Trees are engaged in countless complex cycles and they constantly struggle for water, light and their own survival. This struggle has led to some astonishing abilities: trees communicate with one another, give each other assistance, collaborate with fungi and other creatures, have memories and have even developed their own version of the internet!
Nature, Environment, Video, Plant and Animal Photography, Trees in Biological Sciences, Nature Conservation, Science, Biology, Ecology, Plants, Natural History
Introduction: Discover a hidden world.
A book about trees?! Those boring, green-and-brown things that do nothing but stand there? That, at best, provide some shade or a convenient place to hang a hammock?
Well, yes! Thing is, trees aren’t as boring as you might think. Or did you already know that they have their own kind of electric internet that enables them to warn friends and relatives, from miles away, about an insect attack? Or that, when there’s not enough nitrogen in the ground, they collaborate with fungi? Or that they have different personalities and choose when to shed their leaves?
You see, there is a lot to discover in the woods. In their million-year-long history, trees have developed astonishing abilities that help them in the lifelong struggle to secure water, light and nutrients. They have occupied the craziest ecological niches and have established friendships, antagonisms and alliances with every other possible living thing.
No one is better suited to tell you all this than Peter Wohlleben, who has spent his entire life among the trees. As a child, he already wanted to work in environmental protection, and now has worked for more than 30 years with trees. He practices a natural kind of forestry that is productive and humane – one that he’s honed with his unbelievable eye for detail and his immense factual knowledge. His awe of the woods and its inhabitants is legible in every line that he writes. In these summaries, join him on a walk through the forest that will put our green friends in an entirely new light.
These summaries also explain
- what happens when aphids drink a tree’s blood;
- how a mushroom can kill more living things than a shower of bombs;
- how trees go to the toilet.
Our planet’s lungs: Trees play a vital role in global water and carbon dioxide cycles.
Before diving into the fascinating abilities of trees, let’s take a brief look at their general importance.
Humanity owes them a great deal: they clean the air we breathe and help ensure the availability of water, even in the world’s most remote locations. In fact, without trees humans would be unable to survive.
If there were no trees, large swathes of the earth would dry out. As you probably remember from school, the way the global water cycle works is that water evaporates from the oceans, condenses into clouds that blow onto dry land, where it then rains down and trickles into streams and rivers that flow back into the ocean.
However, this straightforward explanation omits one crucial fact: without trees, every cloud would rain down within 600 kilometers of the coast, leaving the inner parts of continents bone dry. Trees essentially act as gigantic water pumps, transporting water further inland. When it rains in a forest near the coast, much of the rain remains on the leaves of trees and the forest floor. This water then evaporates, forming new clouds that make their way further inland, where they rain down.
In addition to hydrating the inner reaches of continents, trees also clean the air of carbon dioxide, thereby protecting the climate. They gather CO2 from the air and store it, and when they die, some of this gas is re-released into the atmosphere, but much of it remains in the tree.
By burning these dead trees, whether in the form of coal or gas, we’re releasing this CO2 back into the atmosphere and contributing to global warming. What’s more, we’re producing so much CO2 that the trees can’t keep up – they are unable to store it.
As you can see, without trees the earth and its climate would be much less hospitable to humanity.
These roots run deep: Trees interact in many different ways with the forest soil.
Trees aren’t only an essential part of our global climate; they’re also the basis for the soil in which we grow our food. And they interact with the earth’s soil in many different ways.
Surprisingly, the majority of the earth’s soil is comprised of trees. When our planet was formed, there wasn’t much around: there were minerals (for instance, cliffs and rocks), and air and water. Beaten by wind and rain, sections of these cliffs eroded and, over time, became a kind of gravel or sand. This is where single-celled organisms and algae settled.
Small plants and eventually the first trees followed these initial biomasses. When these plants and trees died, they decomposed and, with the help of small organisms, turned into humus (or topsoil), returning to the earth from which they sprang. So, in the form of oil and coal, this ground contains the trees that died long ago.
Living trees are connected to the soil in many ways as well. They are anchored to it via their roots, through which they absorb water that is then pumped to their leaves for photosynthesis. And these roots also connect trees to one another; it is not unusual for two adjacent trees to share nutrients and information through them. Furthermore, trees develop underground connections to fungi, a phenomenon that we’ll examine later.
By dropping their leaves in the fall, trees give back nutrients to the ground, nutrients that are then used by forest-dwellers. Among these nutrients are also all sorts of material that the tree has no more use for. When a tree loses its leaves, it’s basically using the toilet.
A high rate of child mortality: Young trees live dangerous lives.
Without trees, we’d have no water to drink, only bad air to breathe and hardly any soil. High time, then, to get acquainted with our arboreal friends! What does the life of a tree look like?
Let’s start at the beginning of a tree’s life. Every type of tree follows a different reproductive strategy. Some have small seeds that are disseminated by the wind. Others, such as oaks and chestnut trees, have larger seeds, which are usually transported with the help of animals.
Where a seed ends up is dictated by chance. And each type of tree also prefers a different location. Of course, these preferences are mostly based on whether the tree will get the right amount of light and water, be protected from the wind and situated in the right type of soil.
There are types of trees that prefer growing next to one another, like birch trees, which need the protection of the forest. And others, like poplar trees, that often stand alone in meadows, where they can enjoy a whole day of sun and a lot of space. But, of course, this exposes them to storms and other dangers.
Unfortunately, the chances of a tree seed surviving are very low. Often, they land in places where they can’t thrive or come up against conditions that prevent their growth. For example, many land in water (too much moisture!) or on asphalt (too little moisture!) or are brought to caves by animals, where there isn’t enough sunlight.
And even when the seed lands in a place where it could grow, other dangers lurk. It may be eaten by deer or other animals, trampled to death, crushed by a storm or damaged by hail. In fact, over the course of every tree’s life (which can, by the way, last for several hundred years) each tree will only raise one child to adulthood. But as soon as a small tree has managed to take root and survive its first years, it will show the kind of astounding abilities that all trees possess.
Nursery school: Trees have personalities and are able to learn.
Young trees don’t just grow; they develop a personality and, as the years pass, learn more about their environment and how they should best behave in it.
Personality, just as among people, varies among trees. Some are anxious, some bold. We tend to think of trees as doing nothing more than what we see them do. The weather turns cold, trees lose their leaves. Spring arrives, trees sprout buds and leaves. But it’s not that simple.
On the author’s land, for example, there are three oak trees growing close together. Each tree’s trunk almost touches that of its neighbor. In autumn, one of the oak trees always starts to shed its leaves two weeks earlier than the others. Since they all experience the same temperature, the same soil and the same length of day, such variables can’t be the explanation. So what’s happening? Well, this tree is simply more careful than the others. Whoever holds on to their leaves longer can do more photosynthesis and store more nutrients. However, the longer a tree keeps its leaves, the higher the risk of injury: a tree will get hurt if it still has its leaves during a frost or a freeze.
Trees learn from experience. They have to make a lot of decisions throughout their lives. When do they shed their leaves? Where do they let their roots grow –toward the east, where there might be more nutrients in the earth, or toward the west, where there is more moisture?
Not only do trees make their own decisions; they also learn from their mistakes. A tree that, for example, kept its leaves too long during one year will never make this mistake again. This leads to several other conclusions: trees must notice the temperature and the length of the day and be able to save their experiences somewhere. Obviously, trees don’t have brains, but it is thought that in the sensitive tips of their roots they keep track of information and experiences.
But trees aren’t only clever when it comes to caring for themselves. They also support each other.
Chatterbox: Trees communicate in different ways, both with their own kind and with other creatures.
It’s good that trees can learn to cope with the threat of danger because they also like to talk about what they’ve learned. They do this in two ways: with scents and… with e-mail!
Trees can contact not only their own kind but also other creatures by using scent. Depending on the situation, they release different pheromones. They can be quite ingenious: when, for example, an elm tree or a Scots pine tree suffers a caterpillar infestation, the tree releases a scent that attracts a species of tiny wasp. These wasps fly to the affected tree and lay their eggs in the caterpillars; when these eggs hatch, the larvae attack and eat the caterpillars.
Here’s another exciting example: trees have a way of identifying what kind of creature is trying to eat their leaves – by tasting that creature’s saliva!
Information travels even faster through the forest’s own internet. Electric pulses can only spread very slowly within a tree itself. If, for example, a caterpillar starts munching on a leaf, the leaf’s fibers send out electric signals; these signals travel along the fibers at a rip-roaring pace of one centimeter per minute.
Under the ground, however, almost every tree is linked to countless fungal threads, which can transmit electric signals much more quickly. One single fungus can spread itself over several miles and thus connect many trees with each other. We know that trees can send specific electric signals to the fungi and thus inform other trees nearby about insects or drought or other dangers.
How that actually happens is still not fully understood. But it is being studied!
My friend the tree: Trees help one another.
Trees aren’t dumb. They are dependent on their ecosystems and conspecifics and can get in touch with them. No wonder, then, that they help each other out whenever there’s trouble.
Trees often warn their conspecifics of potential dangers, by using scent and the “fungi-internet.” Mostly, that works very well. For example, on the African Savanna, where giraffes like to eat umbrella acacias. Within minutes of this happening to an acacia, it will release poison in its leaves and, at the same time, emits a warning gas – ethanol – that alerts other trees within a 100-meter radius of the attack.
The giraffes know this game well; within a few minutes, they head off to another tree that’s about 100 meters away, or to trees that are upwind, and continue eating. But for a bit the immediate surroundings of the umbrella acacia is protected against further attacks.
And not only do trees warn their tree friends about dangers; they also take care of sick and weak conspecifics with nutrients. For example, one time the author found a very old tree stump. Its insides had rotted a long time ago to topsoil, a clear sign that the tree had been felled over 400 years ago. But the wood on the outside of the stump was still living. How was this possible? After all, the stump didn’t have its own leaves to do its own photosynthesis.
Well, the stump was nourished by its neighbors with nutrients from the root system, and had been for at least 400 years! There’s no way for this stump to heal, but for trees which are only badly hurt, this system can save lives.
Why do trees do such a thing? It’s simple: it’s better together. Trees need the forest; it protects them from storms, provides the right microclimate and warns them of attacks. So they help each other out.
Lucky mushrooms: Trees intentionally work with fungi.
We already saw that trees use fungi to spread information in the forest. The fungi do this via their mycelium – a network of thread-like filaments. But that’s not all: trees and fungi work together on other levels, too.
For instance, they help each other procure water and nutrients, a collaboration that starts when a fungus lets a few filaments grow into the roots of the tree. Then the process gets started: the fungus helps the tree absorb more water. Sometimes the ground is rather dry, and the tree can’t get enough water with its roots alone. The filaments of the fungus are much finer, allowing them to permeate more ground, drawing water and nutrients and passing them on to “their” tree.
In return, the fungi get sugar – produced through photosynthesis – from the tree. It’s a good trade: trees that work with fungi store twice as much nitrogen and phosphorus – both important for life – than those without fungal friends.
This may sound perfectly harmonious. However, when the going gets tough, the fungi can resort to drastic, deadly measures. If, for example, the quantity of nitrogen in the ground sinks below a certain point, certain fungi can produce a poison that kills off all the microorganisms in the surrounding topsoil. These animals die and release the nitrogen that they stored in their bodies, so that it is available for the fungi and the tree.
So fungi can make life much easier for a tree. At the same time, they are also sometimes very dangerous, as we’ll learn in the next chapter.
That hurts! Trees protect themselves from injury.
We’ve seen how trees warn each other when, for example, beetles, giraffes or, a bit closer to home, deer attack. The injuries that trees sustain during such attacks are painful – and so, quite naturally, they want to avoid them at any cost.
There are different types of injury that a tree can suffer. And a major inflictor of these injuries are, of course, animals: deer eat young shoots, woodpeckers peck holes into the trunk and bark beetles drill through the bark and eat almost all of the living wood.
Smaller animals can also do great damage. Aphids attach themselves to the leaves and drink out their fluid, which contains sugar. Unfortunately, this fluid, the blood of trees, contains so little sugar that the aphids have to drink a lot. And all that liquid has to go somewhere! Maybe you’ve parked under a tree that aphids are drinking from, and found your car covered in a sticky mess.
The other big danger is weather. A storm can break branches or split the trunk. Rain, snow and hoar frost also pose a threat. Extreme weight or cold can strain the limbs so severely that they snap off, leaving an open wound.
Trees have thus developed different strategies for dealing with these types of dangers. Spruce trees, for example, have arranged their branches so that, when strained by the weight of snow, they bend downward, virtually lying atop one another.
Trees luckily don’t catch bacterial infections or viruses like we humans, but every injury includes the risk of a different kind of infection: fungus. As soon as the bark is opened, whether by a woodpecker or a broken branch, fungus can enter the tree. The tree tries right away to close the opening with new wood (we see the result of these efforts – a bulge in the bark – at the edges of holes left behind by fallen branches). Unfortunately, this isn’t exactly a speedy process.
And if the fungus gets in, the tree, even if it manages to heal itself, won’t survive for more than 100 years; once the fungus makes it inside, the wood starts to rot and the tree will inevitably, though maybe slowly, die.
Respect: Trees should be treated humanely, as animals are.
The image most people have of trees is dated. Unfortunately, the forestry industry is also behind the times. Traditional forestry, practiced almost everywhere in Germany today, does a few things wrong.
Of course, in the forestry industry, it’s mostly about producing wood. Foresters have long had a misconception: they thought that younger trees produce more wood faster than old trees. But this is untrue. Younger trees actually grow slower than older ones.
In most forests trees are felled when they are about 100 years old. But, for instance, beech trees aren’t sexually mature until anywhere between the ages of 80 and 150. No functioning ecosystem can exist in forests that only contain a few types of trees that are supposed to be harvested so quickly. Many things that we learned in the last chapter simply don’t work there. Trees don’t build partnerships with fungi or warn each other of danger, for instance. This results in unhealthy forests that are prone to pests and are generally unproductive.
A natural forest is more productive. Also, a forestry industry that mimics nature produces both higher amounts of, and better quality wood. Let’s think back to the example of that tree stump, the one that was taken care of for years by its neighbors. This kind of cooperation only takes place in naturally grown forests. Only there do organisms live in balance.
So we should cut down trees later, and, when we do, we should do it carefully and not just walk through the forest with chainsaws. Whoever understands that trees have a memory, that they can feel and live together with their children, won’t be able to fell trees whenever it’s convenient. This person will look for the trees that have fulfilled their duty in the ecosystem, those occupying a position that other trees can soon grow into and take over.
The key message in this book:
Trees are undervalued organisms. They can do much more than is often realized: they communicate and help each other out, they have senses and are designed to fit well into their place in their ecosystem. We should show trees respect and care about their welfare, just as we do with animals.
About the author
Peter Wohlleben spent over twenty years working for the forestry commission in Germany before leaving to put his ideas of ecology into practice. He now runs an environmentally-friendly woodland in Germany, where he is working for the return of primeval forests. He is the author of numerous books about the natural world including The Hidden Life of Trees, The Inner Lives of Animals, and The Secret Wisdom of Nature, which together make up his bestselling The Mysteries of Nature Series. He has also written numerous books for children including Can You Hear the Trees Talking? and Peter and the Tree Children.
Tim Flannery is a scientist, explorer and conservationist. He is a leading writer on climate change and his books include Atmosphere of Hope and The Weather Makers.
Jane Billinghurst’s career has been in book publishing in the UK, the US, and Canada, as an editor, publisher, writer, and translator. She is the translator of the New York Times-bestseller The Hidden Life of Trees by German forester Peter Wohlleben.
Table of Contents
Foreword by Tim Flannery
Introduction to the English Edition
1 / Friendships
2 / The Language of Trees
3 / Social Security
4 / Love
5 / The Tree Lottery
6 / Slowly Does It
7 / Forest Etiquette
8 / Tree School
9 / United We Stand, Divided We Fall
10 / The Mysteries of Moving Water
11 / Trees Aging Gracefully
12 / Mighty Oak or Mighty Wimp?
13 / Specialists
14 / Tree or Not Tree?
15 / In the Realm of Darkness
16 / Carbon Dioxide Vacuums
17 / Woody Climate Control
18 / The Forest as Water Pump
19 / Yours or Mine?
20 / Community Housing Projects
21 / Mother Ships of Biodiversity
22 / Hibernation
23 / A Sense of Time
24 / A Question of Character
25 / The Sick Tree
26 / Let There Be Light
27 / Street Kids
28 / Burnout
29 / Destination North!
30 / Tough Customers
31 / Turbulent Times
32 / Immigrants
33 / Healthy Forest Air
34 / Why Is the Forest Green?
35 / Set Free
36 / More Than Just a Commodity
Note from a Forest Scientist by Dr. Suzanne Simard
Are trees social beings? In The Hidden Life of Trees forester and author Peter Wohlleben convincingly makes the case that, yes, the forest is a social network. He draws on groundbreaking scientific discoveries to describe how trees are like human families: tree parents live together with their children, communicate with them, support them as they grow, share nutrients with those who are sick or struggling, and even warn each other of impending dangers. Wohlleben also shares his deep love of woods and forests, explaining the amazing processes of life, death, and regeneration that he has observed in his woodland.
Video and Podcast
“Warmly avuncular, storybook simple, and heavily dusted with the glitter of wonderment.” – The New Yorker
“The matter-of-fact Mr. Wohlleben has delighted readers and talk-show audiences alike with the news — long known to biologists — that trees in the forest are social beings.” – Sally McGrane, The New York Times
“This fascinating book will intrigue readers who love a walk through the woods.” – Publishers Weekly
“If you read this book, I believe that forests will become magical places for you, too.” – Tim Flannery
“In this spirited exploration, [Wohlleben] guarantees that readers will never look at these life forms in quite the same way again.” – Library Journal
“A paradigm-smashing chronicle of joyous entanglement that will make you joyously acknowledge your own entanglement in the ancient and ever-new web of being.” – Charles Foster, author of Being a Beast: Adventures Across the Species Divide
“Soon after we begin to recognize trees for what they are — gigantic beings thriving against incredible odds for hundreds of years — we naturally come to ask, ‘How do they do it?’ This charming book tells how — not as a lecture, more like a warm conversation with a favorite friend.” – Hope Jahren, author of Lab Girl
“A powerful reminder to slow down and tune into the language of nature.” – Rachel Sussman, author of The Oldest Living Things in the World
“Charming, provocative, fascinating. In the tradition of Jean-Henri Fabre and other great naturalist story-tellers, Wohlleben relates imaginative, enthralling tales of ecology.” – David George Haskell, author of The Forest Unseen, Pulitzer finalist
“Wohlleben’s book is at once romantic and scientific, beautifully articulating his personal relationship with the trees he has dedicated his life to. His view of the forest calls on us all to reevaluate our relationships with the plant world.” – Daniel Chamovitz, PhD, author of What a Plant Knows
“With colorful and engaging descriptions of little-known phenomena in our natural world, Wohlleben helps readers appreciate the exciting processes at work in the forests around them.” – Dr. Richard Karban, University of California, Davis, author of Plant Sensing and Communication
“You will never look at a tree the same way after reading Peter Wohlleben’s The Hidden Life of Trees, which reveals the mind-boggling properties and behavior of these terrestrial giants. Read this electrifying book, then go out and hug a tree — with admiration and gratitude.” – David Suzuki
Read an Excerpt/PDF Preview
WE READ IN fairy tales of trees with human faces, trees that can talk, and sometimes walk. This enchanted forest is the kind of place, I feel sure, that Peter Wohlleben inhabits. His deep understanding of the lives of trees, reached through decades of careful observation and study, reveals a world so astonishing that if you read his book, I believe that forests will become magical places for you, too.
One reason that many of us fail to understand trees is that they live on a different time scale than us. One of the oldest trees on Earth, a spruce in Sweden, is more than 9,500 years old. That’s 115 times longer than the average human lifetime. Creatures with such a luxury of time on their hands can afford to take things at a leisurely pace. The ele; ctrical impulses that pass through the roots of trees, for example, move at the slow rate of one third of an inch per second. But why, you might ask, do trees pass electrical impulses through their tissues at all?
The answer is that trees need to communicate, and electrical impulses are just one of their many means of communication. Trees also use the senses of smell and taste for communication. If a giraffe starts eating an African acacia, the tree releases a chemical into the air that signals that a threat is at hand. As the chemical drifts through the air and reaches other trees, they “smell” it and are warned of the danger. Even before the giraffe reaches them, they begin producing toxic chemicals. Insect pests are dealt with slightly differently. The saliva of leaf-eating insects can be “tasted” by the leaf being eaten. In response, the tree sends out a chemical signal that attracts predators that feed on that particular leaf-eating insect. Life in the slow lane is clearly not always dull.
But the most astonishing thing about trees is how social they are. The trees in a forest care for each other, sometimes even going so far as to nourish the stump of a felled tree for centuries after it was cut down by feeding it sugars and other nutrients, and so keeping it alive. Only some stumps are thus nourished. Perhaps they are the parents of the trees that make up the forest of today. A tree’s most important means of staying connected to other trees is a “wood wide web” of soil fungi that connects vegetation in an intimate network that allows the sharing of an enormous amount of information and goods. Scientific research aimed at understanding the astonishing abilities of this partnership between fungi and plant has only just begun.
The reason trees share food and communicate is that they need each other. It takes a forest to create a microclimate suitable for tree growth and sustenance. So it’s not surprising that isolated trees have far shorter lives than those living connected together in forests. Perhaps the saddest plants of all are those we have enslaved in our agricultural systems. They seem to have lost the ability to communicate, and, as Wohlleben says, are thus rendered deaf and dumb. “Perhaps farmers can learn from the forests and breed a little more wildness back into their grain and potatoes,” he advocates, “so that they’ll be more talkative in the future.”
Opening this book, you are about to enter a wonderland. Enjoy it.
THE ENGLISH EDITION
WHEN I WROTE this book, I wanted to describe my experiences in the forest I manage in the Eifel mountains in Germany and record what the trees had taught me. As soon as the German edition of the book was published, it was clear that the story I had to tell struck a chord with many, many people. My message, though grounded in a forest I interact with almost every day, is a message that applies to forests and woodlands around the world.
I am most familiar with the struggles and strategies of beeches and oaks, and with the contrast between deciduous forests that plan their own futures and coniferous forests planted for commercial gain. However, the struggles and strategies in forests left to their own devices, and the tension created when forests are planted instead of evolving at their own pace, are issues that resonate far beyond my experiences in Hümmel.
I encourage you to look around where you live. What dramas are being played out in wooded areas you can explore? How are commerce and survival balanced in the forests and woodlands you know? This book is a lens to help you take a closer look at what you might have taken for granted. Slow down, breathe deep, and look around. What can you hear? What do you see? How do you feel?
My story also explains why forests matter on a global scale. Trees are important, but when trees unite to create a fully functioning forest, you really can say that the whole is greater than its parts. Your trees may not function exactly as my trees do, and your forest might look a little different, but the underlying narrative is the same: forests matter at a more fundamental level than most of us realize.
Before you plunge into this book to find out what I have discovered just by stepping outside my back door, I want to tell you a story about Yellowstone National Park in the United States to show just how vital undisturbed forests and woodlands are to the future of our planet and how our appreciation for trees affects the way we interact with the world around us.
It all starts with the wolves. Wolves disappeared from Yellowstone, the world’s first national park, in the 1920s. When they left, the entire ecosystem changed. Elk herds in the park increased their numbers and began to make quite a meal of the aspens, willows, and cottonwoods that lined the streams. Vegetation declined and animals that depended on the trees left. The wolves were absent for seventy years. When they returned, the elks’ languorous browsing days were over. As the wolf packs kept the herds on the move, browsing diminished, and the trees sprang back. The roots of cottonwoods and willows once again stabilized stream banks and slowed the flow of water. This, in turn, created space for animals such as beavers to return. These industrious builders could now find the materials they needed to construct their lodges and raise their families. The animals that depended on the riparian meadows came back, as well. The wolves turned out to be better stewards of the land than people, creating conditions that allowed the trees to grow and exert their influence on the landscape.
My hope is that the wolves’ stewardship of natural processes in Yellowstone will help people appreciate the complex ways that trees interact with their environment, how our interactions with forests affect their success, and the role forests play in making our world the kind of place where we want to live. Apart from that, forests hide wonders that we are only just beginning to explore. I invite you to enter my world.
WHEN I BEGAN my professional career as a forester, I knew about as much about the hidden life of trees as a butcher knows about the emotional life of animals. The modern forestry industry produces lumber. That is to say, it fells trees and then plants new seedlings. If you read the professional literature, you quickly get the impression that the well-being of the forest is only of interest insofar as it is necessary for optimizing the lumber industry. That is enough for what foresters do day to day, and eventually it distorts the way they look at trees. Because it was my job to look at hundreds of trees every day—spruce, beeches, oaks, and pines—to assess their suitability for the lumber mill and their market value, my appreciation of trees was also restricted to this narrow point of view.
About twenty years ago, I began to organize survival training and log-cabin tours for tourists. Then I added a place in the forest where people can be buried as an alternative to traditional graveyards, and an ancient forest preserve. In conversations with the many visitors who came, my view of the forest changed once again. Visitors were enchanted by crooked, gnarled trees I would previously have dismissed because of their low commercial value. Walking with my visitors, I learned to pay attention to more than just the quality of the trees’ trunks. I began to notice bizarre root shapes, peculiar growth patterns, and mossy cushions on bark. My love of Nature—something I’ve had since I was six years old—was reignited. Suddenly, I was aware of countless wonders I could hardly explain even to myself. At the same time, Aachen University (RWTH Aachen) began conducting regular scientific research programs in the forest I manage. During the course of this research, many questions were answered, but many more emerged.
Life as a forester became exciting once again. Every day in the forest was a day of discovery. This led me to unusual ways of managing the forest. When you know that trees experience pain and have memories and that tree parents live together with their children, then you can no longer just chop them down and disrupt their lives with large machines. Machines have been banned from the forest for a couple of decades now, and if a few individual trees need to be harvested from time to time, the work is done with care by foresters using horses instead. A healthier—perhaps you could even say happier—forest is considerably more productive, and that means it is also more profitable.
This argument convinced my employer, the community of Hümmel, and now this tiny village in the Eifel mountains will not consider any other way of managing their forest. The trees are breathing a collective sigh of relief and revealing even more of their secrets, especially those stands growing in the newly established preserves, where they are left completely undisturbed. I will never stop learning from them, but even what I have learned so far under their leafy canopy exceeds anything I could ever have dreamed of.
I invite you to share with me the joy trees can bring us. And, who knows, perhaps on your next walk in the forest, you will discover for yourself wonders great and small.
— FRIENDSHIPS —
YEARS AGO, I stumbled across a patch of strange-looking mossy stones in one of the preserves of old beech trees that grows in the forest I manage. Casting my mind back, I realized I had passed by them many times before without paying them any heed. But that day, I stopped and bent down to take a good look. The stones were an unusual shape: they were gently curved with hollowed-out areas. Carefully, I lifted the moss on one of the stones. What I found underneath was tree bark. So, these were not stones, after all, but old wood. I was surprised at how hard the “stone” was, because it usually takes only a few years for beechwood lying on damp ground to decompose. But what surprised me most was that I couldn’t lift the wood. It was obviously attached to the ground in some way.
I took out my pocketknife and carefully scraped away some of the bark until I got down to a greenish layer. Green? This color is found only in chlorophyll, which makes new leaves green; reserves of chlorophyll are also stored in the trunks of living trees. That could mean only one thing: this piece of wood was still alive! I suddenly noticed that the remaining “stones” formed a distinct pattern: they were arranged in a circle with a diameter of about 5 feet. What I had stumbled upon were the gnarled remains of an enormous ancient tree stump. All that was left were vestiges of the outermost edge. The interior had completely rotted into humus long ago—a clear indication that the tree must have been felled at least four or five hundred years earlier. But how could the remains have clung onto life for so long?
Living cells must have food in the form of sugar, they must breathe, and they must grow, at least a little. But without leaves—and therefore without photosynthesis—that’s impossible. No being on our planet can maintain a centuries-long fast, not even the remains of a tree, and certainly not a stump that has had to survive on its own. It was clear that something else was happening with this stump. It must be getting assistance from neighboring trees, specifically from their roots. Scientists investigating similar situations have discovered that assistance may either be delivered remotely by fungal networks around the root tips—which facilitate nutrient exchange between trees1—or the roots themselves may be interconnected.2 In the case of the stump I had stumbled upon, I couldn’t find out what was going on, because I didn’t want to injure the old stump by digging around it, but one thing was clear: the surrounding beeches were pumping sugar to the stump to keep it alive.
If you look at roadside embankments, you might be able to see how trees connect with each other through their root systems. On these slopes, rain often washes away the soil, leaving the underground networks exposed. Scientists in the Harz mountains in Germany have discovered that this really is a case of interdependence, and most individual trees of the same species growing in the same stand are connected to each other through their root systems. 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.
Of course, it makes sense to ask whether tree roots are simply wandering around aimlessly underground and connecting up when they happen to bump into roots of their own kind. Once connected, they have no choice but to exchange nutrients. They create what looks like a social network, but what they are experiencing is nothing more than a purely accidental give and take. In this scenario, chance encounters replace the more emotionally charged image of active support, though even chance encounters offer benefits for the forest ecosystem. But Nature is more complicated than that. According to Massimo Maffei from the University of Turin, plants—and that includes trees—are perfectly capable of distinguishing their own roots from the roots of other species and even from the roots of related individuals.3
But why are trees such social beings? Why do they share food with their own species and sometimes even go so far as to nourish their competitors? The reasons are the same as for human communities: there are advantages to working together. 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. To get to this point, the community must remain intact no matter what. If every tree were looking out only for itself, then quite a few of them would never reach old age. Regular fatalities would result in many large gaps in the tree canopy, which would make it easier for storms to get inside the forest and uproot more trees. The heat of summer would reach the forest floor and dry it out. Every tree would suffer.
Every tree, therefore, is valuable to the community and worth keeping around for as long as possible. And that is why even sick individuals are supported and nourished until they recover. Next time, perhaps it will be the other way round, and the supporting tree might be the one in need of assistance. When thick silver-gray beeches behave like this, they remind me of a herd of elephants. Like the herd, they, too, look after their own, and they help their sick and weak back up onto their feet. They are even reluctant to abandon their dead.
Every tree is a member of this community, but there are different levels of membership. For example, most stumps rot away into humus and disappear within a couple of hundred years (which is not very long for a tree). Only a few individuals are kept alive over the centuries, like the mossy “stones” I’ve just described. What’s the difference? Do tree societies have second-class citizens just like human societies? It seems they do, though the idea of “class” doesn’t quite fit. It is rather the degree of connection—or maybe even affection—that decides how helpful a tree’s colleagues will be.
You can check this out for yourself simply by looking up into the forest canopy. The average tree grows its branches out until it encounters the branch tips of a neighboring tree of the same height. It doesn’t grow any wider because the air and better light in this space are already taken. However, it heavily reinforces the branches it has extended, so you get the impression that there’s quite a shoving match going on up there. But a pair of true friends is careful right from the outset not to grow overly thick branches in each other’s direction. The trees don’t want to take anything away from each other, and so they develop sturdy branches only at the outer edges of their crowns, that is to say, only in the direction of “non-friends.” Such partners are often so tightly connected at the roots that sometimes they even die together.
As a rule, friendships that extend to looking after stumps can only be established in undisturbed forests. It could well be that all trees do this and not just beeches. I myself have observed oak, fir, spruce, and Douglas fir stumps that were still alive long after the trees had been cut down. Planted forests, which is what most of the coniferous forests in Central Europe are, behave more like the street kids I describe in chapter 27. Because their roots are irreparably damaged when they are planted, they seem almost incapable of networking with one another. As a rule, trees in planted forests like these behave like loners and suffer from their isolation. Most of them never have the opportunity to grow old anyway. Depending on the species, these trees are considered ready to harvest when they are only about a hundred years old.
— THE LANGUAGE OF TREES —
ACCORDING TO THE dictionary definition, language is what people use when we talk to each other. Looked at this way, we are the only beings who can use language, because the concept is limited to our species. But wouldn’t it be interesting to know whether trees can also talk to each other? But how? They definitely don’t produce sounds, so there’s nothing we can hear. Branches creak as they rub against one another and leaves rustle, but these sounds are caused by the wind and the tree has no control over them. Trees, it turns out, have a completely different way of communicating: they use scent.
Scent as a means of communication? The concept is not totally unfamiliar to us. Why else would we use deodorants and perfumes? And even when we’re not using these products, our own smell says something to other people, both consciously and subconsciously. There are some people who seem to have no smell at all; we are strongly attracted to others because of their aroma. Scientists believe pheromones in sweat are a decisive factor when we choose our partners—in other words, those with whom we wish to procreate. So it seems fair to say that we possess a secret language of scent, and trees have demonstrated that they do as well.
For example, four decades ago, scientists noticed something on the African savannah. The giraffes there were feeding on umbrella thorn acacias, and the trees didn’t like this one bit. It took the acacias mere minutes to start pumping toxic substances into their leaves to rid themselves of the large herbivores. The giraffes got the message and moved on to other trees in the vicinity. But did they move on to trees close by? No, for the time being, they walked right by a few trees and resumed their meal only when they had moved about 100 yards away.
The reason for this behavior is astonishing. The acacia trees that were being eaten gave off a warning gas (specifically, ethylene) that signaled to neighboring trees of the same species that a crisis was at hand. Right away, all the forewarned trees also pumped toxins into their leaves to prepare themselves. The giraffes were wise to this game and therefore moved farther away to a part of the savannah where they could find trees that were oblivious to what was going on. Or else they moved upwind. For the scent messages are carried to nearby trees on the breeze, and if the animals walked upwind, they could find acacias close by that had no idea the giraffes were there.
Similar processes are at work in our forests here at home. Beeches, spruce, and oaks all register pain as soon as some creature starts nibbling on them. When a caterpillar takes a hearty bite out of a leaf, the tissue around the site of the damage changes. In addition, the leaf tissue sends out electrical signals, just as human tissue does when it is hurt. However, the signal is not transmitted in milliseconds, as human signals are; instead, the plant signal travels at the slow speed of a third of an inch per minute.4 Accordingly, it takes an hour or so before defensive compounds reach the leaves to spoil the pest’s meal. Trees live their lives in the really slow lane, even when they are in danger. But this slow tempo doesn’t mean that a tree is not on top of what is happening in different parts of its structure. If the roots find themselves in trouble, this information is broadcast throughout the tree, which can trigger the leaves to release scent compounds. And not just any old scent compounds, but compounds that are specifically formulated for the task at hand.
This ability to produce different compounds is another feature that helps trees fend off attack for a while. 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. The beneficial predators help trees by eagerly devouring the insects that are bothering them. For example, elms and pines call on small parasitic wasps that lay their eggs inside leaf-eating caterpillars.5 As the wasp larvae develop, they devour the larger caterpillars bit by bit from the inside out. Not a nice way to die. The result, however, is that the trees are saved from bothersome pests and can keep growing with no further damage. The fact trees can recognize saliva is, incidentally, evidence for yet another skill they must have. For if they can identify saliva, they must also have a sense of taste.
A drawback of scent compounds is that they disperse quickly in the air. Often they can be detected only within a range of about 100 yards. Quick dispersal, however, also has advantages. As the transmission of signals inside the tree is very slow, a tree can cover long distances much more quickly through the air if it wants to warn distant parts of its own structure that danger lurks. A specialized distress call is not always necessary when a tree needs to mount a defense against insects. The animal world simply registers the tree’s basic chemical alarm call. It then knows some kind of attack is taking place and predatory species should mobilize. Whoever is hungry for the kinds of critters that attack trees just can’t stay away.
Trees can also mount their own defense. Oaks, for example, carry bitter, toxic tannins in their bark and leaves. These either kill chewing insects outright or at least affect the leaves’ taste to such an extent that instead of being deliciously crunchy, they become biliously bitter. Willows produce the defensive compound salicylic acid, which works in much the same way. But not on us. Salicylic acid is a precursor of aspirin, and tea made from willow bark can relieve headaches and bring down fevers. Such defense mechanisms, of course, take time. Therefore, a combined approach is crucially important for arboreal early-warning systems.
Trees don’t rely exclusively on dispersal in the air, for if they did, some neighbors would not get wind of the danger. Dr. Suzanne Simard of the University of British Columbia in Vancouver has discovered that they also warn each other using chemical signals sent through the fungal networks around their root tips, which operate no matter what the weather.6 Surprisingly, news bulletins are sent via the roots not only by means of chemical compounds but also by means of electrical impulses that travel at the speed of a third of an inch per second. In comparison with our bodies, it is, admittedly, extremely slow. However, there are species in the animal kingdom, such as jellyfish and worms, whose nervous systems conduct impulses at a similar speed.7 Once the latest news has been broadcast, all oaks in the area promptly pump tannins through their veins.
Tree roots extend a long way, more than twice the spread of the crown. So the root systems of neighboring trees inevitably intersect and grow into one another—though there are always some exceptions. Even in a forest, there are loners, would-be hermits who want little to do with others. Can such antisocial trees block alarm calls simply by not participating? Luckily, they can’t. For usually there are fungi present that act as intermediaries to guarantee quick dissemination of news. These fungi operate like fiber-optic Internet cables. Their thin filaments penetrate the ground, weaving through it in almost unbelievable density. One teaspoon of forest soil contains many miles of these “hyphae.”8 Over centuries, a single fungus can cover many square miles and network an entire forest. The fungal connections transmit signals from one tree to the next, helping the trees exchange news about insects, drought, and other dangers. Science has adopted a term first coined by the journal Nature for Dr. Simard’s discovery of the “wood wide web” pervading our forests.9 What and how much information is exchanged are subjects we have only just begun to research. For instance, Simard discovered that different tree species are in contact with one another, even when they regard each other as competitors.10 And the fungi are pursuing their own agendas and appear to be very much in favor of conciliation and equitable distribution of information and resources.11
If trees are weakened, it could be that they lose their conversational skills along with their ability to defend themselves. Otherwise, it’s difficult to explain why insect pests specifically seek out trees whose health is already compromised. It’s conceivable that to do this, insects listen to trees’ urgent chemical warnings and then test trees that don’t pass the message on by taking a bite out of their leaves or bark. A tree’s silence could be because of a serious illness or, perhaps, the loss of its fungal network, which would leave the tree completely cut off from the latest news. The tree no longer registers approaching disaster, and the doors are open for the caterpillar and beetle buffet. The loners I just mentioned are similarly susceptible—they might look healthy, but they have no idea what is going on around them.
In the symbiotic community of the forest, not only trees but also shrubs and grasses—and possibly all plant species—exchange information this way. However, when we step into farm fields, the vegetation becomes very quiet. Thanks to selective breeding, our cultivated plants have, for the most part, lost their ability to communicate above or below ground—you could say they are deaf and dumb—and therefore they are easy prey for insect pests.12 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.
Communication between trees and insects doesn’t have to be all about defense and illness. Thanks to your sense of smell, you’ve probably picked up on many feel-good messages exchanged between these distinctly different life-forms. I am referring to the pleasantly perfumed invitations sent out by tree blossoms. Blossoms do not release scent at random or to please us. Fruit trees, willows, and chestnuts use their olfactory missives to draw attention to themselves and invite passing bees to sate themselves. Sweet nectar, a sugar-rich liquid, is the reward the insects get in exchange for the incidental dusting they receive while they visit. The form and color of blossoms are signals, as well. They act somewhat like a billboard that stands out against the general green of the tree canopy and points the way to a snack.
So trees communicate by means of olfactory, visual, and electrical signals. (The electrical signals travel via a form of nerve cell at the tips of the roots.) What about sounds? Let’s get back to hearing and speech. When I said at the beginning of this chapter that trees are definitely silent, the latest scientific research casts doubt even on this statement. Along with colleagues from Bristol and Florence, Dr. Monica Gagliano from the University of Western Australia has, quite literally, had her ear to the ground.13 It’s not practical to study trees in the laboratory; therefore, researchers substitute grain seedlings because they are easier to handle. They started listening, and it didn’t take them long to discover that their measuring apparatus was registering roots crackling quietly at a frequency of 220 hertz. Crackling roots? That doesn’t necessarily mean anything. After all, even dead wood crackles when it’s burned in a stove. But the noises discovered in the laboratory caused the researchers to sit up and pay attention. For the roots of seedlings not directly involved in the experiment reacted. Whenever the seedlings’ roots were exposed to a crackling at 220 hertz, they oriented their tips in that direction. That means the grasses were registering this frequency, so it makes sense to say they “heard” it.
Plants communicating by means of sound waves? That makes me curious to know more, because people also communicate using sound waves. Might this be a key to getting to know trees better? To say nothing of what it would mean if we could hear whether all was well with beeches, oaks, and pines, or whether something was up. Unfortunately, we are not that far advanced, and research in this field is just beginning. But if you hear a light crackling the next time you take a walk in the forest, perhaps it won’t be just the wind…
— SOCIAL SECURITY —
GARDENERS OFTEN ASK me if their trees are growing too close together. Won’t they deprive each other of light and water? This concern comes from the forestry industry. In commercial forests, trees are supposed to grow thick trunks and be harvest-ready as quickly as possible. And to do that, they need a lot of space and large, symmetrical, rounded crowns. In regular five-year cycles, any supposed competition is cut down so that the remaining trees are free to grow. Because these trees will never grow old—they are destined for the sawmill when they are only about a hundred—the negative effects of this management practice are barely noticeable.
What negative effects? Doesn’t it sound logical that a tree will grow better if bothersome competitors are removed so that there’s plenty of sunlight available for its crown and plenty of water for its roots? And for trees belonging to different species that is indeed the case. They really do struggle with each other for local resources. But it’s different for trees of the same species. I’ve already mentioned that beeches are capable of friendship and go so far as to feed each other. It is obviously not in a forest’s best interest to lose its weaker members. If that were to happen, it would leave gaps that would disrupt the forest’s sensitive microclimate with its dim light and high humidity. If it weren’t for the gap issue, every tree could develop freely and lead its own life. I say “could” because beeches, at least, seem to set a great deal of store by sharing resources.
Students at the Institute for Environmental Research at RWTH Aachen discovered something amazing about photosynthesis in undisturbed beech forests. Apparently, the trees synchronize their performance so that they are all equally successful. And that is not what one would expect. Each beech tree grows in a unique location, and conditions can vary greatly in just a few yards. The soil can be stony or loose. It can retain a great deal of water or almost no water. It can be full of nutrients or extremely barren. Accordingly, each tree experiences different growing conditions; therefore, each tree grows more quickly or more slowly and produces more or less sugar or wood, and thus you would expect every tree to be photosynthesizing at a different rate.
And that’s what makes the research results so astounding. The rate of photosynthesis is the same for all the trees. The trees, it seems, are equalizing differences between the strong and the weak. Whether they are thick or thin, all members of the same species are using light to produce the same amount of sugar per leaf. This equalization is taking place underground through the roots. There’s obviously a lively exchange going on down there. 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. It’s a bit like the way social security systems operate to ensure individual members of society don’t fall too far behind.14
In such a system, it is not possible for the trees to grow too close to each other. Quite the opposite. Huddling together is desirable and the trunks are often spaced no more than 3 feet apart. Because of this, the crowns remain small and cramped, and even many foresters believe this is not good for the trees. Therefore, the trees are spaced out through felling, meaning that supposedly excess trees are removed. However, colleagues from Lübeck in northern Germany have discovered that a beech forest is more productive when the trees are packed together. A clear annual increase in biomass, above all wood, is proof of the health of the forest throng.15
When trees grow together, nutrients and water can be optimally divided among them all so that each tree can grow into the best tree it can be. If you “help” individual trees by getting rid of their supposed competition, the remaining trees are bereft. They send messages out to their neighbors in vain, because nothing remains but stumps. Every tree now muddles along on its own, giving rise to great differences in productivity. Some individuals photosynthesize like mad until sugar positively bubbles along their trunk. As a result, they are fit and grow better, but they aren’t particularly long-lived. This is because a tree can be only as strong as the forest that surrounds it. And there are now a lot of losers in the forest. Weaker members, who would once have been supported by the stronger ones, suddenly fall behind. Whether the reason for their decline is their location and lack of nutrients, a passing malaise, or genetic makeup, they now fall prey to insects and fungi.
But isn’t that how evolution works? you ask. The survival of the fittest? Trees would just shake their heads—or rather their crowns. Their well-being depends on their community, and when the supposedly feeble trees disappear, the others lose as well. When that happens, the forest is no longer a single closed unit. Hot sun and swirling winds can now penetrate to the forest floor and disrupt the moist, cool climate. Even strong trees get sick a lot over the course of their lives. When this happens, they depend on their weaker neighbors for support. If they are no longer there, then all it takes is what would once have been a harmless insect attack to seal the fate even of giants.
In former times, I myself instigated an exceptional case of assistance. In my first years as a forester, I had young trees girdled. In this process, a strip of bark 3 feet wide is removed all around the trunk to kill the tree. Basically, this is a method of thinning, where trees are not cut down, but desiccated trunks remain as standing deadwood in the forest. Even though the trees are still standing, they make more room for living trees, because their leafless crowns allow a great deal of light to reach their neighbors. Do you think this method sounds brutal? I think it does, because death comes slowly over a few years and, therefore, in the future, I wouldn’t manage forests this way. I observed how hard the beeches fought and, amazingly enough, how some of them survive to this day.
In the normal course of events, such survival would not be possible, because without bark the tree cannot transport sugar from its leaves to its roots. As the roots starve, they shut down their pumping mechanisms, and because water no longer flows through the trunk up to the crown, the whole tree dries out. However, many of the trees I girdled continued to grow with more or less vigor. I know now that this was only possible with the help of intact neighboring trees. Thanks to the underground network, neighbors took over the disrupted task of provisioning the roots and thus made it possible for their buddies to survive. Some trees even managed to bridge the gap in their bark with new growth, and I’ll admit it: I am always a bit ashamed when I see what I wrought back then. Nevertheless, I have learned from this just how powerful a community of trees can be. “A chain is only as strong as its weakest link.” Trees could have come up with this old craftsperson’s saying. And because they know this intuitively, they do not hesitate to help each other out.
— LOVE —
THE LEISURELY PACE at which trees live their lives is also apparent when it comes to procreation. Reproduction is planned at least a year in advance. Whether tree love happens every spring depends on the species. Whereas conifers send their seeds out into the world at least once a year, deciduous trees have a completely different strategy. Before they bloom, they agree among themselves. Should they go for it next spring, or would it be better to wait a year or two? Trees in a forest prefer to bloom at the same time so that the genes of many individual trees can be well mixed. Conifers and deciduous trees agree on this, but deciduous trees have one other factor to consider: browsers such as wild boar and deer.
Boar and deer are extremely partial to beechnuts and acorns, both of which help them put on a protective layer of fat for winter. They seek out these nuts because they contain up to 50 percent oil and starch—more than any other food. Often whole areas of forest are picked clean down to the last morsel in the fall so that, come spring, hardly any beech and oak seedlings sprout. And that’s why the trees agree in advance. If they don’t bloom every year, then the herbivores cannot count on them. The next generation is kept in check because over the winter the pregnant animals must endure a long stretch with little food, and many of them will not survive. When the beeches or oaks finally all bloom at the same time and set fruit, then it is not possible for the few herbivores left to demolish everything, so there are always enough undiscovered seeds left over to sprout.
“Mast years” is an old term used to describe years when beeches and oaks set seed. In these years of plenty, wild boar can triple their birth rate because they find enough to eat in the forests over the winter. In earlier times, European peasants used the windfall for the wild boar’s tame relatives, domestic pigs, which they herded into the woods. The idea was that the herds of domestic pigs would gorge on the wild nuts and fatten up nicely before they were slaughtered. The year following a mast year, wild boar numbers usually crash because the beeches and oaks are taking a time-out and the forest floor is bare once again.
When beeches and oaks put blooming on hold for a number of years, this has grave consequences for insects, as well—especially for bees. It’s the same for bees as it is for wild boar: a multi-year hiatus causes their populations to collapse. Or, more accurately, could cause them to collapse, because bees never build up large populations in deciduous forests in the first place. The reason is that true forest trees couldn’t care less about these little helpers. What use are the few pollinators left after barren years when you then unfurl millions upon millions of blossoms over hundreds of square miles? If you are a beech or an oak, you have to come up with a more reliable method of pollination, perhaps even one that doesn’t exact payment. And what could be more natural than using the wind? Wind blows the powdery pollen out of the blossoms and carries it over to neighboring trees. The wind has a further advantage. It still blows when temperatures fall, even when they drop below 53 degrees Fahrenheit, which is when it gets too chilly for bees and they stay home.
Conifers bloom almost every year, which means bees are an option for pollination because they would always find food. However, conifers are native to northern forests, which are too chilly for bees to be out and about while the trees are blooming, and that is probably why conifers, like beeches and oaks, prefer to rely on the wind. Conifers don’t need to worry about taking breaks from blooming, like beeches or oaks, because they have no reason to fear deer and wild boar. The small seeds inside the cones of Spruce & Co. just don’t offer an attractive source of nutrition. True, there are birds such as red crossbills, which pick off cones with the tips of their powerful crossed bills and eat the seeds inside, but in general, birds don’t seem to be a big problem. And because there is almost no animal that likes to store conifer seeds for winter food, the trees release their potential heirs into the world on tiny wings. Thus equipped, their seeds float slowly down from the tips of their branches and can easily be carried away on a breath of wind.
Spruce & Co. produce huge quantities of pollen, almost as though they wanted to outdo deciduous trees in the mating department. They produce such huge quantities that even in a light breeze, enormous dusty clouds billow over coniferous forests in bloom, giving the impression of a fire smoldering beneath the treetops. This raises the inevitable question about how inbreeding can be avoided in such chaotic conditions. Trees have survived until today only because there is a great deal of genetic diversity within each species. If they all release their pollen at the same time, then the tiny grains of pollen from all the trees mix together and drift through the canopy. And because a tree’s own pollen is particularly concentrated around its own branches, there’s a real danger its pollen will end up fertilizing its own female flowers. But, as I just mentioned, that is precisely what the trees want to avoid. To reduce this possibility, trees have come up with a number of different strategies.
Some species—like spruce—rely on timing. Male and female blossoms open a few days apart so that, most of the time, the latter will be dusted with the foreign pollen of other spruce. This is not an option for trees like bird cherries, which rely on insects. Bird cherries produce male and female sex organs in the same blossom, and they are one of the few species of true forest trees that allow themselves to be pollinated by bees. As the bees make their way through the whole crown, they cannot help but spread the tree’s own pollen. But the bird cherry is alert and senses when the danger of inbreeding looms. When a pollen grain lands on a stigma, its genes are activated and it grows a delicate tube down to the ovary in search of an egg. As it is doing this, the tree tests the genetic makeup of the pollen and, if it matches its own, blocks the tube, which then dries up. Only foreign genes, that is to say, genes that promise future success, are allowed entry to form seeds and fruit. How does the bird cherry distinguish between “mine” and “yours”? We don’t know exactly. What we do know is that the genes must be activated, and they must pass the tree’s test. You could say, the tree can “feel” them. You might say that we, too, experience the physical act of love as more than just the secretions of neurotransmitters that activate our bodies’ secrets, though what mating feels like for trees is something that will remain in the realm of speculation for a long time to come.
Some species have a particularly effective way of avoiding inbreeding: each individual has only one gender. For example, there are both male and female willows, which means they can never mate with themselves but only procreate with other willows. But willows, it must be said, aren’t true forest trees. They colonize pioneer sites, areas that are not yet forested. Because there are thousands of wild flowers and shrubs blooming in such places, and they attract bees, willows, like bird cherries, also rely on insects for pollination. But here a problem arises. The bees must first fly to the male willows, collect pollen there, and then transport the pollen to the female trees. If it was the other way around, there would be no fertilization. How does a tree manage this if both sexes have to bloom at the same time? Scientists have discovered that all willows secrete an alluring scent to attract bees. Once the insects arrive in the target area, the willows switch to visual signals. With this in mind, male willows put a lot of effort into their catkins and make them bright yellow. This attracts the bees to them first. Once the bees have had their first meal of sugary nectar, they leave and visit the inconspicuous greenish flowers of the female trees.16
Inbreeding as we know it in mammals—that is to say, breeding between populations that are related to one another—is, of course, still possible in all three cases I have mentioned. And here, wind and bees come into play equally. As both bridge large distances, they ensure that at least some of the trees receive pollen from distant relations, and so the local gene pool is constantly refreshed. However, completely isolated stands of rare species of trees, where only a few trees grow, can lose their genetic diversity. When they do, they weaken and, after a few centuries, they disappear altogether.
— THE TREE LOTTERY —
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.
Species that blossom every year plan for this Herculean task by carefully calibrating their energy levels. However, species that blossom only every three to five years, such as beeches or oaks, are thrown off kilter by such events. Most of their energy has already been earmarked for other tasks, but they need to produce such enormous numbers of beechnuts and acorns that everything else must now take second place. The battle for the branches begins. There’s not a speck of space for the blossoms, so a corresponding number of leaves must vacate their posts. In the years when the leaves shrivel and fall off, the trees look unusually bare, so it’s no surprise that reports on the condition of forests where the affected trees are growing describe the tree canopy as being in a pitiful state. Because all the trees are going through this process at the same time, to a casual observer the forest looks sick. The forest is not sick, but it is vulnerable. The trees use the last of their energy reserves to produce the mass of blossoms, and to compound the problem, they are left with fewer leaves, so they produce less sugar than they normally do. Furthermore, most of the sugar they do produce is converted into oil and starch in the seeds, so there is hardly any left over for the trees’ daily needs and winter stores—to say nothing of the energy reserves intended to defend against sickness.
Many insects have been waiting for just this moment. For example, the beech leaf-mining weevil lays millions upon millions of eggs in the fresh, defenseless foliage. Here, the tiny larvae eat away flat tunnels between the top and bottom surfaces of the leaves, leaving brown papery trails as they feed. The adult beetles chew holes in the leaves until they look as though a hunter has blasted them with a shotgun. Some years, the infestations are so severe that, from afar, the beeches look more brown than green. Normally, the trees would fight back by making the insects’ meal extremely bitter—literally. But after producing all those blossoms, they are out of steam, and so this season they must endure the attack without responding.
Healthy trees get over this, especially because afterward there will be a number of years for them to recover. However, if a beech tree is already sickly before the attack, then such an infestation can sound its death knell. Even if the tree knew this, it would not produce fewer blossoms. We know from times of high forest mortality that it is usually the particularly battered individuals that burst into bloom. If they die, their genetic legacy might disappear, and so they probably want to reproduce right away to make sure it continues. Something similar happens after unusually hot summers. After extreme droughts bring many trees to the brink of death, they all bloom together the following year, which goes to show that large quantities of beechnuts and acorns don’t indicate that the next winter will be particularly harsh. As blossoms are set the summer before, the abundance of fruit reflects what happened the previous year and has nothing to do with what will happen in the future. The effect of weak defenses shows up again in the fall, this time in the seeds. The beech leaf miners bore into fruit buds as well as leaves. Consequently, although beechnuts form, they remain empty, and therefore, they are barren and worthless.
When a seed falls from a tree, each species has its own strategy as to when the seed sprouts. So how does that work? If a seed lands on soft, damp soil, it has no choice but to sprout as soon as it is warmed by the sun in the spring, for every day the embryonic tree lies around on the ground unprotected it is in great danger—come spring, wild boar and deer are always hungry. And this is just what the large seeds of species such as beeches and oaks do. The next generation emerges from beechnuts and acorns as quickly as it can so that it is less attractive to herbivores. And because this is their one and only plan, the seeds don’t have long-term defense strategies against fungi and bacteria. The seeds slough off their protective casings, which lie around on the forest floor through the summer and rot away by the following spring.
Many other species, however, give their seeds the opportunity to wait one or more years until they start to grow. Of course, this means a higher risk of being eaten, but it also offers substantial advantages. For example, seedlings can die of thirst in a dry spring, and when that happens, all the energy put into the next generation is wasted. Or when a deer has its territory and main feeding ground in exactly the spot where the seed lands, it takes no more than a few days for the seedling’s tasty new leaves to end up in the deer’s stomach. In contrast, if some of the seeds do not germinate for a year or more, then the risk is spread out so that at least a few little trees are likely to make it.
Bird cherries adopt this strategy: their seeds can lie dormant for up to five years, waiting for the right time to sprout. This is a good strategy for this typical pioneer species. Beechnuts and acorns always fall under their mother trees, so the seedlings grow in a predictable, pleasant forest microclimate, but little bird cherries can end up anywhere. Birds that gobble the tart fruit make random deposits of seeds wrapped in their own little packages of fertilizer. If a package lands out in the open in a year when the weather is extreme, temperatures will be hotter and water supplies scarcer than in the cool, damp shadows of a mature forest. Then it’s advantageous if at least some of the stowaways wait a few years before waking to their new life.
And after they wake? What are the youngsters’ chances of growing up and producing another generation? That’s a relatively easy calculation to make. Statistically speaking, each tree raises exactly one adult offspring to take its place. For those that don’t make it, seeds may germinate and young seedlings may vegetate for a few years, or even for a few decades, in the shadows, but sooner or later, they run out of steam. They are not alone. Dozens of offspring from other years also stand at their mothers’ feet, and by and by, most give up and return to humus. Eventually, a few of the lucky ones that have been carried to open spaces on the forest floor by the wind or by animals get a good start in life and grow to adulthood.
Back to the odds. 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, but we’ll put that aside for the moment). 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. All the other hopeful embryos are either eaten by animals or broken down into humus by fungi or bacteria.
Using the same formula, let’s calculate the odds that await tree offspring in the least favorable circumstances. Let’s consider the poplar. The mother trees each produce up to 54 million seeds—every year.17 How their little ones would love to change places with the beech tree youngsters. For until the old ones hand over the reins to the next generation, they produce more than a billion seeds. Wrapped in their fluffy packaging, these seeds strike out via airmail in search of new pastures. But even for them, based purely on statistics, there can be only one winner.
— SLOWLY DOES IT —
FOR A LONG time, even I did not know how slowly trees grew. In the forest I manage, there are beeches that are between 3 and 7 feet tall. In the past, I would have estimated them to be ten years old at most. But when I began to investigate mysteries outside the realm of commercial forestry, I took a closer look.
An easy way to estimate the age of a young beech tree is to count the small nodes on its branches. These nodes are tiny swellings that look like a bunch of fine wrinkles. They form every year underneath the buds, and when these grow the following spring and the branch gets longer, the nodes remain behind. Every year, the same thing happens, and so the number of nodes corresponds with the age of the tree. When the branch gets thicker than about a tenth of an inch, the nodes disappear into the expanding bark.
When I examined one of my young beech trees, it turned out that a single 8-inch-long twig already had twenty-five of these swellings. I could find no other indicator of the tree’s age on its tiny trunk, which was no more than a third of an inch in diameter, but when I carefully extrapolated the age of the tree from the age of the branch, I discovered that the tree must have been at least eighty years old, maybe more. That seemed unbelievable at the time, until I continued my investigations into ancient forests. Now I know: it is absolutely normal.
Young trees are so keen on growing quickly that it would be no problem at all for them to grow about 18 inches taller per season. Unfortunately for them, their own mothers do not approve of rapid growth. They shade their offspring with their enormous crowns, and the crowns of all the mature trees close up to form a thick canopy over the forest floor. This canopy lets only 3 percent of available sunlight reach the ground and, therefore, their children’s leaves. Three percent—that’s practically nothing. With that amount of sunlight, a tree can photosynthesize just enough to keep its own body from dying. There’s nothing left to fuel a decent drive upward or even a thicker trunk. And rebellion against this strict upbringing is impossible, because there’s no energy to sustain it. Upbringing? you ask. Yes, I am indeed talking about a pedagogical method that ensures the well-being of the little ones. And I didn’t just come up with the term out of the blue—it’s been used by generations of foresters to refer to this kind of behavior.
The method used in this upbringing is light deprivation. But what purpose does this restriction serve? Don’t parents want their offspring to become independent as quickly as possible? Trees, at least, would answer this question with a resounding no, and recent science backs them up. 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.
Under natural conditions, trees that age are no thicker than a pencil and no taller than a person. Thanks to slow growth, their inner woody cells are tiny and contain almost no air. That makes the trees flexible and resistant to breaking in storms. Even more important is their heightened resistance to fungi, which have difficulty spreading through the tough little trunks. Injuries are no big deal for such trees, either, because they can easily compartmentalize the wounds—that is to say, close them up by growing bark over them—before any decay occurs.
A good upbringing is necessary for a long life, but sometimes the patience of the young trees is sorely tested. As I mentioned in chapter 5, “Tree Lottery,” acorns and beechnuts fall at the feet of large “mother trees.” Dr. Suzanne Simard, who helped discover maternal instincts in trees, describes mother trees as dominant trees widely linked to other trees in the forest through their fungal–root connections. These trees pass their legacy on to the next generation and exert their influence in the upbringing of the youngsters.18 “My” small beech trees, which have by now been waiting for at least eighty years, are standing under mother trees that are about two hundred years old—the equivalent of forty-year-olds in human terms. The stunted trees can probably expect another two hundred years of twiddling their thumbs before it is finally their turn. The wait time is, however, made bearable. Their mothers are in contact with them through their root systems, and they pass along sugar and other nutrients. You might even say they are nursing their babies.
You can observe for yourself whether young trees are playing the waiting game or putting on a growth spurt. Take a look at the branches of a small silver fir or beech. If the tree is obviously wider than it is tall, then the young tree is in waiting mode. The light it is getting is not sufficient to create the energy it needs to grow a taller trunk, and therefore, the youngster is trying to catch the few leftover rays of sunlight as efficiently as possible. To do this, it lengthens its branches out sideways and grows special ultra-sensitive leaves or needles that are adapted to shade. Often you can’t even make out the main shoot on trees like these; they resemble flat-topped bonsai.
One day, it’s finally time. The mother tree reaches the end of her life or becomes ill. The showdown might take place during a summer storm. As torrents of rain pour down, the brittle trunk can no longer support the weight of several tons of crown, and it shatters. As the tree hits the ground, it snaps a couple of waiting seedlings. The gap that has opened up in the canopy gives the remaining members of the kindergarten the green light, and they can begin photosynthesizing to their hearts’ content. Now their metabolism gets into gear, and the trees grow sturdier leaves and needles that can withstand and metabolize bright light.
This stage lasts between one and three years. Once it is over, it’s time to get a move on. All the youngsters want to grow now, and only those that go for it and grow straight as an arrow toward the sky are still in the race. The cards are stacked against those free spirits who think they can meander right or left as the mood takes them and dawdle before they stretch upward. Overtaken by their comrades, they find themselves in the shadows once again. The difference is that it is even darker under the leaves of their cohort that has pulled ahead than it was under their mothers. The teenagers use up the greater part of what weak light remains; the stragglers give up the ghost and become humus once again.
Further dangers are lurking on the way to the top. As soon as the bright sunlight increases the rate of photosynthesis and stimulates growth, the buds of those who have shot up receive more sugar. While they were waiting in the wings, their buds were tough, bitter pills, but now they are sweet, tasty treats—at least as far as the deer are concerned. Because of this, some of the young trees fall victim to these herbivores, ensuring the deers’ survival over the coming winter, thanks to the additional calories. But as the crowd of trees is enormous, there are still plenty that keep on growing.
Wherever there is suddenly more light, flowering plants also try their luck, including honeysuckle. Using its tendrils, it makes its way up around the little trunks, always twining in a clockwise direction. By coiling itself around the trunk, it can keep up with the growth of the young tree and its flowers can bask in the sun. However, as the years progress, the coiling vine cuts into the expanding bark and slowly strangles the little tree. Now it is a question of timing: Will the canopy formed by the old trees close soon and plunge the little tree into darkness once again? If it does, the honeysuckle will wither away, leaving only scars. But if there is plenty of light for a while longer, perhaps because the dying mother tree was particularly large and so left a correspondingly large gap, then the young tree in the honeysuckle’s embrace can be smothered. Its untimely end, though unfortunate for the tree, brings us some pleasure when we craft its bizarrely twisted wood into walking sticks.
The young trees that overcome all obstacles and continue to grow beautifully tall and slender will, however, have their patience tested yet again before another twenty years have passed. For this is how long it takes for the dead mother’s neighbors to grow their branches out into the gap she left when she fell. They take advantage of the opportunity to build out their crowns and gain a little additional space for photosynthesis in their old age. Once the upper story grows over, it is dark once again down below. The young beeches, firs, and pines that have put the first half of their journey behind them must now wait once again until one of these large neighbors throws in the towel. That can take many decades, but even though it takes time, in this particular arena, the die has already been cast. All the trees that have made it as far as the middle story are no longer threatened by competitors. They are now the crown princes and princesses who, at the next opportunity, will finally be allowed to grow up.
— FOREST ETIQUETTE —
IN THE FOREST, there are unwritten guidelines for tree etiquette. These guidelines lay down the proper appearance for upright members of ancient forests and acceptable forms of behavior. This is what a mature, well-behaved deciduous tree looks like. It has a ramrod-straight trunk with a regular, orderly arrangement of wood fibers. The roots stretch out evenly in all directions and reach down into the earth under the tree. In its youth, the tree had narrow branches extending sideways from its trunk. They died back a long time ago, and the tree sealed them off with fresh bark and new wood so that what you see now is a long, smooth column. Only when you get to the top do you see a symmetrical crown formed of strong branches angling upward like arms raised to heaven. An ideally formed tree such as this can grow to be very old. Similar rules hold for conifers, except that the topmost branches should be horizontal or bent slightly downward.
And what is the point of all this? Deep down inside, do trees secretly appreciate beauty? Unfortunately, I cannot say, but what I can tell you is that there is a good reason for this ideal appearance: stability. The large crowns of mature trees are exposed to turbulent winds, torrential rains, and heavy loads of snow. The tree must cushion the impact of these forces, which travel down the trunk to the roots. The roots must hold out under the onslaught so that the tree doesn’t topple over. To avoid this, the roots cling to the earth and to rocks. The redirected power of a windstorm can tear at the base of the trunk with a force equivalent to a weight of 220 tons.19 If there is a weak spot anywhere in the tree, it will crack. In the worst-case scenario, the trunk breaks off completely and the whole crown tumbles down. Evenly formed trees absorb the shock of buffeting forces, using their shape to direct and divide these forces evenly throughout their structure.
Trees that don’t follow the etiquette manual find themselves in trouble. For example, if a trunk is curved, it has difficulties even when it is just standing there. The enormous weight of the crown is not evenly divided over the diameter of the trunk but weighs more heavily on the wood on one side. To prevent the trunk from giving way, the tree must reinforce the wood in this area. This reinforcement shows up as particularly dark areas in the growth rings, which indicate places where the tree has laid down less air and more wood.
Forked trees are even more precarious. In forked trees, at a certain point, two main shoots form, and they continue to grow alongside each other. Each side of the fork creates its own crown, so in a heavy wind, both sides sway back and forth in different directions, putting a great strain on the trunk where the two parted company. If this transition point is in the shape of a tuning fork or U, then usually nothing happens. Woe betide the tree, however, that has a fork in the shape of a V, with the two sides joining at a narrow angle. The fork always breaks at its narrowest point, where the two sides diverge. Because the break causes the tree distress, it tries to form thick bulges of wood to prevent further damage. Usually, however, this tactic doesn’t work, and bacteria-blackened liquid constantly bleeds from the wound. To make matters worse, the place where one side of the fork broke off gathers water, which penetrates the tear in the bark and causes rot. Sooner or later, a forked tree usually breaks apart, leaving the more stable half standing. This half-tree survives for a few more decades but not much longer. The large gaping wound never heals, and fungi begin to devour the tree slowly from the inside out.
Some trees appear to have chosen the banana as a model for their trunks. The lower part sticks out at an angle, and then the trunk seems to have taken a while to orient itself vertically. Trees like this are completely ignoring the manual, but they don’t seem to be alone. Often whole sections of a forest are shaped this way. Are the rules of Nature being set aside here? Not at all. It is Nature herself that forces the trees to adopt such growth patterns.
Take, for example, trees on high mountain slopes just below the tree line. In winter, the snow frequently lies many feet deep, and it is often on the move. And not just in avalanches. Even when it is at rest, snow is sliding at a glacial pace down toward the valleys, even though we can’t detect the movement with our eyes. And while the snow is doing that, it’s bending trees—the young ones, at least. That’s not the end of the world for the smallest among them. They just spring back up again without any ill effects after the snow has melted. However, the trunks of half-grown trees already 10 feet or so tall are damaged. In the most severe cases, the trunk breaks. If it doesn’t break, it remains at an angle. From this position, the tree tries to get back to vertical. And because a tree grows only from its tip, the lower part remains crooked. The following winter, the tree is once more pressed out of alignment. Next year’s growth points vertically once again. If this game continues for a number of years, gradually you get a tree that is bent into the shape of a saber, or curved sword. It is only with increasing age that the trunk thickens and becomes solid enough that a normal amount of snow can no longer wreak havoc. The lower “saber” keeps its shape, while the upper part of the trunk, left undisturbed, is nice and straight like a normal tree.
Something similar can happen to trees even in the absence of snow, though also on hillsides. In these cases, it is sometimes the ground itself that is sliding extremely slowly down to the valley over the course of many years, often at a rate of no more than an inch or two a year. When this happens, the trees slip slowly along with the ground and tilt over while they continue to grow vertically. You can see extreme cases of this in Alaska and Siberia, where climate change is causing the permafrost to thaw. Trees are losing their footing and being thrown completely off balance in the mushy subsoil. And because every individual tree is tipped in a different direction, the forest looks like a group of drunks staggering around. Accordingly, scientists call these “drunken forests.”
At the edge of the forest, the rules for straight trunk growth are not quite so strict. Here, light comes in from the side, from a meadow or a lake—places where trees just don’t grow. Smaller trees can get out from under larger ones by growing in the direction of the open area. Deciduous trees, in particular, take advantage of this. If they allow their main shoot to grow almost horizontally, they can increase the size of their crowns by up to 30 feet, thanks to their radically angled trunks. Of course, the trees then risk snapping off, especially after a heavy snowfall, when the laws of physics come into play and the lever principle exacts its tribute. Still, a shorter life-span with enough light for procreation is better than no life at all.
Whereas most deciduous trees leap at chances to grab more light, most conifers stubbornly refuse. They vow to grow straight or not at all. And off they go, always opposing gravity, directly up in a vertical direction so that the trunk is perfectly formed and stable. Lateral branches encountering light at the forest’s edge are permitted to put on noticeable girth, but that’s it. Only the pine has the cheek to greedily redirect its crown toward the light. No wonder the pine is the conifer with the highest rate of breakage because of snow.
— TREE SCHOOL —
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.20 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.
In winter, there’s more than enough rain, and the tree is not consuming water, because almost all plants take a break from growing at that time of year. Together with belowground 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.
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 keeps reopening. 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!
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.
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. However, every couple of years, a group of forestry workers or a harvesting machine moves in to harvest 10 percent of the trees in commercial forests in Central Europe. And 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 so it takes three to ten 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 ten 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.
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, once again, along comes the Australian scientist Dr. 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.21
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.22 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 cords 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.
— UNITED WE STAND, —
DIVIDED WE FALL
TREES ARE VERY social beings, and they help each other out. But that is not sufficient for successful survival in the forest ecosystem. Every species of tree tries to procure more space for itself, to optimize its performance, and, in this way, to crowd out other species. After the fight for light, it is the fight for water that finally decides who wins. Tree roots are very good at tapping into damp ground and growing fine hairs to increase their surface area so that they can suck up as much water as possible. Under normal circumstances, that is sufficient, but more is always better. And that is why, for millions of years, trees have paired up with fungi.
Fungi are amazing. They don’t really conform to the one-size-fits-all system we use to classify living organisms as either animals or plants. By definition, plants create their own food out of inanimate material, and therefore, they can survive completely independently. It’s no wonder that green vegetation must sprout on barren, empty ground before animals can move in, for animals can survive only if they eat other living things. Incidentally, neither grass nor young trees like it very much when cattle or deer munch on them. Whether it’s a wolf ripping apart a wild boar or a deer eating an oak seedling, in both cases there is pain and death. Fungi are in between animals and plants. Their cell walls are made of chitin—a substance never found in plants—which makes them more like insects. In addition, they cannot photosynthesize and depend on organic connections with other living beings they can feed on.
Over decades, a fungus’s underground cottony web, known as mycelium, expands. There is a honey fungus in Switzerland that covers almost 120 acres and is about a thousand years old.23 Another in Oregon is estimated to be 2,400 years old, extends for 2,000 acres, and weighs 660 tons.24 That makes fungi the largest known living organisms in the world. The two aforementioned giants are not tree friendly; they kill them as they prowl the forest in search of edible tissue. So let’s take a look instead at amicable teamwork between fungi and trees. With the help of mycelium of an appropriate species for each tree—for instance, the oak milkcap and the oak—a tree can greatly increase its functional root surface so that it can suck up considerably more water and nutrients. You find twice the amount of life-giving nitrogen and phosphorus in plants that cooperate with fungal partners than in plants that tap the soil with their roots alone.
To enter into a partnership with one of the many thousands of kinds of fungi, a tree must be very open—literally—because the fungal threads grow into its soft root hairs. There’s no research into whether this is painful or not, but as it is something the tree wants, I imagine it gives rise to positive feelings. However the tree feels, from then on, the two partners work together. 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 (see chapter 3, “Social Security”) and even information—such as an impending insect attack.
This connection makes fungi something like the forest Internet. And such a connection has its price. As we know, these organisms—more like animals in many ways—depend on other species for food. Without a supply of food, they would, quite simply, starve. Therefore, they demand payment in the form of sugar and other carbohydrates, which their partner tree has to deliver. And fungi are not exactly dainty in their requirements. They demand up to a third of the tree’s total food production in return for their services.25 It makes sense, in a situation where you are so dependent on another species, to leave nothing to chance. And so the delicate fibers begin to manipulate the root tips they envelop. First, the fungi listen in on what the tree has to say through its underground structures. Depending on whether that information is useful for them, the fungi begin to produce plant hormones that direct the tree’s cell growth to their advantage.26
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. These diverted pollutants turn up every fall in the pretty fruiting bodies we bring home in the form of porcini, cèpe, or bolete mushrooms. No wonder radioactive cesium, which was found in soil even before the nuclear reactor disaster in Chernobyl in 1986, is mostly found in mushrooms.
Medical services are also part of the package. The delicate fungal fibers ward off all intruders, including attacks by bacteria or destructive fellow fungi. Together with their trees, fungi can live to be many hundreds of years old, as long as they are healthy. But if conditions in their environment change, for instance, as a result of air pollution, then they breathe their last. Their tree partner, however, does not mourn for long. It wastes no time hooking up with the next species that settles in at its feet. Every tree has multiple options for fungi, and it is only when the last of these passes away that it is really in trouble.
Fungi are much more sensitive. Many species seek out trees that suit them, and once they have reserved them for themselves, they are joined to them for better or for worse. Species that like only birches or larches, for instance, are called “host specific.” Others, such as chanterelles, get along with many different trees: oaks, birches, and spruce. What is important is whether there is still a bit of room underground. And competition is fierce. In oak forests alone, more than a hundred different species of fungi may be present in different parts of the roots of the same tree. From the oaks’ point of view, this is a very practical arrangement. If one fungus drops out because environmental conditions change, the next suitor is already at the door.
Researchers have discovered that fungi also hedge their bets. Dr. Suzanne Simard discovered that their networks are connected not only to a specific tree species but also to trees of different species.27 Simard injected into a birch tree radioactive carbon that moved through the soil and into the fungal network of a neighboring Douglas fir. Although many species of tree fight each other mercilessly above ground and even try to crowd out each other’s root systems, the fungi that populate them seem to be intent on compromise. Whether they actually want to support foreign host trees or only fellow fungi in need of help (which these fungi then pass on to their trees) is as yet unclear.
I suspect fungi are a little more forward “thinking” than their larger partners. Among trees, each species fights other species. Let’s assume the beeches native to Central Europe could emerge victorious in most forests there. Would this really be an advantage? What would happen if a new pathogen came along that infected most of the beeches and killed them? In that case, wouldn’t it be more advantageous if there were a certain number of other species around—oaks, maples, ashes, or firs—that would continue to grow and provide the shade needed for a new generation of young beeches to sprout and grow up? Diversity provides security for ancient forests. Because fungi are also very dependent on stable conditions, they support other species underground and protect them from complete collapse to ensure that one species of tree doesn’t manage to dominate.
If things become dire for the fungi and their trees despite all this support, then the fungi can take radical action, as in the case of the pine and its partner Laccaria bicolor, or the bicolored deceiver. When there is a lack of nitrogen, the latter releases a deadly toxin into the soil, which causes minute organisms such as springtails to die and release the nitrogen tied up in their bodies, forcing them to become fertilizer for both the trees and the fungi.28
I have introduced you to the most important tree helpers; however, there are many more. Consider the woodpeckers. I wouldn’t call them real helpers, but they are of at least some benefit to trees. When bark beetles infest spruce, for example, things get dicey. The tiny insects multiply so rapidly they can kill a tree very quickly by consuming its life-giving cambium layer. If a great spotted woodpecker gets wind of this, it’s on the spot right away. Like an oxpecker on a rhinoceros, it climbs up and down the trunk looking for the voracious, fat white larvae. It pecks these out (not thinking particularly of the tree), sending chunks of bark flying. Sometimes this can save the spruce from further damage. Even if the tree doesn’t come through this procedure alive, its fellow trees are still protected because now there won’t be any adult beetles hatching and flying around. The woodpecker is not in the slightest bit interested in the well-being of the tree, and you can see this particularly clearly in its nesting cavities. It often makes these in healthy trees, severely wounding them as it hacks away. Although the woodpecker frees many trees of pests—for instance, oaks from woodboring beetles—it is more a side effect of its behavior than its intent.
Woodboring beetles can be a threat to thirsty trees in dry years, because the trees are in no position to defend themselves from their attackers. Salvation can come in the form of the black-headed cardinal beetle. In its adult form, it is harmless, feeding on aphid honeydew and plant juices. Its offspring, however, need flesh, and they get this in the form of beetle larvae that live under the bark of deciduous trees. So some oaks have cardinal beetles to thank for their survival. And things can get dire for the beetles as well: once all the children of other species of beetles have been eaten, the larvae turn on their own kind.
— THE MYSTERIES OF —
HOW DOES WATER make its way up from the soil into the tree’s leaves? For me, the way this question is answered sums up our current approach to what we know about the forest. For water transport is a relatively simple phenomenon to research—simpler at any rate than investigating whether trees feel pain or how they communicate with one another—and because it appears to be so uninteresting and obvious, university professors have been offering simplistic explanations for decades. This is one reason why I always have fun discussing this topic with students. Here are the accepted answers: capillary action and transpiration.
You can study capillary action every morning at breakfast. Capillary action is what makes the surface of your coffee stand a few fractions of an inch higher than the edge of your cup. Without this force, the surface of the liquid would be completely flat. The narrower the vessel, the higher the liquid can rise against gravity. And the vessels that transport water in deciduous trees are very narrow indeed: they measure barely 0.02 inches across. Conifers restrict the diameter of their vessels even more, to 0.0008 inches. Narrow vessels, however, are not enough to explain how water reaches the crown of trees that are more than 300 feet tall. In even the narrowest of vessels, there is only enough force to account for a rise of 3 feet at most.29
Ah, but we have another candidate: transpiration. In the warmer part of the year, leaves and needles transpire by steadily breathing out water vapor. In the case of a mature beech, the tree exhales hundreds of gallons of water a day. This 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. Bonding forces cause the water molecules to adhere to each other, and because they are strung together like links in a chain, as soon as space becomes available in the leaf thanks to transpiration, the bonded molecules pull each other a little higher up the trunk.
And because even this is not enough, osmosis also comes into play. When the concentration of sugar in one cell is higher than in the neighboring cell, water flows through the cell walls into the more sugary solution until both cells contain the same percentage of water. And when that happens from cell to cell up into the crown, water makes its way up to the top of the tree.
Hmm. When you measure water pressure in trees, you find it is highest shortly before the leaves open up in the spring. At this time of year, water shoots up the trunk with such force that if you place a stethoscope against the tree, you can actually hear it. In the northeastern U.S. and Canada, people make use of this phenomenon to harvest syrup from sugar maples, which are often tapped just as the snow is melting. This is the only time of the year when the coveted sap can be harvested. This early in the year, there are no leaves on deciduous trees, which means there can be no transpiration. And capillary action can be only a partial contributor because the aforementioned rise of 3 feet is hardly worth mentioning. Yet at precisely this time, the trunk is full to bursting. So that leaves us with osmosis, but this seems equally unlikely to me. After all, osmosis works only in the roots and leaves, not in the trunk, which consists not of cells attached one to the other but of long, continuous tubes for transporting water.
So where does that leave us? We don’t know. But recent research has discovered something that at least calls into question the effects of transpiration and the forces of cohesion. Scientists from three institutions (the University of Bern; the Swiss Federal Institute for Forest, Snow, and Landscape Research; and the Swiss Federal Institute of Technology in Zurich) listened more closely—literally. They registered a soft murmur in the trees. Above all, at night. At this time of day, most of the water is stored in the trunk, as the crown takes a break from photosynthesis and hardly transpires at all. The trees pump themselves so full of water their trunks sometimes increase in diameter. The water is held almost completely immobile in the inner transportation tubes. Nothing flows. So where are the noises coming from? The researchers think they are coming from tiny bubbles of carbon dioxide in the narrow water-filled tubes.30 Bubbles in the pipes? That means the supposedly continuous column of water is interrupted thousands of times. And if that is the case, transpiration, cohesion, and capillary action contribute very little to water transport.
So many questions remain unanswered. Perhaps we are poorer for having lost a possible explanation or richer for having gained a mystery. But aren’t both possibilities equally intriguing?
— TREES AGING GRACEFULLY —
BEFORE I TALK about age, I would like to take a detour into the subject of skin. Trees and skin? First let’s approach the subject from the human point of view. Our skin is a barrier that protects our innermost parts from the outer world. It holds in fluids. It stops our insides from falling out. And all the while it releases and absorbs gas and moisture. In addition, it blocks pathogens that would just love to spread through our circulatory system. Aside from that, it is sensitive to contact, which is either pleasant and gives rise to the desire for more, or painful and elicits a defensive response.
Annoyingly, this complicated structure doesn’t stay the same forever but gradually sags as we age. Folds and wrinkles appear so that our contemporaries can playfully guess how old we are, give or take a few years. The necessary process of regeneration is not exactly pleasant, either, when looked at close up. Each of us sheds about 0.05 ounces of skin cells a day, which adds up to about a pound a year. The numbers are impressive: 10 billion particles flake off us every day.31 That doesn’t sound very attractive, but sloughing off dead skin is necessary to keep our outer organ in good condition. And in childhood we need to shed skin so that we can grow. Without the ability to renew and expand the covering Nature gives us, sooner or later, we would burst.
And how does this relate to trees? It’s just the same with them. The biggest difference is simply the vocabulary we use. The skin of Beeches, Oaks, Spruce & Co. is called bark. It fulfills exactly the same function and protects trees’ sensitive inner organs from an aggressive outer world. Without bark, a tree would dry out. And right after the loss of fluid, fungi—which have no chance of survival in healthy, moist wood—would go to town and start breaking everything down. Insects also need lower moisture levels, and if the bark is intact, they are doomed. A tree contains almost as much liquid inside it as we do, and so it’s unappealing to pests because they would, quite simply, suffocate.
A break in its bark, then, 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. Every year, a tree in its prime adds between 0.5 to 1 inch to its girth. Surely this would make the bark split? It should. To make sure that doesn’t happen, the giants constantly renew their skin while shedding enormous quantities of skin cells. In keeping with trees’ size in comparison to ours, these flakes are correspondingly larger and measure up to 8 inches across. If you take a look around on the ground under trunks in windy, rainy weather, you will see the remains lying there. The red bark of pines is particularly easy to spot.
But not every tree sheds in the same way. There are species that shed constantly (fastidious people would recommend an anti-dandruff shampoo for such cases). Then there are others that flake with restraint. You can see who’s doing what when you look at the exterior of a tree. What you see is the outer layer of bark, which is dead and forms an impervious exterior shell. This outer layer of bark also happens to be a good way of telling different species apart. This works for older trees, anyway, for the distinguishing characteristics have to do with the shapes of the cracks or, you could say, with the folds and wrinkles in a tree’s skin. In young trees of all species, the outer bark is as smooth as a baby’s bottom. As trees age, wrinkles gradually appear (beginning from below), and they steadily deepen as the years progress. Just how quickly this process plays out depends on the species. Pines, oaks, birches, and Douglas firs start early, whereas beeches and silver firs stay smooth for a long time. It all depends on the speed of shedding.
For beeches, whose silver-gray bark remains smooth until they are two hundred years old, the rate of renewal is very high. Because of this, their skin remains thin and fits their age—that is to say, their girth—exactly and, therefore, doesn’t need to crack in order to expand. It’s the same for silver firs. Pines and the like, however, drag their feet when it comes to external makeovers. For some reason, they don’t like to be parted from their baggage, perhaps because of the additional security a thick skin provides. Whatever the reason, they shed so slowly that they build up really thick outer bark and their exterior layers can be decades old. This means the outer layers originated at a time when the trees were still young and slim, and as the trees age and increase in girth, the outer layers crack way down into the youngest layer of bark that—like the bark of the beeches—fits the girth of the tree as it is now. So, the deeper the cracks, the more reluctant the tree is to shed its bark, and this behavior increases markedly with age.
The same fate catches up with beeches when they pass middle age. This is when their bark starts to get wrinkles, start…