The Giant Polypore Fungus, Meripilus giganteus: The Never Ending Love for Old Trees
The Giant Polypore
Many fungi are generally considered plant enemies, mostly because they parasitize them. However, not all fungi do that and there are any examples of symbiotic relations between fungi and plants, e.g. mycorrhiza. Some of you may have seen some big light-coloured fungi attached to an old tree trunk and wondered how long will that tree live. You might think that the sight of so many fungi caps may be an indication that the tree is sick and it does not have many more years ahead. Well, you could be wrong. This may be one of those situations in which things are not really as they seem. In fact the fungus could be the tree’s saviour, allowing it to live some more years that would not be possible in other way. As awkward as it may seem, especially to the common sense, many trees that host this common cold climate fungus are in fact privileged. Meripilus giganteus is the scientific name of this fungus, in the Meripilaceae family. It causes a white rot in various species of broadleaved trees, from beech (Fagus) to fir (Abies), spruce (Picea), pine (Pinus), oak (Quercus) and elm (Ulmus). Hence it shows its parasitic side in some cases. However, it seems to grow on beech and oak trees especially. It is a polypore fungus, i.e. it does not have a stalk like the common mushrooms and the spore-bearing tissue continuous along the underside of the mushroom is also absent. This bracket fungus, or shelf fungus, that looks like a giant clam, is usually known as the giant polypore or black-staining polypore, is often found in large clumps at the base of trees, although fruiting bodies are sometimes found some distance away from the trunk, attached the roots mostly. Meripilus giganteus is mainly distributed all over the cold regions of the northern Hemisphere, and is widely distributed in Europe. In the field, it is recognizable by the large, multi-capped fruiting body, as well as its pore surface that quickly darkens black when bruised or injured. It is edible, although some say that is should be eaten while fresh and young. As it matures it hardens and it becomes bitter and more acid. It is popular in Japan along with the similarly looking and more tasty species Grifola frondosa. Meripilus giganteus can reach considerable dimensions up to 90 cm wide, 5 cm thick and weighing up to 25 kg. Usually it presents of numerous rosette-like flattened fan-shaped caps, of different sizes, arising from a common yet very short basal stem.
A Partnership for Life
So how does the tree benefit from such companionship? Well, in many cases the fungus Meripilus giganteusdoes live on the tree living tissue. And when not parasitizing, it feeds on the dead tissue only that composes the center of the tree trunk. As with all trees, and especially the old ones, the major fraction of its trunk and branches is dead tissue, what we usually call wood. Cells that ceased to function and all that it is left it is their lignified cell walls, rich in cellulose, i.e. carbon. The living part of the tree trunk is a thin layer of tissue, mostly vascular, right bellow the bark. It grows and it is replaced every growing season, thus enlarging the trunk diameter – this is called the secondary growth in plants. So, as the tree grows old the previous layers of vascular tissue are accumulated and comprise the tree trunk, usually forming distinguishable rings from which is possible to determine the age of the tree, as you may already know. It is on this dead tissue that Meripilus giganteus feeds and lives when being saprophytic only. For this reason it is classified as a lignicolous fungus.
All this dead part of the trunk and of the branches is what sustains the whole tree structure giving the familiar form that we all know. Growing big as many trees that we usually see has in fact a high price that is hard to keep as trees get older. Here is when Meripilus giganteus plays its important role. It only takes a single microscopic spore of Meripilus giganteus, dispersed by wind, to penetrate in a crack in the bark of the trunk. It germinates and becomes a thin shred, called a hypha that migrates through the living tissue layer of the trunk, underneath the bark, until it reaches the deadwood. Once there, it proliferates tremendously. Some fungi that similarly feed on wood digest cellulose only. Others are also capable of digesting lignin and cellulose. Some Meripilus giganteus specimens will stay deep inside the tree somehow dormant waiting for the tree to grow old and weakened by age. It is also possible that the tree gets severely damaged when stroke by lightning or by strong winds during a storm. It is at this time that Meripilus giganteus shows its splendour. Its hyphae start to multiply prodigiously, feeding and spreading throughout the old trunk so fast and efficiently that the inner core of the trunk starts to disintegrate and decays. It is only at this time that we can see the fungus as the fruiting bodies, the caps, start to emerge from the tree trunk.
It is estimated that a big cap can produce over 20 million spores per minute that spread from its bottom surface. This can go on for as much as 5 months if favourable conditions are met. Looking at these caps they seem to smoke as the sun light hits the spores. As this reproductive and commonly seen stage happens in very old trees only, it is common to assume that the fungus infected the tree causing some severe illness, condemning the tree future. Very far from hurting the tree, in fact, Meripilus giganteus gives great benefit, and not only to the tree. First, the decaying wood resulting from the fungus action will turn out to be a supplement of organic nutrients that otherwise would not be accessible to the tree. As it falls and accumulates on the soil under the hollow trunk, adventitious roots spread from the hollow trunk down to that enriched soil. As they accumulate on the soil bellow it will increase the supply of organic matter that the tree will happily make benefit of. Second, a hollow trunk will be a convenient and attractive habitat for small animals looking for shelter, e.g. owls, bats, squirrels. Last but not least, a hollow trunk changes the way as the trunk itself, as the tree main support structure, responds to pressure. It becomes more elastic and more stable compared to a non-hollow trunk. Adding to that, the absence of an enormous mass of wood attenuates the pressure over the old root system, also a target of the fungus action. Consequently, as it is commonly observed, many old hollow trees survive better stormy weather than younger and more robust trees. It is not unusual to observe 500 or more year old oaks standing next to fallen trees with a quarter of that age. With mutual benefits, as we say in our human world, Meripilus giganteus, and many fungus alike, and trees have found a most convenient way of living together. Some authors claim that this type of association may not be that old in the history of both species, however it became a highly successful partnership.
One final note to say that due to its dual behaviour the sight of Meripilus giganteus on itslef does not assure that it is damaging or benefiting the tree. A more detailed observation and analysis is needed to determine its damaging potential. Great part of its action also depends on the tree itself and on its health status when it was firstly infected. There are many studies and observations of its symbiotic nature and also many observations of its parasitic nature. These dual behaviour is quite common in the fungi of this genus Meripilus as is their misidentification among the five species and with other species of the same family Meripilaceae. In fact this dual behaviour of the fungus and the widely different responses from its hosts can be a sign that this peculiar association may not be that old in the evolutionary history of the fungi and of its hosts.
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