I’ve talked about how the fungus-root network that ties all the trees in an area together functions as a communication and nutrient distribution system, but trees also communicate in other ways. With recorders that can pick up sounds too high or too low for human hearing, we have discovered that an elephant can communicate with another elephant as far away as six miles, and whales can hear other whales as far away as 4,000 miles. Now we learn that trees make sounds that other trees can “hear”.

The roots of the trees click and clack at a frequency well below our hearing. We’re not sure what the trees are “saying” to each other, but the sounds may be associated with stress. We also know that the roots of trees respond to the SOUND of water flowing. This may be why they force their way into water and sewer pipes. They are growing toward the SOUND of flowing water!

Trees also have weak electrical signals that travel through the tree to the roots and through the roots to other trees. These are usually in response to stress on the tree. What it does for the other trees we don’t know.

Trees also communicate with each other through the air. Most flowers give off odors that attract pollinators and trees are no different. Soon the linden trees in town will be filling the air with their sweet smells, but trees and other plants also give off other odors called pheromones. A pheromone is a hormone- like chemical that causes a reaction in some other organism. For example, when leaf caterpillars attack elm trees, the elms release a pheromone that attracts wasps, which kill and eat the caterpillars.

Probably the most studied tree interactions are the acacia trees in Africa. Acacia are the preferred food of giraffes. When a giraffe starts to eat the leaves, the acacia tree responds by releasing ethylene gas that other acacia trees within 50 meters are able to detect. The tree being attacked and the trees responding to the ethylene, produce tannins and pump them into the leaves. The tannins make the leaves bad tasting, thus protecting them from the giraffe. The ethylene is carried in the wind so the giraffes, in response, have learned to get around this defense mechanism by grazing into the wind.

In Japan research has shown that many forest trees and plants release pheromones called phytoncides.

These chemicals apparently inhibit the growth of bacteria and repel insects that feed on the trees. The phytoncides also affect humans.

Studies have shown that breathing the air of the forest; air filled with various pheromones like the phytoncides has positive health benefits. The pheromones reduce stress hormones in our blood, lower our heart rate and blood pressure, and increase our white blood counts. In Japan they call this “forest bathing”.

It is very popular and just walking through the forest, breathing the air, is an accepted medical treatment that is even covered by insurance! So take a walk around the Ehmen Park Arboretum, enjoy the many different trees, and take deep breaths, apparently it’s good for you.