Did you know that trees can talk to each other, sharing environmental cues, nutrients, and warnings of danger?
In 2018, Smithsonian journalist Richard Grant met with Peter Wohlleben, the author of the popular and controversial 2016 book, The Hidden Life of Trees: What They Feel, How They Communicate. Grant came away with an understanding that trees and other plants are actually far more alert, social, sophisticated, and intelligent than we give them credit for.
“Wohlleben takes me to two massive beech trees growing next to each other,” writes Grant. “He points up at their skeletal winter crowns, which appear careful not to encroach into each other’s space. ‘These two are old friends,’ he says. ‘They are very considerate in sharing the sunlight, and their root systems are closely connected. In cases like this, when one dies, the other usually dies soon afterwards, because they are dependent on each other.’”
The idea that trees are able to communicate is not a new one. As far back as the early 1980s, researchers reported damaged poplar seedlings releasing airborne cues. They posited that these biochemical signals or cues could be stimulating biochemical changes in neighbouring plants. This is turn affects insect feeding and growth.
Similarly, in 2006 researchers reported that plants may “eavesdrop” on volatile organic compounds released by neighbours under attack from herbivores, enabling the plants to activate their defences before being attacked themselves.
In their 2009 scientific paper, Explaining evolution of plant communication by airborne signals, researchers Martin Heil and Richard Karban build on this. They noted that despite initial doubts regarding whether trees could “talk” to one another, the recognition of airborne biochemical signals by plants from neighbouring plants was at that stage well-described.
Grant notes that there is a considerable body of scientific work demonstrating that tree communication takes place. Furthermore trees of the same species are communal, even forming alliances with other species. “Forest trees have evolved to live in cooperative, interdependent relationships, maintained by communication and a collective intelligence similar to an insect colony,” he writes.
Researchers are now focusing beneath the ground to find out more about communication between trees. Suzanne Simard, a professor of forest ecology at the University of British Columbia in Canada, highlights a symbiotic relationship between trees and fungi that makes many of these incredibly complex interactions possible.
In her 2018 paper, Mycorrhizal Networks Facilitate Tree Communication, Learning, and Memory, Simard explains that fungal networks in the roots of forest trees facilitate communication between them to influence a sophisticated structure of behaviour between neighbours. “These tree behaviours have cognitive qualities, including capabilities in perception, learning, and memory, and they influence plant traits indicative of fitness,” she writes, adding that the structure and properties of subterranean fungal networks actually resemble neural networks.
She believes that biochemical signals transmitted between trees resemble neurotransmitters and that fungal networks exhibit properties and strategies commonly linked to intelligence, including memory-based learning.
Simard goes even further, giving examples of “collective memory-based interactions among trees, fungi, salmon, bears, and people that enhance the health of the whole forest ecosystem”. Using the term “tree cognition”, Simard advocates for a more holistic approach to studying ecosystems and greater empathy and caring for beings that exhibit a different form of intelligence to our own.
Wohlleben agrees that trees in healthy forests are connected by these underground fungal networks, telling Grant: “Trees share water and nutrients through the networks, and also use them to communicate. They send distress signals about drought and disease, for example, or insect attacks, and other trees alter their behavior when they receive these messages.”
These underground networks are just one part of the puzzle, though, as already shown by the fact that neighbouring plants can also pick up airborne biochemical signals. Wohlleben relates a perfect example to Grant that most South Africans can relate to: the relationship between acacia trees and giraffes. When giraffes feed on an acacia, the tree emits a distress signal: ethylene gas. When other acacias in the area detect the gas, says Wohlleben, they pump tannins into their leaves that, when there are enough of them, can make large herbivores sick… or even kill them.
Because giraffes have evolved with the acacias, however, they browse into the wind to avoid the warning gas reaching 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,” writes Grant.
He points to another example from a study conducted by researchers at Leipzig University and the German Centre for Integrative Biodiversity Research, which shows that trees can recognise the taste of deer saliva. The study analysed the differences between clipped branches with and without deer saliva applied, and found that “the application of deer saliva elicited different responses than clipping alone.”
This, Wohlleben tells Grant, is really significant: “‘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.’”
However, we should be careful not to ascribe too much to these interactions or anthropomorphise trees. It is true that anthropomorphic perceptions of non-human entities can generate pro-environmental outcomes in some circumstances. However, according to psychologist Patrica Ganea from Toronto University, it can also potentially “lead to an inaccurate understanding of biological processes in the natural world”.
So, are trees consciously “talking” to each other? Probably not, but that doesn’t mean that tree communication isn’t an important part of our understanding of these complex organisms that play a vital role in terrestrial ecosystems.
As Grant recognises, even though Wohlleben speaks of how trees relate to each other in human terms, the so-called “tree whisperer” understands this is not really the case: “He makes these blunders sound like conscious, sentient decisions,” Grant writes about actions by trees that are doomed to fail, “when they’re really variations in the way that natural selection has arranged the tree’s unthinking hormonal command system.”
“Wohlleben knows this, of course, but his main purpose is to get people interested in the lives of trees, in the hope that they will defend forests from destructive logging and other threats.”
This is very much his point: if we can empathise with trees, we will have more of a reason to protect them… and that can only be a good thing. Maybe it’s time to start listening to the trees before it’s too late.