Some trees love (and need) salmon…

Organisms are complex beings made up of hundreds to millions of units working together to form a whole. If one unit goes down the whole organism can be impacted. The organelles of a cell, the cells of an organ, the organs of an animal… the species of an ecosystem.

We generally don’t think of ecosystems as living organisms, but scientific research continues to convince me we should.

To understand the link between salmon and the growth of a forest we need to dig into the story of an unexpected player: fungus.

The invisible underground network

An unbreakable bond: Fungus and Trees

Trees and fungi form a symbiotic relationship that has been evolving for over 400 million years. Without each other, neither would be able to survive, but together they thrive.

In the temperate forests of Canada, biologists have been researching this relationship and its role in the health of this wooded ecosystem as a whole. You might assume that studying some of the other native species such as the bald eagle or grizzly bear would be more interesting, but what they’ve discovered underground is pretty 😲  !

Amongst the tree roots lives an almost invisible fungus (about 1/10 the size of an eye lash), which forms an extensive underground labyrinth. Tiny fungus tubes intermingle with tree roots, branching out in a vast network of tunnels like an elaborate subway system.

This hidden transportation hub is remarkably vast. One study found up to 7 miles of this threading in just one pinch of dirt, and it spans the subterranean soil connecting trees and shrubs like an internet for plants.

Why do the trees and fungus need each other?

On the surface, the relationship between fungus and plant is a simple exchange. The tree has carbon (the sugar of life), the fungus provides valuable nutrients. But underneath this basic principle of symbiosis scientists lies an impressive level of complexity.

Trees contribute carbon > fungus contribute minerals and nutrients

Trees soak in sunshine, carbon dioxide, and water, and through photosynthesis they extract the carbon and release oxygen. Trees turn this carbon into sugar which it uses to make its trunk, branches, and other critical tree parts.

Carbon is an essential building block of life, but alone it’s not of much value to vegetation. To achieve their great might, trees also need minerals like nitrogen, phosphorus, magnesium, potassium, copper, etc., all of which can be found in the soil. Without these critical nutrients trees would struggle to grow taller than a foot.

But trees aren’t as good at absorbing these vital nutrients as we all believe. Trees can only soak up these minerals and waters through the tiniest tips of their roots which isn’t nearly enough to meet their needs.

Enter fungus.

Fungus has mastered the transportation of nutrients and water with its vast network of tubes (and as we’ll see in a bit, they’ve also mastered mining, hunting, and scavenging for nutrients). But fungus can’t just live on soil nutrients and water. It needs the sugar of life too.

Because fungus can’t photosynthesize, they rely on trees to do the work. So, in an underground economy of sorts, trees and fungus trade sugar for water and nutrients. (We are almost to the part about salmon!)

The great exchange

To initiate this essential relationship fungal tubes stretch up toward new tree roots and start a chemical conversation of sorts. The chemical signals emitted by fungi prompt a tree’s roots to soften, allowing the fungus to intertwine itself into the plant’s foundation.

This interconnectivity of the two species allows them to exchange packets of sugar, water, and nutrients on a cellular level. One sugar for one nutrient. One nutrient for one water. And so on…

How do the fungi get the minerals?

Symbiotic relationships are awesome, but this one gets even better. The most fascinating part of this story lies in the way fungi hunt, mine, and scavenge for the nutrients they need in order to hold up their end of the deal.


Fungi are expert miners. Their tubes worm their way back and forth through the soil searching, until they bump into some soil particles – packets of minerals. Once they strike rock they secrete an acid that starts to dissolve the mineral.

The fungus continues this process, tunneling inside the rock to reach little packets of minerals they can then mine and transport. When you look at these rock particles under the microscope you can actually see the little tunnels bored out by fungus.


Fungi are hunters too. Their main target (in this northern temperate region) is a small insect called a springtail. Springtails are flea sized insects that spend most of their time munching on leaves of the forest floor. They get their name from their curly appendage on their backside that allows them spring themselves into the air… really high. Fun fact: their self-catapult is equivalent to a human jumping over the Effeil tower.

Springtails also eat is fungus, however, which is what prompted an experiment in which scientists gave springtails some fungus to eat. Their results were unexpected. Instead of eating the fungus, the fungus ate the springtails.

With closer examination, the scientists saw little tubes had grown within the springtails. Furthermore, several of the springtails that had the tubes inside them were still alive! Fungus were mining nitrogen from the living.

In a similar study scientists found that 25% of the nitrogen from springtails ended up in the trees. But springtails aren’t the only source of Nitrogen.

They’ve also traced nitrogen in tree rings back to salmon, which is why scientists also refer to fungi as scavengers.


Bears on the northwest are famous for their salmon fishing abilities. After a catch they often wonder into the forest and find a place to sit and enjoy their meal.

Bears can be pretty picky eaters when they have a plentiful supply of salmon. They’ve been observed peeling the skin off fish to reach the fillet and even opening the stomach to reach an egg sac, leaving behind the rest of the fish.

The result of this seasonal feeding frenzy is a transportation of salmon from stream to forest floor (and eventually up into the canopy).

This is where fungus comes back into the picture.

After the carcass has seeped into the soil from decomposition the fungus sucks up the nitrogen into its network and distributes it to the trees. Salmon are such a rich source of nitrogen for vegetation. In some areas up to 75% of the nitrogen in the trees originated from fish.

If you take away the fish, you take away an essential building block for vegetation.

Closing remarks

The more I learn about our earth’s ecosystems, the more I see our world as one big organism. As with any organism, whether it be a cell, organ, or species, if we neglect the health of it’s components we neglect the health of our planet.

This story of fungus, trees, and salmon is quite simplified, but even a broad understanding of how our world helps us make conscious decisions in our daily lives. Are we going stop to contributing to the problem or just keep ignoring it? I don’t suggest we stop eating fish, but maybe it’s time we widen our perspective and be more aware of the impact our decisions have.

For even more detail on the underground network of fungus see Radio Lab’s podcast From Tree to Shining Tree. Their half hour narrative is worth every minute.

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