The secret life of trees | Book Excerpt: The Heartbeat of Trees by Peter Wohlleben

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July 11, 2021 2:30 AM

Discover how trees can respond to various stimuli such as voices, music and touch, as this excerpt reveals surprising facts about plants, including the fact that they have a heartbeat

The root tips of a tree feel, taste, test, and decide where and how far the roots will travelThe root tips of a tree feel, taste, test, and decide where and how far the roots will travel

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.

If you were hoping to hug a tree and get a hug back, this information must be disappointing. 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. András 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.

Pages 71-74

Excerpted with permission of Penguin Random House

The Heartbeat of Trees
Peter Wohlleben
Penguin Random House
Pp 240, Rs 599

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