Despite all this, I am not anxious when I think about the future of our forests. For on large continents (and the Eurasian continent is the largest one of all) species have to come to grips with new arrivals all the time. Migrating birds bring small animals, fungal spores, or the seeds of new species of trees tucked in their feathers, or these organisms are blown in by turbulent storms. A five-hundred-year-old tree has surely had a few surprises in its life. And thanks to the great genetic diversity in a single species of tree, there is always a sufficient number of individuals that can rise to a new challenge.
If you live in or have traveled to Germany, you might well have already noticed some of the new “naturalized” avian citizens that have turned up without any help from people. Perhaps the Eurasian collared dove, which arrived in Germany from the Mediterranean in the 1930s. Then there is the fieldfare, a type of thrush. This gray-brown bird with dark speckles has been migrating westward for two hundred years. It started in the northeast and has now reached France. We don’t yet know what surprises these birds might have brought with them in their feathers.
A decisive factor in how robust native forests are in the face of such changes is how unspoiled they are. The more intact the social connections and the more moderated the microclimate under the trees, the more difficult it is for foreign invaders to get established. Plants that make headlines are classic examples of this. Take giant hogweed (also known as wild parsnip or wild rhubarb). It originally came from the Caucasus and grows more than 10 feet tall. The white flower heads can measure up to 18 inches across, and because they are so pretty, the plant was imported into Central Europe and elsewhere in the nineteenth century. The plants escaped out of the gardens where they were planted and since then have been spreading across the countryside with ease.
Giant hogweed is considered extremely dangerous because its sap, in combination with ultraviolet light, can burn human skin. Every year, millions are spent digging up plants and destroying them, without any great success. However, hogweed can spread only because the original forested meadows along the banks of rivers and streams no longer exist. If these forests were to return, it would be so dark under the forest canopy that hogweed would disappear. The same goes for Himalayan balsam and Japanese knotweed, which also grow on the riverbanks in the absence of the forests. Trees could solve the problem if people trying to improve things would only allow them to take over.
I have written so much about nonnative species that this might be the place to address the question of what the term “native” means. We tend to call species native if they occur naturally within a country’s borders. A classic example from the animal world is the wolf, which reappeared in most countries in Central Europe in the 1990s and since then has been considered a permanent part of the fauna. It was found in Italy, France, and Poland much earlier than that. This means that the wolf has been native to Europe for a long time, just not in each individual country. But isn’t even this geographic unit too broad? When we talk about porpoises being native to Germany, does that mean they also make their home in the upper reaches of the Rhine? As you can see, that definition wouldn’t make any sense. Native must be understood on a much smaller scale and be based not on human borders but on habitats.
Habitats are defined by their features (water, terrain, topography) and by the local climate. After the last ice age, trees moved into habitats where they found conditions that suited them. That means, for example, that spruce occur naturally (and, therefore, can be considered native) at an elevation of 4,000 feet in the Bavarian Forest, but they do not occur naturally (and, therefore, cannot be considered native) 1,300 feet lower and only half a mile away, where beeches and firs hold sway. Specialists have come up with the term “habitat specific,” which simply means each species has habitats where they are happy to grow. In contrast to our large-scale country borders, habitat borders for species are like a proliferation of small states. When people ignore these boundaries and bring spruce and pines down to warmer elevations, these conifers are not natives in this new location; they are immigrants. And with that we have arrived at my favorite example: red wood ants.
In Europe, red wood ants are icons of nature conservation. In many locations they are mapped, protected, and in cases of conflict, resettled. There can be no objection to this because what we are talking about here is a threatened species. Threatened? And yet red wood ants are immigrants, too, and therefore, I would argue that no special efforts are necessary for their protection. They travel on the coattails of commercially grown spruce and pines. You could say they hang on to the needles for dear life, for without the conifers’ spiny, narrow needles, they can’t build their anthills. And this proves that they were not present in the original native deciduous forests. Moreover, they love the sun, and they need it to shine on their nests for at least a few hours a day. Especially in spring and fall, when it is bitterly cold, a few warm rays ensure additional days when the ants can rummage around. Dark beech woods are, therefore, ruled out as habitat, and red wood ants are forever thankful to foresters for planting huge expanses of spruce and pines.
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— HEALTHY FOREST AIR —
FOREST AIR IS the epitome of healthy air. People who want to take a deep breath of fresh air or engage in physical activity in a particularly agreeable atmosphere step out into the forest. There’s every reason to do so. The air truly is considerably cleaner under the trees, because the trees act as huge air filters. Their leaves and needles hang in a steady breeze, catching large and small particles as they float by. Per year and square mile this can amount to 20,000 tons of material. 65 Trees trap so much because their canopy presents such a large surface area. In comparison with a meadow of a similar size, the surface area of the forest is hundreds of times larger, mostly because of the size difference between trees and grass. The filtered particles contain not only pollutants such as soot but also pollen and dust blown up from the ground. It is the filtered particles from human activity, however, that are particularly harmful. Acids, toxic hydrocarbons, and nitrogen compounds accumulate in the trees like fat in the filter of an exhaust fan above a kitchen stove. But not only do trees filter materials out of the air, they also pump substances into it. They exchange scent-mails and, of course, pump out phytoncides, both of which I have already mentioned.
Forests differ a great deal from one another depending on the species of trees they contain. Coniferous forests noticeably reduce the number of germs in the air, which feels particularly good to people who suffer from allergies. However, reforestation programs introduce spruce and pines to areas where they are not native, and the newcomers experience substantial problems in their new habitats. Usually, they are brought to low elevations that are too warm and dry for conifers to thrive. As a result, the air is dustier, as you can clearly see when the dust motes are backlit by sun streaming down on a summer’s day. And because the spruce and pines are constantly in danger of dying of thirst, they are easy prey for bark beetles, which come along to make a meal of them. At this point, frantic scent-mails begin to swirl around in the canopy. The trees are “screaming” for help and activating their arsenal of chemical defenses. You absorb all of this with every breath of forest air you take into your lungs. Is it possible that you could unconsciously register the trees’ state of alarm?
Consider this. Threatened forests are inherently unstable, and therefore, they are not appropriate places for human beings to live. And because our Stone Age ancestors were always on the lookout for ideal places to set up camp, it would make sense if we could intuitively pick up on the state of our surroundings. There is a scientific observation that speaks to this: the blood pressure of forest visitors rises when they are under conifers, whereas it calms down and falls in stands of oaks.66 Why don’t you take the test for yourself and see in what type of forest you feel most comfortable?
Whether we can somehow listen in on tree talk is a subject that was recently addressed in the specialized literature.67 Korean scientists have
been tracking older women as they walk through forests and urban areas. The result? When the women were walking in the forest, their blood pressure, their lung capacity, and the elasticity of their arteries improved, whereas an excursion into town showed none of these changes. It’s possible that phytoncides have a beneficial effect on our immune systems as well as the trees’ health, because they kill germs. Personally, however, I think the swirling cocktail of tree talk is the reason we enjoy being out in the forest so much. At least when we are out in undisturbed forests.
Walkers who visit one of the ancient deciduous preserves in the forest I manage always report that their heart feels lighter and they feel right at home. If they walk instead through coniferous forests, which in Central Europe are mostly planted and are, therefore, more fragile, artificial places, they don’t experience such feelings. Possibly it’s because in ancient beech forests, fewer “alarm calls” go out, and therefore, most messages exchanged between trees are contented ones, and these messages reach our brains as well, via our noses. I am convinced that we intuitively register the forest’s health. Why don’t you give it a try?
Contrary to popular opinion, the air in the forest is not always particularly rich in oxygen. This essential gas is released when water and carbon dioxide are broken down during photosynthesis. Every day in summer, trees release about 29 tons of oxygen into the air per square mile of forest. A person breathes in nearly 2 pounds of oxygen a day, so that’s the daily requirement for about ten thousand people. Every walk in the forest is like taking a shower in oxygen. But only during the day. Trees manufacture large amounts of carbohydrates not only to lay them down as wood but also to satisfy their hunger. Trees use carbohydrates as fuel, just as we do, and when they do, they convert sugar into energy and carbon dioxide. During the day, this doesn’t affect the air much because after all the additions and subtractions, there is still that surplus oxygen I just mentioned. At night, however, the trees don’t photosynthesize, and so they don’t break down carbon dioxide. Quite the opposite, in fact. In the darkness, it’s all about using carbohydrates, burning sugar in the cells’ power-generating stations, and releasing carbon dioxide. But don’t worry, you won’t suffocate if you take a nighttime ramble! A steady movement of air through the forest ensures that all the gases are well mixed at all times, and so the drop in oxygen near the ground is not particularly noticeable.
How does a tree breathe anyway? You can see a part of its “lungs.” These are the needles or leaves. They have narrow slits on their undersides that look a bit like tiny mouths. The tree uses these openings to exhale oxygen and breathe in carbon dioxide. At night, when the tree is not photosynthesizing, it does the reverse. It’s a long way from the leaves, down the trunk, to the roots, and that’s why tree roots can breathe as well. If they didn’t, deciduous trees would die in winter when they discard their aboveground lungs. But the trees keep ticking over and their roots even grow a little, so energy must be produced with the help of the trees’ reserves, and for this the trees need oxygen. And that is why it is so awful for a tree if the soil around its trunk has been so compacted that the small air pockets in the soil have been crushed. The tree’s roots suffocate, or at least have difficulty breathing, with the result that the tree gets sick.
But let’s get back to breathing at night. It’s not only the trees that are exhaling large amounts of carbon dioxide in the dark. In leaves, in dead wood, and in other rotting plant material, microscopic creatures, fungi, and bacteria are busy in a round-the-clock feeding frenzy, digesting everything edible and then excreting it as humus. In winter, the situation gets even tougher. This, of course, is when the trees are hibernating and even during the day, the oxygen levels are not being topped up, while the soil organisms continue merrily working away underground. They generate so much heat that even in a hard frost, the ground doesn’t freeze down more than 2 inches. Does that mean that a forest in winter is dangerous? What saves us is the global circulation of air, which constantly blows fresh marine air over the continent. A multitude of algae live in salt water. Thanks to them, large amounts of oxygen bubble up out of the ocean year round. Algal activity in the oceans balances the oxygen deficit in Central European forests in the winter so well that we can breathe deeply even when we are standing under beeches and spruce covered in snow.
On the subject of sleep: have you ever considered whether this is something trees even need? What would happen if we wanted to help them, and so we provided them with light at night as well as during the day so that they could manufacture more sugar? According to current research, that would be a bad idea. It seems trees need their rest just as much as we do, and sleep deprivation is as detrimental to trees as it is to us.
In 1981, the German journal Gartenamt reported that 4 percent of oak deaths in one American city happened because the trees were subjected to light every night. And what about the long period of hibernation? This has already been tested unwittingly by some forest fans. I wrote about this in chapter 22, “Hibernation.” They brought young oaks and beeches into their houses, where they kept them in pots on windowsills. In cozy living rooms there’s no such thing as winter as far as the temperature is concerned, which means most of the young trees couldn’t take a breather and just continued to grow. But at some point, lack of sleep exerted its revenge and the plants, which had seemed so full of life, died. Now you could argue that some winters aren’t really very wintery, and at least at lower elevations, there are hardly any frosty days. Despite this, deciduous trees still lose their leaves and don’t grow them again until spring, because, as I have already mentioned, they also measure day length. But isn’t that also the case for the little trees on the windowsill? It might be the case if the heating were turned off and winter evenings were spent in the dark, but hardly any of us are willing to give up the comfortable temperatures (around 70 degrees Fahrenheit) and warm white electric light that conjure up artificial summers inside our houses. And no Central European forest tree can endure eternal summer.
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— WHY IS THE FOREST GREEN? —
WHY DO WE find it so much more difficult to understand plants than animals? It’s because of the history of evolution, which split us off from vegetation very early on. All our senses developed differently, and so we have to use our imaginations to get even the slightest idea of what is going on inside trees. Our color vision is a good example. I love the combination of a bright- blue sky over a canopy of lush green. For me, this color combination is Nature at its most idyllic and the most relaxing color combination I can imagine. Would trees agree with me? Their answer would probably be: “More or less.”
Beeches, spruce, and other species certainly find blue sky, which means lots of sun, equally agreeable. For them, however, the color isn’t so much romantic or moving as it is a flag that signals, “The buffet is open.” For a cloudless firmament means high-intensity light and, therefore, optimal conditions for photosynthesis. Frantic activity for maximum output is the order of the day. Blue means a lot of work. The trees get full as they convert light, carbon dioxide, and water into supplies of sugar, cellulose, and other carbohydrates.
Green, however, has a completely different significance. Before we get to the typical color of most plants, we first have to answer another question: Why is the world full of color anyway? Sunlight is white, and when it is reflected, it is still white. And so we should be surrounded by a clinical-looking, optically pure landscape. That this is not what we see is because every material absorbs light differently or converts it into other kinds of radiation. Only the wavelengths that remain are refracted and reach our eyes. Therefore, the color of organisms and objects is dictated by the color of the reflected light. And in the case of leaves on trees, this color is green.
But why don’t we see leaves as black? Why don’t they absorb all the light? Chlorophyll helps leaves process light. If trees processed light super-efficiently, there would be hardly any left over—and the forest would then look as dark during the day
as it does at night. Chlorophyll, however, has one disadvantage. It has a so-called green gap, and because it cannot use this part of the color spectrum, it has to reflect it back unused. This weak spot means that we can see this photosynthetic leftover, and that’s why almost all plants look deep green to us. What we are really seeing is waste light, the rejected part that trees cannot use. Beautiful for us; useless for the trees. Nature that we find pleasing because it reflects trash? Whether trees feel the same way about this I don’t know, but one thing is for certain: hungry beeches and spruce are as happy to see blue sky as I am.
The color gap in chlorophyll is also responsible for another phenomenon: green shadows. If beeches allow no more than 3 percent of sunlight to reach the forest floor, it should be almost dark down there during the day. But it isn’t, as you can see for yourself when you take a walk in the forest. Yet hardly any other plants grow here. The reason is that shadows are not all the same color. Although many shades of color are filtered out in the forest canopy—for example, very little red and blue make their way through—this is not the case for the “trash” color green. Because the trees can’t use it, some of it reaches the ground. Therefore, the forest is transfused with a subdued green light that just happens to have a relaxing effect on the human psyche.
The Hidden Life of Trees: What They Feel, How They CommunicateDiscoveries from a Secret World Page 18
