Tree roots extend a long way, more than twice the spread of the crown. So the root systems of neighboring trees inevitably intersect and grow into one anotherthough there are always some exceptions. Even in a forest, there are loners, would-be hermits who want little to do with others. Can such antisocial trees block alarm calls simply by not participating? Luckily, they cant. For usually there are fungi present that act as intermediaries to guarantee quick dissemination of news. These fungi operate like fiber-optic Internet cables. Their thin filaments penetrate the ground, weaving through it in almost unbelievable density. One teaspoon of forest soil contains many miles of these hyphae.8 Over centuries, a single fungus can cover many square miles and network an entire forest. The fungal connections transmit signals from one tree to the next, helping the trees exchange news about insects, drought, and other dangers. Science has adopted a term first coined by the journal Nature for Dr. Simards discovery of the wood wide web pervading our forests.9 What and how much information is exchanged are subjects we have only just begun to research. For instance, Simard discovered that different tree species are in contact with one another, even when they regard each other as competitors.10 And the fungi are pursuing their own agendas and appear to be very much in favor of conciliation and equitable distribution of information and resources.11
If trees are weakened, it could be that they lose their conversational skills along with their ability to defend themselves. Otherwise, its difficult to explain why insect pests specifically seek out trees whose health is already compromised. Its conceivable that to do this, insects listen to trees urgent chemical warnings and then test trees that dont pass the message on by taking a bite out of their leaves or bark. A trees silence could be because of a serious illness or, perhaps, the loss of its fungal network, which would leave the tree completely cut off from the latest news. The tree no longer registers approaching disaster, and the doors are open for the caterpillar and beetle buffet. The loners I just mentioned are similarly susceptiblethey might look healthy, but they have no idea what is going on around them.
In the symbiotic community of the forest, not only trees but also shrubs and grassesand possibly all plant speciesexchange information this way. However, when we step into farm fields, the vegetation becomes very quiet. Thanks to selective breeding, our cultivated plants have, for the most part, lost their ability to communicate above or below ground. Isolated by their silence, they are easy prey for insect pests.12 That is one reason why modern agriculture uses so many pesticides. Perhaps farmers can learn from the forests and breed a little more wildness back into their grain and potatoes so that theyll be more talkative in the future.
Communication between trees and insects doesnt have to be all about defense and illness. Thanks to your sense of smell, youve probably picked up on many feel-good messages exchanged between these distinctly different life-forms. I am referring to the pleasantly perfumed invitations sent out by tree blossoms. Blossoms do not release scent at random or to please us. Fruit trees, willows, and chestnuts use their olfactory missives to draw attention to themselves and invite passing bees to sate themselves. Sweet nectar, a sugar-rich liquid, is the reward the insects get in exchange for the incidental dusting they receive while they visit. The form and color of blossoms are signals, as well. They act somewhat like a billboard that stands out against the general green of the tree canopy and points the way to a snack.
So trees communicate by means of olfactory, visual, and electrical signals. (The electrical signals travel via a form of nerve cell at the tips of the roots.) What about sounds? Lets get back to hearing and speech. When I said at the beginning of this chapter that trees are definitely silent, the latest scientific research casts doubt even on this statement. Along with colleagues from Bristol and Florence, Dr. Monica Gagliano from the University of Western Australia has, quite literally, had her ear to the ground.13 Its not practical to study trees in the laboratory; therefore, researchers substitute grain seedlings because they are easier to handle. They started listening, and it didnt take them long to discover that their measuring apparatus was registering roots crackling quietly at a frequency of 220 hertz. Crackling roots? That doesnt necessarily mean anything. After all, even dead wood crackles when its burned in a stove. But the noises discovered in the laboratory caused the researchers to sit up and pay attention. For the roots of seedlings not directly involved in the experiment reacted. Whenever the seedlings roots were exposed to a crackling at 220 hertz, they oriented their tips in that direction. That means the grasses were registering this frequency, so it makes sense to say they heard it.
Plants communicating by means of sound waves? That makes me curious to know more, because people also communicate using sound waves. Might this be a key to getting to know trees better? To say nothing of what it would mean if we could hear whether all was well with beeches, oaks, and pines, or whether something was up. Unfortunately, we are not that far advanced, and research in this field is just beginning. But if you hear a light crackling the next time you take a walk in the forest, perhaps it wont be just the wind ...
3
SOCIAL SECURITY
GARDENERS OFTEN ASK me if their trees are growing too close together. Wont they deprive each other of light and water? This concern comes from the forestry industry. In commercial forests, trees are supposed to grow thick trunks and be harvest-ready as quickly as possible. And to do that, they need a lot of space and large, symmetrical, rounded crowns. In regular five-year cycles, any supposed competition is cut down so that the remaining trees are free to grow. Because these trees will never grow oldthey are destined for the sawmill when they are only about a hundredthe negative effects of this management practice are barely noticeable.
What negative effects? Doesnt it sound logical that a tree will grow better if bothersome competitors are removed so that theres plenty of sunlight available for its crown and plenty of water for its roots? And for trees belonging to different species that is indeed the case. They really do struggle with each other for local resources. But its different for trees of the same species. Ive already mentioned that beeches are capable of friendship and go so far as to feed each other. It is obviously not in a forests best interest to lose its weaker members. If that were to happen, it would leave gaps that would disrupt the forests sensitive microclimate with its dim light and high humidity. If it werent for the gap issue, every tree could develop freely and lead its own life. I say could because beeches, at least, seem to set a great deal of store by sharing resources.
Students at the Institute for Environmental Research at RWTH Aachen discovered something amazing about photosynthesis in undisturbed beech forests. Apparently, the trees synchronize their performance so that they are all equally successful. And that is not what one would expect. Each beech tree grows in a unique location, and conditions can vary greatly in just a few yards. The soil can be stony or loose. It can retain a great deal of water or almost no water. It can be full of nutrients or extremely barren. Accordingly, each tree experiences different growing conditions; therefore, each tree grows more quickly or more slowly and produces more or less sugar or wood, and thus you would expect every tree to be photosynthesizing at a different rate.
Students at the Institute for Environmental Research at RWTH Aachen discovered something amazing about photosynthesis in undisturbed beech forests. Apparently, the trees synchronize their performance so that they are all equally successful. And that is not what one would expect. Each beech tree grows in a unique location, and conditions can vary greatly in just a few yards. The soil can be stony or loose. It can retain a great deal of water or almost no water. It can be full of nutrients or extremely barren. Accordingly, each tree experiences different growing conditions; therefore, each tree grows more quickly or more slowly and produces more or less sugar or wood, and thus you would expect every tree to be photosynthesizing at a different rate.
And thats what makes the research results so astounding. The rate of photosynthesis is the same for all the trees. The trees, it seems, are equalizing differences between the strong and the weak. Whether they are thick or thin, all members of the same species are using light to produce the same amount of sugar per leaf. This equalization is taking place underground through the roots. Theres obviously a lively exchange going on down there. Whoever has an abundance of sugar hands some over; whoever is running short gets help. Once again, fungi are involved. Their enormous networks act as gigantic redistribution mechanisms. Its a bit like the way social security systems operate to ensure individual members of society dont fall too far behind.14
In such a system, it is not possible for the trees to grow too close to each other. Quite the opposite. Huddling together is desirable and the trunks are often spaced no more than 3 feet apart. Because of this, the crowns remain small and cramped, and even many foresters believe this is not good for the trees. Therefore, the trees are spaced out through felling, meaning that supposedly excess trees are removed. However, colleagues from Lübeck in northern Germany have discovered that a beech forest is more productive when the trees are packed together. A clear annual increase in biomass, above all wood, is proof of the health of the forest throng.15
When trees grow together, nutrients and water can be optimally divided among them all so that each tree can grow into the best tree it can be. If you help individual trees by getting rid of their supposed competition, the remaining trees are bereft. They send messages out to their neighbors in vain, because nothing remains but stumps. Every tree now muddles along on its own, giving rise to great differences in productivity. Some individuals photosynthesize like mad until sugar positively bubbles along their trunk. As a result, they are fit and grow better, but they arent particularly long-lived. This is because a tree can be only as strong as the forest that surrounds it. And there are now a lot of losers in the forest. Weaker members, who would once have been supported by the stronger ones, suddenly fall behind. Whether the reason for their decline is their location and lack of nutrients, a passing malaise, or genetic makeup, they now fall prey to insects and fungi.
But isnt that how evolution works? you ask. The survival of the fittest? Trees would just shake their headsor rather their crowns. Their well-being depends on their community, and when the supposedly feeble trees disappear, the others lose as well. When that happens, the forest is no longer a single closed unit. Hot sun and swirling winds can now penetrate to the forest floor and disrupt the moist, cool climate. Even strong trees get sick a lot over the course of their lives. When this happens, they depend on their weaker neighbors for support. If they are no longer there, then all it takes is what would once have been a harmless insect attack to seal the fate even of giants.
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 wouldnt 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 Ill 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 craftspersons 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 starchmore 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 thats why the trees agree in advance. If they dont 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 boars 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.