The Hidden Life of Trees: The International Bestseller – What They Feel, How They Communicate

8

— TREE SCHOOL —

THIRST IS HARDER for trees to endure than hunger, because they can satisfy their hunger whenever they want. Like a baker who always has enough bread, a tree can satisfy a rumbling stomach right away using photosynthesis. But even the best baker cannot bake without water, and the same goes for a tree: without moisture, food production stops.

A mature beech tree can send more than 130 gallons of water a day coursing through its branches and leaves, and this is what it does as long as it can draw enough water up from below.20 However, the moisture in the soil would soon run out if the tree were to do that every day in summer. In the warmer seasons, it doesn’t rain nearly enough to replenish water levels in the desiccated soil. Therefore, the tree stockpiles water in winter.

In winter, there’s more than enough rain, and the tree is not consuming water, because almost all plants take a break from growing at that time of year. Together with belowground accumulation of spring showers, the stockpiled water usually lasts until the onset of summer. But in many years, water then gets scarce. After a couple of weeks of high temperatures and no rain, forests usually begin to suffer. The most severely affected trees are those that grow in soils where moisture is usually particularly abundant. These trees don’t know the meaning of restraint and are lavish in their water use, and it is usually the largest and most vigorous trees that pay the price for this behavior.

In the forest I manage, the stricken trees are usually spruce, which burst not at every seam but certainly along their trunks. If the ground has dried out and the needles high up in the crown are still demanding water, at some point, the tension in the drying wood simply becomes too much for the tree to bear. It crackles and pops, and a tear about 3 feet long opens in its bark. This tear penetrates deep into the tissue and severely injures the tree. Fungal spores immediately take advantage of the tear to invade the innermost parts of the tree, where they begin their destructive work. In the years to come, the spruce will try to repair the wound, but the tear keeps reopening. From some distance away, you can see a black channel streaked with pitch that bears witness to this painful process.

And with that, we have arrived at the heart of tree school. Unfortunately, this is a place where a certain amount of physical punishment is still the order of the day, for Nature is a strict teacher. If a tree does not pay attention and do what it’s told, it will suffer. Splits in its wood, in its bark, in its extremely sensitive cambium (the life-giving layer under the bark): it doesn’t get any worse than this for a tree. It has to react, and it does this not only by attempting to seal the wound. From then on, it will also do a better job of rationing water instead of pumping whatever is available out of the ground as soon as spring hits without giving a second thought to waste. The tree takes the lesson to heart, and from then on it will stick with this new, thrifty behavior, even when the ground has plenty of moisture—after all, you never know!

It’s no surprise that it is spruce growing in areas with abundant moisture that are affected in this way: they are spoiled. Barely half a mile away, on a dry, stony, south-facing slope, things look very different. At first, I had expected damage to the spruce trees here because of severe summer drought. What I observed was just the opposite. The tough trees that grow on this slope are well versed in the practices of denial and can withstand far worse conditions than their colleagues who are spoiled for water. Even though there is much less water available here year round—because the soil retains less water and the sun burns much hotter—the spruce growing here are thriving. They grow considerably more slowly, clearly make better use of what little water there is, and survive even extreme years fairly well.

A much more obvious lesson in tree school is how trees learn to support themselves. Trees don’t like to make things unnecessarily difficult. Why bother to grow a thick, sturdy trunk if you can lean comfortably against your neighbors? As long as they remain standing, not much can go wrong. However, every couple of years, a group of forestry workers or a harvesting machine moves in to harvest 10 percent of the trees in commercial forests in Central Europe. And in natural forests, it is the death from old age of a mighty mother tree that leaves surrounding trees without support. That’s how gaps in the canopy open up, and how formerly comfortable beeches or spruce find themselves suddenly wobbling on their own two feet—or rather, on their own root systems. Trees are not known for their speed, and so it takes three to ten years before they stand firm once again after such disruptions.

The process of learning stability is triggered by painful micro-tears that occur when the trees bend way over in the wind, first in one direction and then in the other. Wherever it hurts, that’s where the tree must strengthen its support structure. This takes a whole lot of energy, which is then unavailable for growing upward. A small consolation is the additional light that is now available for the tree’s own crown, thanks to the loss of its neighbor. But, here again, it takes a number of years for the tree to take full advantage of this. So far, the tree’s leaves have been adapted for low light, and so they are very tender and particularly sensitive to light. If the bright sun were to shine directly on them now, they would be scorched—ouch, that hurts! And because the buds for the coming year are formed the previous spring and summer, it takes a deciduous tree at least two growing seasons to adjust. Conifers take even longer, because their needles stay on their branches for up to ten years. The situation remains tense until all the green leaves and needles have been replaced.

The thickness and stability of a trunk, therefore, build up as the tree responds to a series of aches and pains. In a natural forest, this little game can be repeated many times over the lifetime of a tree. Once the gap opened by the loss of another tree is overcome and everyone has extended their crowns so far out that the window of light into the forest is, once again, closed, then everyone can go back to leaning on everyone else. When that happens, more energy is put into growing trunks tall instead of wide, with predictable consequences when, decades later, the next tree breathes its last.

So, let’s return to the idea of school. If trees are capable of learning (and you can see they are just by observing them), then the question becomes: Where do they store what they have learned and how do they access this information? After all, they don’t have brains to function as databases and manage processes. It’s the same for all plants, and that’s why some scientists are skeptical and why many of them banish to the realm of fantasy the idea of plants’ ability to learn. But, once again, along comes the Australian scientist Dr. Monica Gagliano.

Gagliano studies mimosas, also called “sensitive plants.” Mimosas are tropical creeping herbs. They make particularly good research subjects, because it is easy to get them a bit riled up and they are easier to study in the laboratory than trees are. When they are 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 apply their lesson weeks later, even without any further tests.21

It’s a shame you can’t transport entire beeches or oaks into the laboratory to find out more about learning. But, at least as far as water is concerned, there is research in the field that reveals more than just behavioral changes: when trees are really thirsty, they begin to scream. If you’re out in the forest, you won’t be able to hear them, because this all takes place at ultrasonic levels. Scientists at the Swiss Federal Institute for Forest, Snow, and Landscape 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.22 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 would 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 I mentioned earlier, 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.

9

— UNITED WE STAND, —DIVIDED WE FALL

TREES ARE VERY social beings, and they help each other out. But that is not sufficient for successful survival in the forest ecosystem. Every species of tree tries to procure more space for itself, to optimize its performance, and, in this way, to crowd out other species. After the fight for light, it is the fight for water that finally decides who wins. Tree roots are very good at tapping into damp ground and growing fine hairs to increase their surface area so that they can suck up as much water as possible. Under normal circumstances, that is sufficient, but more is always better. And that is why, for millions of years, trees have paired up with fungi.

Fungi are amazing. They don’t really conform to the one-size-fits-all system we use to classify living organisms as either animals or plants. By definition, plants create their own food out of inanimate material, and therefore, they can survive completely independently. It’s no wonder that green vegetation must sprout on barren, empty ground before animals can move in, for animals can survive only if they eat other living things. Incidentally, neither grass nor young trees like it very much when cattle or deer munch on them. Whether it’s a wolf ripping apart a wild boar or a deer eating an oak seedling, in both cases there is pain and death. Fungi are in between animals and plants. Their cell walls are made of chitin—a substance never found in plants—which makes them more like insects. In addition, they cannot photosynthesize and depend on organic connections with other living beings they can feed on.

Over decades, a fungus’s underground cottony web, known as mycelium, expands. There is a honey fungus in Switzerland that covers almost 120 acres and is about a thousand years old.23 Another in Oregon is estimated to be 2,400 years old, extends for 2,000 acres, and weighs 660 tons.24 That makes fungi the largest known living organisms in the world. The two aforementioned giants are not tree friendly; they kill them as they prowl the forest in search of edible tissue. So let’s take a look instead at amicable teamwork between fungi and trees. With the help of mycelium of an appropriate species for each tree—for instance, the oak milkcap and the oak—a tree can greatly increase its functional root surface so that it can suck up considerably more water and nutrients. You find twice the amount of life-giving nitrogen and phosphorus in plants that cooperate with fungal partners than in plants that tap the soil with their roots alone.

To enter into a partnership with one of the many thousands of kinds of fungi, a tree must be very open—literally—because the fungal threads grow into its soft root hairs. There’s no research into whether this is painful or not, but as it is something the tree wants, I imagine it gives rise to positive feelings. However the tree feels, from then on, the two partners work together. The fungus not only penetrates and envelops the tree’s roots, but also allows its web to roam through the surrounding forest floor. In so doing, it extends the reach of the tree’s own roots as the web grows out toward other trees. Here, it connects with other trees’ fungal partners and roots. And so a network is created, and now it’s easy for the trees to exchange vital nutrients (see chapter 3, “Social Security”) and even information—such as an impending insect attack.

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 carbohydrates, which their partner tree has to deliver. And fungi are not exactly dainty in their requirements. They demand up to a third of the tree’s total food production in return for their services.25 It makes sense, in a situation where you are 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 information is useful for them, the fungi begin to produce plant hormones that direct the tree’s cell growth to their advantage.26

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. These diverted pollutants turn up every fall in the pretty fruiting bodies we bring home in the form of porcini, cèpe, or bolete mushrooms. No wonder radioactive cesium, which was found in soil even before the nuclear reactor disaster in Chernobyl in 1986, is mostly found in mushrooms.

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 are 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, does not 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 is only when the last of these passes away that it is really in trouble.

Fungi are much more sensitive. Many species seek out trees that suit them, and once they have reserved them for themselves, they are joined to them for better or for worse. Species that like only birches or larches, for instance, are called “host specific.” Others, such as chanterelles, get along with many different trees: oaks, birches, and spruce. What is important is whether there is still a bit of room underground. And competition is fierce. In oak forests alone, more than a hundred different species of fungi may be present in different parts of the roots of the same tree. From the oaks’ point of view, this is a very practical arrangement. If one fungus drops out because environmental conditions change, the next suitor is already at the door.

Researchers have discovered that fungi also hedge their bets. Dr. Suzanne Simard discovered that their networks are connected not only to a specific tree species but also to trees of different species.27 Simard injected into a birch tree radioactive carbon that moved through the soil and into the fungal network of a neighboring Douglas fir. Although many species of tree fight each other mercilessly above ground and even try to crowd out each other’s root systems, the fungi that populate them seem to be intent on compromise. Whether they actually want to support foreign host trees or only fellow fungi in need of help (which these fungi then pass on to their trees) is as yet unclear.

I suspect fungi are a little more forward “thinking” than their larger partners. Among trees, each species fights other species. Let’s assume the beeches native to Central Europe could emerge victorious in most forests there. Would this really be an advantage? What would happen if a new pathogen came along that infected most of the beeches and killed them? In that case, wouldn’t it be more advantageous if there were a certain number of other species around—oaks, maples, ashes, or firs—that would continue to grow and provide the shade needed for a new generation of young beeches to sprout and grow up? Diversity provides security for ancient forests. Because fungi are also very dependent on stable conditions, they support other species underground and protect them from complete collapse to ensure that one species of tree doesn’t manage to dominate.

If things become dire for the fungi and their trees despite all this support, then the fungi can take radical action, as in the case of the pine and its partner Laccaria bicolor, or the bicolored deceiver. When there is a lack of nitrogen, the latter releases a deadly toxin into the soil, which causes minute organisms such as springtails to die and release the nitrogen tied up in their bodies, forcing them to become fertilizer for both the trees and the fungi.28

I have introduced you to the most important tree helpers; however, there are many more. Consider the woodpeckers. I wouldn’t call them real helpers, but they are of at least some benefit to trees. When bark beetles infest spruce, for example, things get dicey. The tiny insects multiply so rapidly they can kill a tree very quickly by consuming its life-giving cambium layer. If a great spotted woodpecker gets wind of this, it’s on the spot right away. Like an oxpecker on a rhinoceros, it climbs up and down the trunk looking for the voracious, fat white larvae. It pecks these out (not thinking particularly of the tree), sending chunks of bark flying. Sometimes this can save the spruce from further damage. Even if the tree doesn’t come through this procedure alive, its fellow trees are still protected because now there won’t be any adult beetles hatching and flying around. The woodpecker is not in the slightest bit interested in the well-being of the tree, and you can see this particularly clearly in its nesting cavities. It often makes these in healthy trees, severely wounding them as it hacks away. Although the woodpecker frees many trees of pests—for instance, oaks from woodboring beetles—it is more a side effect of its behavior than its intent.

Woodboring beetles can be a threat to thirsty trees in dry years, because the trees are in no position to defend themselves from their attackers. Salvation can come in the form of the black-headed cardinal beetle. In its adult form, it is harmless, feeding on aphid honeydew and plant juices. Its offspring, however, need flesh, and they get this in the form of beetle larvae that live under the bark of deciduous trees. So some oaks have cardinal beetles to thank for their survival. And things can get dire for the beetles as well: once all the children of other species of beetles have been eaten, the larvae turn on their own kind.

10

— THE MYSTERIES OF —MOVING WATER

HOW DOES WATER make its way up from the soil into the tree’s leaves? For me, the way this question is answered sums up our current approach to what we know about the forest. For water transport is a relatively simple phenomenon to research—simpler at any rate than investigating whether trees feel pain or how they communicate with one another—and because it appears to be so uninteresting and obvious, university professors have been offering simplistic explanations for decades. This is one reason why I always have fun discussing this topic with students. Here are the accepted answers: capillary action and transpiration.

You can study capillary action every morning at breakfast. Capillary action is what makes the surface of your coffee stand a few fractions of an inch higher than the edge of your cup. Without this force, the surface of the liquid would be completely flat. The narrower the vessel, the higher the liquid can rise against gravity. And the vessels that transport water in deciduous trees are very narrow indeed: they measure barely 0.02 inches across. Conifers restrict the diameter of their vessels even more, to 0.0008 inches. Narrow vessels, however, are not enough to explain how water reaches the crown of trees that are more than 300 feet tall. In even the narrowest of vessels, there is only enough force to account for a rise of 3 feet at most.29

Ah, but we have another candidate: transpiration. In the warmer part of the year, leaves and needles transpire by steadily breathing out water vapor. In the case of a mature beech, the tree exhales hundreds of gallons of water a day. This exhalation causes suction, which pulls a constant supply of water up through the transportation pathways in the tree. Suction works as long as the columns of water are continuous. Bonding forces cause the water molecules to adhere to each other, and because they are strung together like links in a chain, as soon as space becomes available in the leaf thanks to transpiration, the bonded molecules pull each other a little higher up the trunk.

And because even this is not enough, osmosis also comes into play. When the concentration of sugar in one cell is higher than in the neighboring cell, water flows through the cell walls into the more sugary solution until both cells contain the same percentage of water. And when that happens from cell to cell up into the crown, water makes its way up to the top of the tree.

Hmm. When you measure water pressure in trees, you find it is highest shortly before the leaves open up in the spring. At this time of year, water shoots up the trunk with such force that if you place a stethoscope against the tree, you can actually hear it. In the northeastern U.S. and Canada, people make use of this phenomenon to harvest syrup from sugar maples, which are often tapped just as the snow is melting. This is the only time of the year when the coveted sap can be harvested. This early in the year, there are no leaves on deciduous trees, which means there can be no transpiration. And capillary action can be only a partial contributor because the aforementioned rise of 3 feet is hardly worth mentioning. Yet at precisely this time, the trunk is full to bursting. So that leaves us with osmosis, but this seems equally unlikely to me. After all, osmosis works only in the roots and leaves, not in the trunk, which consists not of cells attached one to the other but of long, continuous tubes for transporting water.