Forest Decomposers

Fall is a good time to look for fungi on the forest floor.  This is particularly true this year because of the wet weather.  We recognize them when we see their fruiting bodies, the typical toadstool or mushroom for example, but these are only a small part of the fungal existence.  Fungi are actually present the year around and exist as a mat of fungal cells called hyphae.  The mat itself is referred to as a mycelium and can spread over a large distance through the ground or through a tree trunk or fallen tree.  The mycelium absorbs nutrients by digesting rotting vegetation or fallen tree trunks and may sometimes infect living trees.  The ability to do this is due to the unique ability of fungi to produce an enzyme which can digest lignin.  Lignin is a complex polymer which makes tree trunks and branches hard and adds stiffness to tree leaves.  While many organisms can digest the cellulose in plants and leaves, fungi are unique in the ability to digest lignin.  Bacteria cannot digest lignin.  In consequence, fungi are a critical part of the forest ecosystem as decomposers of fallen tree trunks, branches, and leaves.  A mat of decomposing leaves will typically have a network of mycelium throughout.  Fungi therefore ultimately return the nutrients and energy from the trees to the soil.  There is a theory that fungi with the ability to digest lignin evolved at the end of the Carboniferous period.  Prior to that, fallen trees were not easily decomposed and accumulated, ultimately forming the coal and oil deposits we utilize today.  After the evolution of lignin-digesting fungi, fallen trees were  recycled and coal formation ceased.

However, the mycelia are not easily seen with the naked eye, so we are more familiar with their fruiting bodies.  During spring and particularly fall, fungi engage in sexual reproduction.  Hyphae from mycelia which encounter each other intertwine and exchange genetic material.  The fruiting body is produced as a dense collection of hyphal cells and appears as the toadstools and puffballs we are familiar with.  A walk in the forest in fall (in this case along the Peak Trail) let me see a number of typical fungal fruiting bodies growing from decomposing fallen logs.  Most of the typical fruiting body types that we see growing in soil can also be seen on fallen tree trunks.

The fruiting bodies we most commonly associate with tree trunk decomposition are bracket (or shelf) fungi.  Bracket fungus fruiting bodies form dense shelf-like structures called conks which are hard like wood but are formed of fungal hyphae.  The spores are released from the underside of the conk.  This is the typical appearance on the end of a fallen log.  The mycelium is propagated throughout the log.



I also found a log with what looks like the initial stages of growing a new bracket fungus.



This appears to be the new emergence of hyphal structures which should eventually turn into mature conks similar to those seen in the first image, although not necessarily the same species. 

Other fungal fruiting body types can also be seen on rotting logs.  Those in the picture below are in the form of the familiar puffballs.


Mature fruiting bodies in the center have already ruptured and are releasing their spores into the air.  New fruiting bodies are emerging around the periphery. 

There are also fruiting bodies which look like typical mushrooms and release their spores from gills or pores under the mushroom cap as seen below.



Decomposition does not always seem like a pleasant topic, but is essential to the health of the forest by recirculating nutrients back into the soil.  Along with other decomposing organisms such as bacteria and insects, the decomposers form a critical part of the overall food and energy web of the forest environment.

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