One of the largest organisms on Earth is thought to be a fungus: the mycelium of a dark honey fungus (Armillaria ostoyae) in the Malheur National Forest in eastern Oregon, USA. It covers almost 10 square kilometres of forest, and its age is estimated at thousands of years. Very little of it can be seen above ground – clusters of fruiting bodies in autumn and dying trees. Everything else, the actual organism, hides in the soil and under the bark.
How do you even measure a fungus?
You cannot simply look at a fungus and measure it with a tape. Its body is the mycelium – a network of fine threads called hyphae that grow through soil, roots and wood. What we put in the basket are only the fruiting bodies, which the mycelium produces to spread its spores.
In the case of the Oregon honey fungus, scientists collected samples from many infected trees over a large area of forest and compared their genetic material. It turned out that many of them belonged to a single, genetically identical individual. Based on that, they mapped the approximate extent of the mycelium.
How big and how old is the Oregon honey fungus?
- Area: sources usually give about 9–10 km², an area that could fit more than a thousand football pitches.
- Age: this is an estimate, not a measurement. It is calculated from the rate at which the mycelium spreads through the soil. The range you will most often see is from about 2,400 to more than 8,000 years.
- Continuity: the fact that the mycelium is one genetic individual does not have to mean that all of its parts are physically connected today. Over time it may have split into fragments that are still “the same” fungus.
Is it really the largest organism?
It depends on how you count. If the criterion is area, the Oregon honey fungus is right at the top. If it is mass, the answer is often Pando, a clonal colony of aspens in Utah, where thousands of trunks grow from a single root system. For comparison, the largest animal is the blue whale. These records show that an “organism” in nature can be something much bigger and stranger than a single creature.
The Oregon honey fungus is not the only one, either. Earlier, a huge honey fungus mycelium in Michigan made headlines, and more extensive individuals have been described in forests around the world. Honey fungi simply seem to have a talent for it.
Why honey fungus of all things?
Honey fungi have an exceptionally effective life strategy.
Black “bootlaces” under the bark
Honey fungi produce rhizomorphs: dark, tough cords made of tightly packed hyphae. They look like black bootlaces and can be found under the bark of dead trunks or in the soil. With them, the mycelium moves from tree to tree, carrying water and nutrients, even across areas where there is nothing to feed on.
Parasite and decomposer in one
Honey fungi can attack living trees. They cause rot of the roots and the base of the trunk. When the tree dies, the fungus does not leave: it goes on breaking down the dead wood. In Oregon the mycelium gradually infected one conifer after another, and in the forest this shows up as patches of dying trees.
Honey fungi in Poland
Polish forests are home to, among others, the dark honey fungus, the same species as the Oregon giant, and the honey fungus. Their clusters appear in autumn on stumps, roots and at the base of trees. Foragers know them well, but be careful: the deadly poisonous funeral bell grows in similar places, and so do poisonous sulphur tufts (Hypholoma). You must never rely on a single feature such as “grows in a cluster on a stump”.
What does this mean for foragers?
The Oregon honey fungus is a useful reminder of how much of a forest is out of sight. Under every forest where we pick mushrooms lives a huge, invisible network of mycelium. It is the mycelium that:
- forms mycorrhiza with trees and helps them take up water and minerals,
- breaks down dead leaves, needles and wood, returning nutrients to the soil,
- produces the fruiting bodies we pick, year after year, as long as we don’t destroy it.
How not to destroy the mycelium
- Don’t rake the leaf litter or dig through it looking for mushrooms. The mycelium in the top layer of soil is delicate and dries out easily.
- Pick gently. Twist the fruiting body out or lift it carefully, and after checking the stem base, cover the spot with litter again.
- Don’t kick or trample mushrooms you are not picking, poisonous ones included. They are part of the ecosystem and spread spores.
- Leave old and overgrown fruiting bodies. They won’t end up in the basket anyway, and they can still spread spores.
- Don’t pick protected species. In many countries some fungi are protected by law. Leave them in the forest.
- Move through the forest with care. The less trampled the litter, the better for the mycelium.
Research and long-term observations suggest that careful picking of fruiting bodies on its own usually does less harm to the mycelium than trampling and digging up the ground. Good forager habits are about safety, and also about making sure mushrooms grow in the same place next year and in ten years. You will find more rules in our guide to mushroom safety.
The fungus you cannot see
The next time you see a cluster of honey fungi on an old stump in the forest, remember that it is only the visible part. Beneath your feet there may be a network of mycelium that was growing long before there was a road or village here. Track Points and trophies in Gribo are fun, but the reward that matters is a forest that will produce mushrooms again next year.
A rule that applies to every species, honey fungi included: whether a mushroom can be eaten is decided by a mycologist, not by a photo or an app.
Frequently asked questions
What is the largest organism on Earth?
By area, the one most often named is the dark honey fungus mycelium in the Malheur National Forest in Oregon, which stretches over almost 10 km². By mass, the answer is often Pando, a clonal aspen colony in Utah – it all depends on the definition you use.
How old is the Oregon honey fungus?
Its exact age is not known. Estimates are based on how fast the mycelium grows and usually range from about 2,400 to more than 8,000 years.
Does honey fungus harm trees?
Yes, honey fungi can be parasites. They attack the roots and the base of the trunk, causing rot, and can kill weakened and sometimes even healthy trees. On dead wood they live on as saprotrophs that break it down.
Does picking mushrooms destroy the mycelium?
Careful picking of fruiting bodies usually does less harm to the mycelium than trampling, digging up and raking the leaf litter. That is why it is worth picking gently and leaving the spot the way you found it.