The Language of Trees II

A few months ago I wrote about the communication of plants and trees using hormones, which was often when a member species was under threat. However, this is not the complete picture as more research starts to come through of the influence of sound on plants as well as sound being produced by the plants themselves, and the production and measurement of electrical signals.

Lyall Watson (1939 – 2008) earned degrees in botany, zoology, geology, chemistry, marine biology, ecology, anthropology and a doctorate in ethology

This is not a new concept – South African born zoologist Lyall Watson  in 1973 wrote about the electromagnetic signals of plants connected to a lie-detector which could measure their ‘emotions’ which was dismissed by the scientific community. There is also the Federation of Damanhur, a communal ecovillage in Northern Italy, which have been conducting experiments on plants since 1975. The researchers there have used a variation of a simple electric circuit, known as a Wheatstone bridge to measure unknown electrical resistance. They found that the greatest signal variation occurred in the plants upon the arrival of the person who tended the specific plant most regularly.

Is there more going on when we water plants? (Getty Images)

Regarding sound, a recent paper1 shows  how young roots grow towards sound at 220 Hz which suggests they exhibit a frequency selective sensitivity. Moreover, they recorded the production of sound by plants, measured using a laser Doppler vibrometer, had definite spike-like structures which could not be explained by tension release in their water transport system. The researchers posit that sound could be particular useful for short range communication as well as being much cheaper biologically for plants to produce than hormonal chemicals. They also noted that the reception of sound underpins the behavioural organisation of all living organisms and the relationship with their environment – thus it should not be necessarily deemed as irregular in the life of plants.

Roots of young plants grew towards the specific sound source of 220 Hz (1)

What this suggests is that there is much still to learn regarding plants and trees, and these investigations may require interdisciplinary approaches. It also raises questions about the effect of the frequency of human vocal chords on plants, and if specific frequencies of sound can be used to help plants grow, as well as to encourage growth in particular areas of landscape.

  1.  Towards understanding plant bioacoustics. Gagliano et al. Trends in Plant Science, Vol. 17, Issue 6. pg 323-325, 22 March 2012

The Language of Trees

The Forest of Dean. Much more is going on between trees than we can see. (CORBIS)

As much as tree evolution and adaptability is affected by selective pressure from the environment, we must not forget that trees have evolved communication processes over hundreds of millions of years which are more forgiving than playing life’s deadly experiments alone. Each tree produces hormones, (with evidence of many being undiscovered) and these molecules act as signals and allow communication with their selves and fellows in the area. Common science talks about 5 major hormones: auxin, cytokinin, gibberellins, abscisic acid and ethylene. These hormones switch on or off chemical pathways and affect responses not only by the type of hormone, but also by their concentration. In this way the same hormone may induce two different responses. And some of these responses are very interesting.

The tent caterpillar

In 1979, David Rhoades, a zoologist at the University of Washington, was investigating the effect of tent caterpillar attacks on willow trees. He monitored two groups of trees in a field in Seattle, one with no caterpillars as a control, and the other infested. Two weeks later he plucked leaves from the infested tree and fed them to caterpillars in a laboratory and found that they grew slower than usual. What was interesting was that plucked leaves from the nearby control group of willows also inhibited caterpillar growth. The willows flood their leaves with unsavoury chemicals (normally phenolics) which discourage insect growth, however, this defence tactic had also been used by the control group, suggesting that communication must have happened via chemicals in the air from the infested tree. Since then similar results have been found with poplars and by using isolation chambers to have a third control which prevents diffusing molecules from being in contact with other trees, and with no resulting phenolic increase.

Air pollution of Mexico City. Not an easy environment for a tree.

Unfortunately trees in urban environments are exposed to many more chemicals and hormonal communication can be somewhat confusing. Thus root growth may be mismatched to foliage growth, or early blooming may occur from unnatural local ethylene concentration. This is the price trees pay living with us until we can sufficiently increase our air quality. And this is a price they pay while they absorb carbon dioxide and provide us with oxygen, shade, and the natural beauty of their presence.