What do trees say to each other?

The scientific revolution has progressively dismantled humanity’s egocentric illusions. We once thought we were the center of the universe; we’re not. We once thought we were special creations separate from the rest of nature; we’re not. Now we’re learning that our most prized trait—namely, our intelligence—even that doesn’t make us unique.

Through mirror recognition tests, we’ve learned that more animals possess self-awareness than we ever suspected: not just great apes, but (arguably) dolphins, elephants, magpies, manta rays, bears. We’ve learned that bees recognize human faces, that crows use tools and that parrots understand abstraction.

Further afield

The more we search, the more intelligence we find in nature. So, why not look even further afield? Specifically, what if we venture outside the animal kingdom?

For example, how about trees? Could they be thinking deep, slow thoughts, in tune with the turn of the earth and the pulse of the seasons? Could they possess a hidden sentience that we don’t recognize because they’re too large, too slow, and too dissimilar to us?

It sounds ridiculous. But the excellent book The Hidden Life of Trees, by Peter Wohlleben, argues that trees have more intelligence than we might guess. Implausible as it sounds, they’re capable of clever, strategic, and even altruistic behavior.

Wohlleben narrates the experience of finding a ring of mossy stones in a preserve of old beech trees—except when he inspected the “stones” more closely, he realized they were wood. They were remnants of an ancient stump, felled so long ago that the heartwood had rotted away to nothing, leaving only the outermost edge of where the tree had been.

When he looked closer, he found something astonishing: the remnants were still alive. There was green, living wood under the bark. How can life persist for centuries in a rotted-out stump, with no leaves and no chlorophyll to synthesize the sugars the living cells need?

The mycorrhizal network

When you see a mushroom sprouting from soil or dead wood, that’s not the entire fungal organism. It’s just the “fruiting body”, the part that makes the spores. The mushroom is to the fungus as the apple is to the apple tree. The heart of the fungus is the mycelium: an underground network of fine branching hairs called hyphae.

Mycelia can be enormous, sometimes spreading for many miles. They play a crucial role in decomposing organic material. In a healthy ecosystem, they intertwine with the roots of trees and other plants. They connect them together in a so-called mycorrhizal network (derived from the Greek words for “fungus” and “root”), although scientists cheekily nicknamed it the “wood wide web”.

It’s a beneficial symbiosis. Trees synthesize sugar to feed the fungi, and the fungi return the favor with nutrients like phosphorus and nitrogen, which they can extract from soil and rock with decomposer enzymes that trees don’t have. (There are even carnivorous fungi that secrete toxins to kill microscopic animals so their decaying bodies enrich the soil. Nature isn’t nice.)

Trees can also communicate with each other, both directly and through the mycorrhizal network. When one tree is attacked by leaf-munching animals, it can signal its neighbors to ramp up the protection of defensive chemicals. These can be tannins and other bitter-tasting molecules, or pheromones that attract beneficial predators, like wasps which attack leaf-eating insects. This isn’t a reflex reaction, either. Trees do it selectively, depending on what‘s damaging them:

When elms and pines come under attack by leaf-eating caterpillars, for example, they detect the caterpillar saliva, and release pheromones that attract parasitic wasps. The wasps lay their eggs inside the caterpillars, and the wasp larvae eat the caterpillars from the inside out. “Very unpleasant for the caterpillars,” says Wohlleben. “Very clever of the trees.”

A recent study from Leipzig University and the German Centre for Integrative Biodiversity Research shows that trees know the taste of deer saliva. “When a deer is biting a branch, the tree brings defending chemicals to make the leaves taste bad,” he says. “When a human breaks the branch with his hands, the tree knows the difference, and brings in substances to heal the wound.”

“Do Trees Talk to Each Other?” Richard Grant, Smithsonian, March 2018.

Trees can even send water and nutrients to each other through the network. That’s what was happening with Wohlleben’s living beech stump. The neighbors of this ancient, mostly-dead tree were keeping it alive with regular transfusions of sugar. Similar behaviors have been observed in other species of trees, including oak, fir, spruce and Douglas fir.

Trees know what they’re doing

You might say that this doesn’t signify intent, that it’s just a set of chemical reactions, as mechanical as water flowing downhill. But that’s not so. Trees can control what they send through the mycorrhizal network. They can distinguish members of their own species from trees of other species and give their own kind preferential treatment.

In some cases, trees make alliances that can only be described as strategic. According to research by ecologist Dr. Suzanne Simard, paper birch and Douglas fir growing together in Canadian forests are able to cooperate. In the summer, the deciduous birches make more sugar, and they send their excess to the firs. In the winter, evergreen firs make more, and the flow reverses direction.

They can even distinguish more or less closely related trees of the same species, favoring kin over non-kin. “Mother” trees direct more nutrients to saplings grown from their own seeds. Thriving trees can nurse sick ones back to health. Dying trees can give up their stored nutrients for the benefit of the community.

This web of interconnection is so complex that there’s debate about whether trees and fungi are socialist, sharing their resources for the benefit of all, or capitalist, competing with each other and making deals in their own self-interest. (For what it’s worth, the only answer that seems right to me is both. Nature is a push and pull between self-interest and cooperation, and both incentives can be operating in different ways at the same time.)

What is intelligence?

Of course, we can stretch a metaphor too far. Trees have nothing to think with, in the way we understand thinking. They can send and receive chemical messages, perceive changes in the environment and take actions in response, but that’s not the same as consciousness… is it?

On the other hand, you could argue that intelligence is as intelligence does. When a mother bird stages a distraction display, pretending to be injured to lure a predator away from her nest, does she know what she’s doing? Or is she a mere creature of instinct, enacting a strategy programmed into her genes by natural selection, with no conscious design to achieve a goal?

This is a slippery, and ultimately unproductive, argument of semantics. Natural selection will produce living things capable of intelligent, goal-directed behavior, whether it’s embodied in the individual organism or in the collective evolutionary history of the species. This is what Daniel Dannett calls the intentional stance: if describing a thing as an intelligent agent with beliefs and desires has predictive power, there’s no further fact to be discovered about whether it’s “really” conscious or not.

A wealth of intelligence

If intelligence is everywhere in nature, that makes humanity less unique. But it doesn’t make our existence less meaningful or less wonderful. In fact, it’s the opposite. That knowledge makes the natural world more awe-inspiring and all the more worthy of our protection.

When I walk through an autumn forest, it deepens my appreciation to know that it’s not an inert landscape, but a web of living, growing, communicating intelligence. With each step I take, there are chemical messages passing back and forth beneath my feet, spreading from root to root like nerve signals crossing synapses. Some of the carbon dioxide I exhale with each breath is taken up by the trees and joins the reservoir of resources shared by the forest.

It’s not consciousness as we know it, but perhaps it’s consciousness as we don’t know it. I wonder if, in their own dim and diffuse way, the woods are aware of my presence. And, if so, I wonder what they’re saying to each other about me.