The Hidden Life of Trees: What They Feel, How They Communicate—Discoveries from a Secret World

by Peter Wohlleben

  The process of strangulation is even more apparent in another species: the honeysuckle. This plant, with its pretty lilylike flowers, prefers to climb up younger trees. The honeysuckle wraps itself so tightly around the little trunks that as they grow, they develop deep spiral-shaped indentations. As I’ve mentioned already, people like to sell these deformed trees as bizarrely shaped walking sticks, which is fine as the trees wouldn’t have survived much longer out in Nature anyway. Because their growth has been slowed, trees hugged by honeysuckle fall behind the other youngsters. Even if they do manage to grow up, sooner or later a passing storm will break their twisted trunks.

  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 are 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 much help to the tree. Over the years, the number of mistletoes in its crown multiplies. You can recognize affected trees—deciduous trees, anyway—in the cold season. Some trees 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 getting increasingly 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.

  Other plants that simply use the trees for support are less damaging. These would be the mosses. Many species have no roots to sink into soil, or branches; instead they have small hairlike structures, and these are what they use to hold on to the bark. Very little light, no nutrient uptake, no water from the ground, and no tapping of the tree for help: how does that work? It only works if you are extremely frugal. The soft cushions of moss catch water from mist, fog, or rain and store it. Often that is not enough, as the trees either act like umbrellas (Spruce & Co.) or their branches funnel the water down to their roots (deciduous trees). In the latter case, the solution is simple: 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 is why you can’t rely on moss if you want to figure out compass directions. In climates where there is rain year round, moss supposedly indicates the weather side of the tree, 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.

  If the bark is rough as well, moisture remains in its tiny fissures for a particularly long time. Rough bark begins at the bottom of the tree and keeps moving upward in the direction of the crown as the tree ages. That’s why you find moss growing only an inch or two above the ground on young trees, whereas later it encases the lower trunk like a knee-high sock. Moss doesn’t damage the tree, and the tiny plants compensate for the small amount of water they divert by releasing moisture as well, so their influence on the forest climate is positive.

  We’re left with the question of where moss gets its food. If food doesn’t come from the ground, the only place it can come from is the air. And a whole lot of dust is blown through forests every year. A mature tree can filter out more than 200 pounds, which rain flushes down the trunk. Mosses soak up the dusty mixture and filter out what they can use. That deals with the food, and now the only thing missing is light.

  In bright pine or oak forests, light isn’t a problem, but it is in those eternally dark spruce forests. Even the most abstemious must give these a miss, and that’s why particularly dense stands of young trees in coniferous forests are most often completely moss free. It is only as the trees age, when here and there gaps appear in the canopy, that enough sunlight filters through for the trees to get a covering of green. Things are rather different in old beech forests, for here the mosses benefit from leaf-free interludes in spring and fall. It gets too dark again in summer, but the plants are adapted to cycles of hunger and thirst. Sometimes there’s no rain for months on end. If you run your fingers over a cushion of moss in a dry spell, you’ll find it is completely desiccated. Most plants would die at this stage, but not moss. It swells with the next heavy rain shower—and life continues.

  Lichen are even more frugal. These small gray-green growths are a symbiotic combination of fungi and algae. To hold together, they need some kind of a substrate, and in the forest, this is provided by trees. In contrast to moss, they climb much higher up the trunks, where their already extremely slow growth is slowed still further by the leafy canopy. Often it takes them many years to grow a moldy-looking coating over the bark, which prompts many visitors to my forest to ask whether the trees are sick. The trees are not sick; lichen doesn’t do them any harm, and the trees are probably completely indifferent to their presence. These tiny growths balance their snail’s pace when growing with extreme longevity. They can survive to be hundreds of years old, showing that these organisms are perfectly suited to the slow rhythms of life in ancient forests.

  27

  — STREET KIDS —

  HAVE YOU EVER wondered why giant redwoods in Europe never grow particularly tall? Even though quite a lot of them are more than 150 years old, very few have yet topped 160 feet. In their homeland— forests on the western slopes of the Sierra Nevada mountains in California—they easily grow more than twice that size. Why don’t they do that in Europe? If we think back to tree kindergarten, to their extremely long and drawn-out youth, we might be tempted to say: They’re still children. What do you expect? But that doesn’t jibe with the enormous diameters of the older giant redwoods in Europe, which often exceed 8 feet (measured at chest height). Clearly, they know how to grow. They just seem to be putting their energy into growing in the wrong direction.

  Their location gives a clue as to why this might be the case. They were often planted in city parks by princes and politicians as exotic trophies. What is missing here, above all, is the forest, or—more specifically—relatives. At 150 years old, they are, when you consider a potential life-span of many thousands of years, indeed only children, growing up here in Europe far from their home and without their parents. No uncles, no aunts, no cheerful nursery school—no, they have lived all their lives out on a lonely limb. And what about the many other trees in the park? Don’t they form something like a forest, and couldn’t they act like surrogate parents? They usually would have been planted at the same time and so could offer the little redwoods no assistance or protection. In addition, they are very, very different kinds of trees. To let lindens, oaks, or beeches bring up a redwood would be like leaving human children in the care of mice, kangaroos, or humpback whales. It just doesn’t work, and the little Americans have had to fend for themselves. No mother to nurse them or keep a strict eye out to make sure the little ones didn’t grow too quickly. No cozy, calm, moist forest around them. Nothing but solitude.

  And if that weren’t enough, in most cases, the soil is a complete disaster. Whereas the old-growth forest offers soft, crumbly, humus-rich, and constantly moist soil for their delicate roots, European parks offer hard surfaces that have been depleted of nutrients and compacted after years of urbanization. What’s more, members of the public like to walk up to the trees, touch their bark, and relax in the shadow of their crowns. Over the decades, constant trampling around the base
of the trees leads to further soil compaction, which means that rain drains away far too quickly, and in winter, the trees cannot build up a supply of water to last the summer.

  The mechanics of planting also haunt the trees for the rest of their lives. They are kept alive and handled in nurseries for years before being moved to their final locations. Every fall, their roots are trimmed to keep them compact in the nursery beds so that they can later be moved more easily. The root ball, which for a 10-foot-tall tree grows to about 20 feet in diameter if left to its own devices, is cut back to about 20 inches, and to make sure the crown doesn’t wilt from thirst thanks to the root reduction, it too is heavily reduced. All this is done not to improve the health of the tree but simply to make it easier to handle. Unfortunately, when the roots are pruned, the brain-like structures are cut off along with the sensitive tips. Ouch! After that, it is as if this interference makes the trees lose their sense of direction underground. They stop growing roots down into the soil and form a flat plate of roots near the surface instead, severely restricting the trees’ ability to find water and food.

  At first, the young trees don’t seem to mind. They stuff themselves with sugary treats because they can photosynthesize as much as they like in full sun. It’s so easy to get over the loss of a mother’s tender care. And in the early years, the water problems in a rock-hard soil are barely noticeable. After all, the saplings are being lovingly cared for and watered by gardeners when they get dry. But above all, there is no strict discipline. No “Take it easy,” no “Just wait a couple of hundred years,” no punishing light deprivation if you don’t grow up really straight. Every young tree can do just as it likes. So, every year, they go at it as though they were in a race, and every year, they put on a growth spurt. After a certain height, the childhood bonus seems to run out. Irrigating 65-foot-tall trees takes an enormous amount of water and time. To thoroughly moisten the roots, the gardeners must spray many gallons of water out of their hoses—per tree! And so, one day, the care simply stops.

  At first, the giant redwoods don’t really notice. They’ve lived high on the hog for decades and done whatever they wanted. Their thick trunks are like paunches attesting to an orgy of solar indulgence. In the early years, it doesn’t really matter much that the cells inside their trunks are very large, contain a lot of air, and therefore are susceptible to fungal infections. Their side branches also show signs of their loutish behavior. The trees in the park know nothing about the etiquette manual that guides the old-growth forest, calling for thin branches in the lower regions of the trunk, or even for no branches at all. Thanks to the generous amounts of light that reach right to the ground, the redwoods grow thick side branches that later increase their girth so much that the image the trees bring to mind is that of doped-up body builders. True, all the branches on the lower 6 to 10 feet of the trees are usually sawn off by the gardeners to give visitors an unobstructed view of the park, but when compared with old-growth forests, where thicker branches are not allowed below 65 and sometimes not even below 165 feet, the trees’ growth is brazenly decadent.

  What the trees end up with are short, thick trunks topped with crowns. Extreme examples of park trees seem to be nothing but crown. Their roots don’t penetrate more than 20 inches down into the heavily trampled soil, and therefore, they offer little in the way of support. That’s very risky, and trees of a normal height would be much too wobbly. The growth habit of redwoods in the far-off old-growth forests ensures a low center of gravity, so they are pretty stable. It takes a huge storm to upset their equilibrium.

  Once European redwoods have passed the hundred-year mark (the trees are now the age of schoolchildren), that’s the end of easy living. The topmost branches wither away, and no matter how hard the trees try to grow up again, they have reached the end of the road. Their wood is impregnated with natural fungicides, so they can hold out for many more decades despite injuries to their bark. It’s quite different with other species of tree. Beeches, for example, react badly when thick branches are sawn off. Take a closer look the next time you take a walk in a park. You’ll find hardly any large deciduous trees that don’t show signs of having branches trimmed, sawn off, or interfered with in some other way. This “pruning” (it’s actually more like a massacre) is often only for aesthetics, which dictates that the crowns of trees lining a walk or driveway are all the same size and shape.

  A severely pruned crown is a severe blow for the roots, which grow to a size optimally suited to serve the above-ground parts of the tree. If a large percentage of the branches is removed and the level of photosynthesis drops, then just as large a percentage of the underground part of the tree starves. Fungi now penetrate the dead ends where branches have been removed and the trunk has been sawn off. The wood is filled with air pockets, thanks to the tree’s quick growth as a youngster, and fungi have a field day. After only a few decades—which is incredibly fast for a tree—this inner rot can also be seen on the outside of the tree. Complete sections of the crown die off, until the local authorities cut the crown off completely so that it no longer poses a safety hazard for visitors, leaving huge wounds where the tree has been topped. The waxy substance painted over the damaged trunk, supposedly to protect it, often hastens the tree’s demise because it traps moisture inside, creating the damp conditions fungi love. In the end, all that remains is an empty shell that cannot be saved and one day will be chopped down. And because there are no family members who can rush to help these urban trees, the stump will die quickly and completely. A little while later, a new tree will be planted and the drama will begin all over again.

  Urban trees are the street kids of the forest. And some are growing in locations that make the name an even better fit—right on the street. The first few decades of their lives are similar to those of their colleagues in the park. They are pampered and primped. Sometimes they even have their own personal irrigation lines and customized watering schedules. When their roots want to go out and get established in their new territory, they’re in for a big surprise. The soil under the street or pedestrian walkway is harder even than the soil in parks, because it has been compacted by machines using large vibrating metal plates. That’s a huge disappointment for the tree. The roots of forest trees don’t actually grow very deep. Few species grow deeper than 5 feet, and most call a halt to downward growth much sooner. That’s not a problem in the forest, where there is almost no limit as to how wide the roots can grow. Unfortunately, this isn’t the case on the side of the street. The roadway restricts growth in one direction, there are pipes under the pedestrian zones, and soil has been compacted during construction.

  When trees are planted in these restricted spaces, conflicts are inevitable. In such places, plane trees, maples, and lindens like to feel out underground wastewater pipes. We notice the damage when the next storm comes and the streets fill with water. Then specialists armed with root probes investigate to see which tree has caused the blockage. The culprit is sentenced to death for its excursion under the sidewalk and into what it thought was paradise. The offending tree is cut down, and its successor is planted in a built-in root cage to discourage such behavior in the future.

  Why do trees grow into pipes in the first place? For a long time, city engineers thought the roots were somehow attracted by moisture seeping from loose connections between the pipes or by nutrients in the wastewater. However, the results of an extensive applied study by the Ruhr University Bochum point in a completely different direction. The study found the roots in the pipes were growing above the water table and did not seem interested in extra nutrients. What was attracting them was loose soil that had not been fully compacted after construction. Here, the roots found room to breathe and grow. It was only incidentally that they penetrated the seals between individual sections of pipe and eventually ran riot inside them.58 What this means is that when trees in urban areas run up against ground as hard as concrete wherever they turn, they get desperate, and it is only as an absolutely last resort
that they finally find a way out into sloppily backfilled trenches. Once they get there, they are a problem.

  There is no remedial support for the trees, only for the pipes, which are now reburied in especially well-tamped-down soil so that the tree roots can no longer find a footing there. Are you surprised that summer storms topple a particularly large number of street trees? Their puny underground anchoring systems—which in Nature could cover more than 700 square yards and are now restricted to an area shrunk to a tiny percentage of that—are not capable of supporting trunks that weigh many tons.

  But there is even more these tough plants have to bear. The urban microclimate is heavily influenced by heat-inducing asphalt and concrete. Whereas forests cool themselves on hot summer nights, streets and buildings radiate the heat they soaked up during the day, keeping temperatures elevated. Radiated heat makes the air extremely dry. Not only that, but it’s full of exhaust fumes. Many of the companions that look after trees’ well-being in the forest (such as the microorganisms that make humus) are missing. Mycorrhizal fungi that help collect water and food are present only in low numbers. Urban trees, therefore, have to go it alone under the harshest conditions.

  As if that were not enough, they also have to deal with unsolicited extra fertilizers. Above all, from dogs, which lift their legs at every available trunk. Their urine can burn bark and kill roots. Winter salt leads to similar damage. Depending on the severity of the cold, salt is sometimes applied around trees at the rate of 2.2 pounds per square yard. In addition, the needles on conifers, which are still attached to the branches in winter, have to deal with the salt spray thrown up by car tires. At least 10 percent of the salt ends up in the air and falls back down on trees—among other resting places—where it burns the foliage. These painful injuries show up as small yellow and brown spots on the needles. The burns reduce the trees’ ability to photosynthesize the next summer and, therefore, weaken the trees.