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The Hidden Life of Trees book summary

The Hidden Life of Trees book cover

This book has given me an even deeper sense of appreciation for trees, plants and nature. The book is written in short chapters, with each one tackling an aspect of tree life, like tree families, how they communicate, energy and water, tree design, animal interactions etc. I've taken some of the most interesting snippets and collated them here, though I really feel that they don't do the book justice!

Communication and support

  • It appears that nutrient exchange and helping neighbors in times of need is the rule, and this leads to the conclusion that forests are superorganisms with interconnections much like ant colonies.
  • A tree is not a forest. On its own, a tree cannot establish a consistent local climate. It is at the mercy of wind and weather. But together, many trees create an ecosystem that moderates extremes of heat and cold, stores a great deal of water, and generates a great deal of humidity. And in this protected environment, trees can live to be very old.
  • A tree can be only as strong as the forest that surrounds it.
  • Trees, it turns out, have a completely different way of communicating: they use scent.
  • When it comes to some species of insects, trees can accurately identify which bad guys they are up against. The saliva of each species is different, and trees can match the saliva to the insect. Indeed, the match can be so precise that trees can release pheromones that summon specific beneficial predators. If they can identify saliva, they must also have a sense of taste.
  • Fungi operate like fiber-optic internet cables. Their thin filaments penetrate the ground, weaving through it in almost unbelievable density.
  • The fungal connections transmit signals from one tree to the next, helping the trees exchange news about insects, drought, and other dangers.
  • Thanks to selective breeding, our cultivated plants have, for the most part, lost their ability to communicate above or below ground. Isolated by their silence, they are easy prey for insect pests. That is one reason why modern agriculture uses so many pesticides. Perhaps farmers can learn from the forests and breed a little more wildness back into their grain and potatoes so that they’ll be more talkative in the future.
  • __Trees belonging to different species struggle with each other for local resources. But it’s different for trees of the same species. Beeches are capable of friendship and go so far as to feed each other.
  • This equalization is taking place underground through the roots. Whoever has an abundance of sugar hands some over; whoever is running short gets help. Once again, fungi are involved. Their enormous networks act as gigantic redistribution mechanisms.__


  • Trees maintain an inner balance. They budget their strength carefully, and they must be economical with energy so that they can meet all their needs. They expend some energy growing. They must lengthen their branches and widen the diameter of their trunks to support their increasing weight. They also hold some energy in reserve so that they can react immediately and activate defensive compounds in their leaves and bark if insects or fungi attack. Finally, there is the question of propagation.
  • Statistically speaking, each tree raises exactly one adult offspring to take its place.
  • Every five years, a beech tree produces at least thirty thousand beechnuts (thanks to climate change, it now does this as often as every two or three years). It is sexually mature at about 80 to 150 years of age, depending on how much light it gets where it’s growing. Assuming it grows to be 400 years old, it can fruit at least sixty times and produce a total of about 1.8 million beechnuts. From these, exactly one will develop into a full-grown tree—and in forest terms, that is a high rate of success, similar to winning the lottery.
  • Scientists have determined that slow growth when the tree is young is a prerequisite if a tree is to live to a ripe old age. As people, we easily lose sight of what is truly old for a tree, because modern forestry targets a maximum age of 80 to 120 years before plantation trees are cut down and turned into cash.


  • A tree has memory. It will do a better job of rationing water after experiencing drought.
  • 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.
  • Exhalation causes suction, which pulls a constant supply of water up through the transportation pathways in the tree. Suction works as long as the columns of water are continuous.


  • A break in its bark is at least as uncomfortable for a tree as a wound in our skin is for us. And, therefore, the tree relies on mechanisms similar to the ones we use to stop this from happening.
  • Small moist wounds have become portals for fungi to enter. The fungi advertise their triumphant advance through the tree by displaying magnificent fruiting bodies that jut out from the trunk in the shape of semicircular saucers that grow larger with each passing year. Inside, the fungi break down all barriers and penetrate deep into the wood at the heart of the tree. There, depending on the species, they consume stored sugar compounds or, even worse, cellulose and lignin, thereby decomposing the tree’s skeleton and reducing it to powder.
  • The fungus not only penetrates and envelops the tree’s roots, but also allows its web to roam through the surrounding forest floor. In so doing, it extends the reach of the tree’s own roots as the web grows out toward other trees. Here, it connects with other trees’ fungal partners and roots. And so a network is created, and now it’s easy for the trees to exchange vital nutrients and even information—such as an impending insect attack.
  • Fungi demand up to a third of the tree’s total food production in return for their services.
  • In exchange for the rich sugary reward, the fungi provide a few complimentary benefits for the tree, such as filtering out heavy metals, which are less detrimental to the fungi than to the tree’s roots.

Crown and roots

  • The highest branches up in a tree’s crown grow less and less with age. After a specific time — one hundred to three hundred years, depending on the species — the annual new growth gets shorter and shorter.
  • In conjunction with his colleagues, František Baluška from the Institute of Cellular and Molecular Botany at the University of Bonn is of the opinion that brain-like structures can be found at root tips. In addition to signaling pathways, there are also numerous systems and molecules similar to those found in animals. When a root feels its way forward in the ground, it is aware of stimuli. The researchers measured electrical signals that led to changes in behavior after they were processed in a “transition zone.”

Soil life

  • There are more life forms in a handful of forest soil than there are people on the planet. A mere teaspoonful contains many miles of fungal filaments. All these work the soil, transform it, and make it so valuable for the trees.
  • If you find weevils, you can be sure the forest has a long uninterrupted history. If the forest was cleared in the Middle Ages and later replanted, you won’t find these insects, because it would simply have been too far for them to walk to the next old forest.


  • Today, hardly any coal is being formed because forests are constantly being cleared, thanks to modern forest management practices (aka logging).
  • Dig down into the soil a little until you come across a lighter layer. Up to this point, the upper, darker soil is highly enriched with carbon. If the forest were left in peace from now on, this layer would be the precursor of coal, gas, or oil.
  • When the largest coal deposits were formed, in the Carboniferous period, carbon dioxide concentrations were much higher — nine times today’s levels — before prehistoric forests, among other factors, reduced carbon dioxide to a level that was still triple the concentration we have today.
  • There has been a measurable fertilizing effect as the levels of carbon dioxide in the atmosphere have risen. The latest forest inventories document that trees are growing more quickly than they used to. The spreadsheets that estimate lumber production need to be adjusted now that one third more biomass is accruing than a few decades ago. If you are a tree however, slow growth is the key to growing old.

Water cycle

  • The more living and dead wood there is in the forest, the thicker the layer of humus on the ground and the more water is stored in the total forest mass. Evaporation leads to cooling, which, in turn, leads to less evaporation.
  • During a severe storm, a mature tree can down an additional couple of hundred gallons of water that, thanks to its construction, it funnels to its roots. There, the water is stored in the surrounding soil, where it can help the tree over the next few dry spells.
  • It was always the trees that were transferring life-giving moisture into land-locked interiors. Researchers also discovered that the whole process breaks down if coastal forests are cleared. It’s a bit like if you were using an electrical pump to distribute water and you pulled the intake pipe out of the pond.

Animals and trees

  • Although beavers damage the forest around them, they exert a positive influence overall by regulating water supplies. And while they’re at it, they provide habitat for species adapted to large areas of standing water.
  • When they are choosing their trees, deer go for whatever is unusual. Whether they choose spruce, beech, pine, or oak, they will always choose whatever is uncommon locally. Who knows? Perhaps the smell of the shredded bark acts like an exotic perfume. It’s the same with people: it’s the rare things that are most highly prized.
  • Even though the birds are well equipped to hammer away at healthy wood, it would be too much for them to complete the job all at once. And that’s why they take a months-long break after the first phase, hoping fungi will pitch in.
  • Wood fibers conduct sound particularly well, which is why they are used to make musical instruments such as violins and guitars.
  • In total, a fifth of all animal and plant species — that’s about six thousand of the species we know about — depend on dead wood. Dead wood is useful because of its role as a nutrient recycler.

The seasons

  • Trees fuel themselves with energy from the sun, which they use to make sugar and other compounds they can hold in reserve. They store these under their skin just like a bear. Because they can’t get any fatter (only their bones—that is to say, their wood—can grow), the best they can do is fill their tissues with food. And whereas bears can go on eating everything they can find, at some point, the trees get full.
  • Dropping leaves is an effective protective strategy that seems made to measure for the climate in Central European latitudes. It is also an opportunity for trees to finally excrete waste. Just as we take a trip to a quiet little room before we go to bed, trees also rid themselves of substances they do not need and would like to part with. These drift down to the ground in their discarded leaves. Shedding leaves is an active process, so the tree can’t go to sleep yet. After the reserve supplies have been reabsorbed from the leaves back into trunk, the tree grows a layer of cells that closes off the connection between the leaves and the branches. Now all it takes is a light breeze, and the leaves drift down to the ground.
  • In the spring, the young trees leaf out two weeks before the large trees, ensuring themselves a long leisurely breakfast in the sun. But how do the youngsters know when they need to get started? It’s warm temperatures close to the ground that give the game away.
  • How do trees register that the warmer days are because of spring and not late summer? The appropriate reaction is triggered by a combination of day length and temperature. Trees must therefore have a memory. How else could they inwardly compare day lengths or count warm days?

Ways a tree protects itself

  • If the branches were not too thick (up to an inch across), it takes just a few years for the tree to close the gap (if the branch is broken). The tree can then saturate the area with water from the inside, killing the fungi. But if the branches were very thick, this procedure takes too long. The wounds gape open for decades, offering portals through which the fungi can enter and penetrate deep into the wood. The trunk rots and, at the very least, becomes a little less stable. And that is precisely the reason the etiquette manual calls for only thin branches on the lower part of the trunk (where it is more susceptible to breaking).
  • These secret reserves can be activated at any time, and depending on the tree species, they contain a selection of defensive compounds produced by the tree. These so-called phytoncides have antibiotic properties.

Trees in the city

  • Members of the public like to walk up to the trees, touch their bark, and relax in the shadow of their crowns. Over the decades, constant trampling around the base of the trees leads to further soil compaction, which means that rain drains away far too quickly, and in winter, the trees cannot build up a supply of water to last.
  • A severely pruned crown is a severe blow for the roots, which grow to a size optimally suited to serve the above-ground parts of the tree. If a large percentage of the branches is removed and the level of photosynthesis drops, then just as large a percentage of the underground part of the tree starves. Fungi now penetrate the dead ends where branches have been removed and the trunk has been sawn off. The wood is filled with air pockets, thanks to the tree’s quick growth as a youngster, and fungi have a field day.


  • Silver birch bark has another surprise in store. The white color is because of the active ingredient betulin, its primary component. White reflects sunlight and protects the trunk from sunscald. It also guards the trunk against heating up in the warming rays of the winter sun, which could cause unprotected trees to burst. As birches are pioneer trees that often grow all alone in wide-open spaces without any neighbors to shade them, such a feature makes sense. Betulin also has antiviral and antibacterial properties and is an ingredient in medicines and in many skin care products.
  • The quaking aspen takes its name from its leaves, which react to the slightest breath of wind. And although we have sayings that associate this characteristic with fear (“to shake like a leaf”), quaking aspens don’t shake because they are afraid. Their leaves hang from flexible stems and flutter in the breeze, exposing first their upper and then their lower surfaces to the sun. This means both sides of the leaf can photosynthesize. This is in contrast to other species, where the underside is reserved for breathing. Thus, quaking aspens can generate more energy, and they can grow even faster than birches.
  • In short, this is what we call evolution. It helps organisms adapt to changing environmental conditions and, therefore, guarantees the survival of each species. The shorter the interval before the next generation, the more quickly animals and plants can adapt.
  • In a forest that has been left to its own devices, the genetic makeup of each individual tree belonging to the same species is very different. This is in contrast to people, who are genetically very similar.

Forest air

  • Threatened forests are inherently unstable, and therefore, they are not appropriate places for human beings to live. And because our Stone Age ancestors were always on the lookout for ideal places to set up camp, it would make sense if we could intuitively pick up on the state of our surroundings. There is a scientific observation that speaks to this: the blood pressure of forest visitors rises when they are under conifers, whereas it calms down and falls in stands of oaks.
  • Contrary to popular opinion, the air in the forest is not always particularly rich in oxygen. This essential gas is released when water and carbon dioxide are broken down during photosynthesis. Every day in summer, trees release about 29 tons of oxygen into the air per square mile of forest. A person breathes in nearly 2 pounds of oxygen a day, so that’s the daily requirement for about ten thousand people. Every walk in the forest is like taking a shower in oxygen. But only during the day. Trees manufacture large amounts of carbohydrates not only to lay them down as wood but also to satisfy their hunger. Trees use carbohydrates as fuel, just as we do, and when they do, they convert sugar into energy and carbon dioxide. During the day, this doesn’t affect the air much because after all the additions and subtractions, there is still that surplus oxygen I just mentioned. At night, however, the trees don’t photosynthesize, and so they don’t break down carbon dioxide. Quite the opposite, in fact. In the darkness, it’s all about using carbohydrates, burning sugar in the cells’ power-generating stations, and releasing carbon dioxide.
  • How does a tree breathe anyway? You can see a part of its “lungs.” These are the needles or leaves. They have narrow slits on their undersides that look a bit like tiny mouths. The tree uses these openings to exhale oxygen and breathe in carbon dioxide.


  • It seems trees need their rest just as much as we do, and sleep deprivation is as detrimental to trees as it is to us.

Why trees are mostly green

  • Chlorophyll, however, has one disadvantage. It has a so-called green gap, and because it cannot use this part of the color spectrum, it has to reflect it back unused. This weak spot means that we can see this photosynthetic leftover, and that’s why almost all plants look deep green to us. What we are really seeing is waste light, the rejected part that trees cannot use. Beautiful for us; useless for the trees.

Closing notes

  • The main reason we misunderstand trees, however, is that they are so incredibly slow. Their childhood and youth last ten times as long as ours. Their complete life-span is at least five times as long as ours. Active movements such as unfurling leaves or growing new shoots take weeks or even months. And so it seems to us that trees are static beings, only slightly more active than rocks.
  • Not to put too fine a point on it, we use living things killed for our purposes. The real question is whether we help ourselves only to what we need_ from the forest ecosystem, and — analogous to our treatment of animals — whether we spare the trees unnecessary suffering when we do this.

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