Poets have talked for ages about the “whisperings” of the trees in the forests. Fantasy authors like J.R.R. Tolkien and J.K. Rowling have written about trees having moods and even a kind of will, but scientists have been quick to dismiss these ideas.
Recently, however, trees and other plants have been shown to communicate about changes in their environment and their threats. They act in response to stimuli, protecting themselves from threats and responding to opportunities. They also respond to some forms of human communication, including music. Perhaps plants are a bit more sentient than we suspected!
Plants seem to communicate through several means, including:
- Electrical signals (primarily within the plant, between different parts)
- Chemical signals between plants (primarily via volatile organic compounds or VOCs)
- Signals passed indirectly through fungal networks, between root systems
Since the late 1800s, scientists have known that at least some plants generate electrical signals that cause specific actions. For example, sunflowers follow the sun throughout the day. Mimosas (Mimosa pudica) and other typically sensitive plants will quickly fold up their leaves if touched and slowly spread them out again a few minutes later.
The carnivorous Venus flytrap (Dionaea muscipula) has trigger hairs that can count. One touch of a hair will be ignored, for example, so the plant isn’t fooled by dust or debris falling on it. When two contacts are received within 20 seconds, however, the trap snaps shut on its prey. Another five touches are required to begin digestion, so the plant won’t bother with prey that doesn’t move after it gets trapped.
More recently, scientists have learned that many plants have complex responses to changes in their environment. When a plant is wounded, it responds with an increase in healing compounds throughout the plant, which occurs too quickly to be chemically caused. So it appears that plants send electrical signals through pathways we don’t yet understand, just as animals and humans send signals through the nervous system.
Plants don’t just send signals through their bodies; they also send signals to their neighbors.
Researchers in Africa discovered that when giraffes eat leaves off acacia trees, the trees begin to secrete noxious compounds that are toxic to giraffes, helping limit the damage they might otherwise do. They also give off ethylene gas, causing nearby acacia trees to respond in similar ways. As a result of these chemical signals, the response protects an entire grove or community of acacia trees, not just the one being eaten. Not only that, but giraffes seem to know that the trees talk. When they finish browsing one tree, they will walk a hundred meters upwind to find an acacia tree that hasn’t yet heard the news!
The acacia tree is not an isolated example of chemical communication. Trees and other plants give off many kinds of volatile organic compounds (VOCs) that fill the air around them. That’s part of what gives different types of forests or other natural habitats their unique and welcome smell. These VOCs include pheromones, terpenoids, fatty acids, phenylalanines, benzenes, and amino acids. Many cause responses in neighboring plants, whether the same or different species. For example, the dodder weed, a parasitic plant in the morning glory family, will grow toward a tomato plant and avoid a wheat plant based on the VOC signatures produced.
VOCs aren’t the only way individual plants talk to their neighbors. Scientists conducted experiments with garden pea plants; some were intertwined with communicating root systems, and others had root systems separated. Chemical stress that simulated drought was introduced to one of the plants in both groups. In the plants where the roots were in communication, all responded to the stimulus in ways that prepared them to survive with less water, while the plants with roots that were not touching were unable to do so.
Plants can share carbon, nitrogen, and many other nutrients through their roots. Many scientists now suspect that old-growth trees in a forest may provide nutrients, through their roots, to young trees in the same forest, as they cannot yet reach the canopy to get their needed sunlight. The older trees in the forest protect and nourish their young, ensuring the continued health and vitality of the forest beyond the life of even its most senior citizens.
Mycorrhizal (fungal) networks
Mushrooms and fungi have been referred to as “the third kingdom” (after plants and animals) and help reduce, reuse, and recycle all formerly-living things. Scientists are just beginning to understand the role of fungi in our interconnected world, including how important they are in plant-to-plant communication.
A teaspoon of forest soil may contain miles of tiny fibers called mycelium. These tiny threads are a chemical communication network that connects nearby plants, from the smallest grasses to the tallest trees. We think of mushrooms as fungi, but the mushrooms are just the fruiting body. As apples to an apple tree — or elderberries to an elder tree — the mushroom is just the part that gets eaten and passed around. The biomass of most fungi lies invisible below ground until it pops up in a “fairy ring” or pops out of a decaying log as a turkey tail or other visible specimen. Because these mycelium threads are everywhere, they facilitate chemical communication underground between plants of vastly differing species (see Shae’s 2020 Almanac Article, Mycelium in the Garden).
In some cases, fungi may be charging a 30% tax on sugars produced by trees and stored in their roots in order to pass those nutrients on to neighboring trees and plants. The network has been shown to enable sharing of nutrients between trees as wildly different as evergreen Douglas firs and deciduous paper birches. Scientists argue over whether these services should be regarded as a competition, cooperation, or economic bargain, but the fungi just do it.
A good layman’s introduction to some of this interplant communication is The Hidden Life of Trees by Peter Wohlleben. Wohlleben is a German forestry scientist whose life’s work has been to unlock the complex secrets of the forests around us. Trees have occupied this planet much longer than we have, and they have never yet threatened it with extinction. Perhaps, when they speak, we should listen and learn!