Top Quotes: “The Hidden Life of Trees: What They Feel, How They Communicate” — Peter Wohlleben
Tree Families and Friendships
“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 leisurely pace. The electrical impulses that pass through the roots of trees, for example, move at the slow rate of 1/3 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’re the parents of the trees that make up the forest of today.”
“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’ve 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 wilderness back into their grain and potatoes,’ he advocates, ‘so that they’ll be more talkative in the future.’”
“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.”
“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.
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 isn’t a forest. On its own, a tree cannot establish a consistent local climate. It’s at the mercy of wind and weather. But together, many trees create an ecosystem that moderates extremes of hot 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’s why even sick individuals are supported and nourished until they recover.”
“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 hundred years (which isn’t very long for a tree). Only a few individuals are kept alive over the centuries. 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’s rather the degree of connection — or maybe 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 tree 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 well established in undisturbed forests. It could well be that all trees do this and not just beeches. I myself have observed oak, fir, and spruce stumps that were still alive long after all the trees had been cut down. Planted forests, which is what most of the coniferous forests in C. Europe are, behave more like street kids. Because their roots are irreparably damaged when they’re 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’re only about 100 years old.”
Defense and Communication
“Trees have a completely different way of communicating. They use scent.
Scent as a means of communication? The concept isn’t 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’re strongly attracted 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 want 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, 4 decades ago, scientists noticed something on the African savannah. The giraffes there were feeding on umbrella thorn acacias, and 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 trees close by? No, for the time being, they walked right by a few trees and resumed their meal only when they’d moved about 100 yards away.
The reason for this behavior is astonishing. The acacia trees that were being eaten gave off a warning gas 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 prep 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.
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’s hurt. However, the signal isn’t transmitted in milliseconds, as human signals are; instead, the plant signal travels at the slow speed of a third of an inch per second. Accordingly, it takes an hour or so before defensive compounds reach the leaves to spoil the pest’s meal.”
“When it comes to some species of insects, trees can accurately identify which bad guys they’re 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. 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.”
“One teaspoon of forest soil contains many miles of these ‘hyphae.’ 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 the term the ‘wood wide web’ for this. What and how much info is exchanged are subjects we’ve 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. And the fungi are pursuing their own agendas and appear to be very much in favor of conciliation and equitable distribution of info and resources.
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.”
“In the symbiotic community of the forest, not only trees but also shrubs and grasses — and possibly all plant species — exchange info this way. However, when we step into the 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’re deaf and dumb — and therefore they’re easy prey for insect pests. That’s one reason why modern agriculture uses so many pesticides.
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’m referring to the pleasantly-perfumed invites sent out by tree blossoms. Blossoms don’t 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.”
“Dr. Monica Gagliano has, quite literally, had her ear to the ground. It’s not practical to study trees in the lab; therefore, researchers substitute grain seedlings because they’re 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. But the noises discovered in the lab caused the researchers to pay attention. For the roots of the 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.’”
“Students at RWTH Aachen discovered something amazing about photosynthesis in undisturbed beech forests. Apparently, the trees synchronize their performance so that they’re all equally successful. And that’s 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’d 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’re thick or thin, all members of the same species are using light to product 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.”
Tree Love
“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 2? 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 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% 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 birch and oak seedlings sprout. And that’s why the trees agree in advance. If they don’t bloom every year, then the herbivores can’t 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 won’t survive. When the beeches or the oaks finally all bloom at the same time and set fruit, then it’s not possible for the few herbivores left to demolish everything, so there are always enough undiscovered seeds left over to sprout.”
“Trees have survived until today only because there’s 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 that’s precisely what 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 isn’t 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’re 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 can’t 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’s 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.”
“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 aren’t true forest trees. They colonize pioneer sites, areas that aren’t 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 tree. If it was the other way around, there’d be no fertilization. How does a tree manage 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.”
“Completely isolated strands 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.”
“Every 5 years, a beech produces at least 30,000 beechnuts (thanks to climate change, it now does this as often as every 2–3 years). It’s sexually mature at about 80–150 years old, depending on how much light it gets where it’s growing. Assuming it grows to be 400 years old, it can fruit at least 60 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’s a high rate of success. All the other hopeful embryos are either eaten by animals or broken down into humus by fungi or bacteria.”
“Gagliano studies mimosas, also called ‘sensitive plants.’ Mimosas are tropical creeping herbs. They make particularly good research subjects, because it’s easy to get them a bit riled up and they’re easier to study in the lab than trees are. When they’re 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 applytheir lesson weeks later, even without any further tests.
It’s a shame you can’t transport entire beeches or oaks into the lab to find out more about learning. But, at least as far as water is concerned, there’s research that reveals more than just behavioral changes: when trees are really thirsty, they begin to scream. You won’t be able to hear them, because this all takes place at ultrasonic levels. Scientists at the Swiss Institute for Forest 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. 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’d 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, 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.”
Fungi
“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 carbs, which their partner tree has to deliver. And fungi aren’t exactly dainty in their requirements. They demand up to a third of the tree’s total food production in return for their services. It makes sense, in a situation where you’re 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 info is useful for them, the fungi began to produce plant hormones that direct the tree’s cell growth to their advantage.
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.”
“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’re 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, doesn’t 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’s only when the last of these passes away that it’s really in trouble.
Fungi are much more sensitive. Many species seek out trees that suit them, and once they’ve reserved them for themselves, they’re joined to them for better or worse. Species that like only birches of larches, for instances, are called ‘host specific.’ Others, such as chanterelles, get along with many different trees. What’s important is whether there’s still a bit of room underground. And competition is fierce. In oak forests alone, 100+ different species of fungi may be present in different parts of the roots at the same time. From the oaks’ POV, this is a very practical arrangement. If 1 fungus drops out because environmental conditions change, the next suitor is already at the door.”
Aging
“More light, more sun, more UV radiation. The last causes changes in people’s skin, and it appears the same thing happens with trees. Intriguingly, the outer bark on the sunny side of trees is harder, and this means it’s more inflexible and more inclined to crack.”
“Up top, my hair is thinning. And it’s the same with the highest branches up in a tree’s crown. After a specific time — 100–300 years, depending on the species — the annual new growth gets shorter and shorter. In deciduous trees, the successive growth of such short shoots leads to curved, claw-like branches that resemble fingers plagued by arthritis. In conifers, the ramrod-straight trunks end up in topmost shoots or leaders that are gradually reduced to nothing. Whereas spruce in this situation stop growing altogether, silver firs continue to grow — but out instead of up, so they look as though a large bird has built its nest in their upper branches. Pines redirect their growth even earlier so that by the time they reach old age, the whole crown is wide with no identifiable leader.
In any event, every tree gradually stops growing taller. Its roots and vascular system cannot pump water and nutrients any higher because this exertion would be too much for it. Instead, the tree just gets wide (another parallel to many people of advancing years…).”
“The researchers looked at about 700,000 trees on every continent. The surprising result: the older the tree, the more quickly it grows. Trees with trunks 3 feet in diameter generated 3x as much biomass as trees that were only half as wide. So in the case of trees, being old doesn’t mean being weak, bowed, and fragile. Quite the opposite, it means being full of energy and highly productive. This means elders are markedly more productive than young whippersnappers, and when it comes to climate change, they’re important allies for human beings.”
Soil
“Frantisek Baluska is of the opinion that brain-like structures can be found at root tips. In addition to signaling pathways, there are also numerous systems and molecules similar to those found in animals. When a root feels its way forward in the ground, it’s aware of stimuli. The researchers measured electrical signals that led to changes in behavior after they were processed in a ‘transition zone.’ If the root encounters toxic substances, impenetrable stones, or saturated soil, it analyzes the situation and transmits the necessary adjustments to the growing tip. The root tip changes direction as a result of this communication and steers the growing root around the critical areas.”
“There are more life forms in a handful of forest soil than there are people on the planet. A mere teaspoonful contains many miles of fungal filaments. All these work the soil, transform it, and make it so valuable for the trees.
Without soil there would be no forests, because trees must have somewhere to put down roots. Naked rock doesn’t work, and loosely packed stones, even though they offer roots some support, cannot store sufficient quantities of water or food. Geological processes — such as those active in the ice ages with their sub-zero temperatures — cracked open rocks, and glaciers ground the fragments down into sand and dust until, finally, what was left was a loosely packed substrate. After the ice retreated, water washed this material into depressions and valleys, or storms carried it away and laid it down in layers many tens of feet thick.
Life came along later in the form of bacteria, fungi, and plants, and all of which decomposed after death to form humus. Over the course of thousands of years, trees moved into this soil — which only at this stage can be recognized as such — and their presence made it even more precious. Trees stabilized the soil with their roots and protected it against rains and storms. Erosion became a thing of the past, and instead, the layers of humus grew deeper, creating the early stages of bituminous coal.
Erosion is one of the forest’s most dangerous natural enemies. Soil is lost whenever there are more extreme weather events, usually following particularly heavy downpours. If the forest soil cannot absorb all the water right away, the excess runs over the soil surface, taking small particles of soil with it.”
“The forest can lose as much as 2,900 tons per square mile per year. The same area can replace only 290 tons annually through the weathering of stones underground, leading to a huge annual loss of soil. Sooner or later, only the stones remain. Today, you can find many such depleted areas in forests growing in exhausted soils that were cultivated centuries ago. In contrast, forests left undisturbed lose only 1–14 tons of soil per square mile per year. In intact forests, the soil under the tree becomes deeper and richer over time so that growing conditions for trees constantly improve.”
“It’s true that some of carbon dioxide does indeed return to the atmosphere after a tree’s death, but most of it remains locked in the ecosystem forever. The crumbling trunk is gradually gnawed and munched into smaller and smaller pieces and worked, by fractions of inches, more deeply into the soil. The rain takes care of whatever is left, as it flushes organic remnants down into the soil. The farther underground, the cooler it is. And as the temperature falls, life slows down, until it comes almost to a standstill. And so it is that carbon dioxide finds its final resting place in the form of humus, which continues to become more concentrated as it ages. In the far distant future, it might even become bituminous or anthracite coal.
Today’s deposits of these fossil fuels come from trees that died about 300 million years ago.”
“Today, hardly any coal is being formed because forests are constantly being cleared. As a result, warming rays of sunlight reach the ground and help the species living there kick into high gear. This means they consume humus layers even deep down into the soil, releasing the carbon they contain into the atmosphere as gas. The total quantity of climate-changing gases that escapes is roughly equivalent to the amount of timber that has been felled.”
Predators
“It takes a beaver 1 night to bring down a 3–4-inch-thick tree. Larger trees are felled over the course of multiple work shifts. What the beaver is after are twigs and small branches, which it uses for food. It stockpiles enormous quantities in its lodge to last the winter, and as the years pass, the lodge grows by many yards. The branches also camouflage the entrances to the tunnels that lead into the lodge. As an added security feature, the beaver builds these entrances underwater so that predators can’t get in. The rest of the living space is above water and dry.”
“Normally, deer would’t be living in the forest at all because they eat mostly grass. Grass is a rarity in a natural forest and almost never present in the quantities a whole herd requires, and therefore, these majestic animals prefer to live out in the open. But river valleys, where flood ensure open grassland, are where people like to live. Every square yard is used for urban areas or agriculture. And so the deer have retreated the forest, even though they sneak out at night. But as typical plant eaters, they need fiber-rich food around the clock. When there isn’t anything else, in desperation, they turn to tree bark.
When a tree is full of water in the summer, it’s easy to peel off its bark. The deer bite into it with their incisors and pull off whole strips from the bottom up. In winter, when the trees are sleeping and the bark is dry, all the deer can do is tear off chunks. As always, this activity isn’t only really painful for trees but also life-threatening. There’s often a large-scale fungal invasion through the huge gaping wounds, which quickly breaks down the wood. The damage is so extensive the tree can’t close off the wound by quickly walling it off. If the tree grew up in an undisturbed forest — nice and slowly — it can survive even severe setbacks like this. Its wood is made up of the tiniest rings, so it’s tough and dense, which makes things very difficult for the fungi that are trying to work their way into it. I’ve often seen tree youngsters like this that have managed to close wounds, even though it took them decades. It’s quite another story with the planted trees in our commercial forests. Usually, they grow very quickly and their growth rings are huge; therefore, their wood contains a great deal of air. Air and moisture — these are ideal conditions for fungi. And so the inevitable happens: severely damaged trees snap in middle age.”
“In 2009, tree researcher Martin Gossner sprayed the oldest (600 years old) and mightiest (170 feet tall and 6 feet wide at chest height) tree in the Bavarian Forest National Park. The chemical he used, pyrethrum, is an insecticide, which brought any number of spiders and insects tumbling down to the forest floor — dead. The lethal results show how species-rich life is way up high. The scientists counted 2,041 animals belonging to 257 different species.”
Seasons
“Most tree species seem to have large storage areas, and they continue to photosynthesize hungrily and without taking a break right until the first hard frosts. Then they, too, must stop and shut down all activity. One reason for this is water. It must be liquid for the tree to work with it. If a tree’s ‘blood’ freezes, not only does nothing work, but things can also go badly wrong. If wood is too wet when it freezes, it can burst like a frozen water pipe. This is the reason most species begin to gradually reduce the moisture content in their wood — and this means cutting back on activity — as early as July.
But trees can’t switch to winter mode yet, for 2 main reasons. First, unless they’re members of the cherry family, they use the last warm days of summer to store energy, and second, most species still need to fetch energy reserves from the leaves and get them back into their trunk and roots. Above all, they need to break down their green coloring, chlorophyll, into its component parts so that the following sprig they can send large quantities of it back out to the new leaves. As this pigment is pumped out of the leaves, the yellow and grown colors that were there all along predominate. These colors are made of carotene and probably serve as alarm signals. Around this time, aphids and other insects are seeking shelter in cracks in the bark, where they will be protected from low temperatures. Healthy trees advertise their readiness to defend themselves in the coming spring by displaying brightly-colored fall leaves. Aphids & Co recognize these trees as unfavorable places for their offspring because they’ll probably be particularly vigorous about producing toxins. Therefore, they search out weaker, less colorful trees.
But why bother with all this extravagance? Many conifers demonstrate that things can be done differently. They simply keep all their green finery on their branches and thumb their noses at the idea of an annual makeover. To protect its needles from freezing, a conifer fills them with antifreeze. To ensure it doesn’t lose water to transpiration over the winter, it covers the exterior of its needles with a thick layer of wax. As an extra precaution, the skin on its needles is tough and hard, and the small breathing holes on the underside are buried extra deep. All these precautions combine to prevent the tree from losing any significant amount of water. Such a loss would be tragic, because the tree wouldn’t be able to replenish supplies from the frozen ground. It would dry out and could then die of thirst.
In contrast to needles, leaves are soft and delicate — almost defenseless. It’s little wonder beeches and oaks drop them as quickly as they can at the first hint of frost. But why didn’t these trees simply develop thicker skins and antifreeze over the course of their evolution? Does it really make sense to grow millions of new leaves per tree very year, use them for a few months, and then go to the trouble of discarding them again? Apparently, evolution says it does, because when it developed deciduous trees about 100 million years ago, conifers had already been around on this planet for 170 million years. This means deciduous trees are a relatively modern invention. When you take a closer look, their behavior in fall actually makes a lot of sense. By discarding their leaves, they avoid a critical force — winter storms.
When storms blow through forests in C. Europe from October on, it’s a matter of life or death for many trees. Winds blowing at 60+ miles an hour can uproot large trees, and some years, 60 miles an hour is a weekly occurrence. Fall rains soften the forest floor, so it’s difficult for tree roots to find purchase in the muddy soil. The storms pummel mature trunks with forces equivalent to a weight of approximately 220 tons. Any tree unprepared for the onslaught can’t withstand the pressure and falls over. But deciduous trees are well prepared. To be more aerodynamic, they cast off all their solar panels. And so a huge surface area of 1,200 sq. yards disappears and sinks to the forest floor. This is the equivalent of a sailboat with a 130-foot-tall mast dropping a 100-by-130 foot mainsail. And that’s not all. The trunk and branches are shaped so that their combined wind resistance is somewhat less than that of a modern car. Moreover, the whole construction is so flexible that the forces of a strong gust of wind are absorbed and distributed throughout the tree.
These measures all work together to ensure that hardly anything happens to deciduous trees over the winter. If there’s an unusually strong hurricane-force wind — the kind that happens only every 5–10 years in Europe — the tree community stands together to help each individual tree. Every trunk is different. Each has its own pattern of woody fibers, a testament to its unique history. This means that, after the first gust — which bends all the trees in the same direction at the same time — each tree springs back at a different speed. And usually it’s the subsequent gusts that do a tree in, because they catch the tree while it’s still severely bowed and bend it over again, even farther this time. But in an intact forest, every tree gets help. As the crowns swing back up, they hit each other, because each of them is straightening up at its own pace. While some are still moving backwards, others are already swinging forward again. The result is a gentle impact, which slows both trees down. By the the time the next gust of wind comes along, the trees have almost stopped moving altogether and the struggle begins all over again.
With every winter they survive, the trees prove that dropping leaves makes sense and that producing new leaves every year is worth the energy it takes. But it brings up completely different dangers. One of these is snowfall. Snow makes it imperative that deciduous trees drop their leaves in a timely manner. Once the aforementioned 1,200 sq. yards of leaf surface have disappeared, the white blanket has no place to land but on the branches, and this means that most of it falls through onto the ground.”
“Shedding leaves is an active process, so the trees can’t go to sleep yet. After the reserve supplies have been re-absorbed from the leaves back into trunk, the tree grows a layer of cells that closes off the connection between the leaves and the branches. Now all it takes is a light breeze, and the leaves drift down to the ground. Only when that process is complete can trees retire to rest. And this they must do to recuperate from the exertions of the previous season. Sleep deprivation affects trees and people in much the same way: it’s life-threatening. That’s why oaks and beeches can’t survive if we try to grow them in containers in our living rooms. We don’t allow them to get any rest there, and so most of them die within the first year.”
“Spruces, pines, and firs change out their needles because they too must rid themselves of waste materials. They shed the oldest needles, which are damaged and don’t work very well anymore. As long as the trees are healthy, firs always keep 10, spruce 6, and pines 3 years’ worth of needles, as you can tell by taking a look at the annual growth intervals on their branches. Pines especially, which shed about 1/4 of their green needles, can look somewhat sparse in winter. In spring, a new year’s worth of needles is added along with fresh growth, and the crowns look healthy again.”
“It seems logical that warmer days trigger leaf growth, because this is when frozen water in the tree trunk thaws to flow once again. What’s unexpected is that the colder the preceding winter, the earlier the leaves unfurl. Researchers tested this in a climate-controlled lab. The warmer the cold season, the later beech branches greened up — and at first glance, that doesn’t seem logical. After all, in warm years, lots of other plants — wildflowers, for example — often start to grow in January and even begin to flower. Perhaps trees need freezing temperatures to get a restorative sleep in winter and that’s why they don’t get going right away in the spring. Whatever the reason, in these times of climate change, this is a disadvantage, because other species that aren’t so tired and grow their new leaves more quickly will be a step ahead.
How often have we experienced warm spells in January or February without the oaks and beeches greening up? How do they know that it isn’t yet time to start growing again? We’ve begun to solve the puzzle with fruit trees, at least. It seems the trees can count! They wait until a certain number of warm days have passed, and only then do they trust that all is well and classify the warm phase as spring. But warm days alone don’t mean spring has arrived.
Shedding leaves and growing new ones depends not only on temperature but also on how long the days are. Beeches, for example, don’t start growing until it’s light for at least 13 hours a day. That in itself is astounding, because to do this, trees must have some kind of ability to see. It makes sense to look for this ability in the leaves. After all, they come with a kind of solar cell, which makes them well-equipped to receive light waves. And this is just what they do in the summer months, but in April the leaves aren’t yet out. We don’t yet understand the process completely, but it’s probably the buds that are equipped with this ability. The folded leaves are resting peacefully in the buds, which are covered with brown scales to prevent them from drying.”
“How do trees register that the warmer days are because of spring and not late summer? The appropriate reaction is triggered by a combo of day length and temperature. Rising temperatures mean it’s spring. Falling temps mean it’s fall. Trees are aware of that as well. And that’s why species such as oaks or beeches, which are native to the Northern Hemisphere, adapt to reversed cycles in the Southern Hemisphere if they’re exported to New Zealand and planted there. And what this proves as well, by the way, is that trees must have a memory. How else could they inwardly compare day lengths or count warm days?
In particularly warm years, with high fall temps, you can find trees whose sense of time has become confused. Their buds swell in September, and a few trees even put out new leaves. Trees that get in a muddle like this have to suffer the consequences when delayed frosts finally arrive. The fresh growth hasn’t had time to get woody — that is, to get hard and tough for winter — and the leaves are defenseless anyway. And so the new greenery freezes, and that must surely hurt. Worse, the buds for next spring are now lost and costly replacements must be grown. If a tree isn’t careful, it will deplete its energy supplies and be less prepared for the coming season.”
“They’re growing unusually close together: mere inches separate the 100-year-old trunks. That makes them ideal subjects to study, because the environmental conditions for all 3 are identical. Soil, water, local microclimate — there can’t be 3 different sets of each within a few years. This means that if the oaks behave differently, it must be because of their own innate characteristics. And they do, indeed, behave differently!
In winter, when the trees are bare, or in summer, when they’re in full leaf, a driver speeding by wouldn’t even notice 3 separate trees. Their interconnecting crowns form a single large dome. The closely spaced trunks could all be growing from the same root, as happens sometimes if downed trees start to regrow. However, the triad of fall color points to a very different story. Whereas the oak on the right is already turning color, the middle one and the one on the left are still completely green. It takes a couple of weeks for the 2 laggards to follow their colleague into hibernation. But if their growing conditions are identical, what accounts for the behavioral differences? The timing of leaf drop, it seems, really is a question of character.”
A deciduous tree has to shed its leaves. But when is the optimal moment? Trees cannot anticipate the coming winter. They don’t know whether it’s going to be harsh or mild. All they register are shortening days and falling temps. If temps are falling, that is. There are often unseasonably warm days in the fall, and now the 3 oaks find themselves in a dilemma. Should they use these mild days to photosynthesize a while longer and quickly stash away a few extra calories of sugar? Or should they play it safe and drop their leaves in case there’s a sudden frost that forces them into hibernation? Clearly, each of the 3 trees decides differently.
The tree on the right is a bit more anxious, or to put it more positively, more sensible. What good are extra provisions if you can’t shed your leaves and have to spend the winter in mortal danger? So, get rid of the lot in a timely manner and move on! The 2 other oaks are somewhat bolder. Who knows what next spring will bring, or how much energy a sudden insect attack might consume and what reserves will be left over afterward? Therefore, they simply stay green longer and fill the storage tanks under their bark and in the roots to the brim. Until now, this behavior has always paid off for them, but who knows how long it will continue to do so? Thanks to climate change, fall temps are remaining high for longer and longer, and the gamble of holding on to leaves is being drawn out until November. All the while, fall storms are beginning as punctually as ever in October, and so the risk of getting blown over while still in full leaf rises. In my estimation, more cautious trees will have a better chance of surviving in the future.”
Air
“A biologist describes: if you add a pinch of crushed spruce or pine needles to a drop of water that contains protozoa, in less than a second, the protozoa are dead. He also writes that the air in young pine forests is almost germfree, thanks to the phytoncides released by the needles. In essence, then trees disinfect their surroundings. But that isn’t all. Walnuts have compounds in their leaves that deal so effectively with insects that garden lovers are often advised to put a bench under a canopy of walnuts if they want a comfortable place to relax in the garden, because this is where they will have the least chance of being bitten by mosquitoes. The phytoncides in conifers are particularly pungent, and they’re the origin of that heady forest scent that’s especially intense on hot summer days.”
Parasites
“Ivy is the only plant in C. Europe that uses small aboveground roots to anchor itself firmly to bark. Over the course of many decades, it keeps climbing upward until it finally reaches the crown. It can live many hundreds of years up here, though ivy that old is more often found on rocky cliffs or castle walls. Some of the European lit suggests that ivy doesn’t hurt the trees it grows on. After observing the trees growing around our house, I can’t support that view. Quite the opposite, in fact. Pines need a lot of light for their needles, and they particularly resent this competitor taking over in the treetops. Branches begin to die, and this can weaken trees so much that they give up. Ivy vines encircling trunks can grow as thick as small trees, and like boa constrictors winding themselves around their victims, they can squeeze the life out of pines and oaks.”
“Mistletoes save themselves the arduous task of climbing up trees. They prefer to start at the top. To do this, they co-opt thrushes, who deposit the mistletoes’ sticky seeds when they clean off their beaks on the upper branches. But how do plants survive up there with no contact with the ground to get water or food? Now, way up in those lofty heights, there’s water and food aplenty — in the trees. To get at them, the mistletoes sink their roots into the branches they’re sitting on and simply suck out what they need. They’re photosynthesizing for themselves, at least, so the host tree is ‘only’ short water and minerals. That’s why scientists call them ‘hemiparasites,’ and not true parasites. But that’s not so much help to the tree. Over the years, the number of mistletoes in its crown multiplies. You can recognize affected trees — deciduous ones, anyway — in the cold season. Some are absolutely covered with these parasitic plants, and in large quantities they can be dangerous. The constant bloodletting weakens the tree, which, incidentally, is also increasingly getting robbed of light. And as if that were not enough, the mistletoe roots massively weaken the structure of the wood in the branches, which often break after a few years, reducing the size of the crown. Sometimes it all gets too much, and the tree dies.”
“Mosses move into places on the trunk where the water trickles down after a shower. It’s not an even distribution because most trees are tilted slightly to one side. A small stream forms on the upper side of a slight bend, and that’s what the moss taps into. Incidentally, that’s why you can’t rely on moss if you want to figure out compass directions. In climates where there’s rain year round, moss supposedly indicates the weather side of the trees, where the trunk gets wet when the rain hits it; however, in the middle of the forest, where the wind is stilled, rain usually falls vertically. In addition, each tree is bent in a slightly different direction, so if you were to orient yourself according to moss, you’d only end up confused.”
Expanding and Moving
“Silver birch bark has another surprise in store. The white color is because of the active ingredient betulin, its primary component. White reflects sunlight and protects the trunk from sunscald. It also guards the trunk against heating up in the warming rays of the winter sun, which could cause unprotected trees to burst. As birches are pioneer trees that often grow all alone in wide-open spaces without any neighbors to shade them, such a feature makes sense. Betulin also has antiviral and antibacterial properties and is an ingredient in medicines and in many skincare products.”
“The quaking aspen takes its name from its leaves, which react to the slightest breath of wind. Their leaves hang from flexible stems and flutter in the breeze, exposing first their upper and then their lower surfaces to the sun. This means both sides of the leaf can photosynthesize. This is in contrast to other species, where the underside is reserved for breathing. Thus quaking aspens can generate more energy, and they can grow even faster than birches.
When it comes to predators, the quaking aspen pursues a completely different strategy form the birch, relying on stubbornness and size. Even when they’re being nibbled down by deer year after year, they slowly expand their root systems. From their roots, they then grow hundreds of subsidiary shoots, which, as the years progress, develop into decent-sized trunks. Accordingly, a single tree can extend over many hundreds of square yards of ground — or, in extreme cases, even farther. In Fishlake Natl Forest, Utah, there’s a quaking aspen that has taken thousands of years to cover 100+ acres and grow 40,000+ trunks. This organism, which looks like a large forest,has bbeen given the name ‘Pando’ (from the Latin pandere, which means ‘to spread’). You can see something similar in forests and fields in Europe, albeit not on such a grand scale. Once the brush has become sufficiently impenetrable, the a few of the trunks can grow upward undisturbed and develop into large trees in less than 20 years.”
“A competition begins that they will, inevitably, lose. The interloping youngsters gradually grow taller, and after a few decades, they catch up with the trees affording them shade. By this time, their benefactors are burned out, completely spent, and top out their growth at a max of 80 feet.
For Beeches & Co, 80 ft. is nothing. They weave their way through the crowns of the pioneer trees and happily grow up and out over them. With their dense crowns, they’re considerably better at absorbing the light, and now not enough of this precious commodity reaches the birches and poplars they’ve overtaken. The distressed trees put up a fight, especially the silver birches, which have developed a strategy to keep the troublesome competition at bay for at least a few more years: their long, thin, pendulous branches act like whips, and they lash out in all directions in even the lightest breeze. This whipping action damages the crowns of neighboring non-related trees, slaps off their leaves, and new growth, and, at least in the short term, restricts their growth. Despite this, the lowly tenants eventually overtake the birches and poplars and now everything happens relatively quickly. After just a few years, their last reserves used up, the pioneer species die and return to humus.”
“Opening up new places to live is necessary primarily because the climate is always changing. It’s changing very slowly, to be sure, over the course of many hundreds of years, but eventually, despite whatever built-in tolerance trees might have, it will become too warm, too cold, too dry, or too wet for a particular species. Then the trees must depart for other climes, and this means packing up and moving. Such a migration is happening in the C. European forests right now. The reason isn’t just climate change, which has already presented us with a 1.4 degree rise in the average temperature, but also the change from the last ice age to a warmer era.
Ice ages are hugely influential. As the centuries get increasingly colder, trees must retreat to more southerly climes. If the shift takes place slowly over many generations, trees in C. Europe, for example, successfully relocate to the Mediterranean region. But if the ice advances quickly, it buries forests and swallows up species that’ve been dragging their feet.
In C. Europe 3 million years ago, you could find not only the native beeches we have today but also large-leaved beeches. Although beeches managed to make the leap to S. Europe, the less agile large-leaved beeches died out. One reason for their demise was the Alps. This range forms a natural barrier that blocekd the trees’ escape route. To cross the Alps, the trees had first to settle high terrain before descending once more to more comfortable elevations. But higher places are too cold for many trees, even in interglacial periods, so the fortunes of many species ended when they reached the tree line. Today, you can no longer find large-leaved beeches in C. Europe, but you can find them in eastern North America, where they’re known as American beeches. American beeches survived because there’s no inconvenient east-west mountain range blocking movement from N to S on the N. American continent. They could make their way south without hindrance and then move back north after the ice age was over.
Along with a few other tree species, the beeches of C. Europe somehow managed to make it over the Alps and survive in protected locations until our current interglacial period. The road has been open for these relatively few species for thousands of years, and today they’re marching north, still, as it were, following the trail of the melting ice. As soon as the climate warmed up, the germinating seedlings were in luck again. They grew to be mature trees and scattered new seeds that progressed north, mile by mile. The average speed of the beeches’ journey, by the way, is about 1/4 mile — a year.”
“In areas where light now fell because trees had been cut down, other species of trees previously overshadowed by the beeches took over. This severely hindered the post-ice age migration of beeches, and to this day, there are areas in Europe they haven’t yet colonized.
In the past few centuries, hunting has come to European forests as well, which, paradoxically, considerably increased the numbers of deer and wild boar. Thanks to massive feeding programs by hunters, who are mostly interested in increasing the number of antler-bearing stags, the population grew until today it’s up to 5x its natural level. German-speaking regions have one of the highest concentrations of herbivores in the world, so small beeches are finding it harder than ever to survive.”
Genetics
“Once a tree has exhausted its behavioral repertoire, genetics come into play. It takes an extremely long time to produce a new generation of trees. This means speedy adaptation isn’t an option, but other responses are available. In a forest that’s been left to its own devices, the genetic makeup of each individual tree belonging to the same species is very different. This is in contrast to people, who are genetically very similar. The individual beeches growing in a stand near where I live are as far apart genetically as different species of animals. This means each tree has different characteristics. Some deal better with drought than cold. Others have powerful defenses against insects. And yet others are perhaps particularly impervious to wet feet.”
“Douglas firs, which are native to N. America but now grow in C. Europe as well, react in much the same way as oaks, but in their case, their roots seem to be super sensitive. I’ve observed 2 lightning strikes where not only the tree that was struck died, but another 10 Douglas firs within a radius of 50 feet of the strike experienced the same fate. Clearly, the surrounding trees were connected to the victim underground, and that day, instead of life-giving sugar, what they received was a deadly serving of electricity.”
“A small forest of Douglas firs, barely 40 years old, was beginning to die. The culprit? An excess of manganese in the soil, which, apparently, they couldn’t tolerate.
It also turns out there’s no such thing as ‘the Douglas fir,’ as separate varieties with completely different characteristics were imported to Europe. Those from the Pacific coast are the best fit. Their seeds, however, got mixed with seeds from inland species that grow a long way from the ocean. And to complicate the situation further, both crossbreed easily, producing offspring, all of which express characteristics that are completely unpredictable. Unfortunately, it often takes at least 40 years before you can tell whether the trees are healthy or not. If they are, they keep their vivid blue-green needles and thick crowns with tightly packed branches. The trunks of hybrids that contain too many genes from inland trees begin to bleed resin and their needles look distressed. In the end, this is simply a natural correction, albeit a cruel one. Genetic misfits are discarded, even if the process plays out over many decades.”
Air
“Forest air is the epitome of healthy air. People who want to take a deep breath of fresh air or engage in physical activity in a particularly agreeable atmosphere step out into the forest. There’s every reason to do so. The air truly is considerably clearer under the trees, because the trees act as huge air filters. Their leaves and needles hang in a steady breeze, catching large and small particles as they float by. Per year and square mile this can amount to 20,000 tons of material. Trees trap so much because their canopy presents such a large surface area. In comparison with a meadow of a similar size, the surface area of a forest is hundreds of times larger, mostly because of the size difference between trees and grass. The filtered particles contain not only pollutants such as soot but also pollen and dust blown up from the ground. It’s the filtered particles from human activity, however, that are particularly harmful. Acids, toxic hydrocarbons, and nitrogen compounds accumulate in the trees like fat in the filter of an exhaust fan above a kitchen stove. But not only do trees filter materials out of the air, they also pump substances into it. They exchange scent-mails and, of course, pump out phytoncides.”
Conclusion
“Every material absorbs light differently or converts it into other kinds of radiation. Only the wavelengths that remain are refracted and reach our eyes. Therefore, the color of organisms and objects is dictated by the color of the reflected light. And in the case of leaves on trees, the color is green.
But why don’t we see leaves as black? Why don’t they absorb all the light? Chlorophyll helps leaves process light. If trees processed light super-efficiently, there would be hardly any left over — and the forest would then look as dark during the day as it does at night. Chlorophyll, however, has one disadvantage. It has a so-called green gap, and because it cannot use this part of the color spectrum, it has to reflect it back unused. This weak spot means that we can see this photosynthetic leftover, and that’s why almost all plants look deep green to us. What we’re really seeing is waste light, the rejected part that trees cannot use. Beautiful for us; useless for the trees.”
“The remote Great Bear Rainforest in N. British Columbia covers almost 25,000 sq. miles along the rugged coast. Half of this area is forested, including about 9k square miles of old-growth trees. This primeval forest is home to the rare spirit bear, which although it’s white, isn’t a polar bear but a black bear with white fur. First Nations in the area have been fighting since the 90s to protect their homelands. In 2016, an agreement was announced to keep 85% of the forest unlogged, though it does allow for 15% of the trees, mostly old growth at low elevations, to be removed.”
“Researchers in CA have discovered that even fruit flies might dream. Sympathy for flies? That’s quite a stretch for most people, and the emotional path to the forest is even more of a stretch. Indeed, the conceptual gap between flies and trees is well-nigh unbridgeable for most of us.”
“Katsuhiko Matunsaga, a marine chemist, discovered that leaves falling into streams and rivers leach acids into the ocean that stimulate the growth of plankton, the first and most important building block in the food chain. More fish because of the forest? The researcher encouraged the planting of more trees in coastal areas, which did, in fact, lead to higher yields for fisheries and oyster growers.”
