The Hidden Life of Trees: What They Feel, How They CommunicateDiscoveries from a Secret World
Page 12
But why bother with all this extravagance? Many conifers demonstrate that things can be done differently. They simply keep all their green finery on their branches and thumb their noses at the idea of an annual makeover. To protect its needles from freezing, a conifer fills them with antifreeze. To ensure it doesn’t lose water to transpiration over the winter, it covers the exterior of its needles with a thick layer of wax. As an extra precaution, the skin on its needles is tough and hard, and the small breathing holes on the underside are buried extra deep. All these precautions combine to prevent the tree from losing any significant amount of water. Such a loss would be tragic, because the tree wouldn’t be able to replenish supplies from the frozen ground. It would dry out and could then die of thirst.
In contrast to needles, leaves are soft and delicate—in other words, they are almost defenseless. It’s little wonder beeches and oaks drop them as quickly as they can at the first hint of frost. But why didn’t these trees simply develop thicker skins and antifreeze over the course of their evolution? Does it really make sense to grow millions of new leaves per tree every year, use them for a few months, and then go to the trouble of discarding them again? Apparently, evolution says it does, because when it developed deciduous trees about 100 million years ago, conifers had already been around on this planet for 170 million years. This means deciduous trees are a relatively modern invention. When you take a closer look, their behavior in fall actually makes a lot of sense. By discarding their leaves, they avoid a critical force—winter storms.
When storms blow through forests in Central Europe from October on, it’s a matter of life and death for many trees. Winds blowing at more than 60 miles an hour can uproot large trees, and some years, 60 miles an hour is a weekly occurrence. Fall rains soften the forest floor, so it’s difficult for tree roots to find purchase in the muddy soil. The storms pummel mature trunks with forces equivalent to a weight of approximately 220 tons. Any tree unprepared for the onslaught can’t withstand the pressure and falls over. But deciduous trees are well prepared. To be more aerodynamic, they cast off all their solar panels. And so a huge surface area of 1,200 square yards disappears and sinks to the forest floor.52 This is the equivalent of a sailboat with a 130-foot-tall mast dropping a 100-by-130-foot mainsail. And that’s not all. The trunk and branches are shaped so that their combined wind resistance is somewhat less than that of a modern car. Moreover, the whole construction is so flexible that the forces of a strong gust of wind are absorbed and distributed throughout the tree.
These measures all work together to ensure that hardly anything happens to deciduous trees over the winter. If there’s an unusually strong hurricane-force wind—the kind that happens only every five to ten years in Europe—the tree community stands together to help each individual tree. Every trunk is different. Each has its own pattern of woody fibers, a testament to its unique history. This means that, after the first gust—which bends all the trees in the same direction at the same time—each tree springs back at a different speed. And usually it is the subsequent gusts that do a tree in, because they catch the tree while it’s still severely bowed and bend it over again, even farther this time. But in an intact forest, every tree gets help. As the crowns swing back up, they hit each other, because each of them is straightening up at its own pace. While some are still moving backwards, others are already swinging forward again. The result is a gentle impact, which slows both trees down. By the time the next gust of wind comes along, the trees have almost stopped moving altogether and the struggle begins all over again. I never tire of watching tree crowns move back and forth. I can see both the movement of the whole community and the movements of individual trees. Bear in mind, however, that it’s never a good idea to go into the forest during a storm.
Let’s get back to the subject of dropping leaves. With every winter they survive, the trees prove that this makes sense and that producing new leaves every year is worth the energy it takes. But it brings up completely different dangers. One of these is snowfall. Snow makes it imperative that deciduous trees drop their leaves in a timely manner. Once the aforementioned 1,200 square yards of leaf surface have disappeared, the white blanket has no place to land but on the branches, and this means that most of it falls through onto the ground.
Ice can generate even heavier loads than snow. A few years ago, I experienced weather conditions that combined temperatures slightly below freezing with a seemingly harmless drizzle. This unusual weather lasted for three days, and as each hour passed, I became more and more worried about the forest. The light rain landing on freezing branches turned to ice in seconds, quickly weighing the branches down. It looked incredibly beautiful. All the trees were encased in crystal. Whole stands of young birches were bent down under the weight of the ice, and with a heavy heart, I was already giving them up for lost. In the case of mature trees, it was above all the conifers—mostly Douglas firs and pines—that lost up to two thirds of the green branches in their crowns, which broke off with a loud cracking sound. That weakened the trees considerably, and it will take decades for them to completely rebuild their crowns. But the bent-over young birches surprised me. When the ice melted several days later, 95 percent of the trunks stood tall again. Today, a few years later, there’s no sign that anything happened to the trees. Of course, there were a few that didn’t manage to spring back. They died, at some point their rotten little trunks broke, and they are now slowly turning themselves into humus.
So, dropping leaves is an effective protective strategy that seems made to measure for the climate in Central European latitudes. It is also an opportunity for trees to finally excrete waste. Just as we take a trip to a quiet little room before we go to bed, trees also rid themselves of substances they do not need and would like to part with. These drift down to the ground in their discarded leaves. Shedding leaves is an active process, so the tree can’t go to sleep yet. After the reserve supplies have been reabsorbed from the leaves back into trunk, the tree grows a layer of cells that closes off the connection between the leaves and the branches. Now all it takes is a light breeze, and the leaves drift down to the ground. Only when that process is complete can trees retire to rest. And this they must do to recuperate from the exertions of the previous season. Sleep deprivation affects trees and people in much the same way: it is life threatening. That’s why oaks and beeches can’t survive if we try to grow them in containers in our living rooms. We don’t allow them to get any rest there, and so most of them die within the first year.
Young trees standing in their parents’ shadow exhibit a few clear deviations from the standard strategy for shedding leaves. When the mother trees lose their leaves, sunlight suddenly floods the ground. The eager young pups are waiting for just this moment, and they take advantage of the bright light to fill up with lots of energy—and they are usually surprised by the first frosts while they are at it. If temperatures are well below freezing, with nights lower than 23 degrees Fahrenheit, the trees have no option but to start yawning and begin hibernation. Now it’s too late to grow a separating layer of cells, and jettisoning leaves is no longer an option; however, this is no big deal for the tiny trees. Because they are so small, the wind is no threat and even snow is rarely a problem.
In the spring, the young trees exploit a similar opportunity. They leaf out two weeks before the large trees, ensuring themselves a long leisurely breakfast in the sun. But how do the youngsters know when they need to get started? After all, they don’t know the date when the mother trees might leaf out. It’s warm temperatures close to the ground that give the game away. Spring really is rung in here approximately two weeks earlier than it is 100 feet higher up in the canopy. Up high, harsh winds and freezing cold nights delay the warm season for a little while longer. It’s the protective canopy created by the branches of the old trees that keeps heavy, late frosts from reaching the ground. At the same time, the layer of leaves covering the soil acts like a warming compost pile, allowing the thermometer to climb a
couple of degrees. Counting the days they benefited from in the fall, the youngsters can enjoy one month of free growth time—and that’s almost 20 percent of the growing days available to them. Not bad.
Among deciduous trees, there are different approaches to frugal living. Most trees draw energy reserves back into their branches before they shed their leaves, but a few don’t seem to care. Alders, for example, happily drop bright-green leaves onto the ground as though there were no tomorrow. Alders, however, usually grow in swampy, nutrient-rich soil and can, apparently, afford the luxury of producing new chlorophyll every year. Fungi and bacteria at the base of the trees recycle the discarded leaves to produce the raw materials the alders need to build chlorophyll, and all the trees need to do is take these building blocks up through their roots. They don’t even have to worry about recycling nitrogen, thanks to the symbiotic relationship they have with bacteria in nodules on their roots, which constantly provide them with all the nitrogen they need. Per year and square mile of alder forest, these tiny helpers can extract up to 87 tons of nitrogen from the air and make it available to the roots of their tree friends.53 That is more than most farmers spread over their fields as fertilizer.
So, whereas many trees take pains to budget carefully, alders flaunt their wealth. Ash and elders behave in a similar manner. Because these spendthrifts all discard their leaves while they are still green, they don’t contribute anything to the fall colors of the forest. Only the misers, it seems, are colorful. No, that’s not quite true. Yellow, orange, and red come to the fore when chlorophyll is removed, but these carotenes and anthocyanins are also broken down eventually. The oak is such a careful species that it stashes everything away and discards only brown leaves. Thus, trees differ in their spending habits. It’s all over for the beech when its leaves turn brown and yellow, whereas the cheery cherry loses its leaves when they’re red.
Finally, we return to the conifers. I’ve given them rather short shrift so far, but there are three species that drop their leaves like deciduous trees—the larch, the bald cypress, and the dawn redwood. I have no idea why these three conifers are the only ones to follow the deciduous trees’ example. Perhaps in the evolutionary competition the best way to overwinter has simply not yet been decided. Holding on to needles certainly brings advantages in the spring, because the trees can get going immediately without having to wait for new growth. However, many new shoots dry out when the crowns warm up nicely in the spring sun and begin to photosynthesize while the ground is still frozen. Because they can’t put the brakes on transpiration, as soon as they become aware of the danger, the needles go limp—particularly those from last year, which don’t yet have a thick coat of wax.
Apart from that, spruce, pines, firs, and Douglas firs change out their needles because they too must rid themselves of waste materials. They shed the oldest needles, which are damaged and don’t work very well anymore. As long as the trees are healthy, firs always keep ten, spruce six, and pines three years’ worth of needles, as you can tell by taking a look at the annual growth intervals on their branches. Pines especially, which shed about a quarter of their green needles, can look somewhat sparse in winter. In spring, a new year’s worth of needles is added along with fresh growth, and the crowns look the picture of health once again.
ASPEN
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— A SENSE OF TIME —
IN MANY LATITUDES, forests drop leaves in the fall and leaf out in the spring, and we take this cycle for granted. But if we take a closer look, the whole thing is a big mystery, because it means that trees need something very important: a sense of time. How do they know that winter is coming or that rising temperatures aren’t just a brief interlude but an announcement that spring has arrived?
It seems logical that warmer days trigger leaf growth, because this is when frozen water in the tree trunk thaws to flow once again. What is unexpected is that the colder the preceding winter, the earlier the leaves unfurl. Researchers from the Technical University of Munich (TUM) tested this in a climate-controlled laboratory.54 The warmer the cold season, the later beech branches greened up—and at first glance, that doesn’t seem logical. After all, in warm years, lots of other plants—wild flowers, for example—often start to grow in January and even begin to flower, as we are constantly reminded by media headlines. Perhaps trees need freezing temperatures to get a restorative sleep in winter and that’s why they don’t get going right away in the spring. Whatever the reason, in these times of climate change, this is a disadvantage, because other species that are not so tired and grow their new leaves more quickly will be a step ahead.
How often have we experienced warm spells in January or February without the oaks and beeches greening up? How do they know that it isn’t yet time to start growing again? We’ve begun to solve the puzzle with fruit trees, at least. It seems the trees can count! They wait until a certain number of warm days have passed, and only then do they trust that all is well and classify the warm phase as spring.55 But warm days alone do not mean spring has arrived.
Shedding leaves and growing new ones depends not only on temperature but also on how long the days are. Beeches, for example, don’t start growing until it is light for at least thirteen hours a day. That in itself is astounding, because to do this, trees must have some kind of ability to see. It makes sense to look for this ability in the leaves. After all, they come with a kind of solar cell, which makes them well equipped to receive light waves. And this is just what they do in the summer months, but in April the leaves are not yet out. We don’t yet understand the process completely, but it is probably the buds that are equipped with this ability. The folded leaves are resting peacefully in the buds, which are covered with brown scales to prevent them from drying out. Take a closer look at these scales when the leaves start to grow and hold them up to the light. Then you’ll see it. They’re transparent! It probably takes only the tiniest amount of light for the buds to register day length, as we already know from the seeds of some agricultural weeds. Out in the fields, all it takes is the weak light of the moon at night to trigger germination. And a tree trunk can register light as well. Most tree species have tiny dormant buds nestled in their bark. When a neighboring tree dies and falls down, more sun gets in, which in many trees triggers the growth of these buds so that the tree can take advantage of the additional light.
And how do trees register that the warmer days are because of spring and not late summer? The appropriate reaction is triggered by a combination of day length and temperature. Rising temperatures mean it’s spring. Falling temperatures mean it’s fall. Trees are aware of that as well. And that’s why species such as oaks or beeches, which are native to the Northern Hemisphere, adapt to reversed cycles in the Southern Hemisphere if they are exported to New Zealand and planted there. And what this proves as well, by the way, is that trees must have a memory. How else could they inwardly compare day lengths or count warm days?
In particularly warm years, with high fall temperatures, you can find trees whose sense of time has become confused. Their buds swell in September, and a few trees even put out new leaves. Trees that get in a muddle like this have to suffer the consequences when delayed frosts finally arrive. The fresh growth has not had time to get woody—that is, to get hard and tough for winter—and the leaves are defenseless anyway. And so the new greenery freezes, and that must surely hurt. Worse, the buds for next spring are now lost and costly replacements must be grown. If a tree isn’t careful, it will deplete its energy supplies and be less prepared for the coming season.
Trees need a sense of time for more than just their foliage. This sense is equally important for procreation. If their seeds fall to the ground in fall, they mustn’t sprout right away. If they do, two problems present themselves. First, the delicate shoots won’t have time to get woody, which means they will freeze. Second, when the weather is cold, there is very little for deer to eat and they would be only too happy to pounce on the fresh, green growth. So it’s better to sp
rout in the spring along with all the other plants. Therefore, seeds register cold, and only when extended warm periods follow hard frost do the baby trees dare to come out of their protective coverings. Many seeds don’t possess a sophisticated counting mechanism like the one used to trigger leaf growth, and that’s why it works so well when squirrels and jays bury beechnuts and acorns an inch or so deep in the soil. Down here it doesn’t warm up until true spring arrives. Light seeds, such as the seeds of birches, have to pay more attention. With their little wings, they always land on the surface of the soil and just lie there. Depending on where they come to rest, they might end up in bright sunlight, and therefore, these little ones must be able to wait and register the appropriate day length just as their parents do.
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— A QUESTION OF CHARACTER —
ON THE COUNTRY road between my home village of Hümmel and the next small town in the Ahr valley stand three oaks. They are a commanding presence out in the open fields, and the area is named in their honor. They are growing unusually close together: mere inches separate the one-hundred-year-old trunks. That makes them ideal subjects for me to study, because the environmental conditions for all three are identical. Soil, water, local microclimate— there can’t be three different sets of each within a few yards. This means that if the oaks behave differently, it must be because of their own innate characteristics. And they do, indeed, behave differently!
In winter, when the trees are bare, or in summer, when they are in full leaf, the driver of a car speeding by wouldn’t even notice three separate trees. Their interconnecting crowns form a single large dome. The closely spaced trunks could all be growing from the same root, as happens sometimes if downed trees start to regrow. However, the triad of fall color points to a very different story. Whereas the oak on the right is already turning color, the middle one and the one on the left are still completely green. It takes a couple of weeks for the two laggards to follow their colleague into hibernation. But if their growing conditions are identical, what accounts for the differences in their behavior? The timing of leaf drop, it seems, really is a question of character.