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An Excerpt from The Heartbeat of Trees

Available June 2021

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When you hug a tree, nothing electric happens, because, as you now know, your voltages are the same. But might the tree be aware of your touch in some other way? There is one strong contender here—with young trees, at least—and this is a process known as thigmomorphogenesis, which is when plants grow more slowly after being touched. All you have to do, for example, is stroke your tomato plants for a few minutes each day and they slow their upward growth and put their energy into growing thicker stems instead.

This, however, is not the plant saying it loves you, too, but rather the plant reacting to what it likely experiences as a breeze blowing by, because the wind elicits a similar response. The shorter the plant, the less leverage the wind gets and the less pressure there is on the plant’s roots, so a tomato plant with a shorter, thicker stem is more stable. The same is true, of course, for movement caused when animals brush past plants—plants that are less stable are more likely to fall over. Therefore, it may well be that the way tomatoes or small trees respond to this kind of disturbance (not only from the wind) is part of their genetic repertoire.

If you’ve noticed that plants are healthier after you’ve stroked them, you’re right. Scientists have discovered that plants stimulated by touch produce more jasmonic acid. This acid not only regulates height and triggers the growth of thicker stems so the plants are more stable, it also makes the plants more resistant to pests.

The responses I have described are simply a defensive strategy plants employ against external events they view as a threat. Moreover, if the tree is to experience your hug, it must be sensitive enough to touch that it can feel your arms around its trunk. A tree does indeed possess a certain sensitivity to touch, but in completely different circumstances. For example, if a neighboring tree or a metal post presses against its trunk, it will begin to grow around the obstacle. For this to happen, however, the pressure has to be strong and above all persistent over time—two conditions that are not met in a hug. Large trees in particular have thick bark as befits their stature, and nearly all the cells in the outer layers are dead, which means trees feel as much, or as little, with their bark as we feel with our hair.

We do, however, find a great deal of sensitivity in a completely different part of the tree: its roots. At this level, the tree works its way through the ground with its root tips, which contain brain-like structures. The root tips feel, taste, test, and decide where and how far the roots will travel. If there is a stone in the way, the sensitive tips notice and choose a different route. The sensitivity to touch that tree lovers are seeking is therefore to be found not in the trunk but underground. If it is possible to make contact, the roots would be the first place to try. They have the additional advantage that they are easy to reach and, in contrast to the upper parts of the tree, are active even in winter. However, they like neither pressure nor fresh air—and so there’s no point exposing these tender structures because even ten minutes in the sun spells death for their tissue.

The most recent scientific discoveries, however, offer something completely different: the heartbeat of trees. Heartbeat? Trees, of course, do not have hearts like we do, but they need something that performs a similar function or the most important processes in their bodies would not work.

What blood is to people, water is to trees. I have written a lot about how water is transported up into the crown of the tree; exactly how that happens has not yet been adequately explained. The most popular theory, that moisture is drawn to the uppermost twigs by transpiration, doesn’t work. According to this theory, water evaporates out of the leaves and that creates a vacuum in the trunk. This vacuum then draws water up out of the roots and the surrounding soil. Unfortunately for this theory, water pressure in the trunk of deciduous trees is highest in early spring. At this time of year, there isn’t a single green leaf on the tree and so nothing can transpire.

The other attempted explanations (osmosis, capillary action) don’t work either, so we are currently without answers. Or, we were. Dr. Andr.s Zlinszky at the Balaton Limnological Institute in Tihany, Hungary, is shedding some light on the matter. Some years ago, he and colleagues from Finland and Austria noticed that birch trees appear to rest at night. The scientists used lasers to measure trees on calm nights. They noticed the branches hung up to 4 inches (10 centimeters) lower, returning to their normal position when the sun rose. The researchers started talking about sleep behavior in trees.

Clearly, Zlinszky could not get this discovery out of his head, and he decided he needed to investigate further. He and a colleague, Professor Anders Barfod, measured another twenty-two trees of different species. Once again, they documented the rise and fall of the branches, but this time some of the cycles were different. The branches changed position not only morning and night, but also every three to four hours. What could be the reason for this rhythm?

The scientists turned their attention to water transport. Was it conceivable that the trees were making pumping movements at these regular intervals? After all, other researchers had already determined that the diameter of a tree’s trunk sometimes shrinks by about 0.002 inches (0.05 millimeters) before expanding again. Were the scientists on the trail of a heartbeat that used contractions to pump water gradually upward? A heartbeat so slow that no one had noticed it before? Zlinszky and Barfod suggested this as a plausible explanation for their observations, nudging trees one step further toward the animal kingdom.

A heartbeat every three to four hours is, unfortunately, too slow for even the most sensitive person to feel when they hug a tree, and so, once again, we have failed in our search for a noticeable signal from the tree in response to our touch.

There is one last possible way to connect with trees that I would like to look at in more detail: our voices. Our most important form of communication is verbal, and quite a few people try to talk to trees or to their houseplants. Why did I say “try”? This is something people actually do, and they expect the plants to react in some way. There are also wine growers who play a wide variety of music to their vines and believe that certain musical genres lead to better harvests. Is there a grain of truth to this? Can plants even hear?

To that last question, I can answer without hesitation in the affirmative. This was tested years ago with Arabidopsis, a genus of rockcress beloved of scientists. Beloved because it grows well, it reproduces rapidly, and it’s easy to keep track of its genes. Scientists discovered that the roots of Arabidopsis oriented themselves toward clicks in the frequency of 200 hertz and then grew in that direction. They can also produce sounds that function as a kind of Morse code.

Professor Monica Gagliano of the University of Western Australia discovered that pea roots can hear water flowing underground. To test this, she buried three pipes. In the first, she played a recorded swooshing sound, in the second real water flowed, and in the third there was the recorded sound of flowing water but no actual water. The plants were not fooled. Their roots grew only toward the real water. And when they were not thirsty, they showed no signs of movement. But is that really equivalent to hearing? Gagliano and her team thought that if it were, the roots would find white noise irritating, and this is exactly what they observed.

So plants, including trees, can hear. Just like us, they use this skill for a specific purpose. We don’t hear most low-frequency sounds because we don’t need to, and plants also listen to those things that are important to them, such as water underground. But what about all those reports of vineyards filled with classical music to encourage bountiful harvests? What about the testimonials from people who talk to trees? From a sober, scientific perspective, the roots’auditory abilities are excluded from these responses, because roots grow in the relatively well-soundproofed realm beneath the surface. Therefore, we need to look above ground. Let’s look more closely at stems, branches, and leaves. Is there any evidence of acoustic responses in these?

Over a period of many days, a television team from West German Broadcasting in Aachen offered sunflowers at the Jülich Research Center a range of different sounds, including classical music. They discovered no growth differences at all between the plants. Does that mean there is nothing going on acoustically above ground? Let’s not give up so quickly. Perhaps music is the wrong starting point. We’d do better if we looked for sounds that mean something to plants.

How about, for example, the nibbling of caterpillars, an ominous sound to plants of all species? At the University of Missouri, this was exactly what they investigated. Researchers put caterpillars on samples of Arabidopsis. The vibrations caused by the caterpillars munching were enough to shake the plants’ stems, and the researchers used laser beams to record the vibrations. When researchers then played these vibrations to plants that were not being eaten, they produced particularly large quantities of defensive chemicals when they were later attacked. Wind and other sounds with the same frequency did not elicit a reaction from the plants. Arabidopsis, then, can hear, and this makes perfect sense. Thanks to acoustic warnings, it is able to recognize danger some distance away, so it can make appropriate preparations to defend itself. What is particularly important here is that the plants ignore noises that pose no threat to them. These noises probably include human voices as well as different styles of music. What a shame. The reports of crops that can clearly distinguish classical music from rock music sound so delightful.

We still need to monitor one thing, however: music with notes that approximate the sound of munching caterpillars. That could happen, and, if it did, an acoustic misunderstanding might prevent the plants from enjoying Mozart.

I can well understand people’s desire to communicate with trees. To sit under these giants, run your hands over their bark, and feel secure—all this would be even more special if there were an active, positive response to your presence or, even better, to your touch. I am not going to deny that something like that might be possible, but conservative science at least has no proof that it could happen. And even if this were the last word on the subject, does the tree have to respond? Could it not be that people and trees live in completely different worlds? After all, our species has existed for only 0.1 percent of the time that trees have been around.

Although trees clearly feel nothing of all this, we, for our part, definitely experience a physical reaction, one that I will look at in more detail later. For the time being, it should be enough that we feel good around trees—and I hope we can then be content to allow them to live their own wild lives.

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