Eventually we reached an unsigned fork in the road. We chose to head toward a patch of tall, enticing conifers, stopping to admire the colossal trees before continuing on, letting our curiosity guide us through the network of Forest Service roads in search of the perfect camping spot.
It was a hot day in late August. The afternoon sky was a hazy blue, the air still. After setting up camp on an open knoll, I ventured off on my own to explore the surrounding hills on foot. Originally from Vermont, this place posed a puzzle for me. I was relatively unfamiliar with the wildflowers, trees, and grasses of this ecosystem and unused to the drier air.
As my path traced the form of the rolling hilltops, I came across a clump of dead trees perched on the side of a wide ridge, trunks bleached white by the sun. I felt a pang of sadness as I gazed at the tangled knot of skeletons. At the time, I was in the midst of studying how evergreen forests respond to climate change. Seeing the lifeless conifers in the Blues felt like a personal affront.
As a scientist who focuses on how climate change impacts high altitude and high latitude ecosystems, it’s my job to look for warning signs etched into the landscape. In recent years, I’ve felt more sadness as I’ve watched places around me change dramatically in response to unprecedented changes in climate systems. So when I saw the skeletal stand of trees, my mind immediately began to run through scenarios that could explain the trees’ death. Wildfire? Bark beetle infestation? Drought? Evidence of the usual suspects was lacking and I had a sneaking suspicion that I wouldn’t actually be able to see the culprit behind these dead trees.
When observing life in a forest, most of us focus on what we can actually see. A tree’s soaring trunk, the brilliant green display of a fern’s fan, a gnarled burl’s unnatural bulge, or the peeling scales of an Engelmann spruce’s bark.
Our attention is captivated by the forest’s above ground presence. But this is a perspective that can be misleading, and perhaps even dangerous when trying to understand a forest’s inner workings. Beneath our feet, the soil houses a rich and complex world of its own – a world that we can’t see, but one we couldn’t live without.
Soil houses a rich and complex world we can’t see, but couldn’t live without.
I was taught in elementary school that the largest living organism in the world was the blue whale. This is perhaps easier to explain to third graders than the truth. In fact, the largest living organism in the world is a honey fungus. Specifically, an individual of the Armillaria ostoyae species. Colloquially, this specific fungus is known as ‘the humongous fungus.’ By most estimates, this individual spans 3.7 square miles and weighs around 35,000 tons. The weight of this fungal network is equivalent to 233 average-sized blue whales. And it’s located in Oregon, at the southeastern edge of the Blue Mountains.
I learned of the existence of the expansive network laid by Oregon’s humongous fungus three months after watching that August sunset in the Blues. I had been unaware as I sat on the dusty knoll with my face turned up to catch the last rays of sun, curling my bare toes into the fine soil, that I was very close to the largest living organism in the world.
Once I learned about the fungus, I was hooked. I studied some maps and determined that I had been slightly north of the humongous fungus’s sprawling territory; I hadn’t actually been on top of it. But that wouldn’t have mattered. You can’t actually see the world’s largest organism: it exists almost completely underground, silently snaking through the soil. While unlikely that the humongous fungus was responsible for the death of my trees, one much like it very well could have been. And I was curious to learn more about the role of fungi in my beloved forests.
Origins of a killer fungus
Thousands of years ago, two fungal spores mated. Each spore was a tiny capsule with a tough exterior designed to protect the genetic blueprint contained inside. Because of their size, they moved deftly through the world, floating invisibly through the air, riding the tides of wind.
When two spores came in contact with each other under the right conditions, their nuclei fused in the process of reproduction, and the resulting germination product gave rise to this particular Armillaria network. Its first act of survival would have been to extend tiny, threadlike hyphae out into the soil in search of food and water. Encountering trees and other woody material it could digest, the fungus grew, hyphae branching out into different directions. The fungus spread at a rate of one to three feet per year, making it somewhere between 2,000 and 8,000 years old.
Armillaria ostoyae is a pathogen: it preys on conifers and other trees. Taking them down from the roots up it colonises, starves, kills. This predatory pathogen has the perfect alibi: it operates mostly out of sight, underground. Armillaria adopts the form of black, ropy shoestring-like threads: rhizomorphs. These cord-like structures connect food sources, allowing the fungus to shuttle nutrients and water across vast distances and enabling the fungus to extend its sprawl and invade new territory in search of woody material to digest. There’s no doubt that Armillaria – a harbinger of death in forests it moves under – is good at what it does.
There are a few weeks each year when the fungus is visible above ground. During the fall rainy season, honey brown, pillowy mushroom caps sprout up. But these clusters are fragile, self-effacing, almost demure. Not the profile of a massive forest pathogen.
Armillaria does leave some other clues of its presence, like a white spongy film under the tree bark of its hosts. And if you were to dig into the soil around a patch of dead trees you might unearth some black, sinewy rhizomorphs, but that would require an intimacy with this fungus that most of us don’t have. The death this fungus brings to forests is an easier indicator of its presence than the fungus itself. On my first trip, I wasn’t well-versed in the life of the humongous fungus when I encountered those dead trees in the Blue Mountains, so I didn’t know how to look for it.
I’m fascinated by this fungus and its ability to evade detection despite its enormous size. A behemoth underground network. An enormous individual that can’t be seen. An organism that has persisted for thousands of years in spite of human development, evolving ecosystems, changing climates.
While Armillaria is a likely cause of death for the skeletons in the Blues, I’ll never really know. It’s not the neat conclusion that I was hoping for and I’m not even sure it’s fair to view Armillaria as a malicious predator. So was there some larger ecosystem purpose or perhaps weakness at play?
Attempting to sink deeper into this question, I try flipping my perception of the forest upside down, viewing the situation not from the perspective of the trees but from the perspective of the fungus. As the fungus, I look for woody material to digest and try to sense weak spots in the forest that may be easier prey for my stealth predation. In flipping my world view, I have to concede that the fungus is not an intentional killer, but actually a crucial part of this ecosystem.
A fungus acquitted
Fungi are important, if often overlooked, players in both the above- and below-ground world. A diverse subclass of organisms, fungi range in size, complexity, shape, and function. Some are detrimental to plants and animals, bringing disease and death. Others are vital to our survival. Merely by breathing we inhale somewhere between 1,000 and one million spores every day.
One of the major roles of fungus in soil is decomposition, transforming organic material into nutrients and in turn making the nutrients available for growing plants and animals. In this way, fungi help clear dead plant matter from forests and recycle the nutrients of this material back into the ecosystem, spurring new life. Fungi also play a major role in drawing carbon into soil, sequestering it from the atmosphere.
Armillaria itself is opportunistic. It usually targets trees that are already weak and ailing: low hanging fruit. By pruning maladapted trees, Armillaria helps increase the strength of the forest’s gene pool by leaving a higher proportion of healthy trees. But Armillaria isn’t a hoarder. The fungus shares its spoils by digesting woody material for the benefit of the entire ecosystem. As the fungus moves underneath the forest, it is constantly reallocating resources and bolstering the strength of its ecosystem.
As the fungus moves underneath the forest, it is constantly reallocating resources and bolstering the strength of its ecosystem.
But in recent centuries, the forests above the fungus have changed. And we humans have been a driver of that change. Excessive logging and a legacy of fire suppression have created less diverse yet denser forests. In forests that have been protected from the purges of natural fire cycles, we find particularly flammable trees in tight clusters, and more leaf litter and dead branches on the ground, paving the way for catastrophic fires rather than low intensity burning.
Furthermore, warming temperatures, droughts, and shifting precipitation patterns are putting new stress on trees and paving the way for an increased prevalence of invasive insects, undermining the strength and durability of forests. These changes have made our forests weaker: more food for Armillaria.
A human accomplice
Recently – the blink of an eye in the lifetime of this fungus – human activity has inadvertently favored the survival of the fungus over trees. Armillaria’s size may be an indicator of overall forest health: the bigger it is, the worse off forests are. And we may be the unwitting accomplices of the fungus, our activity making it a serial killer.
In a world where all organisms are trying to honor their evolutionary blueprint to survive and procreate, I can’t fault Armillaria for the death of the trees. The fungus may just be capitalizing on the weaker forests that we have played a role in creating.
Our climate is changing. Ecosystem dynamics will continue to shift. And Armillaria will kill more trees. In the midst of this instability and ecological upheaval, we can’t disentangle components of ecosystems from each other. The fungus may currently be thriving but it won’t be able to live without its hosts. It needs the trees. The loss of Armillaria would likely be indicative of broader ecosystem collapse, so I can’t help but root for the fungus even as I mourn the demise of the trees upon which it preys.
When I close my eyes, I can transport myself back to my August night in the Blue Mountains. In hindsight, this memory is tinged not only by the sights and smells of that day – the sun warming my face as it crept down, eventually sinking behind the layers of mountains to the West; scrubby vegetation tickling my legs as I stretched out on my back on top the knoll next to our car to wait for the stars to come out; a light breeze blowing wisps of hair across my face… This memory is now also infused with the knowledge of all that I couldn’t see or feel or taste or touch or smell. Now, looking back, I can imagine a fungus’s ropy hyphae extending throughout the soil beneath me questing for more trees, perhaps hoping for my complicity.