The Wood Wide Web: The Interconnectedness of Trees and How They Communicate

​It’s easy to forget that trees are actually living things. They often fade to the background: big, imposing, inanimate objects in your everyday landscape. We only notice them when they get in the way, or start to die and pose a threat, or maybe just lose a branch and need to be pruned. 

​But trees are very much alive – much more alive than you would think. As it turns out, forest trees talk to and take care of one another. 

​How Do Trees Communicate? 

​There are actually a few different ways trees talk to each other. Research has shown that trees can emit and detect pheromones, as well as chemical signals and slow-pulsing electrical signals. The electrical signal system in a tree is actually very similar to an animal’s nervous system. 

​Some plants may even be able to create and detect sounds. Specifically, a crackling noise in the roots 220 hertz, which is inaudible to human ears.

​Pheromonal Communication Between Trees

​The concept of pheromones is not new; we’ve known for a long time about how various species use different smells to convey different things, like signaling danger or attracting a mate. 

The idea that trees use pheromones too has taken longer to discover, because – not to state the obvious – trees don’t have  noses. However, trees do still have scent receptors in their leaves, which can detect pheromones. 

​A good example of pheromonal communication between trees is the acacia tree in sub-Saharan Africa. When a giraffe munches on acacia leaves, the tree emits a distress pheromone, in the form of ethylene gas. Other acacia trees in the area can detect the gas through their own leaves, and in response, will send extra tannins into their leaves. These tannins, in large enough quantities, can poison and even kill large herbivores like giraffes. 

​So, one acacia tree is hurt, and it tells the other trees around it so they can defend themselves. 

​(Interestingly, the giraffes have evolved along with the acacia trees, and have learned to eat on a path going into the wind, so the warning signal from the tree they’ve finished with can’t reach the tree they’re going to graze next. When there’s no wind, giraffes will eat from one tree, and then walk farther than the ethylene gas can travel to pick another tree.)

​Another example of pheromonal communication actually involves taste, as well! Elm and pine trees are often attacked by caterpillars that eat their leaves. The leaves can actually detect, or ‘taste’ the caterpillar saliva, and in response, will emit a pheromone that attracts parasitic wasps. These wasps will lay their eggs inside the caterpillars, which eventually kills them. It might be too late for the infected tree, but at least the caterpillars won’t spread as readily to others.

​Trees can also detect deer saliva, and will channel certain chemicals to their leaves that make them taste bad to the deer to discourage it. These pheromonal responses are all different depending on what caused the damage to the tree. If a branch is simply broken, like if you break off a twig, the tree does nothing. 

​Communication Through Mycorrhizal Fungi 

​Trees in a forest are all entangled with one another. This is easy to see in the canopy, where their branches interact, but this interconnectedness also extends underground to their root systems. 

All of a tree’s roots taper to a fine, hair-like end. These thin tendrils tangle with one another, and tangle with the roots of other trees, too. And all these roots are joined together by microscopic fungal filaments, called mycorrhizal fungi. The fungi are what make a mess of jumbled roots into a functioning network. 

​Pretty much all plant roots are covered with mycorrhizal fungi in a symbiotic relationship. Basically, the fungi help extract nutrients from the soil around the tree, and in return, the tree gives the fungi sugars it creates through photosynthesis, which the fungi can’t do for themselves. 

​There are lots of different species of fungi that form mycorrhizal relationships with plants, but all of them can be broken down into two main groups: endomycorrhizal and ectomycorrhizal. 

​Endomycorrhizal fungi wrap around plant roots and penetrate into them to exchange nutrients from within the roots. Ectomycorrhizal fungi, on the other hand, form a kind of sheath around the outside of roots and exchange nutrients from the outside. 

​Regardless of which type of mycorrhizal fungi engages with its roots, what really makes a tree’s mycorrhizal relationship special is the mycorrhizal network. 

​What Is a Mycorrhizal Network?

​While most plants just exchange nutrients with their mycorrhizal fungi by themselves, trees are different. The root fungi of trees whose roots are interconnected can also exchange chemical, hormonal and slow-pulsing electrical signals. The network resembles our neural networks in human brains.

​This is another way trees ‘talk’ to one another. One tree emits a signal through its roots, which is then transmitted by its mycorrhizal fungi to the mycorrhizal fungi on the surrounding tree roots, which in turn transmit the information to their own trees and their surrounding mycorrhizal fungi, and so on and so forth. Signals can travel for miles like this, passing from the original tree that sent out the signal and through fungi to fungi, until an entire forest has ‘heard the message.’ 

​These messages are similar to the kinds of things the trees might say with pheromones. For example, in one study, a Douglas fir was injured by insects. The fir sent a chemical signal out through its mycorrhizal network that was received by a nearby pine tree. The pine tree, in turn, produces a defensive enzyme to protect itself against that particular insect. 

​This is all pretty amazing, but the sharing doesn’t stop there. Trees don’t just talk to each other – they take care of each other, too. 

​What Else Do Trees Share?

​Mycorrhizal fungi is characterized by the exchange of nutrients between the fungi and the tree it lives on, so both parties get what they need. But mycorrhizal fungi can also share nutrients with other mycorrhizal fungi. This means that one tree can send out certain nutrients through its roots to the trees around it.

​A perfect example of trees taking care of their neighbors can be found in a gigantic beech tree stump in Germany. The tree itself was felled hundreds of years ago. However, scraping away the surface of the remaining stump revealed green, living tree on the inside! With no leaves at all to speak of for photosynthesis, the stump must be getting the sugars it needs from other trees who can photosynthesize. 

​But this sharing isn’t just automatic – trees don’t just support all the trees around them indiscriminately. Scientists have yet to determine exactly how, but trees definitely know which of their kin need help. For example, a birch tree shading a Douglas fir was found to be sharing large amounts of carbon with that particular fir than in its exchanges with other trees. In doing so, the birch tree helped mitigate the negative effects of its shading on the fir. 

​Similarly, young saplings are often too shaded to photosynthesize enough sugars to survive. The larger ‘mother’ trees around them will support them with carbon donations until they ‘get on their feet.’ These Mother trees (also called Hub trees) are the old, well-established trees in the forest. They won’t exchange between themselves unless one gets sick or injured, but they freely support smaller trees with their excess sugars, nitrogen, and other nutrients. 

​Trees also know their kin, period. That is, trees know which nearby trees are of the same species, and will only favor certain ones. For example, in a large forest study, researchers found that all the cedar and maple trees appeared to be on one network, communicating and sharing nutrients, while all the hemlock and Douglas firs were on another. 

​This isn’t to say that the maples and cedars didn’t share at all with the hemlocks and firs – just that they shared significantly less. 

​Why Would Trees Share Nutrients? 

​In an environment where it’s ‘survival of the fittest,’ why would trees help each other out? 

​Scientists assume it is an effort to maintain a functional ecosystem. There are many benefits to having more trees around: protection against wind and storm, structural integrity, and as mentioned, forewarning of approaching danger. 

​More trees also means more resources for the other species in the area. If all the deer in a forest ate from just a few trees, those trees wouldn’t survive very long. Whereas if those deer eat a little from many trees, all the trees will likely live a long time. 

​In fact, research indicates that when Mother trees in a forest are cut down, the entire forest can noticeably decline. That’s because these larger, older trees produced much more nutrients and sugars than they needed, and were supporting all the younger trees. Without that support, many others may begin to wither. 

​Mother trees will actually continue to support their forest long after they themselves die. It can take decades for a tree to actually die, and in that time, studies have shown that Mother trees will spread about 40% of their carbon to the forest. 

The interconnectedness of trees is a fascinating and extensive field of study that we’re truly just beginning to scratch the surface of. If you want to learn more about how trees communicate and work together, check out The Hidden Life of Trees: What They Feel, How They Communicate—Discoveries from A Secret World by Peter Wohlleben.

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Author

Emily Casuccio is sister and sister-in-law to Rebekah and Scott Rushing, and has over half a decade of experience in copywriting, copyediting, proofreading, and developmental storyboarding. She's worked with both published and undiscovered authors on both fiction and nonfiction, and takes pride in supporting local businesses. Her passion lies in the written word and helping authors of all capacities realize their dreams and achieve their fullest potential. To learn more about her, read samples of her work, or contact her, visit her online portfolio.

Sources:
Smithsonian Magazine
NPR
Mycorrhizal Applications
One Earth
The Mother Tree Project
BBC
Learning With Experts
New Scientist