By Lambert Strether of Corrente.
“If a lion could speak, we couldn’t understand him” –Ludwig Wittgenstein, Philosophical Investigations
Unlike most of my posts, this post will not have a thesis. Starting somewhere, maybe here, in my perambulations through the Twittersphere, I ran across some links on trees communicating — for the moment, we’ll put aside the question of whether trees signal, communicate, or talk, or whether forests do — and I went down a rathole of research, some highlights of which I thought I would share with you. I found the research amazing and beautiful in itself, and the gardening and permaculture contingent in the readership may find it useful in their practice. The three main researchers seem to be Switzerland’s Edward Farmer, Canada’s Suzanne Simard (Ted Talk) and Germany’s Peter Wohlleben (YouTube). America’s George David Haskell also rates a mention (Ted Talk). However, my purpose is not to summarize their research, but to exhibit some bright shiny objects I picked up while going walkabout, and then conclude with some woo woo.
Unlike most of my posts on the biosphere, I will not begin with a classification system or taxonomy for trees. That might be a good thing: San Francisco’s ArboristNow urges that “we know what a tree is – but there is no precise definition which is completely recognized of what a tree is in ordinary language or in botany.” The Arbor Day Foundation disagrees, giving one. But then they would. Here instead is the simplest possible diagram of your standard tree:
I’m now going to make things even simpler, and give examples of how trees communicate above ground (crown and trunk), and below ground (roots).
Communication Above Ground
Forms of communcation above ground seem to fall into two buckets: Volatile Organic Compounds, and electrical signals. (The electrical signals above ground are from leaf-to-leaf in the same tree; they are too slow and weak to travel through the air. That would not be the case for electrical signals send through a tree’s root system, however, as roots entangle below ground.)
Volatile Organic Compounds (VOCs). From Quanta, “The Secret Language of Plants“:
It’s now well established that when bugs chew leaves, plants respond by releasing volatile organic compounds into the air. By [ecologist Richard] Karban’s last count, 40 out of 48 studies of plant communication confirm that other plants detect these airborne signals and ramp up their production of chemical weapons or other defense mechanisms in response. “The evidence that plants release volatiles when damaged by herbivores is as sure as something in science can be,” said Martin Heil, an ecologist at the Mexican research institute Cinvestav Irapuato. “The evidence that plants can somehow perceive these volatiles and respond with a defense response is also very good.”
The VOCs signal not only within a single species of plant, but between them, and even to insects! Quanta once more:
It turns out almost every green plant that’s been studied releases its own cocktail of volatile chemicals, and many species register and respond to these plumes. For example, the smell of cut grass — a blend of alcohols, aldehydes, ketones and esters — may be pleasant to us but to plants signals danger on the way. Heil has found that when wild-growing lima beans are exposed to volatiles from other lima bean plants being eaten by beetles, they grow faster and resist attack. Compounds released from damaged plants prime the defenses of corn seedlings, so that they later mount a more effective counterattack against beet armyworms. : sagebrush induces responses in tobacco; chili peppers and lima beans respond to cucumber emissions, too.
Plants can communicate with insects as well, sending airborne messages that act as distress signals to predatory insects that kill herbivores. Maize attacked by beet armyworms releases a cloud of volatile chemicals that attracts wasps to lay eggs in the caterpillars’ bodies. The emerging picture is that plant-eating bugs, and the insects that feed on them, live in a world we can barely imagine, perfumed by clouds of chemicals rich in information. Ants, microbes, moths, even hummingbirds and tortoises ([plant signaling pioneer Ted Farmer of the University of Lausanne] checked) all detect and react to these blasts.
Trees being plants, the same communication pathways are available to them. From Smithsonian, “Do Trees Talk to Each Other?“:
Trees also communicate through the air, using pheromones and other scent signals. Wohlleben’s favorite example occurs on the hot, dusty savannas of sub-Saharan Africa, where the wide-crowned umbrella thorn acacia is the emblematic tree. When a giraffe starts chewing acacia leaves, the tree notices the injury and emits a distress signal in the form of ethylene gas. Upon detecting this gas, neighboring acacias start pumping tannins into their leaves. In large enough quantities these compounds can sicken or even kill large herbivores.
Electrical Signals. From PNAS, “Identification of cell populations necessary for leaf-to-leaf electrical signaling in a wounded plant“:
Organ-to-organ electrical signaling is a highly conserved feature of land plants. For example, wound-induced electrical signals known as “slow wave potentials” (SWPs; otherwise known as “variation potentials”) have been found in numerous species…. In Arabidopsis thaliana, severe damage triggers electrical activity that propagates from leaf to leaf with apparent velocities in the centimeter-per-minute range… Fast nervous conduction in animals evolved under strong selection from predators (34). Here we argue that pressures from herbivores have led to the evolution of leaf-to-leaf electrical signaling in plants…. Together, our results support the hypothesis that a primary role of SWP in plants is to activate defenses in tissues distal to wounds. Defense-related electrical signaling is therefore common to both the plant and animal kingdoms.
(Instrumenting this signaling is now a line of business, however tiny.) Farmer is a co-author of this piece, and he writes elsewhere of trees being “wounded” by the environment, and responding to wounds. (This makes me wonder what signals, if any, the trees next to or even caught up in PG&E’s power lines might be sending. Nothing pleasant, I would imagine.)
Communication Below Ground
The really sexy topic here is “mycorrhizal networks” (similar, I think, to the “mycelial mat,” see NC here), but first let me follow through on electrical signaling below ground.
Electrical Signals. From Frontiers in Plant Science, “The Integration of Electrical Signals Originating in the Root of Vascular Plants“:
Plants have developed different signaling systems allowing for the integration of environmental cues to coordinate molecular processes associated to both early development and the physiology of the adult plant….. [I]t is well-known that plants have the ability to generate different types of long-range electrical signals in response to different stimuli such as light, temperature variations, wounding, salt stress, or gravitropic stimulation. Presently, it is unclear whether short or long-distance electrical communication in plants is linked to nutrient uptake. This review deals with aspects of sensory input in plant roots and the propagation of discrete signals to the plant body.
“Salt stress”… I can just imagine the trees screaming after the roads get salted in the winter. Or perhaps they are inured/
Mycorrhizal Networks. This is a bit of a grand finale, because Suzanne Simard focuses on that topic and is getting a lot of good press right now. From The New York Times, “The Social Life of Forests” (in one of those horrid mobile-friendly layouts, unfortunately):
Underground, trees and fungi form partnerships known as mycorrhizas: Threadlike fungi envelop and fuse with tree roots, helping them extract water and nutrients like phosphorus and nitrogen in exchange for some of the carbon-rich sugars the trees make through photosynthesis. Research had demonstrated that mycorrhizas also connected plants to one another and that these associations might be ecologically important, but most scientists had studied them in greenhouses and laboratories, not in the wild. For her doctoral thesis, Simard decided to investigate fungal links between Douglas fir and paper birch in the forests of British Columbia.
Amazingly, the “fungal links” enable what we would, if we were anthropomorphizing, call altruism:
By analyzing the DNA in root tips and tracing the movement of molecules through underground conduits, Simard has discovered that fungal threads link nearly every tree in a forest — even trees of different species. Carbon, water, nutrients, alarm signals and hormones can pass from tree to tree through these subterranean circuits. Resources tend to flow from the oldest and biggest trees to the youngest and smallest.
And of course there is communication:
Chemical alarm signals generated by one tree prepare nearby trees for danger. Seedlings severed from the forest’s underground lifelines are much more likely to die than their networked counterparts. And if a tree is on the brink of death, it sometimes bequeaths a substantial share of its carbon to its neighbors.
Mycorrhizal networks can be enormous:
Mycorrhizal networks were abundant in North America’s forests. Most trees were generalists, forming symbioses with dozens to hundreds of fungal species. In one study of six Douglas fir stands measuring about 10,000 square feet each, almost all the trees were connected underground by no more than three degrees of separation; one especially large and old tree was linked to 47 other trees and projected to be connected to at least 250 more; and seedlings that had full access to the fungal network were 26 percent more likely to survive than those that did not.
Here is a network diagram Simard created:
I would argue (as others would not) that we are seeing the typical hub-and-spoke pattern of a scale-free network; the “hub” is the tree at bottom right, labeled “most highly connected,” much as Atlanta is the most highly connected hub airport in the United States. If Mycorrhizal networks are in fact scale-free, that would imply that a hub tree in a forest of thousands of trees would have thousands of connections, and so on. I bet when you walk down the street, looking up at the trees, you never imagined they were all connected and communicating with each other!
I did promise some woo woo — by which I mean material that a Forestry Department focused on yield might have a hard time getting its collective head around — and here, from another Simard article, it is. From Memory and Learning in Plants (PDF), “Mycorrhizal Networks Facilitate Tree Communication, Learning, and Memory.” I will quote the whole abstract, underlining the woo parts:
Mycorrhizal fungal networks linking the roots of trees in forests are increasingly recognized to facilitate inter-tree communication via resource, defense, and kin recognition signaling and thereby influence the sophisticated behavior of neighbors. These tree behaviors have , including capabilities in perception, learning, and memory, and they influence plant traits indicative of fitness. Here, I present evidence that , with scale-free patterns and small-world properties that are correlated with . Moreover, the multiple exploration strategies of interconnecting fungal species have parallels with crystallized and fluid that are important in memory-based learning. The biochemical signals that transmit between trees through the fungal linkages are thought to provide resource subsidies to receivers, particularly among regenerating seedlings, and some of these signals appear to have similarities with . I provide examples of neighboring tree behavioral, learning, and memory responses facilitated by communication through mycorrhizal networks, including, respectively, (1) enhanced understory seedling survival, growth, nutrition, and mycorrhization, (2) increased defense chemistry and kin selection, and (3) among trees, fungi, salmon, bears, and people that enhance the health of the whole forest ecosystem. Viewing this evidence through the lens of , microbiome collaborations, and may contribute to a more holistic approach to studying ecosystems and a greater human empathy and caring for the health of our forests.
“The forest is the computer,” then, as Sun did not quite say. (I should pause to say that I saw Avatar during a long-haul and quite liked it, especially the “neural queue” concept). So if there is such a thing as “tree cognition,” and there is such a thing as “forest intelligence” — not metaphorically, but literally — can we talk to trees? Or talk to a representative tree in a forest? One biologist thinks so. From Quartz, “A biologist believes that trees speak a language we can learn“:
Connection in a network, [biologist George David Haskell] says, necessitates communication and breeds languages; understanding that nature is a network is the first step in hearing trees talk.
For the average global citizen, living far from the forest, that probably seems abstract to the point of absurdity. Haskell points readers to the Amazon rainforest in Ecuador for practical guidance. To the Waorani people living there, nature’s networked character and the idea of communication among all living things seems obvious. In fact, the relationships between trees and other lifeforms are reflected in Waorani language.
In Waorani, . So, for example, any one ceibo tree isn’t a “ceibo tree” but is “the ivy-wrapped ceibo,” and another is “the mossy ceibo with black mushrooms.” In fact, anthropologists trying to classify and translate Waorani words into English struggle because, Haskell writes, “when pressed by interviewers,
Because they relate to the trees as live beings with intimate ties to surrounding people and other creatures, the Waorani aren’t alarmed by the notion that a tree might scream when cut, or surprised that harming a tree should cause trouble for humans. The lesson city-dwellers should take from the Waorani, Haskell says, is that “dogmas of separation fragment the community of life; they wall humans in a lonely room. We must ask the question: ‘can we find an ethic of full earthly belonging?’”
I am all for “greater human empathy and caring” and “an ethic of full earthly belonging,” but I would like to know if anyone has ever, literally, tried to talk to a tree. Was it successful? How would one know? How would the communication take place? What could we say that would be of interest to a tree? More nutrients? No wounding? It sounds like a lifetime project, possibly best conducted by a forest monk. Of course, this is the NC commentariat; perhaps we have a reader who has talked to trees already. Hello? Are you out there?
 The early research on this topic was apparently pretty sketchy, and “The Secret Life of Plants” didn’t help. From Quanta: “The first few “talking tree” papers quickly were shot down as statistically flawed or too artificial, irrelevant to the real-world war between plants and bugs. Research ground to a halt. But the science of plant communication is now staging a comeback. Rigorous, carefully controlled experiments are overcoming those early criticisms with repeated testing in labs, forests and fields.”
 Though I am not at all sure trees would think of themselves that way. How Ents came to walk has always been a sticking point for me in Lord of The Rings. What about their roots?
 An example of a VOC is Limonene, a major component of pine resin and orange peel scents. (I had a simple mental model of one VOC, one scent, but things aren’t so simple.)
 Whichever fool picked 500 for a font-weight also made the article difficult to read in Safari’s Reader View. Acres and acres of bold, ffs.
 I am assuming the hubs are trees and not fungi.
 That is, by their positions within a network.