Review of new findings on “tree communication/intelligence”

The epistemic status of this post: Casual search for journal articles substantiating claims on trees, review of their content and methodology. Feel free to check out the linked sources for yourself. I am not a trained natural scientist nor do I play one online.

A few friends have suggested I check out scientific research on whether trees “communicate”, cooperate “socially”, have memory – and as some people seem to suggest, sentience, intelligence, and even conscious intentions.

This has been getting a lot of attention with the commercial success of books like “The Overstory” and “The Hidden Life of Trees”.

tl;dr: Recent research into tree communication is very interesting and it seems ____scientifically sound?____ to me. More than I knew, information is transmitted within ___and across?___ plants. It is unfounded and misleading to imply (either directly or by using anthropomorphising language) that trees are sentient (the ability to have subjective perceptual experiences, which would require a level of awareness and cognitive ability), intelligent (can recognise patterns and extrapolate from them), have intentions (rather than effective evolved mechanisms for survival) or are even self-aware.

Claims from the article “Do Trees Talk to Each Other?”

I read a Smithsonian Magazine profile of Peter Wohlleben, author of “The Hidden Life of Trees”, which is entitled “Do Trees Talk to Each Other?”. As a first look at the tree science mentioned there, I will pull quotes that contain factual claims from this Smithsonian article and then review the studies.

I will ask myself the following basic questions:

  • Does the research actually say what is claimed in the Smithsonian article? (I’ll paraphrase claims to make sure I understand them myself.)
  • Methodology: How did the researchers find that out?
  • Is the research published in peer-reviewed, well-known journals?
  • And what does the research say in more detail?
    • (What does that say about the questions of plant-internal communication, inter-plant communication, plant ‘intelligence’, ’sentience’, or even self-awareness?)

Where available, I have linked to a freely accessible PDF of the study I reviewed. These links are under each quote from the article: “based on the study by…”.

Claim 1: Injured parts of a plant inform its other parts about the injury by sending electrical signals, and this seems very similar to nervous systems in animals.

[When a plant has a branch cut or broken off] it sends electrical signals like wounded human tissue. (…) Edward Farmer at the University of Lausanne in Switzerland has been studying the electrical pulses, and he has identified a voltage-based signaling system that appears strikingly similar to animal nervous systems (although he does not suggest that plants have neurons or brains). Alarm and distress appear to be the main triggers of tree communication.

based on the study by Farmer et al. (2013)
Acker-Schmalwand Arabidopsis thaliana

Farmer et al. published an article on how this works in one plant, Arabidopsis thaliana, and the roles different genes may play, in the renowned scientific journal Nature.

Using non-invasive electrodes, they mapped slight electrical surface potential changes in the plant “after wounding a leaf and found that membrane depolarisations correlated with jasmonate signalling domains in undamaged leaves.” What does this mean? When they did damage to leaves, they could measure in other leaves that the electrical surface potential changed and that ‘jasmonates’ were emitted. Jasmonates are “potent regulators of defence responses”, a group of substances which activates the plant’s defence mechanisms. One hour after hurting a leaf, jasmonate levels were more than a hundred times higher in all other leaves than before.

(How, and against, what does a plant “defend itself”? Against herbivore animals, the Smithsonian article says, by emitting another substance that will make its leaves taste bitter or might even make them lethal to animals. These are things a later claim speaks to – so more on this below).

The researchers tried walking larvae across the leaves, which did not generate signals. But when the larvae started eating the leaf? That did. Seems to make evolutionary sense. “Since insects release various chemical elicitors in addition to causing wounding,” the researchers write, they also wounded leaves mechanically, to make sure the electrical signals are not an effect of the insects’ chemicals. “Touching the leaf did not generate changes in surface potential, but wounding 40% of the leaf tip resulted in strong and reproducible [electrical] surface potential changes.”

Farmer et al. also found that “current injection elicited jasmonoyl-isoleucine accumulation”, meaning: When they did the injection of electrical currents themselves (instead of making the plant do it, by ripping its leaves), then the plant also accumulated the aforementioned signal substances, jasmonates. This verifies that the electrical current is indeed how the plant transmits its ‘my other leaves need to defend themselves!’ alarm signal.

Also interesting: They placed the measuring electrodes on the leaves they were injuring, each at a 1cm distance from the next electrode toward the plant’s stem (since hurting the leaf tip would have to send a signal that way to reach other leaves). The signals were measured “several seconds” after hurting the leaf and took a few seconds for each centimetre of plant distance. It’s more like a signal crawl, apparently, the rest of the plant does not receive a signal instantly. Changes in amplitude were typically close to ‑70 mV.

The mechanism is called “leaf-to-leaf wound signalling”, and the electrical signals triggered by injuring leaves are called “wound-induced surface potential changes (WASPs)”. At least that one plant seems to do it.

But the finding has more significance than that. Farmer et al. say, even in the study’s title, that this process is regulated by glutamate receptor-like genes. Glutamate receptors are what is important for synaptic activity in animals, including humans. So plants manage pain signals in a way that is very similar to how the same mechanism works in animals.

And the WASPs detected on WT plants were indistinguishable to those on wounded plants that do not use jasmonates to activate defence mechanisms. As they write, this “suggests that the mechanism that produces WASPs is upstream or independent” of the ability to produce jasmonates. And that means the same mechanism should be checked in more plants, because this may be how it works in more plants.

So this plant has a mechanism for sending warning signals within itself, and that rests on glutamate receptor-like (GLR) genes. Those are interesting if we want to know which plants may have the same internal communication mechanism, then. About 20 Glutamate-like receptors have been discovered in this specific plant, Arabidopsis thaliana. 1

Which other plants have those? De Bortoli et al. (2016) found them2 in the genomes of:

  • Chlamydomonas (a tiny kind of green algae consisting of about 325 species),
Chlamydomonas globosa
  • chlorophytes (another group of green algae, 90% of which live in freshwater and marine environments),
  • mosses,
  • ferns,
  • gymnosperms:
Gymnospermae.jpg
Gymnosperms include pines, cypresses, cycads, and Gingko
  • and flowering plants (colloquially: flowers!).

Again, these glutamate-like receptors are like glutamate receptors, which take care of fast pain signal transmission in our own bodies. “GLRs are related to ionotropic glutamate receptors (iGluRs) that are important for fast excitatory synaptic transmission in the vertebrate nervous system.” Note: They are important for it, not the only component in it. But Wudick et al. (2018) warn that these receptors may do different things in animals and plants.3 We should always check, and not just think that all plants with GLRs must have the same electrical alarm signal.

Farmer et al. theorise on why these mechanisms are similar in animals and (potentially, some) plants: “[The iGluRs that animals incl. humans have] and their plant relatives may control signalling mechanisms that existed prior to the divergence of animals and plants. If so, a deeply conserved function for these genes might be to link damage perception to protective responses [in distant parts of a plant or animal organism].” Imagine that: The researchers think this mechanism is so old, it may have existed before animals.4

In other plants, if they even have this, the speed of WASPs, those wound-induced electrical signals, may be different. Would be interesting to have this study replicated by other researchers, and repeated on many plant species, and find out about differences in transmission speed and whether anything other than injuring their leaves sends this signal. Also, which we’ll get to in the review of other claims below, can plants transmit this signal to nearby plants?

So far, this seems credible and plausible. Plant-internal communication via electrical signals seems to be a thing, and it seems like it may generalise beyond this specific kind of plant. Again, the PDF with the study.

Other claims I am going to review:

Monica Gagliano at the University of Western Australia has gathered evidence that some plants may also emit and detect sounds, and in particular, a crackling noise in the roots at a frequency of 220 hertz, inaudible to humans.

An example of pheromone communication 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.

Giraffes are aware of this, however, having evolved with acacias, and this is why they browse into the wind, so the warning gas doesn’t reach the trees ahead of them. If there’s no wind, a giraffe will typically walk 100 yards— farther than ethylene gas can travel in still air—before feeding on the next acacia. Giraffes, you might say, know that the trees are talking to one another.

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.

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.”

One teaspoon of forest soil contains several miles of fungal filaments.

When the biggest, oldest trees are cut down in a forest, the survival rate of younger trees is substantially diminished.

Conclusion

We have lots to learn about trees, and recent findings are exciting: some plants transmit signals within themselves, ____more?________.
I am getting a lot more interested, and do appreciate nature more thanks to the insights into how it operates.

I was and remain sceptical of the promoters of the Intelligent Trees narrative (most notably, Peter Wohlleben), because of:

  1. the eco-spiritualist language some authors use to emotionalize the findings
  2. the intense anthropomorphization of trees, and the unfounded suggestion of tree consciousness/self-awareness
  3. varying degrees of ignorance about why it is important to not mislead others about research.

The current approach, in my view, will do more harm than good to the cause of changing humanity’s relationship o nature.

  1. Ibrahim, Mohamed M., et al: Glutamate Receptor-Like Ion Channels in Arabidopsis thaliana. In: Stress Signaling in Plants: Genomics and Proteomics Perspective, Volume 2. Springer, Cham, 2017. S. 69–81. PDF not freely available via Google Scholar.
  2. De Bortoli, Sara, et al.: Evolutionary insight into the ionotropic glutamate receptor superfamily of photosynthetic organisms. In: Biophysical Chemistry, 2016, 218. Jg., S. 14–26. PDF here.
  3. Wudick, Michael M., et al.: Comparing plant and animal glutamate receptors: common traits but different fates?. In: Journal of Experimental Botany, 2018, 69. Jg., Nr. 17, S. 4151–4163. PDF here.
  4. Related: A famous hypothesis of how the first plant came to be is that a tiny alga cell, by accident and with help from other parasitic bacteria, ate a cyanobacterium (capable of photosynthesis) and spawned a new kind of hybrid species, from which all modern plant cells derive.