*Robert Moor* # Progressive Summary # Definitions # Chapter Notes ## Prologue - Sky > A tree is not just a thing made up of bark and leaves and sap and wood. At its core, a tree is not even really a noun. It is more like a verb. A tree is what a tree does. It is hard for us to perceive this fact, however, because most of the time, a tree does what it does so slowly. > A tree, in other words, is not only a living being. It is also a way of being. Put very simply, that way of being—what philosophers call arborescence—consists of three simultaneous processes: branching, pruning, and gnarling. The result is something inventive, exacting, and long-lasting. Something wise. > Some trees share food and messages beneath the soil, but others emit poison to kill off their neighbors. One species in the Panamanian rainforest, known as the choibá, appears to have evolved an unusually tall crown so it can attract lightning bolts, which it absorbs and then channels to other nearby trees; in the aftermath of a thunderstorm, the surrounding trees eventually die off, but the choibá thrives. The Bodhi tree, that great symbol of peace, is especially murderous: Ficuses begin their lives as parasitic seeds in the crowns of other trees, then they send their roots downward, slowly ripping their host apart. > Before the Second World War, the German playwright Bertolt Brecht wrote that, when one is living through a turbulent moment in history, it is “almost a crime” to “talk about trees” for in doing so “we maintain our silence about so much wrongdoing.” As I wrote this book—as bombs fell and delusions spread and the fires continued to grow—I felt the acid truth of these words eating away at my resolve. Spending all day thinking about trees, secluded in my quiet little patch of forest, sometimes felt like averting my gaze from a world I’d rather not see. As Brecht knew all too well, it is far easier to go for a walk in the woods than it is to stop monsters from marching to power. > I came to see trees as something more than plants; I recognized them as a source of wisdom, a pattern of flourishing, an answer to chaos, which extends from mossy glades to the hard corners of the metropolis. > It was a spring afternoon, and I was in the office of a neuroscientist named Jeff Lichtman at Harvard. He was renowned for creating finely detailed maps of the brain’s “connectome”—the way that one brain cell links to the next, and the next, and the next, and how that all joins up to form the human intellect. > > One of the most surprising findings of his research, Lichtman told me, was how inelegant the mammalian brain is, structurally speaking. Scientists had assumed that, for the sake of efficiency, neurons would find the shortest distance between two points. However, when Lichtman began measuring the length of the axons in the brain of a mouse, he found that some were as much as 25 percent longer than they needed to be—a startling waste of precious space. > With an air of detached amusement, Lichtman acknowledged that the shape of the brain was “a little irrational.” It was important to remember, he said, that a “neural pathway” is not really a pathway. It’s a living thing. And life, as a rule, doesn’t need to be perfect. It only needs to be good enough to survive. > Brains, bodies, branches, the winding course of our lives, the arc of our collective histories: all are forced to hold on to their gnarls. These things aren’t perfect, but they don’t need to be. They only need to find a bit of light, hold it, and pass it along. > Over the course of my research, I eventually came to understand that there are many reasons why trees inspire such powerful feelings of reverence in us. Trees outlive us, expanding our sense of time; they tower over us, putting our lives into perspective; they provide for us, inspiring a sense of gratitude; and they trouble us, challenging us to embrace what John Fowles calls the “green chaos” of the more-than-human world. But there is something else, which, in our tendency to focus solely on the sunny side of arborescence, we too often overlook: A tree is a way of persisting in a world of wounds. > A tree overcomes its limitations by doing what trees do: It arboresces. It begins by branching outward in all directions, growing upward to explore the bright, dry air and downward into the dark, wet soil. It eventually prunes back the limbs that snag too much wind or fail to reach the light. What remains grows stronger over time, as each new layer of wood wraps around the old, locking those curves into place. Branch, prune, gnarl. In this way, a tree learns to live with, and then outgrow, its imperfections. > Darwin’s grand theory—what he called the evolutionary “Tree of Life”—unsettled, and then reshaped, the world. It remains so disturbing that many people still either fail to or refuse to properly understand it. This, argues the evolutionary biologist Robert J. O’Hara, is because it demands what he calls “tree thinking.” Many people are still beholden to what might be called “tower thinking,” which assumes that we humans are the natural apex of evolution. But tree thinking forces us to reject that comforting belief. There is no one pinnacle of evolution; there is a broad crown, and every living being is a part of it. Thinking in this arborescent manner also helped scholars pry themselves away from the oppressive belief that people are arranged in a strict racial hierarchy; instead, all humans are now widely understood to be part of a single (very complicated) family tree. Much of the social progress of the past century has been founded upon this basic shift in thought—from kinds to kin, from a tower to a tree. > Life, in short, is fundamentally arborescent. In order to make sense of it, it helps to think in trees. > “Learn good sense from a tree, O mind.” This line is taken from a Hindi bhajan, or hymn, that Mahatma Gandhi and his followers used to sing. When I ask myself how a person should live, this is the advice I now turn to: Learn to branch out like a tree, to let go like a tree, to weather hardship like a tree, to rise above like a tree, to set down roots like a tree. In a scarred and storm-battered world, we couldn’t hope to find wiser teachers. ## Chapter 1 - Branching > Talking with Lichtman, I had begun to envision the shape of a human life in arboreal terms. Young people are masters of ramification and experimentation: We sprout upward and outward, learning to crawl, to walk, to climb; we try out new skills and acquire new talents; we rise from one grade to the next, into the heights of our chosen careers. Middle age is a time of gnarling: Our strengths amplify, our skin toughens, our flexibility diminishes, and our midsections thicken. Our final years, by contrast, are marked by deletion, followed by decay: We retire; we shed burdens; we slough off obligations; we forget names; we gradually lose our vision and our hearing and our hair and our teeth; we slouch, then we hunch, until we finally collapse into the soil. Looking at a human life in this way reveals our species to be somewhat weedy—short-lived, fast-growing, and physically unimpressive. A tree, by contrast, grows slowly, but it never stops growing throughout its long life. > Our species’ innate tendency to narrow with age is exacerbated by the structure of the modern world. The market pressures us to specialize, maximizing the profit our minds and bodies can produce, sharpening us to its needs. Then, when we are worn down to a nub, it tosses us away. The old person who chooses to grow in a tree-wise manner—to continue trying new things, shedding bad habits and false beliefs, and strengthening what works best—throughout life, even until the very end, stands in defiant opposition to all that. Truly growing old, as opposed to merely getting old, is a guerrilla war waged against the overwhelming forces of gravity, entropy, and social convention. > The human brain does not continue steadily branching and pruning throughout its existence, as a tree does. Rather, when the brain is first forming, it branches out all at once, and then, for the most part, it spends the rest of its life paring those branches back. Between the ages of two and ten, about 50 percent of the brain’s synapses—the nerve junctions where one neuron links to another—are snipped away. > What may seem like a genetic flaw, Lichtman explained, is in fact a rational evolutionary by-product. The human brain evolved to “master” a given set of skills necessary for its survival; there is no clear evolutionary advantage to being able to learn to play the piano at the age of eighty. He asked me to imagine the life of an elder in a hunter-gatherer clan living one hundred thousand years ago. That person would have already learned all of the necessary ecological information for their given area, mastered the skills necessary to harvest food, and (ideally) refined the kinds of social skills that minimize conflict. Their nervous system would have grown to fit the niche of their lives, just like everything else in the forest in which they live. > Lichtman believes the modern desire for an eternally youthful brain is largely a result of us having become evolutionarily maladapted to our own cultural landscape. “The problem with modern old people is that the world is changing far more quickly than our nervous system was designed to adapt,” he said. “Our brains were not designed for a world that was changing when we were adults. We were supposed to be really wise by the time we’re old. Now, old people are not only not wise, we feel stupid!” ## Chapter 2 - Pruning > Biologically speaking, there is not much difference between the soaring Douglas fir trees in the forest behind my house and the tiny sprigs of rosemary growing in my garden. When plants really need to reach higher than their competitors, they tend to evolve into trees. This process of “treeification” has occurred at least nine times throughout history, as different types of plants (club mosses, horsetails, ferns, hazels) evolved to lift themselves higher than their competitors, a phenomenon known as “convergent evolution.” “A tree is really just a growth form,” von Aderkas said. The historian Jared Farmer takes this logic one step further. “Trees are plants that humans call trees—a term of dignity, not botany,” he writes. An intriguingly postmodern idea, but not the most helpful definition. The biologist Colin Tudge came closer to a working definition when he defined a tree, cheekily, as “a big plant with a stick up the middle.” An ecologist named Tom Langen once informed me that, while training to work in forestry for the Peace Corps, he was taught to define a tree as “a self-standing woody plant that a person can climb.” This definition—specifically, the emphasis on the tree’s entanglement with us—strikes me as truer than most. > Trees begin their lives in miniature, as tiny embryos encased within seeds. Before the evolution of seeds, around 360 million years ago, all plants reproduced via spores, single cells packed with genetic information, light enough to float on the breeze. Seeds are like spores, only they are better equipped for survival. Encased in its hard shell and equipped with a tiny larder of sugar and water, a germinated seed can wait for weeks, even years for the proper conditions to sprout. Indeed, in some cases they can wait for millennia. A seed from a Judean date palm—another tree that is not technically a tree, but rather a “large woody herb”—sat for two thousand years in a clay jar, buried among the ruins of the palace of King Herod the Great, until, in 2005, a team of scientists decided to plant it in the ground. Miraculously, it sprouted to life. It is now a full-grown tree, putting out seeds of its own. > A tree typically grows from a seed. But many don’t. Some trees begin their lives as transplants, a single branch trimmed from another tree, which, when planted on a new patch of ground, sets down roots and develops into its own tree. This new tree is not the child of its parent, but a clone, an exact genetic replica. Willow trees perform this trick naturally, dropping their branches into rivers, where they float downstream, wash ashore, take root, and sprout up. I once visited a clone of the Bodhi tree in the city of Anuradhapura, Sri Lanka, which was brought over from India some two thousand years ago and had been growing larger ever since. People there worshipped it precisely as if it were the original Bodhi tree—which, in a sense, it was. > Every breath a tree exhales contains a spritz of water molecules. This allows a tree to cool off its leaves, which prevents them from being scorched and dried out by the sun. (Von Aderkas calls a plant “essentially a water-cooled solar panel.”) A single tree in the tropics can lift a thousand gallons of water from its roots to its leaves on a hot day. One might wonder how a being without muscles can find the strength to lift the equivalent of a liquified killer whale. The answer is that it doesn’t need to. To exhale, trees open tiny pores on their leaves, allowing water to evaporate. Each molecule of water in the veins of a tree is chemically linked to nearby water molecules in a chain—a phenomenon known as “hydrogen bonding,” familiar to anyone who’s ever watched two adjacent water droplets on a tabletop merge into a single blob. So when one water molecule is exhaled, another is automatically pulled up to replace it. As Tudge writes, “Water is not pumped from below but dragged from above by the leaves… not in a crude and turbulent gush but in millions on millions of orderly threads.” In other words, the tree does not lift water to the sky; the sky lifts water through the tree. > A tree is a plant that has learned to grow vast without spreading itself too thin. The earliest plants had not yet mastered this skill. When plants made their way out of the sea and onto land more than 500 million years ago, they were little more than clumps of pond scum. Over time, they slowly grew taller and more complex, playing around with a variety of body types. Then they stumbled onto a useful architectural trick: By splitting themselves apart into multiple branches (and sub-branches (and so on)), they could gather more sunlight. By funneling those branches down to a single stem, they could conserve their building materials. And by fanning out once again into many roots, they could grip the soil and drink up more water. By the time the first trees appeared, about 400 million years ago, their ancestors had already mastered the art of branching. But now, with the use of sturdy wood, trees could grow even taller, reach out farther, and survive longer, casting larger and larger shadows on their competitors down below. > A tree in an open field is a picture of expansiveness, reaching out in all directions. But a tree in a dense forest—standing shoulder to shoulder with other trees, as it were—having only a narrow window of light up above to grow toward, will develop a long, straight, largely branchless trunk. This is possible because as it grows, a tree often sheds its lowest branches, using a form of self-amputation. Indeed, in some sense, trees are better surgeons than we could ever hope to be. When a branch no longer receives enough sunlight, the tree will begin to seal off its withered limb by growing layers of new tissues at its base, then dissolving some of the connective tissues, so that when the branch falls away, it does so cleanly. The wound heals before the wound is ever made. > When a tree grows upward, it typically sends only one branch toward the sky—what’s called the leader branch—while all of the other branches grow out laterally. Logically, it would seem that many branches would compete for the sunlight at the top of the tree. The reason why other branches tend not to challenge the leader’s dominance is because, along with a trickle of nourishing sugars, the leader sends down special hormones, called auxins, which inhibit the vertical growth of the branches lower down on the tree. When the leader branch is broken off in a storm or nibbled down by pests, multiple branches will often compete to become the new leader. A kind of slow anarchy ensues, which can threaten the structural stability of the tree as a whole, until a new leader emerges. > Also, trees can sense the presence of light, via cells called photoreceptors. When a branch is exposed to sunlight, auxins are pumped away from the source of light, causing it to grow faster on the side with more auxins, which bends the branch toward the light. (In a brilliant twist, roots do precisely the opposite: There, the side with more hormones is genetically programmed to grow slower, which means it bends away from the light.) In the 1980s, NASA scientists proved this conclusively by growing a tiny forest of pine seedlings on board a space station, in a zero-gravity environment. They found that they grew more or less normally, with their branches reaching toward the light and their roots reaching down into the soil. As the farmer and author Gene Logsdon notes, “Sunlight seems like such a soft, passive, gentle kind of force, but it is strong enough to bend tree trunks.” > For most of my life, I have been bored by flowers. They always seemed to me to be frivolous, frilly, decorative, nonnutritive—merely beautiful. I was wrong. Flowers, it turns out, are one of the greatest innovations in the history of life on earth. Prior to roughly 140 million years ago, all trees on earth were gymnosperms, which means that they dispersed their pollen using the wind, in the hopes that their genetic information would float through the air and land on a female cone of the same species. By developing seeds hidden within flowers and fruit, a new class of trees—the angiosperms—found a way to reproduce more effectively. Rather than haphazardly releasing their pollen into the wind, the flowers of angiosperms could entice a wide array of animals (insects, birds, certain small mammals) to deliver their pollen to precisely where it needed to go. This tactic proved immensely successful; angiosperms soon spread around the planet, outcompeting their flowerless rivals. It had only one major downside. As the author Tristan Gooley notes, “Animal pollination is much more efficient than wind, but you have to make sure your flowers stand out. It’s like the most ruthless horticultural competition imaginable: If you fail to get a gold medal, your family dies.” > When a tree is bent over by harsh winds, or when another tree falls on it, the tree will react accordingly: It will grow asymmetrically in order to straighten up. This process differs in needled trees and leafy trees: In needled trees, the trunk reacts by growing tougher fibers, called “compression wood,” on its underside, whereas in leafy trees, it grows more flexible wood, called “tension wood,” on its upper side. Both are forms of what is called “corrective growth”: The crown of the tree is gradually pushed (or pulled) upward. But due to the gnarling process, the curvature in a tree’s trunk will forever bear the mark of its trials. Loggers tend to hate bent lumber, because crooked trees can be more dangerous to fell and because they make for less valuable timber. For this reason, the Taoist philosopher Zhuangzi professed a special reverence for twisted, “useless” trees, which, being spared the axe, can endure for centuries. > Trees can be sculpted because trees are mostly made up of wood. Other kinds of plants, like herbs, are able to stand upright only because the water pumped through their vessels keeps them taut; when the water runs out, they wilt. But true trees have evolved wood—a substance made up chiefly of two polymers, flexy cellulose and flinty lignin—which allows them to grow taller. It’s the lignin that makes wood woody. Lignin is also what must be stripped out of wood, when it is pulped and processed, to make paper pliable. My friends have often pointed out the irony that this book about the beauty of trees is printed on the mashed-up corpses of slaughtered trees. It is an additional irony that to make this book of trees, someone first had to remove the very thing—woodiness—that makes a tree a tree. > In the wild, the shape of a tree is the visual record of every moment of that tree’s life—every wound, every ice storm, every new bounty of sunlight. Tristan Gooley writes that trees can also be interpreted to glean information about the land around them: the prevailing winds, the height of the water table, even the location of the magnetic poles. (One of the many handy survival tricks he imparts is that, in the northern hemisphere, one can orient oneself by counting the number of aborted branches on a tree. Since the sun tends to be in the south, trees will grow, and subsequently drop, more branches on their south side.) The shape of trees also reflects their latitude. Tudge notes that trees in the far north and the far south grow “tall and steeple-like to catch the light that comes at them from the side,” whereas trees in the Middle East and Mediterranean acquire flat tops, “aimed at the sunlight beamed from overhead.” Trees in tropical rainforests shoot upward, juiced with ample rainwater, while in drier equatorial habitats, like the African savanna, they “spread themselves like cats.” > Trees are serene in the face of death. Rather than shying away from it, they use it to strengthen themselves. The vascular tissues of a tree are fated to die long before the tree does. The phloem lives only a few months (or, in some cases, years), then dies, dries out, and lines the inner bark, whereas the xylem, which survives for an even shorter amount of time, eventually gets crushed down into heartwood. These layers of dead tissue accumulate, year after year, enclosing one within the next, as the tree grows both taller and thicker. In this way, the tree’s circulatory system gradually becomes both its skin and its spine. By the time a tree reaches maturity, it is roughly 90 percent dead. “And so the tree continues, part dying and part being born,” wrote Aristotle. “Hence their long life.” > Certain trees have evolved to store large amounts of water in the tissues of their trunks, saving it for the dry season. Baobabs, the camels of the arboreal world, can hold the contents of an average backyard swimming pool. Because the water is mostly stored in the tree’s vascular tissues, humans can’t access it—the notion of tapping a baobab in the desert for fresh water is a myth—but thirsty elephants have figured out how to tear hunks off the tree and chew on the wood, sucking out its moisture. Other trees, like magnolias, are said to hold so much water in pockets in their trunks that when arborists use chainsaws to cut into them, they occasionally get sprayed with a thick, red, tannin-rich liquid that eerily resembles blood. > A tree is open-minded by its very nature. Unlike so many of us, it does not solve problems by imagining a single solution and then pursuing it with laser-like focus, nor does it solve it through blind guesswork. Instead, it employs a systematic process of trial and error, or more accurately, trials and errors: by growing many branches simultaneously, dropping some and retaining the rest, which gather their own share of sunlight while also supporting one another and the tree as a whole. ## Chapter 3 - Ascending # Quotes # References