Under the Blue Mountains of Oregon, there lurks something massive and prehistoric. It’s the largest recorded organism on Earth. It weighs in at more than 200 blue whales and dwarfs Pando, Utah’s famous grove of quaking aspens. However, this organism is nearly invisible to the untrained eye. What is it, you may ask? Well, it’s a single, genetically identifiable specimen of honey mushroom, or Armillaria ostoyae, that has been growing for thousands of years.
Aptly referred to as the humongous fungus, the honey mushroom covers nearly 4 square miles within Malheur National Forest and weighs perhaps 7,500 tons or more. The fungus likely attained its record-setting dimensions in part thanks to conditions created by 20th century forest management. 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 dead wood 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.
Researchers decide to look into this phenomena. They hike to the forest and see that there are a few trees here that are more spread out. Some are clearly dying—the work not of the Humongous Fungus but rather a smaller relative. In Malheur, there are several Armillaria specimens, and it’s hard to tell with boots on the ground where one fungus ends and another begins. That’s why the researchers collect samples and map them genetically.
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. The team uses a 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.
They theorize that part of the reason it got so big is because of the history of fire suppression. As one researcher said, “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. For example, the ponderosa pine, which makes a fabulous holiday tree, requires fungal assistance as a vulnerable seedling. This tree can grow to more than 100 feet tall, but it couldn’t make it to one foot without fungi because the fungi help keep surrounding soil moist and ferry nutrients through the soil to the young tree’s roots.
A. 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. 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. As a result of this knowledge, we now understand that this is 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.
Improved knowledge about the positive role of Armillaria is part of an emerging, broader view of forest ecosystems. As scientists learn more about a forest’s natural defenses against fire and other threats, and its ability to regenerate after being damaged, those mechanisms are being incorporated into a new way of forest management. “The definition of forest health is different and more holistic today than it was in previous times of forestry,” Cowan says.
Before humans arrived in what’s now the American West, fires resulting from lightning strikes regularly cleared out scrub and debris in the underbrush. Trees grew further apart, at irregular intervals rather than the neat grid of modern forest plantations, which made it harder for fire and pathogens—even giant fungi—to move unchecked from tree to tree.
The first humans to enter the western forests learned the rhythms of these ecosystems over millennia. In several regions, Native American tribes set fires regularly to remove excess brush and assist fire-adapted species. This traditional approach to forest management minimized surface fuel created by a scrubby, cluttered understory. When a lightning strike ignited a blaze naturally, it would not become intense enough to threaten established trees with thick bark and soaring canopies that provided the roof of the entire ecosystem. The forest continued to protect itself, with an assist from the people who relied on it and respected its natural rhythms.
The cycle broke when European settlers forced Indigenous communities off their lands throughout the West and began managing the forests to suit their own needs, including for cattle grazing and timber. Logging projects left piles of organic detritus on the forest floor as ready fuel. When a fire inevitably broke out, attempts to control it were often disorganized.
As a result, there are now more fir trees in western forests than there should be. Douglas firs and grand firs, specifically, are common—and are not adapted to withstand fires. Though these firs are native, they are proliferating in non-native numbers.
Douglas firs and grand firs have allowed something else to happen. These species are highly susceptible to infections of the fungus A. ostoyae. While the Humongous Fungus predates 20th century forest management through fire suppression by thousands of years, it probably would not have gotten so enormous without it.
The A. ostoyae specimen known as the Humongous Fungus is not alone; in the late 20th century, another outsized Armillaria, this one in Washington State, achieved similar proportions causing researchers to ponder the fact that maybe this is the largest organism recorded, but not in actuality.
Regardless, these giant fungi which are slowly destroying the forest may also be tools to help it recover from a century of problematic fire management—and to protect it from a changing climate that’s hotter, drier, and at greater risk for catastrophic fires.
While it’s unclear whether a fire burning above it would damage the Humongous Fungus itself, trees are spaced further apart and organic material on the ground has been broken down. As the Humongous Fungus and other Armillaria expand at a rate of up to 5 feet per year in all directions, they chomp through the highly susceptible Douglas firs and grand firs—creating space, and filtering nutrients back into the soil, to support the potential growth of species more resistant to fire (and fungus). Eventually, Armillaria could clean out all the overgrowth and natural debris on the forest floor—but not on a timeline that’s acceptable to humans.
Now, more forest management experts are starting to reintroduce fire into the landscape across the American West through small, highly controlled fires known as prescribed burns. This can be politically and ecologically tricky, but it’s the best solution researchers can come up with – for now.
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