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

by Peter Wohlleben

  Hoarfrost is similar to snow, but it’s much more romantic. At least we think so, because the plants and trees look as though they’ve been sprinkled with sugar. When below-freezing temperatures and foggy conditions occur together, fine drops of moisture immediately freeze wherever they touch a branch or a needle. After a few hours, the whole forest looks white, even though not a flake of snow has fallen. If the weather conditions persist for days, hundreds of pounds of frosty ice crystals can accumulate in the treetops. When the sun finally breaks through a hole in the fog, all the trees sparkle as though they were in a fairy tale. But unfortunately for them, they are in the real world, and they are groaning under the weight of the ice and beginning to bend dangerously. Woe to the tree that has a weak spot in its wood. Then a dry crack echoes through the forest like a gunshot, and the whole crown comes tumbling down.

  In Central Europe, hoarfrost occurs on average every ten years, and this means a tree has to endure up to fifty such events in its lifetime. The less integrated the tree is in a community of its own species, the greater the danger. Loners standing unprotected out in the cold fog succumb markedly more often than well-connected individuals in a dense forest who can lean on their neighbors for support. Moreover, the wind tends to blow over dense forest canopies, so mostly it is just the highest branch tips that get heavily blanketed with ice crystals.

  But the weather has still more tricks up its sleeve. There is lightning, for example. An old German saying about storms in the forest, “Eichen sollst du weichen, Buchen sollst du suchen,” translates as “Avoid oaks, seek beeches.” The saying originates in the fact that on some gnarly old oaks you can see a channel a few inches wide extending down the trunk where a lightning strike has split the bark open and penetrated deep into the wood. I’ve never seen a scar like this on the trunk of a beech. But the conclusion that lightning never strikes beech trees is as false as it is dangerous. Large old beeches offer no protection from lightning because they are struck just as often. The main reason there is next to no damage on beech trees is because their bark is so smooth.

  During a thunderstorm, it rains, and the water that sheets down the wrinkle-free surface of beech bark creates a continuous film. When lightning strikes, the electricity travels down the outside of this film because water conducts electricity much better than wood. Oaks, however, have rough bark. The rainwater that runs down their trunks forms little cascades and drips to the ground in hundreds of mini-waterfalls. Therefore, the flow of electricity from the lightning strike is constantly interrupted. When this happens, the point of least resistance becomes the damp wood of the outer growth rings, which the tree uses to transport water. In response to the energy surge from the lightning strike, the sapwood explodes as though it has been shot, and years later, the scar bears witness to the oak’s misfortune.

  Douglas firs, which are native to North America but now grow in Central Europe as well, react in much the same way as oaks, but in their case, their roots seem to be super sensitive. In the forest I manage I’ve observed two lightning strikes where not only the tree that was struck died, but another ten 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.

  In thunderstorms with a lot of lightning something else can happen—fire. I experienced that once in the middle of the night, when fire trucks rushed into our community forest to extinguish a small fire. Lightning had struck a hollow old spruce. The flames inside the tree were protected from the pouring rain, and they were licking their way up the rotten wood. The fire was put out quickly, but even without help, not much would have happened. The surrounding forest was sopping wet and the fire would very likely not have caught hold in the rest of the stand. Nature doesn’t expect fires in native Central European forests. The once-dominant deciduous trees don’t catch fire because their wood doesn’t contain any resins or essential oils. As a result, none of the trees have developed any mechanism to react to heat. The cork oaks of Portugal and Spain are a testament to the fact that such a mechanism even exists. The cork oaks’ thick bark protects them from the heat of grass fires and allows the buds that lie under the bark to start growing again once the fire has passed.

  In Central European latitudes, though, the monocultures of plantation spruce and pines can fall prey to fire when the trees’ discarded needles get bone dry in summer. But why do conifers store so many flammable substances in their bark and needles anyway? If fires are the order of the day in their native latitudes, wouldn’t they be better off if they were highly flame resistant? It wouldn’t be possible for a tree like the Swedish spruce in Dalarna, which is more than eight thousand years old, to reach such a ripe old age if it was engulfed by fire every two hundred years. I think it is careless people—the kind of people who leave their campfires unattended, for example—who have been responsible for fiery disturbances in the forest for thousands of years. The small number of lightning strikes that actually start small localized fires are so rare that European tree species never adapted to them. Pay attention to the cause the next time you hear a news report about a forest fire: most are attributed to human activity.

  In North America, as in Europe, there have been people around since the last ice age, tinkering with fire. And so it’s probable that most forest fires on that continent have also been caused by human activity—a clearing burned for planting subsistence crops here, a carelessly discarded stub of smoldering tobacco there. But Nature has a role to play, too.

  Left to their own devices, North American forests experience natural fire cycles. Where the climate is naturally moist and cool, lightning strikes soon fizzle out on the damp forest floor, and forest fires may occur only every few hundred years. In areas where needles and twigs on the ground are often bone dry, lightning can spark fires as often as every couple of years. Fires left to burn through the forest on this natural cycle usually stay at ground level, getting hot enough to burn away brush in the understory and leaving established trees blackened but unscathed.

  But just as people spark fires, they also rush to put them out. On the floors of forests not swept by a regular cycle of low-intensity fires, piles of kindling build up, just waiting for a spark. In these conditions, instead of staying low and clearing the understory, fires soon escalate and climb up into the canopy. As the crowns ignite, windblown embers land on neighboring trees, and so the fires spread. Low-level ground fires become raging infernos, leaving acres of blackened slopes in their wake.

  Many trees in North America are adapted to natural cycles of ground fires. Ponderosa pines and giant redwoods have evolved thick bark to protect their sensitive cambium. Jackpines have cones that pop open in heat so that their seeds fall onto a forest floor cleared of vegetation, landing on a soft bed of ash that is a perfect place for life to start anew. However, the character of forest fires in North America has been changed by naturally increasing drought conditions and the human practice of fire suppression, and forests that would once have survived, or even thrived, in the face of fire are now threatened by its destructive force.64

  Less dangerous but much more painful to trees is a phenomenon that even I didn’t know about until recently. The forester’s lodge where we live lies on a mountain ridge at an elevation of barely 1,600 feet. The streams all around, which are deeply carved into the landscape, don’t do the forest any harm. Quite the opposite. However, large rivers are something completely different. They regularly overflow their banks, and therefore, very specific ecosystems grow on either side of them: forested riparian meadows. Which species of trees get established in these meadows depends on the kind of high water and how often it happens. If the floods are fast flowing and last for many months of the year, then willows and poplars fit the bill. They can cope with long periods in wet conditions. You usually find these conditions close to the river, and this is where willow and poplar meadows ge
t established. Farther away and often a few yards higher up, floods occur less often, and when they do—in spring after the snow melts—then you find large pools of slow-moving water. By the time the trees leaf out, most of the water has already drained away, and in such conditions, oaks and elms feel right at home. In contrast to the areas where willows and poplars grow, these hardwood meadows are very sensitive to summer flooding. In summer floods, otherwise robust trees can die, because their roots suffocate.

  Some winters, however, the river really causes the trees pain. On a trip I made through a hardwood meadow in the middle of the Elbe River, I noticed loose strips of bark on all the trees. The damage was all at the same height up the trunks: about 6 feet above ground level. I’d never seen anything like it, and I couldn’t figure out what might have caused the damage. The other people on the trip were at as much of a loss as I was, until the staff at the biosphere preserve solved the puzzle: the damage was caused by ice. When the Elbe froze over in particularly cold winters, thick ice floes formed. When the air and the water warmed up in spring, the ice floated between the oaks and elms on the floodwater, bumping up against the tree trunks. As the water was at the same level everywhere on the meadow, the wounds were to be found at the same height on all the trees.

  In the context of climate change, one day the movement of ice on the Elbe will be a thing of the past. But the scars of the older trees—at least the ones that have experienced all kinds of capricious weather since the early twentieth century—will bear testimony to these events for a long time to come.

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  — IMMIGRANTS —

  THANKS TO THE botanical souvenirs early plant hunters brought back to their homelands and more recent arrivals because of the forest industry, a huge number of tree species have been introduced that would never have found their way to Europe on their own. Names like Douglas fir, Japanese larch, and grand fir don’t occur in European folk songs or poems because they have not yet become fixtures in Europe’s shared social memory. The process works in the other direction, as well. European arrivals make their own mark when travelers in search of a new life bring memories of home along in their luggage.

  Immigrants have a special status in the forest. In contrast to tree species that have migrated naturally, they arrive without their typical ecosystems. In some cases, just their seeds were imported, which means that most of the fungi and all of the insects remained back in their homelands. Douglas Fir & Co. could make a completely new start in Europe. That can certainly have its advantages. There are absolutely no illnesses because of pests—at least not in the first decades. People had a similar experience in Antarctica. The air there is almost completely devoid of germs or dust, which would be ideal for people with allergies, if only the continent were not so isolated. When trees hop over to a new continent with our help, it’s like a big breath of fresh air for them. The lucky ones find fungal partners for their roots among the nonspecialists. Beaming with health, the new arrivals grow mighty trunks in European forests, and they do so in very short periods of time. No wonder they seem superior to the native species—at least in some locations.

  Trees that migrate under their own steam can establish themselves only where they feel completely at home. Not only the climate but also the type of soil and the moisture levels must fit their lifestyles if they are going to prevail in the presence of the old trees that already rule the forest. For trees that we humans introduce into the forest, the long-term outcome is a bit like a game of roulette. You never know exactly what’s going to happen. The black cherry, for example, is a deciduous tree from North America that has a wonderfully beautiful trunk and high-quality wood when it grows there. No question—European foresters wanted to have a tree like that in their forests. But after a few decades, disillusionment set in. In their new land, the trees grew crooked and lopsided and hardly got taller than 65 feet, and they barely grew at all under the pines of eastern and northern Germany. The trees fell out of favor, but by now people couldn’t get rid of them. Deer spurned their bitter branches, preferring to nibble away at beeches, oaks, or, if absolutely necessary, even pines. And so the black cherry got the burdensome arboreal competition off its back, and the newcomer keeps expanding its territory.

  The Douglas fir can also tell you a tale or two about the uncertainty of the future. In some places, after growing for more than a hundred years, they have become impressive giants. Other forests, however, have had to be cut down in their entirety before they matured, as I experienced firsthand in my intern year in forestry school. A small forest of Douglas firs, barely forty years old, was beginning to die. Scientists puzzled over this for a long time. Whatever could have caused this decline? It wasn’t fungi, and insects were ruled out as well. The culprit finally turned out to be an excess of manganese in the soil, which, apparently, the Douglas firs couldn’t tolerate.

  It also turns out there is 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 forty 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.

  Our native beeches have had no trouble showing these interlopers the door. They employ the same strategy they use in their competition with oaks. The deciding factor that has allowed beeches to win out over Douglas firs over the course of centuries is their ability to grow in the deepest, darkest shade under large trees. The offspring of the North American mothers need much more light and perish in the kindergartens established by our native deciduous trees. It is only when people lend a helping hand by repeatedly clearing trees so that sunlight reaches the ground that the little Douglas firs stand a chance.

  It’s dangerous when foreigners pop up that are genetically very similar to native species. The Japanese larch is just such a case. When it arrived here, it met the European larch. The European larch often grows crooked and, in addition, quite slowly, and so in the last century it was often replaced with the Japanese tree. Both species cross easily to form hybrids. This raises the danger that one day, a long time from now, the last purebred European larches will disappear. There’s just such a mixing and muddling of genes going on in the forest I manage in the Eifel mountains, where neither species is native. Another candidate for extinction is the black poplar, which mixes with cultivated hybrid poplars that have been crossed with Canadian poplars.

  But most introduced species pose no threat to native trees. Without our help, a number of them would have disappeared again after a couple of hundred years at the most. Even with our help, the survival of the new arrivals is questionable in the long term. For the pests that plague them take advantage of global trade. It is true that there is no active import of these organisms—after all, who would want to introduce damaging pests? Yet, slowly but surely, fungi and insects are making their way across the Atlantic or the Pacific in imported lumber and establishing themselves in Europe. Often they come in packing materials, such as wood pallets that haven’t been heated to sufficiently high temperatures to kill harmful organisms. And parcels sent by private individuals from overseas sometimes contain living insects. I have personal experience of this. I had an antique moccasin from North America shipped to my home in Germany. As I unpacked the leather footwear from its newspaper wrapping, a number of small brown beetles crawled in my direction. I caught them as quickly as I could, squished them, and disposed of them in
the trash. Squishing bugs might sound odd coming from the pen of a conservationist, but introduced insects, once they get established, are life threatening not only for introduced species but also for natives.

  The Asian long-horned beetle poses just such a threat. It probably traveled to Europe and other parts of the world from China in packing crates. The beetle is an inch long and has 2-inch-long antennae. To us, it’s a beautiful-looking beetle. Its dark body is flecked with white, and it has black and white bands on its legs and antennae. Deciduous trees, however, find it decidedly less attractive, because it lays its eggs individually in numerous small splits in their bark. Voracious larvae hatch and feed, and adult beetles drill thumb-sized exit holes in the trunk. These holes are then attacked by fungi, and eventually the trunk breaks. In Europe, the beetles are still concentrated in urban areas, making life even more difficult for the “street kids.” We don’t yet know if they will spread to forested areas away from urban settlements, because the beetles are lazy and prefer to stay within a radius of a few hundred yards of the place where they were born.

  Another import from Asia behaves very differently. This particular fungus, ash dieback fungus, is well on its way to finishing off most of the ash trees in Europe. Its fruiting bodies look harmless, even rather cute. They are just teeny-weeny mushrooms that grow on the stalks of fallen leaves. The fungal filaments themselves, however, run amok in the trees and kill one ash after another. A few ash trees seem to survive the repeated assaults, but it is questionable whether there will be ash forests lining the banks of European streams and rivers in the future. In connection with this, I sometimes wonder if foresters don’t play a role in the spread of the disease. I visited damaged forests in southern Germany, and then afterward, I was out and about in the forest I manage—wearing the same shoes! Might there have been tiny fungal spores on my soles that traveled into the Eifel mountains as stowaways? Whatever the case may be, since then, the first ash trees in Hümmel have also been struck with the disease.