In former times, I myself instigated an exceptional case of assistance. In my first years as a forester, I had young trees girdled. In this process, a strip of bark 3 feet wide is removed all around the trunk to kill the tree. Basically, this is a method of thinning, where trees are not cut down, but desiccated trunks remain as standing deadwood in the forest. Even though the trees are still standing, they make more room for living trees, because their leafless crowns allow a great deal of light to reach their neighbors. Do you think this method sounds brutal? I think it does, because death comes slowly over a few years and, therefore, in the future, I wouldn’t manage forests this way. I observed how hard the beeches fought and, amazingly enough, how some of them survive to this day.
In the normal course of events, such survival would not be possible, because without bark the tree cannot transport sugar from its leaves to its roots. As the roots starve, they shut down their pumping mechanisms, and because water no longer flows through the trunk up to the crown, the whole tree dries out. However, many of the trees I girdled continued to grow with more or less vigor. I know now that this was only possible with the help of intact neighboring trees. Thanks to the underground network, neighbors took over the disrupted task of provisioning the roots and thus made it possible for their buddies to survive. Some trees even managed to bridge the gap in their bark with new growth, and I’ll admit it: I am always a bit ashamed when I see what I wrought back then. Nevertheless, I have learned from this just how powerful a community of trees can be. “A chain is only as strong as its weakest link.” Trees could have come up with this old craftsperson’s saying. And because they know this intuitively, they do not hesitate to help each other out.
4
— LOVE —
THE LEISURELY PACE at which trees live their lives is also apparent when it comes to procreation. Reproduction is planned at least a year in advance. Whether tree love happens every spring depends on the species. Whereas conifers send their seeds out into the world at least once a year, deciduous trees have a completely different strategy. Before they bloom, they agree among themselves. Should they go for it next spring, or would it be better to wait a year or two? Trees in a forest prefer to bloom at the same time so that the genes of many individual trees can be well mixed. Conifers and deciduous trees agree on this, but deciduous trees have one other factor to consider: browsers such as wild boar and deer.
Boar and deer are extremely partial to beechnuts and acorns, both of which help them put on a protective layer of fat for winter. They seek out these nuts because they contain up to 50 percent oil and starch—more than any other food. Often whole areas of forest are picked clean down to the last morsel in the fall so that, come spring, hardly any beech and oak seedlings sprout. And that’s why the trees agree in advance. If they don’t bloom every year, then the herbivores cannot count on them. The next generation is kept in check because over the winter the pregnant animals must endure a long stretch with little food, and many of them will not survive. When the beeches or oaks finally all bloom at the same time and set fruit, then it is not possible for the few herbivores left to demolish everything, so there are always enough undiscovered seeds left over to sprout.
“Mast years” is an old term used to describe years when beeches and oaks set seed. In these years of plenty, wild boar can triple their birth rate because they find enough to eat in the forests over the winter. In earlier times, European peasants used the windfall for the wild boar’s tame relatives, domestic pigs, which they herded into the woods. The idea was that the herds of domestic pigs would gorge on the wild nuts and fatten up nicely before they were slaughtered. The year following a mast year, wild boar numbers usually crash because the beeches and oaks are taking a time-out and the forest floor is bare once again.
When beeches and oaks put blooming on hold for a number of years, this has grave consequences for insects, as well—especially for bees. It’s the same for bees as it is for wild boar: a multi-year hiatus causes their populations to collapse. Or, more accurately, could cause them to collapse, because bees never build up large populations in deciduous forests in the first place. The reason is that true forest trees couldn’t care less about these little helpers. What use are the few pollinators left after barren years when you then unfurl millions upon millions of blossoms over hundreds of square miles? If you are a beech or an oak, you have to come up with a more reliable method of pollination, perhaps even one that doesn’t exact payment. And what could be more natural than using the wind? Wind blows the powdery pollen out of the blossoms and carries it over to neighboring trees. The wind has a further advantage. It still blows when temperatures fall, even when they drop below 53 degrees Fahrenheit, which is when it gets too chilly for bees and they stay home.
Conifers bloom almost every year, which means bees are an option for pollination because they would always find food. However, conifers are native to northern forests, which are too chilly for bees to be out and about while the trees are blooming, and that is probably why conifers, like beeches and oaks, prefer to rely on the wind. Conifers don’t need to worry about taking breaks from blooming, like beeches or oaks, because they have no reason to fear deer and wild boar. The small seeds inside the cones of Spruce & Co. just don’t offer an attractive source of nutrition. True, there are birds such as red crossbills, which pick off cones with the tips of their powerful crossed bills and eat the seeds inside, but in general, birds don’t seem to be a big problem. And because there is almost no animal that likes to store conifer seeds for winter food, the trees release their potential heirs into the world on tiny wings. Thus equipped, their seeds float slowly down from the tips of their branches and can easily be carried away on a breath of wind.
Spruce & Co. produce huge quantities of pollen, almost as though they wanted to outdo deciduous trees in the mating department. They produce such huge quantities that even in a light breeze, enormous dusty clouds billow over coniferous forests in bloom, giving the impression of a fire smoldering beneath the treetops. This raises the inevitable question about how inbreeding can be avoided in such chaotic conditions. Trees have survived until today only because there is a great deal of genetic diversity within each species. If they all release their pollen at the same time, then the tiny grains of pollen from all the trees mix together and drift through the canopy. And because a tree’s own pollen is particularly concentrated around its own branches, there’s a real danger its pollen will end up fertilizing its own female flowers. But, as I just mentioned, that is precisely what the trees want to avoid. To reduce this possibility, trees have come up with a number of different strategies.
Some species—like spruce—rely on timing. Male and female blossoms open a few days apart so that, most of the time, the latter will be dusted with the foreign pollen of other spruce. This is not an option for trees like bird cherries, which rely on insects. Bird cherries produce male and female sex organs in the same blossom, and they are one of the few species of true forest trees that allow themselves to be pollinated by bees. As the bees make their way through the whole crown, they cannot help but spread the tree’s own pollen. But the bird cherry is alert and senses when the danger of inbreeding looms. When a pollen grain lands on a stigma, its genes are activated and it grows a delicate tube down to the ovary in search of an egg. As it is doing this, the tree tests the genetic makeup of the pollen and, if it matches its own, blocks the tube, which then dries up. Only foreign genes, that is to say, genes that promise future success, are allowed entry to form seeds and fruit. How does the bird cherry distinguish between “mine” and “yours”? We don’t know exactly. What we do know is that the genes must be activated, and they must pass the tree’s test. You could say, the tree can “feel” them. You might say that we, too, experience the physical act of love as more than just the secretions of neurotransmitters that activate our bodies’ secrets, though what mating feels like for trees is something that will remain in the realm of speculation for a long time to come.
Inbreeding as we know it in mammals—that is to say, breeding between populations that are related to one another—is, of course, still possible in all three cases I have mentioned. And here, wind and bees come into play equally. As both bridge large distances, they ensure that at least some of the trees receive pollen from distant relations, and so the local gene pool is constantly refreshed. However, completely isolated stands of rare species of trees, where only a few trees grow, can lose their genetic diversity. When they do, they weaken and, after a few centuries, they disappear altogether.
5
— THE TREE LOTTERY —
TREES MAINTAIN AN inner balance. They budget their strength carefully, and they must be economical with energy so that they can meet all their needs. They expend some energy growing. They must lengthen their branches and widen the diameter of their trunks to support their increasing weight. They also hold some energy in reserve so that they can react immediately and activate defensive compounds in their leaves and bark if insects or fungi attack. Finally, there is the question of propagation.
Species that blossom every year plan for this Herculean task by carefully calibrating their energy levels. However, species that blossom only every three to five years, such as beeches or oaks, are thrown off kilter by such events. Most of their energy has already been earmarked for other tasks, but they need to produce such enormous numbers of beechnuts and acorns that everything else must now take second place. The battle for the branches begins. There’s not a speck of space for the blossoms, so a corresponding number of leaves must vacate their posts. In the years when the leaves shrivel and fall off, the trees look unusually bare, so it’s no surprise that reports on the condition of forests where the affected trees are growing describe the tree canopy as being in a pitiful state. Because all the trees are going through this process at the same time, to a casual observer the forest looks sick. The forest is not sick, but it is vulnerable. The trees use the last of their energy reserves to produce the mass of blossoms, and to compound the problem, they are left with fewer leaves, so they produce less sugar than they normally do. Furthermore, most of the sugar they do produce is converted into oil and starch in the seeds, so there is hardly any left over for the trees’ daily needs and winter stores—to say nothing of the energy reserves intended to defend against sickness.
Many insects have been waiting for just this moment. For example, the beech leaf-mining weevil lays millions upon millions of eggs in the fresh, defenseless foliage. Here, the tiny larvae eat away flat tunnels between the top and bottom surfaces of the leaves, leaving brown papery trails as they feed. The adult beetles chew holes in the leaves until they look as though a hunter has blasted them with a shotgun. Some years, the infestations are so severe that, from afar, the beeches look more brown than green. Normally, the trees would fight back by making the insects’ meal extremely bitter—literally. But after producing all those blossoms, they are out of steam, and so this season they must endure the attack without responding.
Healthy trees get over this, especially because afterward there will be a number of years for them to recover. However, if a beech tree is already sickly before the attack, then such an infestation can sound its death knell. Even if the tree knew this, it would not produce fewer blossoms. We know from times of high forest mortality that it is usually the particularly battered individuals that burst into bloom. If they die, their genetic legacy might disappear, and so they probably want to reproduce right away to make sure it continues. Something similar happens after unusually hot summers. After extreme droughts bring many trees to the brink of death, they all bloom together the following year, which goes to show that large quantities of beechnuts and acorns don’t indicate that the next winter will be particularly harsh. As blossoms are set the summer before, the abundance of fruit reflects what happened the previous year and has nothing to do with what will happen in the future. The effect of weak defenses shows up again in the fall, this time in the seeds. The beech leaf miners bore into fruit buds as well as leaves. Consequently, although beechnuts form, they remain empty, and therefore, they are barren and worthless.
When a seed falls from a tree, each species has its own strategy as to when the seed sprouts. So how does that work? If a seed lands on soft, damp soil, it has no choice but to sprout as soon as it is warmed by the sun in the spring, for every day the embryonic tree lies around on the ground unprotected it is in great danger—come spring, wild boar and deer are always hungry. And this is just what the large seeds of species such as beeches and oaks do. The next generation emerges from beechnuts and acorns as quickly as it can so that it is less attractive to herbivores. And because this is their one and only plan, the seeds don’t have long-term defense strategies against fungi and bacteria. The seeds slough off their protective casings, which lie around on the forest floor through the summer and rot away by the following spring.
Many other species, however, give their seeds the opportunity to wait one or more years until they start to grow. Of course, this means a higher risk of being eaten, but it also offers substantial advantages. For example, seedlings can die of thirst in a dry spring, and when that happens, all the energy put into the next generation is wasted. Or when a deer has its territory and main feeding ground in exactly the spot where the seed lands, it takes no more than a few days for the seedling’s tasty new leaves to end up in the deer’s stomach. In contrast, if some of the seeds do not germinate for a year or more, then the risk is spread out so that at least a few little trees are likely to make it.
Bird cherries adopt this strategy: their seeds can lie dormant for up to five years, waiting for the right time to sprout. This is a good strategy for this typical pioneer species. Beechnuts and acorns always fall under their mother trees, so the seedlings grow in a predictable, pleasant forest microclimate, but little bird cherries can end up anywhere. Birds that gobble the tart fruit make random deposits of seeds wrapped in their own little packages of fertilizer. If a package lands out in the open in a year when the weather is extreme, temperatures will be hotter and water supplies scarcer than in the cool, damp shadows of a mature forest. Then it’s advantageous if at least some of the stowaways wait a few years before waking to their new life.
And after they wake? What are the youngsters’ chances of growing up and producing another generation? That’s a relatively easy calculation to make. Statistically speaking, each tree raises exactly one adult offspring to take its place. For those that don’t make it, seeds may germinate and young seedlings may vegetate for a few years, or even for a few decades, in the shadows, but sooner or later, they run out of steam. They are not alone. Dozens of offspring from other years also stand at their mothers’ feet, and by and by, most give up and return to humus. Eventually, a few of the lucky ones that have been carried to open spaces on the forest floor by the wind or by
animals get a good start in life and grow to adulthood.
Back to the odds. Every five years, a beech tree produces at least thirty thousand beechnuts (thanks to climate change, it now does this as often as every two or three years, but we’ll put that aside for the moment). It is sexually mature at about 80 to 150 years of age, depending on how much light it gets where it’s growing. Assuming it grows to be 400 years old, it can fruit at least sixty times and produce a total of about 1.8 million beechnuts. From these, exactly one will develop into a full-grown tree—and in forest terms, that is a high rate of success, similar to winning the lottery. All the other hopeful embryos are either eaten by animals or broken down into humus by fungi or bacteria.
Using the same formula, let’s calculate the odds that await tree offspring in the least favorable circumstances. Let’s consider the poplar. The mother trees each produce up to 54 million seeds—every year.17 How their little ones would love to change places with the beech tree youngsters. For until the old ones hand over the reins to the next generation, they produce more than a billion seeds. Wrapped in their fluffy packaging, these seeds strike out via airmail in search of new pastures. But even for them, based purely on statistics, there can be only one winner.
6
— SLOWLY DOES IT —
FOR A LONG time, even I did not know how slowly trees grew. In the forest I manage, there are beeches that are between 3 and 7 feet tall. In the past, I would have estimated them to be ten years old at most. But when I began to investigate mysteries outside the realm of commercial forestry, I took a closer look.
An easy way to estimate the age of a young beech tree is to count the small nodes on its branches. These nodes are tiny swellings that look like a bunch of fine wrinkles. They form every year underneath the buds, and when these grow the following spring and the branch gets longer, the nodes remain behind. Every year, the same thing happens, and so the number of nodes corresponds with the age of the tree. When the branch gets thicker than about a tenth of an inch, the nodes disappear into the expanding bark.
The Hidden Life of Trees: What They Feel, How They CommunicateDiscoveries from a Secret World Page 3