Whispers In the Forest
by Barb Moore, Island County Master Gardener
[Originally published in the Make a Difference column in the July 14, 2022 edition of the Whidbey Weekly]
Forests are an important part of our environment. Large forests create their own microclimates, influence regional weather patterns, soak up runoff, prevent flooding, block wind, and stabilize and enrich soils. Forests provide habitats for nearly half of the Earth’s known species and now we are learning that trees in a forest can communicate with each other.
Trees in forests use essential processes called transpiration and respiration to pull water from the soil and exchange carbon dioxide from the atmosphere with water vapor and oxygen. The photosynthesis involved creates sugars that feed root systems while reducing greenhouse gases. However, trees require lots of nutrients and water; a small deciduous maple manufactures 35 pounds of leaf canopy each year, and every ounce must be pulled from the air or mined from the soil quickly in the springtime. To accumulate needed nutrients for leaf production, the tree must absorb and transpire approximately 8,000 gallons of water from the soil. That’s enough to fill a tanker truck!
Forests create natural microclimates suitable for tree growth and sustenance. On its own, a tree cannot establish a consistent local climate because it is exposed to wind and weather patterns. Isolated trees generally have far shorter lives than those living in forests, since forests create an ecosystem that moderates temperature extremes, stores water and generates humidity. Further, trees in forests can communicate with each other through tree roots and soil mycorrhizal fungi, enabling transmission of chemical cues, information, and resources. Scientists are just beginning to understand this extraordinary phenomenon.
One remarkable finding is the communal sharing of resources in a forest. Trees may be in nutrient-rich or barren locations (for example, fast draining slopes or rocky outcroppings) within a forest, but the fungal network makes it possible to share nutrients and water, thus helping the whole system maintain a healthy balance. Fungal networks also enable older and taller trees to provide nutrients and water to their own seedlings while they are immature and cannot reach the sunshine above the forest canopy. Evidence of connection across species has also been found.
Beyond their intercommunication through the fungi and root systems, trees also communicate by creating and releasing an airborne chemical called a volatile organic compound (VOC). These compounds can be likened to a scent, though most cannot be detected by the human nose. Trees release VOCs to attract pollinators, as an antiseptic to keep leaves and bark healthy and fight infections and to attract predators of insects that are wounding them.
Trees also employ organic compounds to transfer information among their own organs and to other trees. This intra- and inter-plant signaling can cause physiological changes within trees that make them less palatable to insects. Evidence of this phenomenon was found after a severe attack of tent caterpillars defoliated Sitka willow trees in King County. Researchers fed leaves from surviving willows in the area to tent caterpillars and observed that the caterpillars grew much more slowly and were sicklier than others. There appeared to be a chemical in the leaves that was making them sick. Even more impressively, some of the undamaged trees from which leaves were harvested were located far away from the original tent caterpillar infestation–too far to have been signaled through root-to-root interactions.
To round off their already marvelous communication abilities, trees appear to also communicate by means of slow-pulsing electrical signals. Research going back to the 1970s identified a voltage-based signaling system that strikingly resembles an animal’s nervous system. Simply stated, a leaf in a tree that is being eaten can communicate to other parts of the plant to increase the production of defensive chemicals. Similar to the neurological signaling in the animal kingdom, this tree signaling is extremely slow and travels at the speed of a third of an inch per second. These signals allow the tree–and possibly neighboring trees–to make physiological changes within their chemistry to combat wounding. One study found that a defensive response to an ax chop in one tree was detected in a neighboring tree that was 45 feet away within 20 seconds.
Trees have remarkable environmental and self-preservation abilities through these communication strategies. Our expanding knowledge of this new science may drive increased preservation of natural forested lands, in turn helping biodiversity within species and ecosystems and ultimately increase the health of our planet.