The Hidden Life of Trees: What They Feel, How They Communicate—Discoveries from a Secret World

“When it comes to some species of insects, trees can accurately identify which bad guys they’re up against. The saliva of each species is different, and trees can match the saliva to the insect. Indeed, the match can be so precise that trees can release pheromones that summon specific beneficial predators.”

Trees have evolved the ability to sense which pests are consuming their leaves, and therefore threatening their health. This gives them the enormous advantage of being able to attract predators which will help the trees by preying on their pests. For example, a tree that is being eaten by caterpillars may use its scent compounds to attract wasps that will lay their larvae inside the caterpillars’ bodies, killing them.

“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. Simard’s discovery of the ‘wood wide web’ pervading our forests.”

Like the internet, the “wood wide web” that Wohlleben refers to here can transmit signals and messages. Trees sensitive root tips are capable of sending such messages through the fungal networks that they partner with to be able to link up with other trees’ roots.

“Trees maintain an inner balance. They budget their strength carefully, and they must be economical with energy so they can meet all their needs. They expend some energy growing…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.”

Wohlleben explains that there are many different functions that require a tree’s energy, and for this reason they cannot simply commit all of their energy to growing, since it is crucial to save some in case of an emergency. Without this energy store a single pest or parasitic attack could be life threatening to the tree.

“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.”

Somewhat counterintuitively, Wohlleben informs the reader that sick and dying trees bloom and reproduce as quickly as possible in the spring or summer months. These trees want to make sure they can reproduce before they die, and so they rush their blooming.

“Thirst is harder for trees to endure than hunger, because they can satisfy their hunger whenever they want. Like a baker who always has enough bread, a tree can satisfy a rumbling stomach right away using photosynthesis. But even the best baker cannot bake without water, and the same goes for a tree: without moisture, food production stops.”

Wohlleben emphasizes the importance of water to a tree’s health. He explains that trees can only feed themselves if they have the moisture to make this possible, making times of drought extremely stressful and dangerous for trees.

“Fungi are amazing. They don’t really conform to our one-size-fits-all system we use to classify living organisms as either animals or plants…Their cell walls are made of chitin—a substance never found in plants—which makes them more like insects. In addition, they cannot photosynthesize and depend on organic connections with other living beings they can feed on.”

Fungi are a frequent motif in Wohlleben’s work, since their functions in the forest range from beneficial helpers to parasitic attackers to neutral decomposers. This quotation helps the reader understand what unique life forms fungi are, which constitute their own category and are not plants in spite of what many people think.

“In any event, every tree stops growing taller. Its roots and vascular system cannot pump water and nutrients any higher because this would be too much exertion for the tree. Instead, the tree just gets wider…The tree is also not capable of maintaining its mature height for long because its energy levels diminish slowly over the years.”

Wohlleben explains the last few stages trees experience before they reach the end of their life. Eventually, the tree simply cannot reach any higher and only grows wider in its final years ,before finally waning in energy and being overcome by the elements and parasites.

“It’s no skin off an old oak’s back if a wild boar wants to use its bark as a scratching post.”

Wohlleben shares this English translation of a German expression to illustrate how tough and resilient oak tree bark can become.

“Coppicing was popular because people were so poor in those days that they couldn’t afford to wait any longer for new wood. You can spot these relics of bygone times when you take a walk in a European forest. Look for trees that have numerous bushy trunks or thick callouses at the base where periodic felling has encouraged a proliferation of growth.”

The author explains that “coppicing,” or harvesting the new shoots from a deciduous tree’s trunk, was an important logging technique throughout European history. This method allowed land users to repeatedly harvest small amounts of firewood from tree trunks without killing the tree permanently.

“Research revealed the spruce to be an absolutely unbelievable 9,550 years old […] For me, this inconspicuous small plant is a symbol for how little we understand about forests and trees and how many wonders we have yet to discover.”

Wohlleben explains that coppicing can draw out the life of a tree, since it will continue to send out new shoots from its cut trunk. He notes that one coppiced spruce tree in northern Europe was over 9,000 years old. The author shares that this remarkable tree’s age shows that there is much yet to understand about all the factors that influence tree life.

“There are more life forms in a handful of forest soil than there are people on the planet.”

Wohlleben educates the reader about how soil—and the creatures that live in it—greatly influence trees’ ability to grow and thrive. Most soil life forms are microscopic and cannot be seen by the naked eye. Amazingly, every handful of soil contains billions of these life forms who are an essential aspect of the forest ecosystem.

“The forest is really a gigantic carbon dioxide vacuum that constantly filters out and stores this component of the air.”

Because trees can absorb carbon dioxide, the author compares forests to a vacuum that takes this element out of the air. This function is part of what makes forests a boon to human health and the author adds that for this reason forests are a tool for combating climate change as well.

“The researchers looked at about 700,000 old trees on every continent around the world. The surprising result: the older the tree, the more quickly it grows. So, in the case of trees, being old doesn’t mean being weak, bowed, and fragile.”

Wohlleben debunks the myth that younger trees grow faster than older trees. He writes that humans made this assumption because we perceive trees through their own agist lens. This myth was disproven in this study, which showed that older trees grow more productively than their young counterparts.

“As foresters like to say, the forest creates its own ideal habitat.”

Wohlleben explains to the reader that while there are many factors which influence trees’ lives, they also have their own influence on their environment. By shedding leaves or needles, stabilizing and enriching the soil, and storing water, trees create conditions that will allow them and their companion trees to thrive.

“It didn’t matter if they [scientists] were studying the rainforest or the Siberian taiga, it was always the trees that were transferring life-giving moisture into land-locked interiors. Researchers also discovered that the whole process breaks down if coastal forests are cleared.”

Wohlleben describes the process by which connected forests transfer evaporated water from coastal regions to inland ecosystems. These forest connections can only work as water pumps if they are continuous and are jeopardized if the first link in the chain, the coastal forest, is cut down.

“For the dead trunk is as indispensable for the cycle of life in the forest as the live tree. For centuries, the tree sucked nutrients from the ground and stored them in its wood and bark. And now it is a precious resource for its children.”

Wohlleben calls dead wood “indispensable” because of its role in providing nutrients for young trees. The author goes on to describe how many species of bacteria, fungi, and insects depend on dead wood to live. Only these actors make trees’ decomposition possible, since these species specialize in feeding on different elements of the dead tree, breaking it down and allowing the nutrients to reenter the soil for the young trees to use.

“The anticipated trajectory of a tree’s life can change at any time for any number of reasons. Its health depends on the stability of the forest ecosystem. It’s better if temperature, moisture, and light conditions don’t change abruptly, because trees react extremely slowly. But even when all the external conditions are optimal, insects, fungi, bacteria, and viruses are always lurking, waiting for the chance to strike.”

Sudden changes—even seemingly beneficial ones like extra light—can wreak havoc on a tree’s sense of balance and threaten to interrupt its healthy growth by ruining the predictability of its ecosystem. Wohlleben adds that even in ideal conditions trees must always be prepared for the ever-present threat of bugs, bacteria, and fungi.

“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 armouring is being thrown up at breakneck speed everywhere.”

Wohlleben examines how birch trees manage to infuse their bark with so much bitumen, which is a highly effective antiviral and antibacterial compound. He argues that pioneer species like the birch can create such effective defensive mechanisms because they aren’t trying to grow slowly for hundreds of years like other tree species. Instead, they often exhaust their resources and energy supplies within decades.

“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.”

Wohlleben claims that undisturbed forests stand a better chance of coping with non-native pests and parasites because of temperature moderation, trees’ social connections, and other factors. For this reason, he believes that communities should allow some forests to remain completely undisturbed by human activity.

“Thus ancient Central European forests often strike us as being dull or species poor when we see them for the first time. The diversity of animal life plays out mostly in the microscopic realm, hidden from the eyes of forest visitors. We notice only the larger species, such as birds or mammals, and we don’t see them very often because typical forest dwellers are mostly quiet and very shy.”

People are more likely to perceive open ecosystems such as meadows as more biodiverse because it is easier for humans to notice and interact with the species that live in those environments. He points out that even ecological experts can hold this misconception and explains that it is normal for healthy forests to not have conspicuous, colorful plants and animals.

“In these times of dramatic environmental upheaval, our yearning for undisturbed nature is increasing. Countries around the world are enacting legislation to protect what remains of their original forests.”

The author recognizes that many human communities are beginning to forge new relationships with the forest and recognize their importance. This quotation shows how Wohlleben feels that people have a natural need or “yearning” to enjoy undisturbed forest environments which is in sharp contrast to how we have been interacting with nature.

“Most national parks give in to the clamor of complaint and sell to sawmills the trees they have felled and removed from the forest to combat bark beetle infestations. This is a grave mistake. For the dead spruce and pines are midwives to the new forest.”

Wohlleben laments that public perceptions of ugliness or wastefulness can slow down the recovery of native forests. He explains that, while it does not appear natural or healthy, it is beneficial for the ecosystem to allow the non-native commercial trees to rot down and provide valuable humus for the next generation of young trees.

“What organic farms are to agriculture, continuous cover forests with careful selective cutting are to silviculture.”

The author argues that responsible foresters allow forests to maintain their natural cycles and grow trees of different ages and sizes. These foresters allow some trees to grow old and die naturally, while selectively cutting a small number of trees and using natural methods such as horses or oxen for tree removal.