The Hidden Life of Trees: What They Feel, How They CommunicateDiscoveries from a Secret World
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The thick outer layer of the oak’s bark is also much more robust than the smooth, thin skin of the beech, and it can take a great deal of punishment. This has given rise to a saying in German, “Was schert es eine alte Eiche, wenn sich ein Wildschwein in ihr scheuert?” which roughly translates as: “It’s no skin off an old oak’s back if a wild boar wants to use its bark as a scratching post.”
13
— SPECIALISTS —
TREES CAN GROW in many extreme environments. Can? Indeed, they must! When a seed falls from a tree, its landing site can be changed only if the wind blows or an animal moves it. And once it has sprouted in the spring, the die is cast. From that point forward, the seedling is bound to this little piece of earth for the rest of its life and must take whatever life hands out. And for most tree youngsters, life hands out a whole series of challenges, for the place where a seed happens to end up often turns out to be highly unsuitable.
It’s either too dark, as it is when a light-hungry bird cherry sprouts under large beeches. Or it’s too bright, which is the case for beech youngsters whose delicate foliage gets scorched by the blazing sun in clearings. Marshy forest floors rot the roots of most trees, whereas in dry, sandy soil they die of thirst. Places with no nourishing soil at all—rocks or branch forks in big trees—are particularly unfortunate landing sites. And sometimes luck doesn’t last. Consider seeds that come to rest in the tall stumps left when trees snap. The settled seeds grow into little trees whose roots descend into the moldering wood. But when the first unusually dry summer rolls around, causing the last of the moisture to evaporate even from decaying wood, the would-be winners wither and die.
Many Central European tree species have similar ideas about the ideal place to live, because similar criteria for well-being hold true for most of them. They love nutrient-rich, loose, crumbly soil that is well aerated to a depth of many feet. The ground should be nice and moist, especially in summer. But it shouldn’t get too hot, and in winter, it shouldn’t freeze too much. Snowfall should be moderate but sufficient that when the snow melts, it gives the soil a good soaking. Fall storms should be moderated by sheltering hills or mountain ridges, and the forest shouldn’t harbor too many fungi or insects that attack bark or wood.
If trees could dream of an earthly paradise, this is what it would look like. But apart from a few small pockets, these ideal conditions are nowhere to be found. And that is a good thing for species diversity. If Central Europe were such a paradise, the competition would be won almost exclusively by beeches. They know exactly how to exploit abundance, and they suppress competitors by growing up through the crowns of other trees and then covering the losers with their upper branches. If a tree is going to survive such powerful competition, it has to come up with an alternative strategy, but deviations from the arboreal idea of paradise make life difficult for trees, and any tree that wants to find an ecological niche next to a beech must be ready to practice self-denial in one area or another. But are we really talking about ecological niches? As almost no habitat on Earth offers ideal living conditions, it’s actually got more to do with the tree adapting than the niche being ideal. There are any number of difficult sites, and a tree that can get along in such places can conquer an enormous geographic range. And that’s basically what the spruce has done.
Spruce can gain a foothold everywhere where summers are short and winters are bitterly cold—from the Far North to mountain ranges in Central Europe near the tree line. Because the growing season in Siberia, Canada, and Scandinavia is often only a few weeks long, a beech growing there wouldn’t even have a chance to open up all its leaves before the end of the season. And the winter is so bitterly cold that the tree would have frostbite long before it was over. In such regions, it’s the spruce that prevails.
Spruce store essential oils in their needles and bark, which act like antifreeze. And that’s why they don’t need to jettison their green finery but keep it wrapped around their branches in the cold season. As soon as the weather warms up in the spring, they can start photosynthesizing. Not a day is lost, and even if there are only a few weeks in which sugar and wood can be produced, the tree can still grow an inch or two every year.
However, holding on to needles is also extremely risky. Snow lands on the branches and accumulates until the load is so heavy it can break the tree. The spruce employs two defense mechanisms to avoid this. First, a spruce usually grows an absolutely straight trunk. When a structure is nice and vertical, it is difficult to upset its equilibrium. Second, in summer, the branches stick out horizontally. As soon as snow lands on them, they gradually angle down until they are layered one on top of the other like tiles on a roof. Arranged like this, they mutually support each other, and the tree, when viewed from above, presents a much skinnier profile. This means that most of the snow falls around the tree and not on it. Spruce growing in snowy areas at high altitudes or in the Far North also form very long, narrow crowns with short branches, and these slim the trees down even more.
Holding on to needles flirts with yet another danger. Needles increase the surface area the tree presents to the wind, and therefore, spruce are prone to toppling over in winter storms. The only thing that protects them is their extremely slow rate of growth. Trees hundreds of years old are often no taller than 30 feet, and statistically speaking, the danger of being blown over doesn’t increase significantly until the trees are more than 80 feet tall.
The natural forest in Central European latitudes is overwhelmingly beech, and beech trees allow very little light to reach the ground. The yew, the epitome of frugality and patience, has decided to make the most of these conditions. Because it knows it can’t hold a candle to the beech in the growth department, it has decided to specialize in the forest understory. And here, with the help of the 3 percent of residual light the beeches allow to filter down through their leaves, it grows. Under these conditions, it can take a whole century before a yew reaches 20 to 30 feet and sexual maturity, and a lot can happen to it in this time. Herbivores can nibble it down and set it back by decades. Or worse, a dying beech could knock it over completely. But this tough little tree has taken precautions. Right from the beginning, it puts considerably more energy into building up its root system than other species of trees. Here, it stashes away nutrients, and if misfortune strikes above ground, it grows right back without missing a beat. This often leads to the formation of multiple trunks, which may merge when the tree reaches an advanced age, giving the tree an untidy appearance. And boy can these trees grow old! Living to be a thousand years old or more, they easily outstrip the closest competition, and over the course of centuries, they increasingly get to bask in the sun whenever an old tree growing above them breathes its last. Despite this, yews grow no more than 65 feet tall. They are fine with this, and they don’t strive to reach greater heights.
The hornbeam (which, though you wouldn’t know it from its name, is related to the birch) tries to imitate the yew, but is not quite so frugal in its habits and needs a bit more light. But it does survive under the beeches, even though it doesn’t grow into a large tree here. A hornbeam rarely grows taller than 65 feet anyway, and it reaches that height only when it grows under trees that allow light through, such as oaks. Here, the hornbeam is free to develop, and as it doesn’t get in the way of the larger oaks, at least, there is plenty of room for both species. But often along comes a beech that pulls rank on both of them and grows up and over the oaks. The hornbeam can compete only where there is not only a great deal of shade but also severe drought and heat. Here, beeches have to give up eventually, which means, on dry south-facing slopes at least, the hornbeam stands a chance of emerging the winner.
In swampy ground and standing, oxygen-depleted water, the roots of most trees don’t survive and the trees die off. You find these conditions near springs or along the banks of streams where the flood plain is regularly underwater. Say a beechnut finds itself there by mistake and sprouts. At first, it might grow into an imposing tree. But
sometime during a summer thunderstorm, the tree will fall over when its rotten roots lose their footing. Spruce, pines, hornbeams, and birches run into similar problems when their roots spend some or all of their time in stagnant water. It’s completely the opposite for alders. At around 100 feet, it’s true they don’t grow as tall as their competitors, but they have no problem growing on unpopular swampy ground. Their secret is a system of air ducts inside their roots. These transport oxygen to the tiniest tips, a bit like divers who are connected to the surface via a breathing tube. In addition, the trees have cork cells in the lower parts of their trunks, which allow air to enter. It is only when the water level remains higher than these breathing holes for an extended period of time that the alders weaken sufficiently for their roots to fall victim to aggressive fungi.
14
— TREE OR NOT TREE? —
WHAT IS A tree exactly? The dictionary defines it as a woody plant with a trunk from which branches grow. So the main shoot must be dominant and grow steadily upward or the plant is classified as a shrub, which has many smaller trunks—or rather branches—that originate from a common rootstock. But what about size? Personally, I’m always bothered when I see reports about Mediterranean forests that look to me like a collection of bushes. Trees are, after all, majestic beings, under whose crowns we seem as insignificant as ants in the grass. But then again, on a journey to Lapland, I stumbled upon completely different ambassadors of the tree family that made me feel like Gulliver in Lilliput.
I’m talking about dwarf trees on the tundra, which are sometimes trampled to death by travelers who don’t even know they are there. It can take these trees a hundred years to grow just 8 inches tall. I have to say that science doesn’t recognize them as trees, and it doesn’t accord tree status to the Arctic shrubby birch, either (as you can tell by its name). The latter can grow little trunks up to 10 feet tall, but mostly they remain below eye level and, therefore, are clearly not taken seriously. But if you were to apply the same measure to other trees, then small beeches or mountain ash wouldn’t count as trees either. These two are often browsed on so heavily by large mammals such as deer that they grow multiple shoots like bushes and hold out at a height of 20 inches for decades.
And what if you cut a tree down? Is it then dead? What about the centuries-old stump I introduced you to at the beginning of this book that is still alive today, thanks to its comrades? Is that a tree? And, if it isn’t, then what is it? It gets even more complicated when a new trunk grows out of an old stump. In many woods, this happens all the time. For centuries in Europe, deciduous trees were cut right down to the base of their trunks by charcoal burners, who harvested them to make charcoal. New trunks grew from the base, forming the foundation for the deciduous woods we have today. Oak and hornbeam forests, in particular, originate from this kind of harvesting, which is known as coppicing. In these forests, the cycle of cutting back and allowing the trees to regrow was repeated every few decades, so the trees never grew tall or matured. Coppicing was popular because people were so poor in those days that they couldn’t afford to wait any longer for new wood. You can spot these relics of bygone times when you take a walk in a European forest. Look for trees that have numerous bushy trunks or thick callouses at the base where periodic felling has encouraged a proliferation of growth. Are these trunks now young trees, or alternatively, are they really thousands of years old?
This is a question also asked by scientists, among them a group researching ancient spruce in Dalarna province in Sweden. The oldest spruce in Dalarna has grown a carpet of flat shrubby growth around its single small trunk. All this growth belongs to one tree, and its roots were tested using carbon 14 dating. Carbon 14 is a radioactive carbon that continuously forms in the atmosphere and then gradually decays. This means that the ratio of carbon 14 to other carbon in the atmosphere is always the same. Once carbon 14 is incorporated into inactive biomasses, for instance wood, the process of decay continues unabated, but no new radioactive carbon is accumulated. The lower the amount of radioactive carbon it contains, the older the tissue must be.
Research revealed the spruce to be an absolutely unbelievable 9,550 years old. The individual shoots were younger, but these new growths from the past few centuries were not considered to be stand-alone trees but part of a larger whole.33 And, I think, quite rightly so. The root is certainly a more decisive factor than what is growing above ground. After all, it is the root that looks after the survival of an organism. It is the root that has withstood severe changes in climatic conditions. And it is the root that has regrown trunks time and time again. It is in the roots that centuries of experience are stored, and it is this experience that has allowed the tree’s survival to the present day. As a result of this research on the spruce, a number of scientific schools of thought have been thrown overboard. On the one hand, before this research, no one had any idea that spruce could live for much more than five hundred years; on the other, until then, people had assumed that this conifer first arrived in this part of Sweden two thousand years ago after the ice retreated. For me, this inconspicuous small plant is a symbol for how little we understand about forests and trees and how many wonders we have yet to discover.
So, let’s get back to why the roots are the most important part of a tree. Conceivably, this is where the tree equivalent of a brain is located. Brain? you ask. Isn’t that a bit farfetched? Possibly, but now we know that trees can learn. This means they must store experiences somewhere, and therefore, there must be some kind of a storage mechanism inside the organism. Just where it is, no one knows, but the roots are the part of the tree best suited to the task. The old spruce in Sweden also shows that what grows underground is the most permanent part of the tree—and where else would it store important information over a long period of time? Moreover, current research shows that a tree’s delicate root network is full of surprises.
It is now an accepted fact that the root network is in charge of all chemical activity in the tree. And there’s nothing earth shattering about that. Many of our internal processes are also regulated by chemical messengers. Roots absorb substances and bring them into the tree. In the other direction, they deliver the products of photosynthesis to the tree’s fungal partners and even route warning signals to neighboring trees. But a brain? For there to be something we would recognize as a brain, neurological processes must be involved, and for these, in addition to chemical messages, you need electrical impulses. And these are precisely what we can measure in the tree, and we’ve been able to do so since as far back as the nineteenth century. For some years now, a heated controversy has flared up among scientists. Can plants think? Are they intelligent?
In conjunction with his colleagues, František Baluška from the Institute of Cellular and Molecular Botany at the University of Bonn 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.34 When a root feels its way forward in the ground, it is 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.
Right now, the majority of plant researchers are skeptical about whether such behavior points to a repository for intelligence, the faculty of memory, and emotions. Among other things, they get worked up about carrying over findings in similar situations with animals and, at the end of the day, about how this threatens to blur the boundary between plants and animals. And so what? What would be so awful about that? The distinction between plant and animal is, after all, arbitrary and depends on the way an organism feeds itself: the former photosynthesizes and the latter eats other living beings. Finally, the only other big differenc
e is in the amount of time it takes to process information and translate it into action. Does that mean that beings that live life in the slow lane are automatically worth less than ones on the fast track? Sometimes I suspect we would pay more attention to trees and other vegetation if we could establish beyond a doubt just how similar they are in many ways to animals.
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— IN THE REALM OF —
DARKNESS
FOR US HUMANS, soil is more obscure than water, both literally and metaphorically. Whereas it is generally accepted that we know less about the ocean floor than we know about the surface of the moon,35 we know even less about life in the soil. Sure, there’s a wealth of species and facts that have been discovered and that we can read about. But we know only a tiny fraction of what there is to know about the complex life that busies itself under our feet. Up to half the biomass of a forest is hidden in this lower story. Most lifeforms that bustle about here cannot be seen with the naked eye. And that is probably the reason we are not as interested in them as we are in, say, wolves, black woodpeckers, or fire salamanders. For trees, though, these creatures are probably way more important. A forest would have no problem doing without its larger inhabitants. Deer, wild boar, carnivores, and even most birds wouldn’t leave any yawning gaps in the ecosystem. Even if they were all to disappear at once, the forest would simply go on growing without many adverse effects. Things are completely different when it comes to the tiny creatures under their feet. 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.