In the iconic musical number, “Waving Through a Window,” Evan Hansen asks the age-old question: “If you’re falling in a forest and there’s nobody around, do you ever really crash or even make a sound?” The answer is yes. Trees are all around us… and they are always listening. But they’re not nosy like your next-door neighbour, so you don’t need to worry about them prying into your nonexistent love life and crippling job prospects.
It may sound like trees ‘whisper’ to each other through the wind, but they actually speak to each other with signals transmitting beneath the ground, with messages depending on what stimuli they’re facing. In forests, trees are linked by mycorrhizal or fungal networks linked by the tips of their roots, which establishes a symbiotic relationship between the trees and the fungi that helps form the network. Peter Wohlleben, a German forester and author of the book The Hidden Life of Trees, aptly calls this vast fungal network the Wood-Wide Web. In return for using fungal filaments to send chemical signals to other trees, the host tree allows the fungi to absorb 30% of the sugar that the tree creates using photosynthesis.
Different species of trees that have the same mycorrhizal networks, like birches and fir trees, can form symbiotic relationships. When they grow together they can exchange nutrients and distress signals to increase their mutual chances of survival. Oaks, on the other hand, contain toxic tannins in their leaves which deter or even kill hungry insects. When an oak is being attacked it sends electrical impulses via nerve cells at the root tips, which travel slowly compared to those of a human or animal — but these are plants that can live for thousands of years, so it works out.
The umbrella thorn acacia, which grows predominantly in the African savannah, has an unusual method of protecting its neighbours by sending messages through the air. This acacia is often attacked by hungry giraffes who try to eat its leaves. When this happens, the acacia sends an airborne distress signal of ethylene gas, which reaches other acacias and tells them to inject tannin into their leaves, which are dangerous to giraffes. However, the giraffes have unfortunately found a way of thwarting this clever trick: when they eat facing the wind, or in no wind, the ethylene gas can’t reach the other trees. They also know to walk long distances between trees before they start eating again — about 90m before the ethylene gas is unable to travel further.
Some trees can also detect insects that feed on them, distinguishing the saliva of a specific insect and releasing pheromones to attract predators of that insect. When a caterpillar eats the leaves of an elm or pine, the tree can attract parasitic wasps using these pheromones and encourage the wasp to lay eggs inside the caterpillars, which eat the caterpillars from the inside out. Gruesome, but effective.
In a forest environment, an older tree can nurture saplings that haven’t grown large enough to reach sunlight. These older trees can pump sugar, water, and essential nutrients so that saplings don’t starve from a lack of sunlight. Accordingly, when old trees are cut down, there are negative consequences for the young trees, whose survival rate decreases because they don’t have a reliable source of food. Similarly, Mother trees are connected with their young and develop a greater mycorrhizal network as they age so that their influence continues to grow and become vital for the stability of the forest. The saplings are even subject to favouritism, as the mother tree prioritises sending its nutrients to its young and navigating its root system so that the saplings have space to grow.
When these trees die, it is a process that can take decades. Some 40% of the dying tree’s carbon is sent off to other trees, while the rest filters out through decomposition. However, even after a tree has died, its neighbours don’t necessarily give up on it. Wohlleben recounts that in the Eifel forests of Western Germany he once discovered a birch stump that had died several centuries prior, which still showed the green tinge of chlorophyll. This was caused by other trees around it continuing to share nutrients with the tree, long after its death.
So if Evan Hansen really did fall in a forest, the trees would definitely notice. They would know the difference between a human accidentally breaking a branch and an animal eating the leaves, and would send nutrients to the injured section to help repair and regrow. Unfortunately for Evan, they can’t quite heal his broken arm.
