Occasional paper: Fungal banking

So in the last couple of decades we’ve discovered that many plants rely on networks of soil fungi to bring them critical trace nutrients. This is a symbiotic relationship: the fungal network can access these nutrients much better than plants can, and in return the plants provide the fungus with other stuff — particularly energy, in the form of glucose sugar, made from photosynthesis.

It turns out this relationship is particularly important for large, long-lived trees. That’s because trees spend years as seedlings, struggling in the shade of their bigger relatives. If they’re going to survive, they’ll need help.

The fungal network gives them that help. The fungus not only provides micronutrients, it actually can pump glucose into young seedlings, compensating for the sunlight that they can’t yet reach. This is no small thing, because the fungus can’t produce glucose for itself! Normally it trades nutrients to trees and takes glucose from them in repayment. So it’s reaching into its own stored reserves to keep the baby seedling alive.

Gosh that’s beautiful isn’t Nature great! Well… yes and no.

Because the fungus isn’t doing this selflessly. The nutrients and glucose aren’t a gift. They’re a loan, and the fungus expects to be repaid.



After a certain number of years, if the seedling doesn’t start producing glucose? The fungus cuts it off. No glucose, no nutrients, nothing.  The seedling then dies and falls to the forest floor and rots, and the fungal network gets at least some of its loaned material back.

— Let’s pause here for an obvious question: how does a brainless fungus make a loan?

Well… fungi don’t have brains, true. They don’t even have nerves. But fungi have networks — really, they *are* networks. If you look at a mushroom, you’re only seeing the “fruiting body” of a much bigger organism, 95% of which is underground. The real fungus is a complex web of slender strands that may reach tens or even hundreds of meters away.



We know that fungi can send chemical signals across this network, and we know that the network can grow and change in response to those signals. And we know that a big network following a few simple rules can result in surprising emergent behaviors. (Kind of like, you know… a market.) So, at least some fungi can do some sort of basic information processing. At least some fungi have /behavior/.

So, coming back to the particular behavior in question: does this sound familiar? The fungus makes loans out of its stored capital. and if the loan isn’t repaid on time, it forecloses.

Yes: the fungus is acting like a banker: A mindless fungal network invented capitalism tens of millions of years ago.

And it underpins all the great forests of the world!



For centuries, people noticed that forests tended to expand at their edges.  Let farmland lie fallow for a few years, and first weeds would take over; then shrubs and bushes; then trees, and then bigger trees.  It might take a century or more, but field would turn to forest.  This became known as “ecological succession”.  A bunch of empirical rules were derived by observation: given this sort of soil and this much rainfall, you’d see this group of plants first, and then twenty years later this other group, and so forth.

The problem was, succession leading to forest was a bunch of observations with a big theoretical hole in the center.  Imagine a mid-succession field full of tall grass and bushes and mid-sized shrubs.  Okay, so… how does the seedling of a slow-growing tree species break in?  It should be overshadowed by the shrubs and bushes, and die before it ever has a chance to grow above them.

And the answer is, the fungus.  The forest uses the fungus to pump sugar and nutrients into those seedlings, allowing them to grow until they are overshadowing the tall grasses and shrubs, not vice versa.  The fungus is a tool the forest uses to expand.  Or — looked at another way — the fungus is a venture capitalist, extending startup loans so that its client base can penetrate a new market.

This also answered a bunch of other questions that have puzzled observers for generations.  Like, it’s long been known that certain trees are “nurse trees”, with unusual numbers of seedlings and saplings growing closely nearby.  Turns out: it’s the fungus.  Why some trees do this and not others is unclear, but the ones that do, are using the fungus.  Or: there’s a species of lily that likes to grow near maple trees.  Turns out they’re getting some energy from the maple, through the fungus.  Are the lilies symbiotes, providing some unknown benefit to the maple tree?  Or are they parasites, who are somehow spoofing either the maple or the fungus?  Research is ongoing.

Oh, and the fungus may also force the trees to make changes to their lifestyle… for their own good.  “Mycorrhiza also induce changes in the emission of plant volatiles, making them more attractive to the natural enemies of herbivores, their predators [7], and parasitoids [8], thereby providing plants with additional protection.”  Like a bank requiring that you get insurance and install a security system.

Oh, and plants can send signals through the network — for instance, that one tree is under attack by beetle grubs.  Nearby trees may start diverting energy to produce grub-deterrent toxins, which they wouldn’t normally.  We already knew plants could do that sort of thing using airborne pheromones.  Turns out they can do it through the fungus, too.  Does the fungus “charge” for this service?  Or is it gratis, a public service, a fungal 911?  Research is ongoing.

But wait: there’s more!  There are different sorts of fungi, and they have different preferences as to what kinds of plants they team up with.  (Market partition, if you like.)  Some are specialists, some are generalists.  When there are a bunch of different sorts of fungal networks present, the forest tends to be more diverse and more productive.  Competition is good, right?

Not necessarily.  An individual fungus doesn’t want to be competing.  It wants to be the monopolist, the sole provider.  So, there’s evidence that the networks push their favored species to grow higher and faster, so that they can overshadow the species the fungus doesn’t “like”.  The result may be a forest that is less diverse and productive, but that’s not the fungus’s problem.

This is one of those quietly expanding frontiers, with a steady drumbeat of new papers coming out every year.  How far can signals travel?  What other information might be carried?  How long does a fungal network “remember”?  Just how deep does the rabbit hole go?  (And, of course… is there any way to hack this for human advantage and profit?)

Depending on your point of view, you could say this all shows that banks and finance — in the very broadest sense — are not only natural but hardwired into life; something very similar has been going on, silently but profoundly, for millions of years, and it has shaped the world around us.  Or, you could say this all shows that banks and financiers could be replaced by a sufficiently large fungus.

Anyway.  The next time you take a walk in the woods… yes, the trees really are talking: that’s not poetry or a metaphor.  It’s just, a lot of that conversation probably consists of haggling with their fungal partners.