Fungi often exist out of sight, with most of their mass extending far beyond the visible mushroom. This hidden world includes organisms that challenge traditional notions of size. Understanding these immense fungal networks requires looking beyond the fruiting body to grasp their true extent.
Defining Immense Fungi
The title of “biggest mushroom” depends on the criteria used for measurement, as two different fungi hold records based on their unique growth forms. Armillaria ostoyae claims the record for the largest living organism by area. This single individual spans an impressive 2,385 acres in the Malheur National Forest in eastern Oregon. Estimates suggest this massive fungal colony is between 2,400 and 8,650 years old and weighs as much as 35,000 tons.
Conversely, Bridgeoporus ellipsoideus (formerly Fomitiporia ellipsoidea) holds the record for the largest individual fruiting body, the visible, spore-producing structure. Discovered on Hainan Island, China, this bracket fungus measured 10.85 meters (35 feet 7 inches) long, 84 centimeters (2 feet 9 inches) deep, and 5 centimeters (2 inches) thick. This colossal structure was estimated to weigh between 400 and 500 kilograms. The distinction lies between a vast, interconnected underground network and a single, massive above-ground structure.
Biology Behind Their Scale
The size of these fungi is attributed to biological mechanisms and environmental factors. Armillaria ostoyae achieves its immense spread through an extensive underground network of root-like structures called rhizomorphs, or “shoestrings.” These filaments spread through soil, connecting to new food sources like tree roots. The bulk of this organism exists as mycelium, a network of thread-like filaments extending deep into the soil and under tree bark.
This parasitic fungus invades tree sapwood, drawing nutrients and causing root rot. Its ability to spread slowly, typically between 0.7 to 3.3 feet per year, contributes to its longevity and expansive reach. Stable environmental conditions, consistent moisture, and a continuous supply of host trees allow Armillaria ostoyae to persist and grow for millennia without significant disturbance.
Bridgeoporus ellipsoideus, on the other hand, grows as a perennial bracket fungus on dead wood. Its growth is slow and continuous, adding layers year after year to form a large, woody structure. This species functions as a saprotroph, meaning it obtains nutrients by breaking down dead organic matter. Consistent deadwood availability and lack of disturbance enable its fruiting body to reach such dimensions over decades.
Ecological Role
These fungi play different roles within their ecosystems. Armillaria ostoyae acts as a forest pathogen, causing root disease. It infects and kills conifer trees by colonizing roots and girdling them with mycelial fans under the bark. While causing tree mortality and impacting timber production, it also decomposes dead wood, recycling nutrients into the soil.
Bridgeoporus ellipsoideus functions as a wood-decaying fungus. It breaks down dead logs and stumps, important for nutrient cycling. Decomposing woody material releases nutrients for other organisms and plants, enriching forest soil. Their size allows them to process substantial organic matter, impacting environmental health and nutrient flow.