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

by Peter Wohlleben

  As we learned in previous chapters, 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 is going to be harsh or mild. All they register are shortening days and falling temperatures. If temperatures are falling, that is. There are often unseasonably warm days in the fall, and now the three 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 three trees decides differently.

  The tree on the right is a bit more anxious than the others, 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 whole winter in mortal danger? So, get rid of the lot in a timely manner and move on to dreamland! The two 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 their 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 temperatures 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.

  You can see something similar on the trunks of deciduous trees and silver firs. According to the tree etiquette manual, the trunks should be tall and smooth, and this means no branches on the lower half of the tree. That makes sense because there’s not much light at the bottom. As there are no sunbeams to be processed, unnecessary body parts that would only use up food are simply shut down. It’s a bit like our muscles, which our bodies reduce in size when we don’t use them in order to save calories. But trees cannot remove their branches on their own; they just have to let them die. The rest must be done by fungi, which attack the wood once it is dead. At some point, the branch rots, breaks off, and is finally recycled into humus.

  Now the tree has a problem at the point where the branch broke off. Fungi can easily grow farther into the trunk because there is no protective coating of bark—at least not yet. But the tree can change this. If the branches were not too thick (up to an inch across), it takes just a few years for the tree to close the gap. The tree can then saturate the area with water from the inside, killing the fungi. But if the branches were very thick, this procedure takes too long. The wounds gape open for decades, offering portals through which the fungi can enter and penetrate deep into the wood. The trunk rots and, at the very least, becomes a little less stable. And that is precisely the reason the etiquette manual calls for only thin branches on the lower part of the trunk. Once they have fallen off as the tree grows, under no circumstances are they to be replaced. Yet that is exactly what a few trees do.

  When a neighboring colleague dies, some trees use the light that falls on them to grow out new buds below. They grow thick branches that are very beneficial at first. These trees can now take advantage of the opportunity to photosynthesize in two places at once: at the crown and lower down on the trunk. But one day, perhaps twenty years later, the other trees standing around will have increased the size of their crowns so much that the gap in the canopy closes up. Once again, the lower levels are dark, and the thick branches die. Now the trees pay dearly for their craving for sun. As I’ve just described, fungi now march deep into the trunk of the foolish trees and put them in danger. When you take your next walk into the forest, you can check for yourself to see that such behavior really is an individual choice and, therefore, a question of character. Take a look at the trees growing around a small clearing. All have the same temptation to do something stupid like growing new branches on their trunks, but only a few give in. The rest keep their bark nice and smooth and avoid the predictable risk.

  25

  — THE SICK TREE —

  STATISTICALLY SPEAKING, MOST species of trees can live to a ripe old age. In the burial area of the forest I manage, tree buyers always ask how long their tree might live. Mostly, they choose beeches or oaks, and as far as we know, these trees usually live to be between four hundred and five hundred years old. But what is a statistic worth when you apply it to an individual tree? Just as much as it is worth when you apply it to an individual person—nothing. The anticipated trajectory of a tree’s life can change at any time for any number of reasons. Its health depends on the stability of the forest ecosystem. It’s better if temperature, moisture, and light conditions don’t change abruptly, because trees react extremely slowly. But even when all the external conditions are optimal, insects, fungi, bacteria, and viruses are always lurking, waiting for the chance to strike. That usually happens only when a tree gets out of balance. Under normal circumstances, a tree carefully apportions its energy. The largest portion is used for daily living: the tree has to breathe, “digest” its food, supply its fungal allies with sugar, and grow a bit every day. Then the tree has to keep hidden reserves of energy on hand to fight off pests.

  These secret reserves can be activated at any time, and depending on the tree species, they contain a selection of defensive compounds produced by the tree. These so-called phytoncides have antibiotic properties, and there has been some impressive research done on them. A biologist from Leningrad, Boris Tokin, described them like this back in 1956: 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. In the same paper, Tokin writes that the air in young pine forests is almost germfree, thanks to the phytoncides released by the needles.56 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 are the origin of that heady forest scent that is especially intense on hot summer days.

  If the carefully calibrated balance of energy for growth and defense gets thrown out of alignment, then a tree might get sick. This can happen, for example, when a neighboring tree dies. Suddenly, the crown gets more light, and now what the tree wants more than anything is more photosynthesis. That makes sense because a chance like this comes along only about once every hundred years. The tree, finding itself suddenly bathed in sunlight, forgets about everything else and focuses exclusively on growing branches. It has no option really, because its surrounding cohort is doing the same thing, which means that the gap in the canopy will close again in about twenty years, which, if you are a tree, means you don’t have much time.

  Suddenly, growth speeds up, and instead of adding a few fractions of an inch each year, the tree is adding about 20 inches. This takes energy, which is then not available for fending off illnesses and pests. If the tree is lucky, all goes well, and once the canopy closes again, the tree will have increased the size of its crown. Then it will take a break and settle back into apportioning its energy in a way that suits its lifestyle. But woe betide the tree if something goes wrong during this growth spurt. A fungus might attack the stub left by a fallen branch and, unnoticed, make its way along the dead wood and into the trunk. Or a bark beetle might take an exploratory bite out of a tree busy reaching for the light and discover there is no defensive response. Then the game is up. The trunk, which appears to be in the very best of health, finds itself increasingly under attack because it doesn’t have the energy to mobilize a defense.

  The first reactions to the attacks soon show up in the treetops. In
deciduous trees, the vital topmost growth suddenly dies, leaving thick branch stubs with no side branches sticking up into the sky. The initial reaction of conifers is that their needles don’t last as long. Sick pines, for instance, retain not three but maybe only one or two generations of needles on their branches, which makes their crowns noticeably more open. In spruce you also get what is known in Germany as the “Lametta effect,” where the twigs hang limply from the branches. (Lametta is another name for the tinsel that is draped over the branches of Christmas trees.) A short time later, big flakes of bark break off the trunk. Things can deteriorate quite quickly from this point. Like a deflating hot air balloon, the crown implodes and sinks as it dies, and winter storms break off the dead branches. You can see this even more clearly with spruce, because the desiccated tips of dying trees contrast clearly with the living green of the lower branches.

  Every year, a live tree adds a growth ring to the wood in its trunk because it is, you could say, damned to grow whether it wants to or not. In the growing season, the cambium, that narrow layer of clear cells between the bark and the wood, grows new woody cells on the inside and new bark cells on the outside. If a tree cannot increase its girth, it dies. At least, that is what we thought for a long time. Then researchers noticed pines in Switzerland that looked outwardly healthy and were covered in green needles. On closer inspection—either by cutting the trees down or taking core samples—researchers discovered that a few of them hadn’t created a single new growth ring for more than thirty years.57 Dead pines covered in green needles? The trees had been attacked by an aggressive fungus called annosus root rot, and their cambium had died. But the roots were still pumping water up to the crown through the long narrow transport vessels in the trunk, providing the needles with life-giving moisture. And the roots themselves? When the cambium is dead, the bark is too, which means the tree can no longer pump sugar solutions from its needles back down to its roots. Therefore, healthy neighboring pines must have been helping their dying comrades by supplying their roots with food, as I’ve described in chapter 1, “Friendships.”

  Apart from diseases, a lot of trees suffer injuries over the course of their lives. There are many different ways this can happen. A neighboring tree might fall. In a dense forest, a falling tree cannot avoid hitting a few surrounding comrades. If this happens in winter, when the trees’ relatively dry bark fits tightly around their wood, not much happens. Most often only a few branches break, leaving no visible signs of damage after just a few years. Injuries to the trunk are more serious, and these usually happen during the summer months. This is when the cambium is full of water, crystal clear, and slippery. At this time, it doesn’t take much pressure to loosen the outer layer of the tree, and branches from a falling neighbor scraping by can rip yard-long wounds in the tree’s trunk. Ouch! The damp wound is an ideal landing site for fungal spores, which arrive just minutes later. They grow fungal threads that immediately begin making a meal of the wood and the tree’s food supplies. But they don’t make much progress. There’s simply too much water in the wood. Although fungi like it moist, soaking wet conditions spell death for them. At first, their victory march into the interior of the trunk is slowed by the wet outer wood where sap is flowing; however, the sapwood is now exposed to the air, and its outer layers can dry out. A slow-motion struggle begins.

  The fungus advances as the sapwood loses its moisture, while the tree tries to close up the wound. To do this, the tissues around the injury really get going and start growing together as fast as they can. They can cover up to a third of an inch of injured wood per year. To avoid complications in the future, the tree must get the area sealed once again within five years, tops. Once new bark has closed the old wound, the tree can saturate the damaged sapwood from the inside and kill the fungus. However, if the fungus has made it from the sapwood into the heartwood—the older wood beneath the sapwood that no longer transports water or stores energy reserves—then it’s too late. The decommissioned wood is drier and, therefore, ideal for the attacker, and the tree can’t mount any defenses here. So, the decisive factor in whether the tree has a chance is the size of the wound. Anything much more than an inch is life threatening.

  But even if the fungus wins and makes itself at home inside the tree, all is not yet lost. True, the fungus can get stuck into the wood without further hindrance, but it takes its time. A whole century can pass before everything has been consumed and turned to mush. Even this won’t make the tree the slightest bit less stable, because the fungus cannot expand into the wet outer growth rings of the living sapwood. In extreme cases, the tree gets hollowed out like a stovepipe. And just like a pipe, the tree remains stable. So we shouldn’t feel sorry for a rotten tree, and it doesn’t necessarily feel pain either, because the heartwood is no longer active and usually no longer contains any living cells. The outer growth rings, which are still active, transport water up the trunk and, therefore, are much too wet for fungi.

  If a tree has successfully walled off—that is to say, closed up—an injury to its trunk, then it can usually grow as old as its uninjured companions. But sometimes, especially in cold winters, the old wounds can act up again. Then a crack like a rifle shot echoes through the forest and the trunk splits open along the old injury. This is caused by differences in tension in the frozen wood, because the wood in trees with a history of injury varies greatly in density.

  26

  — LET THERE BE LIGHT —

  I’VE ALREADY TALKED a lot about sunlight, and it’s turned out to be an extremely important factor in the forest. This should come as no surprise. After all, trees are plants and need to photosynthesize to survive. But because enough sun usually shines on our garden beds and lawns, in the home garden, water and fertile soil tend to be more decisive factors for plant growth. In our everyday lives, we don’t notice that light is more important, and because we like to apply our own situations to others, we overlook the fact that an intact forest has completely different priorities.

  In the forest, there’s a battle for every last ray of sunlight, and each species is specialized to grow in a particular niche so that it can soak up some energy, however paltry the amount might be. In the upper story—the executive offices—the mighty beeches, firs, and spruce stretch out and soak up 97 percent of the sunlight. This behavior is cruel and inconsiderate, but doesn’t every species take what it can? Trees have won this competition for the sun because they grow such tall trunks. But a plant can grow a long sturdy trunk only if it lives for a very long time, because an enormous amount of energy is stored in its wood. To grow its trunk, a mature beech needs as much sugar and cellulose as there is in a 2.5-acre field of wheat. Of course, it takes not 1 but 150 years to grow such a mighty structure, but once it’s up there, hardly any other plants—except for other trees—can reach it, and the rest of its life is worry free. Its own offspring are designed to survive in what light remains, and of course, their mothers feed them as well. That is not the case for the rest of the rank and file, and they must come up with other strategies for survival.

  Some plants bloom early. In April, seas of frothy white blooms cover the brown earth under old deciduous trees as wood anemones cast their spell on the forest. Sometimes yellow or violet-blue flowers are mixed in, such as liverworts, so-called because their leaves are shaped a bit like human livers. They earned one of their common names in German—Vorwitzchen, or “cheeky little ones”—because the flowers appear so early in the year. Liverworts are stubborn plants. Once they’ve found a spot, they want to stay there forever, and they spread very slowly by seed. That’s why you find these early bloomers only in deciduous woods that have been around for at least a few hundred years.

  The colorful troupe just about exhausts itself putting on a glorious floral show. The reason for this extravagance is that they want to make the most of the short window of time available to them. While the spring sun warms the forest floor from March to early May, the deciduous trees sleep on. Under the giants’ bare
branches, Liverworts & Co. seize the opportunity to produce the carbohydrates they need for the following year. They store the food in their roots. In addition, the little beauties have to reproduce, which uses up additional energy. It’s a small miracle they can pull it all off in just a month or two. As soon as the buds break on the trees, it gets much too dark again, and the flowers are forced to take another ten months off.

  When I said earlier that hardly any other plant can reach the tree’s height, the emphasis was on “hardly.” For there are some plants that can make it up into the canopy. It’s particularly arduous and tedious to start right from the bottom. Ivy is one plant that does this. Ivy begins as a small seed at the foot of a tree with an open growth habit—those species that are particularly wasteful with sunbeams and allow any number of them to fall to the forest floor unused. Under pines or oaks, that’s enough for a nice thick carpet of ivy to grow—at first, just on the forest floor. Then, one day, a tendril starts to climb up a trunk. Ivy is the only plant in Central Europe that uses small aboveground roots to anchor itself firmly to bark. Over the course of many decades, the ivy 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 literature suggests that ivy doesn’t hurt the trees it grows on. After observing the trees growing around our house, I can’t support this 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.