One honey fungus individual covers 965 hectares

When people speak of the largest organism, it is easy to picture a single body: a whale, a sequoia or an enormous fruiting body. In fungi the boundary of an individual runs differently. What we see above ground is only a short-lived reproductive structure, while the organism proper forms a network of hyphae hidden in soil, roots and wood. In the Malheur National Forest in Oregon, researchers identified a genetically uniform individual of the honey fungus Armillaria ostoyae covering about 965 hectares.

This was not established from the extent of infected trees alone. Samples of the fungus were taken from many places, cultured, and their genetic characters compared. Isolates that matched were treated as fragments of the same individual. That makes it possible to distinguish one extensive mycelium from neighbouring colonies of the same species. The figure of 965 hectares therefore describes the area over which one genotype occurs, not a continuous mat of mycelium filling every centimetre of forest.

The honey fungus is not a gentle giant. It develops in tree roots and causes root disease. Its distinctive rhizomorphs, resembling dark cords, allow it to move between sources of wood. Fruiting bodies appear seasonally, but the organism can persist underground for a very long time. Its age is estimated at thousands of years from its extent and probable growth rate; it is not a direct measurement taken from the beginning of the colony's life.

It is therefore safer to speak of one of the largest known organisms and to state the criterion at once. By area occupied, the Oregon honey fungus holds the record. A record for mass depends on the model and its assumptions, and records for length or height would belong to other organisms. The most striking fact remains unchanged: a forest landscape may in large part be one individual, invisible from the surface. It is also worth remembering that the species name is sometimes given as Armillaria solidipes; the literature uses both for the North American taxon.

What the evidence actually shows

“965 hectares” does not mean one visibly continuous sheet of mycelium. Researchers sampled Armillaria from roots and stems across Oregon’s Blue Mountains and compared somatic compatibility and genetic markers. Those tests delineated the territory occupied by one genotype—a genet of Armillaria ostoyae (now often treated as A. solidipes). Mushrooms emerging far apart are only short-lived reproductive structures of that same extensive organism. Its age and mass were not weighed or directly dated; both are model-based estimates that depend on assumed growth rates, sampling density, and continuity between sampled points. The strongest result is therefore the approximately 965-hectare genetic footprint, not the unusually precise tonnage or multi-millennial ages repeated in popular accounts. The case also shows why a fungal “individual” is defined by genotype rather than by the number of fruiting bodies above ground.