Nature Notes

Vol. II May-July 1986 No. 9


Until just a little over a decade ago the existence of a group of fungi designated mycorrhiza was known but its function not understood. We now know that, rather than the root parasite it was previously suspected of being, it is in fact a root symbiont. It is difficult to imagine that forest monarchs, such as huge Douglas-firs, western red cedars and others, are dependent upon microscopic fungi for their majestic size and incredible longevity. In fact, it turns out that ninety per cent of all vascular plants have a mycorrhizal root symbiont. What is this amazing fungus group, and why is the forest so dependent upon it?

It is best to imagine this fungus, not as a mushroom-like plant, but rather as a tremendous net of root-hair like threads or fibers. These fungi threads are so heavily branched, or ramified, that a single cubic foot of forest soil may have hundreds of feet spread throughout it. Every inch of this net absorbs nitrogen, phosphorous, water, and other soil nutrients and minerals. These are then drawn into a tree's roots, where they are transfered to the tree's root tissue. These raw materials for the photosynthetic process are then transported up the tree's stem, to its canopy, where they are formed into sugars with the sun's energy.

The benefits for the tree are many. The mycorrhiza fungus greatly enhances the tree's nutrient and water uptake. This in turn increases the tree's tolerance to drought. The fungus also helps to deter root pathogens by producing bacterial antitoxins. Consequently, it greatly prolongs root life. In exchange for all of these benefits that the tree receives, the fungus is fed sugars that the tree produces. Remember, fungus has no chlorophyll and in most cases is either a saprophyte or a parasite; not so mycorrhiza.

Mycorrhiza is a very broad description for many different types of fungi root symbionts. Douglas-fir, hemlock, and true fir trees have ectotrophic mycorrhizae. With this type of mycorrhiza the water and nutrient/food exchange takes place in the intercellular space of the tree roots. Cedar trees, on the other hand, have endotrophic mycorrhizae where the nutrient/food exchange occurs actually within the root cells.

Almost all of the mycorrhiza fungi produce a fruiting body or spore forming structure known as a truffle. These are ascomycetes meaning that the spores are internal, or contained within a sack. This contrasts to regular mushrooms which are basidiomycetes. On regular mushrooms the spores form externally on gills. Also in contrast to regular mushrooms, truffles are hypogeous, growing entirely underground or within rotting logs. Mushrooms are epigeous, growing above ground.

How can the spores of a truffle spread and grow if they are underground? Well, truffles are a delicacy, just ask any European. They are highly odoriferous. In Europe people have used dogs and pigs for centuries to hunt for the hidden morsels. Many forest denizens "dig" these little taste treats. Rabbits, mice, deer and even bears all love to eat truffles. Flying squirrels will glide to the forest floor at night to conduct a truffle hunt.

Perhaps the most amazing of all is the red-backed vole which is a vegetarian. Not just a common garden variety vegetarian, but a fungitarian. This little rodent which lives in and under rotting logs relies upon fungi for ninety per cent of its diet. Like all of the other animals it digests only the sack that contains the spores. The spores themselves pass through the animal's digestive system perfectly viable and are then deposited throughout the forest in nice little packages with accompanying fertilizer. These packages then innoculate tree seedlings, beginning the cycle all over again. Thus a tight cycle of interdependence exists between forest giants, small mammals and a microscopic fungus, each dependent upon this relationship for its very existence.

Chris Maun

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