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Thirst is harder for trees to endure than hunger, because they can satisfy their hunger whenever they want. Like a baker who always has enough bread, a tree can satisfy a rumbling stomach right away using photosynthesis. But even the best baker cannot bake without water, and the same goes for a tree: Without moisture, food production stops.

A mature beech tree can send more than 130 gallons of water a day coursing through its branches and leaves, and this is what it does as long as it can draw enough water up from below. However, the moisture in the soil would soon run out if the tree were to do that every day in summer. In the warmer seasons, it doesn’t rain nearly enough to replenish water levels in the desiccated soil. Therefore, the tree stockpiles water in winter.

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Raimund Linke

In winter, the tree is not consuming as much water, because most plants take a break from growing at that time of year. Together with below ground accumulation of spring showers, the stockpiled water usually lasts until the onset of summer. But in many years, water then gets scarce. After a couple of weeks of high temperatures and no rain, forests usually begin to suffer. The most severely affected trees are those that grow in soils where moisture is usually particularly abundant. These trees don’t know the meaning of restraint and are lavish in their water use, and it is usually the largest and most vigorous trees that pay the price for this behavior.

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In the forest I manage, the stricken trees are usually spruce, which burst not at every seam but certainly along their trunks. If the ground has dried out and the needles high up in the crown are still demanding water, at some point, the tension in the drying wood simply becomes too much for the tree to bear. It crackles and pops, and a tear about 3 feet long opens in its bark. This tear penetrates deep into the tissue and severely injures the tree. Fungal spores immediately take advantage of the tear to invade the innermost parts of the tree, where they begin their destructive work. In the years to come, the spruce will try to repair the wound, but the tear doesn’t always heal. From some distance away, you can see a black channel streaked with pitch that bears witness to this painful process.

And with that, we have arrived at the heart of tree school. Unfortunately, this is a place where a certain amount of physical punishment is still the order of the day, for Nature is a strict teacher. If a tree does not pay attention and do what it’s told, it will suffer. Splits in its wood, in its bark, in its extremely sensitive cambium (the life-giving layer under the bark): It doesn’t get any worse than this for a tree. It has to react, and it does this not only by attempting to seal the wound. From then on, it will also do a better job of rationing water instead of pumping whatever is available out of the ground as soon as spring hits without giving a second thought to waste. The tree takes the lesson to heart, and from then on it will stick with this new, thrifty behavior, even when the ground has plenty of moisture—after all, you never know!

When trees are really thirsty, they begin to scream.

It’s no surprise that it is spruce growing in areas with abundant moisture that are affected in this way: They are spoiled. Barely half a mile away, on a dry, stony, south-facing slope, things look very different. At first, I had expected damage to the spruce trees here because of severe summer drought. What I observed was just the opposite. The tough trees that grow on this slope are well versed in the practices of denial and can withstand far worse conditions than their colleagues, who are spoiled for water. Even though there is much less water available here year round—because the soil retains less water and the sun burns much hotter—the spruce growing here are thriving. They grow considerably more slowly, clearly make better use of what little water there is, and survive even extreme years fairly well.

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A much more obvious lesson in tree school is how trees learn to support themselves. Trees don’t like to make things unnecessarily difficult. Why bother to grow a thick, sturdy trunk if you can lean comfortably against your neighbors? As long as they remain standing, not much can go wrong. In natural forests, it is the death from old age of a mighty mother tree that leaves surrounding trees without support. That’s how gaps in the canopy open up, and how formerly comfortable beeches or spruce find themselves suddenly wobbling on their own two feet—or rather, on their own root systems. Trees are not known for their speed, and it takes some species many years before they stand firm once again after such disruptions.

The process of learning stability is triggered by painful micro-tears that occur when the trees bend way over in the wind, first in one direction and then in the other. Wherever it hurts, that’s where the tree must strengthen its support structure. This takes a whole lot of energy, which is then unavailable for growing upward. A small consolation is the additional light that is now available for the tree’s own crown, thanks to the loss of its neighbor. But, here again, it takes a number of years for the tree to take full advantage of this. So far, the tree’s leaves have been adapted for low light, and so they are very tender and particularly sensitive to light. If the bright sun were to shine directly on them now, they would be scorched—ouch, that hurts! And because the buds for the coming year are formed the previous spring and summer, it takes a deciduous tree at least two growing seasons to adjust. Conifers take even longer, because their needles stay on their branches for up to 10 years. The situation remains tense until all the green leaves and needles have been replaced.

The thickness and stability of a trunk, therefore, build up as the tree responds to a series of aches and pains. In a natural forest, this little game can be repeated many times over the lifetime of a tree. Once the gap opened by the loss of another tree is overcome and everyone has extended their crowns so far out that the window of light into the forest is, once again, closed, then everyone can go back to leaning on everyone else. When that happens, more energy is put into growing trunks tall instead of wide, with predictable consequences when, decades later, the next tree breathes its last.

The trees that were being eaten gave off a warning gas that signaled to neighboring trees that a crisis was at hand.

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So, let’s return to the idea of school. If trees are capable of learning (and you can see they are just by observing them), then the question becomes: Where do they store what they have learned and how do they access this information? After all, they don’t have brains to function as databases and manage processes. It’s the same for all plants, and that’s why some scientists are skeptical and why many of them banish to the realm of fantasy the idea of plants’ ability to learn. But along comes the Australian scientist Monica Gagliano.

Gagliano studies mimosas, also called “sensitive plants.” Mimosas are tropical creeping herbs. They make particularly good research subjects, because it is easy to get them a bit riled up and they are easier to study in the laboratory than trees are. When they are touched, they close their feathery little leaves to protect themselves. Gagliano designed an experiment where individual drops of water fell on the plants’ foliage at regular intervals. At first, the anxious leaves closed immediately, but after a while, the little plants learned there was no danger of damage from the water droplets. After that, the leaves remained open despite the drops. Even more surprising for Gagliano was the fact that the mimosas could remember and apply their lesson weeks later, even without any further tests.

It’s a shame you can’t transport entire beeches or oaks into the laboratory to find out more about learning. But, at least as far as water is concerned, there is research in the field that reveals more than just behavioral changes: When trees are really thirsty, they begin to scream. If you’re out in the forest, you won’t be able to hear them, because this all takes place at ultrasonic levels. Scientists at the Swiss Federal Institute for Forest, Snow, and Landscape Research recorded the sounds, and this is how they explain them: Vibrations occur in the trunk when the flow of water from the roots to the leaves is interrupted. This is a purely mechanical event and it probably doesn’t mean anything.1 And yet?

We know how the sounds are produced, and if we were to look through a microscope to examine how humans produce sounds, what we would see wouldn’t be that different: The passage of air down the windpipe causes our vocal chords to vibrate. When I think about the research results, in particular in conjunction with the crackling roots I mentioned earlier, it seems to me that these vibrations could indeed be much more than just vibrations—they could be cries of thirst. The trees might be screaming out a dire warning to their colleagues that water levels are running low.

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Peter Wohlleben studied forestry and was a civil servant in the forestry commission for over 20 years. He holds lectures and seminars and has written books on subjects pertaining to woodlands and nature protection. His bestselling book The Hidden Life of Trees has sold to more than 20 countries.

From the book The Hidden Life of Trees, © 2016, by Peter Wohlleben. Published in 2016 by Greystone Books. Reprinted with permission of the publisher.

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1. Swiss Federal institute for Forest, Snow, and Landscape Research WSL. Rendering ecophysiological processes audible. (2015).

2. Anahäuser, M. The silent scream of the lima bean. Max Planck Research 4 (2007).

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3. Song, Y.Y., Simard, S.W., Carroll, A., Mohn, W.W., & Zheng, R.S. Defoliation of interior Douglas-fir elicits carbon transfer and defense signaling to ponderosa pine neighbors through ectomycorrhizal networks. Nature, Scientific Reports 5, 8495 (2015).

4. Beiler, K.J., Durall, D.M., Simard, S.W., Maxwell, S.A., & Kretzer, A.M. Mapping the wood-wide web: Mycorrhizal networks link multiple Douglas-fir cohorts. New Phytologist 185, 543-553 (2009).

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