pH. Brown rot residues also tend to ameliorate soil temperatures. 

Fig. 11. Types of rot, white rot (top) T. Niemelä, and brown rot (bottom) L. Ryvarden.


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