PDF Summary:The Hidden Life of Trees, by Peter Wohlleben

In The Hidden Life of Trees, forester Peter Wohlleben delves deep into the world of old-growth forests, unveiling secret interactions, intricate networks, and fascinating behaviors among trees. Wohlleben draws on scientific research and his experiences to shed light on the ways trees interact with their environment and one another. The Hidden Life of Trees invites us to look at forests with renewed wonder and appreciation, emphasizing the importance of conserving these vital ecosystems and seeing them not just as resources but as living entities deserving our respect and protection.

In this guide, we’ll explain Wohlleben’s arguments for why trees matter and explore some characteristics of trees and forest that make them so remarkable. We’ll then talk about how we can better support and protect the world’s ancient forests. Throughout the guide, we’ll discuss the scientific basis of Wohlleben’s work and compare his conclusions with those of other ecologists and environmental activists, including Suzanne Simard (Finding the Mother Tree) and Robin Wall Kimmerer (Braiding Sweetgrass).

(continued)...

(Shortform note: To better understand trees’ auditory communication, biologist David Haskell used advanced recording equipment to capture the subtle, often overlooked sounds produced within the cellular structure of trees. For instance, he recorded the sounds of water being pulled up through the tree's transport tissue, the hissing noise caused by internal respiration, and even the minuscule vibrations caused by insect interactions. Haskell refers to the collection of these sounds as a tree's “song.” Haskell's work has opened up a new avenue for ecological monitoring, where deviations in the song of a tree could provide potential early warning signs of environmental stress or disease.)

Trees Are Sentient

Finally, Wohlleben proposes the idea that trees are sentient. While Wohlleben doesn't claim trees have consciousness or emotions in the way humans do, he suggests that they exhibit behaviors and responses that indicate a level of sensitivity to and awareness of their surroundings which could be described as sentience.

According to Wohlleben, trees' root systems have similarities with the human brain. He explains how roots, much like our neurons, transmit signals and share information, forming a sophisticated communication network. This unseen tree “brain” isn't centralized as it is in humans, but spread out below ground, with each root tip acting like a mini control center. This root tip allows trees to detect water, absorb nutrients, and even interact with neighboring trees, sharing resources and sending distress signals.

(Shortform note: Simard (Finding the Mother Tree) elaborates on the similarities between this underground network and the human brain in a 2021 interview. She explains that the underground network of roots and fungi closely resembles a human’s neural network, as both are systems designed for the most efficient transfer of information and resources. Furthermore, one of the chemicals found in trees’ networks is glutamate, a primary brain neurotransmitter in humans. Like Wohlleben, Simard argues that based on her observations of trees’ sophisticated interactions, she can only conclude that they hold a kind of wisdom.)

Wohlleben offers three additional pieces of evidence to support his argument that trees are sentient:

1. Trees are capable of learning.
2. Trees have a sense of time.
3. Trees have distinct personalities.

In the next section, we’ll discuss each piece of evidence in greater depth.

Trees Are Capable of Learning

First, Wohlleben argues that trees are capable of learning based on their ability to adapt to changing environmental conditions. For example, trees that have survived intense droughts store more water in the subsequent years, indicating a type of memory and ability to learn from previous hardships. Wohlleben also notes how trees in crowded forests moderate their growth rates to avoid overshadowing nearby trees, illustrating a sense of spatial awareness and knowledge of their surroundings.

(Shortform note: Further research suggests that trees aren't only capable of adapting to their environment, but they can also make decisions about how to adapt. A 2017 study found that plants respond differently to competition for light depending on their environment. The experiment showed that plants grew taller if surrounded by similarly sized, dense neighbors, exhibited greater shade tolerance when surrounded by taller, denser neighbors, and grew wider when surrounded by taller, sparser neighbors. These findings suggest that plants can tailor their growth according to both current circumstances and probable future ones.)

Trees Have a Sense of Time

Wohlleben also argues trees have a sense of time rooted in their innate biological rhythms and cycles. He points out that trees follow a yearly cycle of growth and dormancy that aligns with the changing seasons, suggesting they have an internal clock that guides their activities. For instance, trees know when to sprout and drop their seeds to ensure the maximum chance of survival. Trees of the same species even produce seeds at the same time to ensure the genes of individual trees are well-mixed. Similarly, trees know when to shed their leaves in the fall to conserve resources for the winter, and when to resume growth in the spring when resources are abundant.

(Shortform note: In Braiding Sweetgrass, Kimmerer expands on trees' collective sense of time using the example of pecan trees' mast fruiting. Kimmerer explains how all pecan trees in a region time their seed production collectively. The oversupply of nuts causes a surge in the squirrel population, which triggers an influx of predators, causing a subsequent decline in the squirrel numbers. The pecan trees then begin producing seeds again, strategically timed when squirrel numbers are low, increasing their seeds' chances of germination. Kimmerer’s analysis of pecan trees’ synchronized masting behavior shows how the biological clock of trees influences not only their personal growth cycles but also broader, communal survival strategies.)

Trees Have Distinct Personalities

Finally, making his case for tree sentience, Wohlleben also argues that trees, like humans or other animals, have distinct personalities. He observes that individual trees, even when growing in the same soil and under the same microclimate, sometimes change color and shed their leaves at different times. This variation, he suggests, isn't merely a response to environmental factors, but a matter of individual choice, suggesting a unique character or personality.

(Shortform note: J.R.R. Tolkien’s portrayal of Ents—the ancient tree creatures in The Lord of the Rings trilogy—provides a striking fictional echo to Wohlleben's views on tree sentience and individuality. In Tolkien's world, Ents aren't simply animated trees but beings with personalities, wisdom, and complex emotions. While Ents only exist in the realm of fantasy, they serve to highlight a perspective that Wohlleben encourages his readers to consider—that trees are more than just interchangeable lifeforms; they carry an essence of personality and individuality.)

Similarly, says Wohlleben, while it is generally more advantageous for trees to invest energy into branches at their crown, where they can absorb more sunlight, not all trees follow this strategy. In a forest clearing, for instance, some trees choose to sprout new branches on their trunk, while others don't. Wohlleben interprets this as another sign of individual choice or character, further supporting his argument that trees exhibit a form of sentience.

(Shortform note: While Wohlleben interprets trees’ different growth patterns as evidence of individual choice, other scientists argue these differences are simply innate adaptations that maximize survival in different ecological settings. For example, a tree in a forest clearing may sprout branches on its trunk because it receives more sunlight due to the break in canopy cover. It could also be a response to competition from other flora. When faced with heavy undergrowth, trees need to optimize their leaf surface area for maximum photosynthesis. This means the existence of distinct personalities isn't the only possible explanation for Wohlleben’s observations.)

Why Does the Complexity of Trees Matter?

According to Wohllben, trees’ ability to cooperate, communicate, and respond to their environment in ways that resemble intelligence should compel us to re-evaluate our perception of forests. First, there are practical implications for how we manage forests. For example, if trees are interdependent and communicate, then the felling of one tree isn't simply the removal of an individual, but a disturbance to a community. Furthermore, acknowledging trees’ sentience could pivot forestry practices toward more sustainable logging, land usage, and conservation efforts that respect and prioritize the well-being of trees and forests.

Beyond the pragmatic, Wohlleben's ideas extend into the philosophical realm, encouraging us to broaden our understanding of community and cooperation beyond the confines of the human and animal spheres. This perspective invites a rethinking of our relationship with nature and our roles as environmental stewards and promotes a heightened respect for the lives of trees.

What Do Critics Say About Wohlleben’s Ideas?

Wohlleben is explicit that his goal in writing The Hidden Life of Trees was to get people to think differently about the innate value of trees. However, many in the scientific community, including ecologists and tree physiologists, take issue with Wohlleben's approach.

Critics of Wohlleben pinpoint several issues with the book. First, many object to Wohlleben’s anthropomorphization of trees. Even Simard, whom Wohlleben cites repeatedly, is quoted as saying that his anthropomorphization was “too much.” Additionally, some argue that Wohlleben cherry-picks scientific evidence to support his views, disregarding conflicting research that doesn't align with his narrative. For example, Wohlleben's portrayal of forests as cooperative and interconnected ignores the fierce competition that also exists within ecosystems.

However, in his review of The Hidden Life of Trees, New Yorker writer Robert Moor suggests that the soft science underpinning the book matters less than the book’s impact. He makes the case that Wohlleben has brought much-needed attention to a previously neglected issue and that this perhaps justifies the parts of the book that read more like fiction than science.

The Importance of Trees

While Wohlleben argues that trees are innately valuable, he also explains how they’re crucial to the survival of numerous species, including humans. In this section, we’ll highlight trees’ important role in the global ecosystem and explain the sometimes unexpected ways in which humans, and other species, depend on them to survive.

Forests Help Control Our Climate

First of all, Wohlleben explains, forests help regulate our climate on a regional and global level.

Forests help maintain regional climates through their role in the hydrological cycle, or the way water moves around the world. Trees absorb vast amounts of water from the soil through their root systems and release it into the atmosphere via transpiration. This action serves as a natural water pump, converting groundwater into water vapor, which forms clouds that can travel great distances to deliver rain to even the remotest areas. Without this forest-driven process and without healthy coastal forests, where the process begins, many landlocked regions would become arid deserts.

(Shortform note: Some countries have tried using forests to shift their regional climates with the aim of improving environmental conditions and mitigating the effects of climate change. One notable example is the "Green Great Wall," a large-scale forest belt in northern China spanning nearly 4,500 kilometers (2,800 miles). The goal of the project was to substantially enhance climatic and hydrological conditions in the area. It was anticipated that the afforestation efforts could increase precipitation by up to 20% and reduce temperatures, offering relief from the prevailing arid conditions. While the project undoubtedly made significant progress in afforestation and land restoration, its overall success is still being evaluated.)

Finally, Wohlleben says mature forests mitigate climate change on a global scale, primarily due to their significant capacity for carbon sequestration. As trees age and grow, they store vast amounts of carbon in their trunks, branches, leaves, and roots. Furthermore, the forest floor in mature forests accumulates thick layers of organic matter and fallen leaves, leading to the formation of a rich soil, known as humus, which stores additional carbon. However, if these trees are cut down and burned, the carbon is rereleased into the atmosphere as carbon dioxide (CO₂).

What Is the Role of Carbon Dioxide in Climate Change?

Carbon dioxide contributes to global warming by intensifying the greenhouse effect—a natural process in which certain gases in the Earth's atmosphere trap the sun's heat, much like a blanket, preventing it from escaping back into space. Although CO₂ naturally occurs, human activities such as industrial processes, deforestation, and the burning of fossil fuels pump excess CO₂ into the atmosphere, thereby enhancing the greenhouse effect and resulting in global warming.

As Wohlleben explains, forests play a crucial role in carbon sequestration, the process by which CO₂ is absorbed from the atmosphere and stored for extended periods. However, on a per unit area basis, other ecosystems like oceans, wetlands, and grasslands might have a greater capacity for carbon storage. For instance, coastal and marine ecosystems such as mangroves, salt marshes, and seagrass can store up to 10 times more carbon than terrestrial forests. Nonetheless, due to their vast expanses, forests still hold a significant share of the world's sequestered carbon.

Forests Are Key to Biodiversity

According to Wohlleben, trees are also crucial to sustaining our planet's biodiversity, the variety of life on earth. Old-growth forests are particularly rich in biodiversity, providing the ideal habitat for myriad plants, animals, fungi, and microorganisms.

Trees serve as essential physical habitats for numerous species. Caterpillars munch on their leaves, birds nest on their branches, and fungi colonize their roots. Some species, such as woodpeckers and beetles, make their homes inside trunks, and others like mosses and lichens grow on their bark. Trees even continue to provide habitats after they’re dead: When trees die and decompose, they become nurse logs, providing nutrition and housing for a range of organisms from fungi and mosses to insects and small mammals.

The Cascading Impact of Habitat Loss

Predictions indicate that habitat loss and degradation could set off a cascade of extinctions, leading to substantial biodiversity loss. Studies anticipate that, on average, Earth's ecosystems might see a 6-10% decrease in biodiversity by 2050, and this could rise to as high as 27% by the end of the century. The most drastic changes are likely to happen before the middle of the century, indicating the urgent need to address this issue. Importantly, these predictions account for co-extinctions and the ripple effect that the loss of one species can have on those that are interdependent, elements that previous models often overlooked.

A microcosm of this issue can be observed in Borneo, where logging and palm oil plantations have caused severe tree habitat loss, leading to a steep decline in the Bornean orangutan population, which depends on the trees for survival. As a result, fewer seeds are being dispersed and orangutan predators, like the clouded leopard, go hungry.

However, Wohlleben adds, trees don’t just provide physical spaces; they also create friendly microclimates. By forming dense canopies, trees moderate temperature fluctuations, control humidity levels, and reduce wind speeds, effectively giving rise to environments that can support a diverse range of organisms not usually found in the open.

(Shortform note: According to the Environmental Protection Agency, trees play a crucial role as natural climate moderators in increasingly hot urban settings. First, trees combat the urban heat island effect, a phenomenon characterized by higher temperatures in city environments due to heat absorption and slow release by materials like asphalt and concrete. Additionally, trees contribute to temperature reduction through evapotranspiration, a process in which they release water vapor, producing a natural cooling effect. According to research, urban forested areas are on average 2.9°F cooler than non-forested urban areas.)

Additionally, trees play a crucial role in nutrient cycling. Wohlleben explains that as they drop their leaves, the leaves decompose and return the extracted nutrients back into the soil. This ultimately makes these nutrients accessible to the diverse life forms living beneath the forest floor. According to Wohlleben, there are as many organisms in a small handful of soil as there are people in the world, and still more yet to be discovered. Soil-dwelling species, from microscopic organisms to larger creatures like mites, play indispensable roles in maintaining a healthy ecosystem by decomposing organic matter and recycling nutrients.

(Shortform note: While Wohlleben focuses on the benefits of fallen leaves in old-growth forests, dead leaves can also help improve the ecosystem of your backyard. According to naturalists, leaving leaves on your lawn through the autumn can promote biodiversity, as they serve as natural mulch and provide a habitat for insects and other small creatures. Dead leaves can also enhance the soil by decomposing into organic matter. This method serves as a natural cycle of fertilization and benefits your garden by improving soil structure—a much-needed counter mechanism in urban areas, where the soil often lacks essential nutrients.)

However, says Wohlleben, when old-growth forests are harvested or replaced by more commercially viable mono-cropped tree plantations, these intricate and balanced soil communities are dismantled. Soil compaction, pollution, and alterations to the microhabitat from traditional forestry practices additionally jeopardize the survival of these organisms, which disrupts nutrient cycling and reduces the overall biodiversity of the region. Less biodiversity makes the ecosystem less resilient to changes, like diseases or climate change, which can further degrade the environment.

(Shortform note: The intricate network of microorganisms found in old-growth forests, in many ways, mirrors our internal human microbiome, as highlighted by Ed Yong in I Contain Multitudes. Within the forest's soil, tree bark, and decaying matter, microorganisms not only interact, compete, and cooperate, but they also engage in vital processes such as nutrient cycling and decomposition. These interactions form intricate webs of relationships that sustain the overall resilience and function of the forest ecosystem. Similarly, within our own bodies, the interactions between trillions of microbes play a crucial role in our overall health and well-being.)

Our Responsibility to Forests

Ultimately, The Hidden Life of Trees is a call to action. Despite trees' remarkable ability to adapt to massive shifts in climate and their high degree of genetic variation, which guards against total extinction, Wohlleben argues trees still need our help. He advocates for a more holistic, empathetic, and sustainable approach to our stewardship of forests.

(Shortform note: Despite their resilience, many tree species are facing extinction threats. According to a 2022 study, 1 in 6 tree species native to the US are at risk. Yet, only eight species are federally recognized as endangered or threatened, leaving others without conservation support. Climate change, exemplified by increased wildfires, poses a major threat. For instance, California's sequoias, long considered invincible, lost up to 10,600 mature trees to the 2020 Castle Fire, illustrating the vulnerability of even robust species to environmental changes.)

First, if logging is a necessity, Wohlleben strongly advises implementing sustainable forestry practices. He stresses the importance of fostering forests with a mix of tree species and ages, as this strengthens biodiversity and protects against disease. He also advocates for minimal human intervention. Instead of clear-cutting, or removing large amounts of trees at once, Wohlleben recommends selective logging, a method that only removes a few trees at a time to preserve the overall health of the forest and allows trees to grow, age, and decay naturally. Moreover, selective, intermittent logging allows trees sufficient time for maturation and regeneration.

(Shortform note: Though not directly addressed by Wohlleben, fire management is a crucial sustainable forestry practice, particularly in certain ecosystems. Fires naturally promote tree regeneration and help maintain overall forest health. While suppressing fires is often necessary to protect human lives and property, it can inadvertently lead to an excessive buildup of vegetation, ultimately leading to more severe wildfires. Prescribed or controlled burns, which have long been practiced by indigenous communities in North America, mimic natural fire cycles and help safely manage forest ecosystems.)

Wohlleben offers additional innovative ways to protect forests and ensure their continued usefulness. One method gaining traction in some countries is the introduction of arboreal mortuaries, where cremated human remains are interred at the base of existing trees, providing nutrients to the tree and preserving the forest's natural integrity. Arboreal mortuaries repurpose forests as cemeteries and protect them from unnecessary logging. Wohlleben also advocates for educational tourism opportunities that allow people to experience and learn about forest ecosystems, further fostering an appreciation that drives forest preservation.

(Shortform note: The increasing awareness of the environmental impact of traditional burial practices has led to a rise in interest in "green burials," like those described by Wohlleben. Green burials not only reduce your carbon footprint and preserve natural habitats, but they can also save families thousands of dollars on funeral expenses. Funeral professionals say the biggest challenge for green burials is a lack of awareness and resources, but there are over 150 green cemeteries in the US and Canada. The Green Burial Council offers resources and a list of potential providers for people interested in a green burial.)

Finally, Wohlleben argues that the best thing we can do for forests is to recognize their innate value and leave them alone to live out their lives with dignity.

Do We Need an Alternative Economic Model?

Wohlleben argues that changing how we perceive trees and letting them be has the power to change the balance in our current environmental crisis. In his review in The New Yorker, Moor argues that Wohlleben fails to address the underlying cause of the crisis—an economic system predicated on growth.

In Doughnut Economics, Kate Raworth argues that an economy based primarily on growth (and therefore intensive extraction of natural resources) is unsustainable. Raworth offers an alternative model. The central idea of the “doughnut economy” is to grow the economy enough to support the basic needs of all humans, but not so much that the economic growth exceeds the planet’s ability to regenerate resources that support life on Earth. Raworth’s conception of the post-growth economy requires the following changes: a negative-interest currency, tax reform, and breaking the culture of consumerism. She recommends specific policies to reform the global economy and de-emphasize growth.

Applying the principles of doughnut economics to Wohlleben's argument would require shifting our perception of trees from resources to essential components of a sustainable ecosystem. This would mean implementing policies and incentives that discourage clear-cutting and unsustainable extraction and instead encourage practices such as selective logging or agroforestry.