Weakness equals pests. It’s easier for scales and aphids to strike, because street trees have limited resources they can put toward defending themselves. High urban temperatures are a contributing factor. Hot summers and warm winters favor the insects, which survive in larger numbers. In Central Europe, one species constantly makes headlines because it is a menace to the human population as well: the oak processionary. This moth gets its name because after feeding in the crown, its caterpillars crawl nose to tail down the trunk in long lines. They protect themselves from predators using thick webs, where they retreat to molt as they grow. People fear the little pests because they are covered in fine stinging hairs, which break off when you touch them and make their way under the skin. There, like stinging nettles, they release substances that itch and cause welts and can even trigger acute allergic reactions. The stinging hairs on the shed skins remain hanging in the webs and can inflict damage for up to ten years. In urban areas, the arrival of these insects can spoil a whole summer, yet ultimately, they are not the ones at fault.
The oak processionary is relatively rare in Nature. Just a few decades ago, it was on the list for critically endangered species, and now everyone everywhere wants to get rid of them. Population explosions have been described for more than two hundred years. The German Federal Agency for Nature Conservation doesn’t attribute these infestations to climate change and rising temperatures but to the presence of attractive food sources for the moth.59 They love warm crowns drenched in sunlight. In the middle of the forest, these are hard to find. The few oaks that grow in the forest are mixed in with beeches, and only their topmost tips reach the light. In the city, however, oaks stand out in the open, where they are warmed by the sun all day long. The caterpillars love this. And as the whole “forest” in urban areas offers such perfect conditions, it’s no surprise that there are population explosions, which are a stern reminder that oaks and other species growing along the streets and between houses have to fight for their lives.
At the end of the day, the stresses the trees must bear are so great that most of them die prematurely. Even though they can do whatever they want when they’re young, this freedom is not enough to compensate for the disadvantages they face later in life. One consolation is that because streets and pathways are often planted with rows of the same species of trees, at least they are able to communicate with other members of their species. Plane trees—recognizable by their attractive bark, which peels off in colorful flakes—are a popular choice for these regimented plantings. Whatever it is these street kids talk to each other about through their scent-mail—and whether the tone of these messages is as rough as their lives—the street gangs are keeping this information strictly to themselves.
28
— BURNOUT —
STREET KIDS ARE denied the cozy atmosphere of the forest. And because they are trapped where they have been planted, they have no choice. There are, however, a few species of tree that couldn’t care less about the forest’s comforts and social interactions and prefer to strike out on their own. These are the so-called pioneer tree species (that sounds much better), which like to grow up as far away from their mothers as they can. Accordingly, their seeds are capable of flying long distances. They are very small and padded or equipped with tiny wings so that powerful storms can carry them for miles. Their goal is to land outside the forest, where they can colonize new areas.
The site of a devastating landslide or a recent volcanic eruption that spewed enormous quantities of ash, areas torched by forest fire—all have potential as long as there aren’t any large trees there already. There’s a reason for this: pioneer species hate shade. Shade slows their upward growth, and a tree that grows slowly has already lost. A race for a place in the sun erupts among the Johnny-on-the-spots. These eager beavers include different species of poplar, such as quaking aspen, and silver birch and pussy willow. In contrast to small beeches and pines, whose annual growth is measured in fractions of an inch per year, the pioneers sometimes grow more than 3 feet taller in the same period. In just ten years, they can transform land that once lay fallow into a young forest rustling in the breeze. And most of these quick starters are blooming by then to give their seeds a head start in the search for new realms to conquer and a chance to occupy the last remaining open patches of ground around them.
An open space, however, is attractive to herbivores, because it’s not only trees that try their luck here but also grasses and wild flowers, which don’t do well in the forest understory. Deer—or, in earlier times, wild horses, aurochs, and bison—are drawn to these plants. Grasses are adapted to constant grazing and are relieved that the young trees that threaten their existence are being polished off in the process. Many shrubs that would dearly love to grow taller than the grasses have developed dangerous thorns to protect themselves from the voracious beasts. Blackthorn is so vicious that its pointed protrusions persist on dead plants for years to impale rubber boots and even car tires, to say nothing of the hides and hooves of animals.
Pioneer trees seek to defend themselves in other ways, as well. They grow quickly, so their trunks get thick fast, and they put on a massive layer of rough outer bark. You can see evidence of this rapid growth on silver birches, where black fissures split their smooth white exteriors. Not only do browsers break their teeth on the tough bark, but they are also revolted by the taste of its oil-saturated fibers. This oil, by the way, is the reason even green birch bark burns so wonderfully well and is great for lighting campfires. (If you’re going to try this, pull off only the outermost layer of bark so that you don’t harm the tree.)
Silver birch bark has another surprise in store. The white color is because of the active ingredient betulin, its primary component. White reflects sunlight and protects the trunk from sunscald. It also guards the trunk against heating up in the warming rays of the winter sun, which could cause unprotected trees to burst. As birches are pioneer trees that often grow all alone in wide-open spaces without any neighbors to shade them, such a feature makes sense. Betulin also has antiviral and antibacterial properties and is an ingredient in medicines and in many skin care products.60
What’s really surprising is how much betulin there is in birch bark. A tree that makes its bark primarily out of defensive compounds is a tree that is constantly on the alert. In such a tree there is no carefully calibrated balance between growth and healing compounds. Instead, defensive armoring is being thrown up at a breakneck pace everywhere. Why doesn’t every species of tree do that? Wouldn’t it make sense to be so thoroughly prepared against attack that potential aggressors would breathe their last the moment they took the first bite? Species that live in social groups don’t entertain this option because every individual belongs to a community that will look after it in times of need, warn it of impending dangers, and feed it when it is sick or in distress. Cutting back on defense saves energy, which the tree can then invest in producing wood, leaves, and fruit. Not so with the birches, which must be completely self-reliant if they are to survive. But they, too, grow wood—and indeed, they do so a lot faster—and they, too, want to, and do, reproduce. Where does all their energy come from? Can this species somehow photosynthesize more efficiently than others? No. The secret, it turns out, lies in wildly overtaxing their resources. Birches rush through life, live beyond their means, and eventually wear themselves out. But before we take a look at the results of this behavior, allow me to introduce you to another unsettled spirit: the quaking aspen.
The quaking aspen takes its name from its leaves, which react to the slightest breath of wind. And although we have sayings that associate this characteristic with fear (“to shake like a leaf”), quaking aspens don’t shake because they are afraid. Their leaves hang from flexible stems and flutter in the breeze, exposing first their upper and then their lower surfaces to the sun. This means both sides of the leaf can photosynthesize. This is in contrast to other species, where the underside is reserved for breathing. Thus, quaking aspens can gen
erate more energy, and they can grow even faster than birches.
When it comes to predators, the quaking aspen pursues a completely different strategy from the birch, relying on stubbornness and size. Even when they are being nibbled down by deer year after year, they slowly expand their root systems. From their roots, they then grow hundreds of subsidiary shoots, which, as the years progress, develop into decent-sized trunks. Accordingly, a single tree can extend over many hundreds of square yards of ground—or, in extreme cases, even farther. In Fishlake National Forest, Utah, there is a quaking aspen that has taken thousands of years to cover more than 100 acres and grow more than forty thousand trunks. This organism, which looks like a large forest, has been given the name “Pando” (from the Latin “pandere,” which means to spread).61 You can see something similar in forests and fields in Europe, albeit not on such a grand scale. Once the brush has become sufficiently impenetrable, then a few of the trunks can grow upward undisturbed and develop into large trees in less than twenty years.
It goes without saying that constant struggle and rapid growth exact their toll. After the first three decades, exhaustion sets in. The topmost branches, a yardstick for the vitality of pioneer tree species, thin out. That in itself wouldn’t be too worrisome, but trouble is brewing under the poplars, birches, and willows. Because they let a lot of light shine through their crowns and reach the ground unused, Johnny-come-latelies can get a foothold. These would be the slower-growing maples, beeches, hornbeams, or even silver firs, which prefer to spend their childhoods in the shade anyway. The pioneer species have no choice but to shade them, and when they do, they are signing their own death warrants. A competition begins that they will, inevitably, lose. The interloping youngsters gradually grow taller, and after a few decades, they catch up with the trees affording them shade. By this time, their benefactors are burned out, completely spent, and top out their growth at a maximum of 80 feet.
For Beeches & Co., 80 feet is nothing. They weave their way through the crowns of the pioneer trees and happily grow up and out over them. With their dense crowns, they are considerably better at exploiting the light, and now not enough of this precious commodity reaches the birches and poplars they have overtaken. The distressed trees put up a fight, especially the silver birches, which have developed a strategy to keep the troublesome competition at bay for at least a few more years: their long, thin, pendulous branches act like whips, and they lash out in all directions in even the lightest breeze. This whipping action damages the crowns of neighboring non-related trees, slaps off their leaves and new growth, and, at least in the short term, restricts their growth. Despite this, the lowly tenants eventually overtake the birches and poplars and now everything happens relatively quickly. After just a few years, their last reserves used up, the pioneer species die and return to humus.
But their lives would be relatively short compared with other forest trees even without the hard-hitting competition. As their upward growth slows, their defenses against fungi disappear. One broken-off branch is enough to provide a port of entry. Because their wood is composed of large cells grown in haste, it contains a lot of air, and so the destructive fungal filaments can spread quickly. The trunk rots big time, and because pioneer species often stand out in the open alone, it’s not long until the next fall storm topples the tree. This is not a tragedy for the species itself. Its goal of rapid dispersal was achieved a long time ago, as soon as it quickly reached sexual maturity and propagated.
29
— DESTINATION NORTH! —
TREES CAN’T WALK. Everyone knows that. Be that as it may, they need to hit the road somehow. But how can they do this without feet? The answer lies in the transition to the next generation. Every tree has to stay where it put down roots as a seedling. However, it can reproduce, and in that brief moment when tree embryos are still packed into seeds, they are free. The moment they fall from the tree, the journey can begin.
Some species are in a big hurry. They equip their offspring with fine hairs so that they can drift off on the next wind, light as a feather. Species that rely on this strategy have to grow tiny seeds so that they are light enough to float away. Poplars and willows produce minute fliers like this and send them off on half-mile-long journeys. The advantage of long-distance travel is offset by the disadvantage that the seeds contain hardly any provisions. The sprouting seed quickly uses up its energy reserves, making it highly susceptible to starvation and thirst. The seeds of birches, maples, hornbeams, ash, and conifers are somewhat heavier. At this weight, flight in a feathery coating is no longer practical, so these trees equip their fruit with flying aids. Some species, such as conifers, have an efficient winged design for their seeds, which works well to slow the seeds as they fall. If a storm blows through when the seeds are falling, they can travel about a mile. Species that produce heavy fruit, such as oaks, chestnuts, or beeches, could never cover such distances. Therefore, they avoid any kind of structural assistance and instead enter into an alliance with the animal world.
Mice, squirrels, and jays love oily, starchy seeds. They tuck them into the forest floor as winter provisions, and there the seeds often stay, lost or no longer needed. Sometimes a hungry tawny owl swoops down and a yellow-necked mouse ends up as a meal itself. And so the little rodent makes its contribution to the next generation of trees, small though it might be. These mice often bury their winter stores directly at the base of the trunk of the mighty beech whose nuts they gather. There are lots of small dry holes among the roots, and little creatures love to live in them. If a mouse has moved in, you’ll find husks of completely consumed beechnuts piled in front. At least a few of these stockpiles are buried a few yards from the tree on the open forest floor. After the death of the mouse, they sprout the following spring and become the new forest.
The jay transports heavy seeds the farthest. It carries acorns and beechnuts a few miles away. The squirrel manages only a few hundred yards, whereas mice bury their supplies barely more than 30 feet from the tree. So if you are a heavy-fruited species, you’re certainly not going anywhere quickly. However, the large reserves of food in the seed are a cushion to ensure the seedling has a good chance of surviving its first year.
This means that light-seeded poplars and willows can open up new habitats much more quickly—for example, when a volcanic eruption shuffles the cards in the deck of life and the game starts over. But because these trees don’t get very old and allow a lot of light to reach the ground, tree species that arrive on the scene later eventually take over. But why make the journey at all? Couldn’t the forest just stay right where it is, where things are comfortable and pleasant?
Opening up new places to live is necessary primarily because the climate is always changing. It’s changing very slowly, to be sure, over the course of many hundreds of years, but eventually, despite whatever built-in tolerance trees might have, it will become too warm, too cold, too dry, or too wet for a particular species. Then the trees must depart for other climes, and this means packing up and moving. Such a migration is happening in Central European forests right now. The reason is not just climate change, which has already presented us with a 1.4-degree Fahrenheit rise in the average temperature, but also the change from the last ice age to a warmer era.
Ice ages are hugely influential. As the centuries get increasingly colder, trees must retreat to more southerly climes. If the shift takes place slowly over many generations, trees in Central Europe, for example, successfully relocate to the Mediterranean region. But if the ice advances quickly, it buries forests and swallows up species that have been dragging their feet.
In Central Europe 3 million years ago, you could find not only the native beeches we have today but also large-leaved beeches. Although beeches managed to make the leap to southern Europe, the less agile large-leaved beeches died out. One reason for their demise was the Alps. This mountain range forms a natural barrier that blocked the trees’ escape route. To cross the Alps, the trees had first to settle high t
errain before descending once more to more comfortable elevations. But higher places are too cold for many trees, even in interglacial periods, so the fortunes of many species ended when they reached the tree line. Today, you can no longer find large-leaved beeches in Central Europe, but you can find them in eastern North America, where they are known, simply, as American beeches. (The reference to their large leaves can be found in their Latin name, Fagus grandifolia—“grandis” means big and “folia” means leaves.) American beeches survived because there is no inconvenient east-west mountain range blocking movement from north to south on the North American continent. They could make their way south without hindrance and then move back north after the ice age was over.
Along with a few other tree species, the beeches of Central Europe somehow managed to make it over the Alps and survive in protected locations until our current interglacial period. The road has been open for these relatively few species for thousands of years, and today they are marching north, still, as it were, following the trail of the melting ice. As soon as the climate warmed up, the germinating seedlings were in luck again. They grew to be mature trees and scattered new seeds that progressed north, mile by mile. The average speed of the beeches’ journey, by the way, is about a quarter mile—a year.
The Hidden Life of Trees: What They Feel, How They CommunicateDiscoveries from a Secret World Page 15