Polypore

Polypores, also called bracket or shelf fungi, are a morphological group of basidiomycete-like gilled mushrooms and hydnoid fungi that form large fruiting bodies called conks, which are typically woody, circular, shelf- or bracket-shaped, with pores or tubes on the underside. Conks lie in a close planar grouping of separate or interconnected horizontal rows. Brackets can range from only a single row of a few caps, to dozens of rows of caps that can weigh several hundred pounds. They are mainly found on trees (living and dead) and coarse woody debris, and may resemble mushrooms. Some form annual fruiting bodies while others are perennial and grow larger year after year. Bracket fungi are typically tough and sturdy and produce their spores, called basidiospores, within the pores that typically make up the undersurface. Most polypores inhabit tree trunks or branches consuming the wood, but some soil-inhabiting species form mycorrhiza with trees. Polypores and the related corticioid fungi are the most important agents of wood decay, playing a very significant role in nutrient cycling and aiding carbon dioxide absorption by forest ecosystems. Several polypore species are serious pathogens of plantation trees and are major causes of timber spoilage. As polypores are much more diverse in old natural forests with abundant dead wood than in younger managed forests or plantations, a number of species have declined drastically and are under threat of extinction due to logging and deforestation. Polypores are used in traditional medicine, and they are actively studied for various industrial applications.

Taxonomy

Because bracket fungi are defined by their growth form rather than phylogeny, the group contains members of multiple clades. The beefsteak fungus, a well-known bracket fungus, is actually a member of the agarics. Other examples of bracket fungi include chicken of the woods (or sulphur shelf), birch bracket, dryad's saddle, artist's conk, and turkey tail. The name polypores is often used for a group that includes many of the hard or leathery fungi, which often lack a stipe, growing straight out of wood. "Polypore" is derived from the Greek words poly, meaning "much" or "many", and poros, meaning "pore".

Classification For most of 20th century polypores were treated as a family, the Polyporaceae. Reconstructions of family trees of fungi show that the poroid fruiting body has evolved numerous times in the past. Modern DNA-based evolutionary classification places polypores to at least 12 orders. The orders containing most polypore species are the Polyporales (genera such as Fomes, Polyporus and Trametes) and Hymenochaetales (e.g. Oxyporus, Phellinus and Trichaptum). Economically perhaps the most significant polypores Heterobasidion spp., pests of conifer plantations, belong to the Russulales. Other polypore orders are the Agaricales, Amylocorticiales, Auriculariales, Boletales, Cantharellales, Gloeophyllales, Sebacinales, Thelephorales and Trechisporales. The Polyporales in the modern sense are not only polypores but also other fruiting body types such as crust fungi, hydnoid fungi and agaricoid mushrooms. Currently polypores are divided into about 170 genera.

Description

The fungal individual that develops the fruit bodies that are identified as polypores resides in soil or wood as mycelium. Polypores are often restricted to either deciduous (angiosperm) or conifer (gymnosperm) host trees. Some species depend on a single tree genus (e.g. Piptoporus betulinus on birch, Perenniporia corticola on dipterocarps). Forms of polypore fruit bodies range from mushroom-shaped to thin effused patches (crusts) that develop on dead wood. Perennial fruit bodies of some species growing on living trees can grow over 80 years old (e.g. Phellinus igniarius). Most species of polypores develop new, short-lived fruit bodies annually or several times every year. Abundant fruit takes place during the autumn or rainy season. Structure of the fruit bodies is simple. Effused or resupinate fruit bodies typically consist of two layers - a tube layer of vertically arranged tubes that open downwards, and supporting layer called subiculum that supports and attached the tubes to substrate. In fruit bodies with a cap (pileate fruit bodies) the tissue between upper surface and the pore layer is called context. A few polypores (e.g. Fomes fomentarius and Inocutis rhaedes) also have a core between context and substrate. A minority of polypores also have a stalk (stipe) that attach to the cap either laterally or centrally depending on the species. Polypore tubes are a honeycomb-like structure, where the individual tubes have fused together. Their sides are covered with a spore-forming surface, the hymenium. The tubes offer shelter for developing spores and help to increase the area of the spore-producing surface. Pore size and shape vary a lot between species, but little within a species – some Hexagonia spp. have 5 mm wide pores whereas pores of Antrodiella spp. are invisible to naked eye with 15 pores per mm. Generally the larger the pores, the larger the spores. A few polypores produce asexual spores (chlamydospores or conidia) in the upper surface of their cap (e.g. Echinopora aculeifera, Oligoporus ptychogaster) or without the presence of a sexual fruit body (e.g. Inonotus rickii, Heterobasidion spp.).

Delimitation (morphology) Most polypores have a poroid hymenium, but not all species. A few, for instance Elmerina holophaea and Lenzites betulina, form gills like agarics but are still considered polypores, since in all other respects they are similar to closely related polypores, forming tough fruiting bodies on the wood. A couple of species where the tubes have not fused together in a honey-comb manner are variably classified as polypores or not (e.g. Porotheleum fimbriatum). There's no clear distinction between polypores and hydnoid fungi - some polypores with irregularly poroid lower surface have been considered both polypores and hydnoid fungi (e.g. Echinodontium tinctorium, Irpex lacteus). Bolete mushrooms are a separate morphological group not included in polypores even though they have tubes. Fleshy fruiting bodies with a stalk and microscopic characters separate boletes from polypores.

Ecology

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