So where does that leave us? We don’t know. But recent research has discovered something that at least calls into question the effects of transpiration and the forces of cohesion. Scientists from three institutions (the University of Bern; the Swiss Federal Institute for Forest, Snow, and Landscape Research; and the Swiss Federal Institute of Technology in Zurich) listened more closely—literally. They registered a soft murmur in the trees. Above all, at night. At this time of day, most of the water is stored in the trunk, as the crown takes a break from photosynthesis and hardly transpires at all. The trees pump themselves so full of water their trunks sometimes increase in diameter. The water is held almost completely immobile in the inner transportation tubes. Nothing flows. So where are the noises coming from? The researchers think they are coming from tiny bubbles of carbon dioxide in the narrow water-filled tubes.30 Bubbles in the pipes? That means the supposedly continuous column of water is interrupted thousands of times. And if that is the case, transpiration, cohesion, and capillary action contribute very little to water transport.
So many questions remain unanswered. Perhaps we are poorer for having lost a possible explanation or richer for having gained a mystery. But aren’t both possibilities equally intriguing?
11
— TREES AGING GRACEFULLY —
BEFORE I TALK about age, I would like to take a detour into the subject of skin. Trees and skin? First let’s approach the subject from the human point of view. Our skin is a barrier that protects our innermost parts from the outer world. It holds in fluids. It stops our insides from falling out. And all the while it releases and absorbs gas and moisture. In addition, it blocks pathogens that would just love to spread through our circulatory system. Aside from that, it is sensitive to contact, which is either pleasant and gives rise to the desire for more, or painful and elicits a defensive response.
Annoyingly, this complicated structure doesn’t stay the same forever but gradually sags as we age. Folds and wrinkles appear so that our contemporaries can playfully guess how old we are, give or take a few years. The necessary process of regeneration is not exactly pleasant, either, when looked at close up. Each of us sheds about 0.05 ounces of skin cells a day, which adds up to about a pound a year. The numbers are impressive: 10 billion particles flake off us every day.31 That doesn’t sound very attractive, but sloughing off dead skin is necessary to keep our outer organ in good condition. And in childhood we need to shed skin so that we can grow. Without the ability to renew and expand the covering Nature gives us, sooner or later, we would burst.
And how does this relate to trees? It’s just the same with them. The biggest difference is simply the vocabulary we use. The skin of Beeches, Oaks, Spruce & Co. is called bark. It fulfills exactly the same function and protects trees’ sensitive inner organs from an aggressive outer world. Without bark, a tree would dry out. And right after the loss of fluid, fungi—which have no chance of survival in healthy, moist wood—would go to town and start breaking everything down. Insects also need lower moisture levels, and if the bark is intact, they are doomed. A tree contains almost as much liquid inside it as we do, and so it’s unappealing to pests because they would, quite simply, suffocate.
A break in its bark, then, is at least as uncomfortable for a tree as a wound in our skin is for us. And, therefore, the tree relies on mechanisms similar to the ones we use to stop this from happening. Every year, a tree in its prime adds between 0.5 to 1 inch to its girth. Surely this would make the bark split? It should. To make sure that doesn’t happen, the giants constantly renew their skin while shedding enormous quantities of skin cells. In keeping with trees’ size in comparison to ours, these flakes are correspondingly larger and measure up to 8 inches across. If you take a look around on the ground under trunks in windy, rainy weather, you will see the remains lying there. The red bark of pines is particularly easy to spot.
But not every tree sheds in the same way. There are species that shed constantly (fastidious people would recommend an anti-dandruff shampoo for such cases). Then there are others that flake with restraint. You can see who’s doing what when you look at the exterior of a tree. What you see is the outer layer of bark, which is dead and forms an impervious exterior shell. This outer layer of bark also happens to be a good way of telling different species apart. This works for older trees, anyway, for the distinguishing characteristics have to do with the shapes of the cracks or, you could say, with the folds and wrinkles in a tree’s skin. In young trees of all species, the outer bark is as smooth as a baby’s bottom. As trees age, wrinkles gradually appear (beginning from below), and they steadily deepen as the years progress. Just how quickly this process plays out depends on the species. Pines, oaks, birches, and Douglas firs start early, whereas beeches and silver firs stay smooth for a long time. It all depends on the speed of shedding.
For beeches, whose silver-gray bark remains smooth until they are two hundred years old, the rate of renewal is very high. Because of this, their skin remains thin and fits their age—that is to say, their girth—exactly and, therefore, doesn’t need to crack in order to expand. It’s the same for silver firs. Pines and the like, however, drag their feet when it comes to external makeovers. For some reason, they don’t like to be parted from their baggage, perhaps because of the additional security a thick skin provides. Whatever the reason, they shed so slowly that they build up really thick outer bark and their exterior layers can be decades old. This means the outer layers originated at a time when the trees were still young and slim, and as the trees age and increase in girth, the outer layers crack way down into the youngest layer of bark that—like the bark of the beeches—fits the girth of the tree as it is now. So, the deeper the cracks, the more reluctant the tree is to shed its bark, and this behavior increases markedly with age.
The same fate catches up with beeches when they pass middle age. This is when their bark starts to get wrinkles, starting from the bottom up. As though they want to broadcast this event, they set to work getting mosses to colonize these nooks and crannies, where moisture from recent rains lingers to soak the plush cushions. You can estimate the age of beech forests from quite a distance: the higher the green growth is up the trunk, the older the trees.
Trees are individuals, and their predisposition to wrinkles varies. Some trees acquire their wrinkles at a younger age than their contemporaries. I have a few beech trees in the forest I manage that at the age of one hundred are covered from top to bottom with rough outer bark. Usually, it takes another 150 years for this to happen. There’s no research to show whether this is purely because of genetics or whether a lifetime of excess also plays a role. At least a few factors are, once again, similar to the human condition. The pines in our garden are definitely deeply fissured. This cannot be because of age alone. At about one hundred, they have just outgrown their youth. Since 1934, the year our forester’s lodge was built, they have been growing in a particularly sunny spot. Part of the property was cleared to build the lodge, and since then the pines left standing have had more light. More light, more sun, more ultraviolet radiation. The last causes changes in people’s skin, and it appears the same thing happens with trees. Intriguingly, the outer bark on the sunny side of the trees is harder, and this means it is more inflexible and more inclined to crack.
The changes I have mentioned, however, can also be because of “skin diseases.” In the same way teenage acne often leaves lifelong scars, an attack by bark flies can leave a tree with a rough exterior. In this case, there are no wrinkles; instead, there are thousands of tiny pits and pustules that never disappear no matter how long the tree lives. Sick trees can also develop festering, moist wounds. Bacteria move into these damp areas and stain them black. So, it is not only in people that the skin is a mirror to the soul (or state of well-being).
Old trees can perform another very specific function in the forest ecosystem. In Central Europe, there are no longer any true old-growth forests. The largest extensive stand of trees is between two
hundred and three hundred years old. Until these forest preserves become old-growth forests once again, we must look to the West Coast of Canada to understand the role played by ancient trees. There, Dr. Zoë Lindo of McGill University in Montreal researched Sitka spruce that were at least five hundred years old. First of all, she discovered large quantities of moss on the branches and in the branch forks of trees of this advanced age. Blue-green algae had colonized the trees’ mossy cushions. These algae capture nitrogen from the air and process it into a form the trees can use. Rain then washes this natural fertilizer down the trunks, making it available to the roots. Thus, old trees fertilize the forest and help their offspring get a better start in life. The youngsters don’t have their own moss because moss grows very slowly and takes decades to get established.32
Apart from wrinkled skin and mossy growths, there are other physical changes that indicate a tree’s age. Take, for example, the crown, which I can compare with something I have as well. Up top, my hair is thinning. It just doesn’t grow like it did when I was young. And it’s the same with the highest branches up in a tree’s crown. After a specific time—one hundred to three hundred years, depending on the species—the annual new growth gets shorter and shorter. In deciduous trees, the successive growth of such short shoots leads to curved, claw-like branches that resemble fingers plagued by arthritis. In conifers, the ramrod-straight trunks end in topmost shoots or leaders that are gradually reduced to nothing. Whereas spruce in this situation stop growing altogether, silver firs continue to grow—but out instead of up, so they look as though a large bird has built its nest in their upper branches. In Germany, where stork nests are a common sight, experts call this phenomenon “stork nest crown.” Pines redirect their growth even earlier so that by the time they reach old age, the whole crown is wide with no identifiable leader.
In any event, every tree gradually stops growing taller. Its roots and vascular system cannot pump water and nutrients any higher because this exertion would be too much for the tree. Instead, the tree just gets wider (another parallel to many people of advancing years…). The tree is also not capable of maintaining its mature height for long because its energy levels diminish slowly over the years. At first, it can no longer manage to feed its topmost twigs, and these die off. And so, just as an old person gradually loses body mass, an old tree does too. The next storm sweeps the dead twigs out of the crown, and after this cleanup, the tree looks a little fresher for a while. The process is repeated each year, reducing the crown so gradually we barely notice. Once all the topmost twigs and small branches are lost, only the thicker lower branches remain. Eventually, they die too, though they are not so easily dislodged. Now the tree can no longer hide its advanced age or its infirmity.
It’s at this time, if not before, that the bark comes into play once again. Small moist wounds have become portals for fungi to enter. The fungi advertise their triumphant advance through the tree by displaying magnificent fruiting bodies that jut out from the trunk in the shape of semicircular saucers that grow larger with each passing year. Inside, the fungi break down all barriers and penetrate deep into the wood at the heart of the tree. There, depending on the species, they consume stored sugar compounds or, even worse, cellulose and lignin, thereby decomposing the tree’s skeleton and reducing it to powder, even though the tree has been bravely resisting this process for decades. On either side of the wounds, which continue to expand, the tree grows new wood, which it builds up into thick stabilizing ridges. For a while, that helps to shore up the ruined structure against powerful winter storms. Then one day, it’s all over. The trunk snaps and the tree’s life is at an end. “Finally,” you can almost hear the young trees-in-waiting sigh. In the years to come, they will quickly push their way up past the crumbling remains. But service in the forest doesn’t end when life ends. The rotting cadaver continues to play an important role in the ecosystem for hundreds of years. But more on that later.
12
— MIGHTY OAK OR —
MIGHTY WIMP?
WHEN I WALK through the forest I manage, I often see oaks in distress. And sometimes they are very distressed indeed. Anxious suckers sprouting at the base of the trunk are a dead giveaway. These spindly tufts of growth ring the tree and usually quickly wither away. They indicate that the tree is engaged in an extended fight to the death, and it is panicking. It doesn’t make any sense for the oak to attempt to grow leaves so close to the ground, because oaks are trees that need light. They need very bright conditions to photosynthesize. Their ground-hugging solar panels don’t produce any energy in the twilight of the understory, and the superfluous arrays are quickly done away with.
A healthy tree doesn’t bother to sink energy into developing that kind of growth, preferring instead to extend the reach of its crown up above. At least, that’s what it does when it is left in peace. However, oaks in Central European forests are not left in peace, because this is the homeland of the beech. Now, the beech is an amazingly socially oriented tree, but only when it comes to its own kind. Beeches harass other species, such as oaks, to such an extent that they weaken.
It all starts quite slowly and harmlessly when a jay buries a beechnut at the feet of a mighty oak. Because the jay has enough food cached away elsewhere, this beechnut lies undisturbed, and come spring, it sprouts. Slowly, over the course of a number of decades, the sapling grows upward, quietly and unnoticed. True, the young tree doesn’t have its mother, but at least the old oak provides shade, and so it helps to raise the youngster at a measured, healthy pace. What looks harmonious above ground turns out to be the beginning of a fight for survival below the surface.
The beech roots penetrate every space the oak is not using, burrowing beneath the old trunk and snapping up water and food the big tree had counted on using for itself. This causes a subtle weakening in the oak. After about 150 years, the little beech tree has grown so tall that it is gradually growing into the crown of the oak. Into, and after a few more decades, through and past, for in contrast to its competitor, the beech can extend its crown and keep growing almost all its life.
By now, the beech leaves are getting direct sunlight, so the tree has all kinds of energy to expand. It grows an impressive crown, which catches 97 percent of the sunlight, just as beech crowns always do. The oak finds itself relegated to the second tier, where its leaves make a vain attempt to snatch some light. Sugar production is drastically reduced, reserves are used up, and the oak slowly starves.
The oak realizes it cannot beat this stiff competition and will never be able to grow tall shoots to overtake the beech. In its time of need, perhaps in the face of rising panic, it does something that goes against all the rules: it grows new shoots and leaves way down at its base. The leaves are particularly large and soft and can manage with less light than the leaves up in the crown. However, 3 percent is simply not enough. An oak is not a beech. Therefore, these sprouts of anxiety wither, and the precious energy it took to grow them has been squandered. At this stage of starvation, the oak can hold out for a few more decades, but at some point, it gives up. Its powers are waning, and woodboring beetles might come along and put it out of its misery. The beetles lay their eggs under the bark, and the wriggling larvae make short work of feeding on the tree’s skin and ending the defenseless tree’s life. So, is the mighty oak really a mighty wimp? How did such a weakling of a tree become the symbol for fortitude and longevity?
No matter how badly this tree might fare in most forests in comparison with beeches, an oak can be very tough if it doesn’t have any competition. Consider open spaces—specifically our cultivated landscape. Whereas beeches last barely more than two hundred years outside the cozy atmosphere of their native forests, oaks growing near old farmyards or out in pastures easily live for more than five hundred. And what if an oak gets a deep wound or a wide crack in its trunk as a result of a lightning strike? That doesn’t matter to the oak, because its wood is permeated with substances that discourage fungi and severely slow down f
ungal decomposition. These tannins also scare off most insects and, incidentally and inadvertently, improve the taste of wine—should a barrel ever be made from the tree. (Think “oaked” wine.) Even severely damaged trees with major branches broken off can grow replacement crowns and live for a few hundred years longer. Most beech trees wouldn’t be able to do that, and they certainly wouldn’t be able to do it outside the forest without their beloved networked connections. A storm-battered beech is able to hang on for no more than a couple of decades.
In the forest I manage, oaks show they are made from very stern stuff. On a particularly warm south-facing slope, there are quite a few oaks clawing at naked rock with their roots. When the summer sun heats the stones unbearably, the last drops of water evaporate. In winter, bone-chilling frost penetrates deeply in the absence of the thick protective layer of earth mixed with copious quantities of rotting leaves that you find on the forest floor. The slightest wind catches the slope, so only a few meager lichens grow there, and they do little to moderate temperature extremes. The result? After a century, the trees, or rather miniature trees, are no thicker than your wrist and barely more than 15 feet tall. While their colleagues have grown substantial trunks and more than 100 feet tall in the cozy environment of the forest, these frugal trees make do and content themselves with growing no taller than shrubs. But they do survive! The advantage of this hardscrabble existence is that other species gave up long ago. So, it seems, there can be advantages to a life of deprivation if it means you don’t have to worry about competition from other trees.
The Hidden Life of Trees: What They Feel, How They CommunicateDiscoveries from a Secret World Page 6