A new book, The Hidden Life of Trees, claims that trees talk to one another. But is this really the case? The simple answer is that plants certainly exchange information with one another and other organisms such as insects. Think of the scents of newly mowed grass or crushed sage. Some of the chemicals that make up these aromas will tell other plants to prepare for an attack or summon predatory insects to defend them. These evocative smells could be seen as cries of warning or screams for help.
When plants are damaged by infection or by being eaten, they release a range of volatile molecules into the air around them. After exposure to some of these chemicals, nearby plants of the same species and even other species become less vulnerable to attack, for example by producing toxins or substances that make themselves harder to digest. These changes don’t usually happen straight away but the genes needed turn on much more quickly when they are needed.
There is also evidence that the chemicals released by plants in a particular location are subtly different from those released elsewhere by the same species. Consequently, it seems that if plants talk, they even have languages or at least regional accents.
But is this really communication, as humans understand it? It really isn’t clear whether a plant releasing chemicals intends to pass on information to another plant by doing so. I respond to the chemicals released by frying onions but that doesn’t mean that the onions are talking to me. So are these really messages or just the opportunist use of chemical information in the environment?
It seems more likely that these signals started out not as a way to send information to other trees but to get messages quickly and efficiently to other parts of the same plant. Pests or infections will often jump from one branch of a tree to the ones closest to it. But a warning telling those branches to prepare for an imminent attack might have to travel most of the way through the tree and then back up it if the message had to move through the body of the plant. This could be a journey of tens of metres in a tall tree.
A signal that can travel through the air, meanwhile, can go directly to the branches closest to the attack. A consequence of these volatile signals, however, is that they can be “overheard” by any plants the chemicals reach. So when other trees respond by also beefing up their defences, is it communication or eavesdropping?
Perhaps it is a bit of both. Maybe an internal messaging system became co-opted to help plants close enough to “listen in” as they would often be related to the tree sending the message in a classic example of evolutionary “kin selection”. However, releasing chemicals into the environment is indiscriminate and other plants and organisms can take advantage. Sometimes these chemical “messages” can attract pests or parasites. The smell of crushed sage doesn’t protect it from humans, for example … rather the opposite.
Not all transfer of information between plants is through the air. The vast majority of plants live in symbiotic relationships with soil fungi. We tend to think of forest fungi as mushrooms and toadstools above the ground but these only pop up after sexual reproduction. The real fungus is a mat of elongated cells spreading through the forest floor.
The trees provide the fungi with sugar and the fungi help the tree to gather water and soil nutrients. And many plants can be joined underground by cells of the same individual fungus. Sometimes when one plant suffers damage, other plants connected to it through their soil fungi protect themselves against future attacks while other plants equally near that aren’t “plugged in” don’t. This fungal network is another carrier for information, a true Wood Wide Web.
But who is in control? The messages are relayed by the fungus and perhaps it is the one really using the information, gathering it from one of its host plants and passing it on to the others to protect its “revenue”. The fungus helps the plants to communicate but may do it for its own purposes, and that might include preferentially helping its best producers, whether they are related to the tree sending the message or not. Information intended for family and friends may end up being passed on to unrelated third parties to profit the carrier of the message. In this way, fungi is a bit like a social media company, listening into and benefiting from its users’ posts.
So we return to the question of whether any of these examples are communication in the sense that we would mean it. Anything that makes people think more about plants is good, but perhaps making trees seem more like us can lead us to overlook their essential nature. As a slightly hippy student, what attracted me to plant science was the way that trees and other plants fluidly adjust to their environment. Perhaps using the chemicals that reach them to shape their adaptation is just another facet of this. Worrying about whether trees communicate actually says more about us than them.