World’s Largest Organism and the Forest Health

 In Oregon, the Humongous Fungus plays a complex role in an ecosystem reshaped by humans

By Colin Hogan

Under the blue mountains of Oregon lurks something massive and prehistoric. Yet the largest recorded organism on Earth, weighing more than 200 blue whales and dwarfing even Pando, Utah’s famous grove of quaking aspens, is nearly invisible to the untrained eye. It’s a single, genetically identifiable specimen of honey mushroom or Armillaria ostoyae that has been growing for thousands of years.

Nicknamed the Humongous Fungus, it covers nearly four square miles within Malheur National Forest and weighs perhaps 7,500 tons. The fungus likely attained its record-setting dimensions in part thanks to conditions created by 20th-century forest management.

And it continues to grow, expanding mostly underground in networks of thin filaments called mycelia. As the fungus spreads, it moves up into trees, hidden beneath their bark. It then slowly eats away at its host, often killing the tree and then continuing to munch on the deadwood for decades.

More than just an insidious parasite, the Humongous Fungus is a symbol of an ailing, at-risk forest, unintended consequences of fire suppression, and the challenge of restoring an ecosystem’s health.

“If there were no trees dying, I wouldn’t have a job,” says forest pathologist Mike McWilliams, who calls himself the unofficial tour guide of the massive fungus. “But I like this thing because it’s super interesting.”

McWilliams continues driving, following dirt roads deeper into the forest, where the trees become smaller and closer together. The ground is littered with fallen trees and brush, what foresters call surface fuel. Then, at last, the tour arrives at the main attraction: the Humongous Fungus.

A stand of ponderosa pine in Oregon’s Malheur National Forest

It’s easier to see the decay that Malheur’s most famous resident leaves behind than the fungus itself. What should be a thick and thriving forest is instead a collection of toppled trees, with many more dying. McWilliams uses his Pulaski, an ax-like forestry tool, to chip away at bark and reveal subtle, cream-colored fans on the exposed wood: evidence of the fungus spreading within an infected fir.

“Part of the reason [the Humongous Fungus] got so big is because of the history of fire suppression,” McWilliams says, referring to the dominant tenet of the last century of forest management. “Fires would have reduced the proportion of highly susceptible hosts, and you’d have a functional, healthy forest there.”

Just as fire has an important role in a forest ecosystem, so do various species of fungus. Simply put, terrestrial forests couldn’t exist without fungi. Some fungi exchange nutrients with plant roots in return for the sugars that come from photosynthesis. The ponderosa pine, a fire-resistant tree with reddish bark and a distinctive butterscotch smell, requires fungal assistance as a vulnerable seedling: It can grow to more than 100 feet tall, but it couldn’t make it to one foot without fungi, which help keep surrounding soil moist and ferry nutrients through the soil to the young tree’s roots.

  1. ostoyae, the Humongous Fungus species, isn’t one of these beneficial fungi—at least, not for the trees, it infects during the parasitic stage of its life cycle, eventually killing them. But during its saprophytic stage, when it feeds off its dead host, armillaria, like many other fungi, facilitates the crucial process of decomposition and helps return resources to the soil; we now know it’s what makes the fungus important to the overall ecosystem.

“There’s been an increase in understanding of how fungal pathogens play an important role in the forest: They remove the weakened trees and aid a resistant and vigorous pool of tree genetics,” says Oregon State University regional wildland fire specialist Ariel Cowan, who studies the intersection of soil health, wildfires, and fungi.

(Courtesy: Atlas Obscura)

Image courtesy of (Image Courtesy: Atlas Obscura)

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