‘Exceptional’
Robin Lane
Fox
‘Perception-
THE GENIUS OF TREES
THE GENIUS OF TREES
How Trees Mastered the Elements and Shaped the World
HARRIET RIX
THE BODLEY HEAD LO NDON BH
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First published by The Bodley Head in 2025
Copyright © Harriet Rix 2025
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Most of the images contained in this book are the property of the author or her father, Martyn Rix. Fossilised Archaeopteris root courtesy of William Stein.
From Revolt Against the Sun: The Selected Poetry of Nazik al-Mala’ika: A Bilingual Reader translated by Emily Drumsta reprinted by kind permission of Westbourne Publishers Limited.
From Basti by Intizar Hussein translated by Frances Pritchett, reprinted by kind permission of the New York Review of Books
From Memoirs by Pablo Neruda, translated from the Spanish by Hardie St Martin, reprinted by kind permission of Agencia Balcells.
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For my mother and the Goatchers, who grew countless trees
Chapter
Chapter
Chapter
Chapter
Chapter
Introduction
There is a power that has been since all eternity, and that force and potentiality is ‘viriditas’, the greening.
Hildegard of Bingen
I’ve never been good at saying no. As a result, I found myself in Iraq for the first time in June 2014, staying with Assyrian friends in the town of Amedi, an ancient and magnificent walled town on a mesa – a flat-topped hill bounded by steep escarpments – just south of the mountains dividing Iraq and Turkey. It was a feast day to celebrate Noah landing the ark safely on a mountain top, so the whole extended family came together to eat pacha, a sort of Assyrian haggis, uncomfortably and very obviously stomach. The talk was troubled – ISIS was entering Mosul, only 80 km away – but just as we started to eat, a breath of jollity arrived: the son of the house, who had been unsuccessfully hunting wild boar. There are lots of them in the mountains, he said, they eat the acorns. I was naively amazed – I had never really thought about trees in connection with Iraq.
That evening we wandered through the oak trees below the escarpment. There seemed to be many different species, and different combinations of leaf shape and bark texture, branch angle and acorn size. One big tree had a thick hollow trunk, charred inside but still living, with long, smooth dark-green leaves, and covered in dark-brown spherical oak galls. These, my host told me, were the reason Amedi had been rich in the twelfth century: the galls contained chemicals, tannins, that the tree used to defend itself against insects, but which humans harvested as the key ingredient in early
ink.* Another tree, short and with many stems because it had been coppiced, had small, spiky, silvery-green leaves covered with down. Another, the trunk draped over a large stone as if there was no need for earth, had enormous, sharply serrated leaves, and bunches of Velcro-like hooks hanging down. In the autumn, I was told, they would develop into enormous acorns that humans would eat. All very different from the European oaks I had grown up with in Devon. But they made me feel at home.
When I got back to England I went to stay with my dendrologist grandmother, and was put firmly in my place. Not only did she know of people eating acorns in the US and Greece, but she had done it herself in the war, and found them not bad, she said, just rather bitter (those tannins again). She was interested to hear about the trees I’d seen. Oak species vary considerably, she explained, and have been growing back into their old haunts since being driven into retreat by the last ice age. Oak trees now stretched almost continuously right around the globe, she went on, in a belt of interrelated species. They dominated forests from North America to the Zagros mountains in Iraq, and the ones I’d seen were probably trying out the best leaf-shapes for the areas they hadn’t moved back to so far. So far? She said it as though more than 12,000 years wasn’t long at all.
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When modern humans evolved about 40,000 years ago, there were an estimated six trillion trees on the planet. By the time we appeared on the scene, trees had already altered the planet’s air, changed the flow of water, used fire as a tool and built relationships with the plants and animals around them. For almost 400 million years, trees have been some of the largest organisms on dry land, physically blocking airflows with their branches, channelling waterflows with their roots and acting as architects for other segments of nature; a mosaic of microhabitats. We
* Amedi was a place of scholarship and had also had one of the most famous libraries in the region. My host was sparing my feelings – he did not mention that it had been destroyed in wanton violence by British troops in 1919.
can see this above the ground when we take the time to observe closely: a tree or leaf is a condensation of the place it comes from, and bears the marks of its experiences. The leaf of a ginkgo, for example, has veins optimised 385 million years ago, a broad fan-shape which nearly led to its extinction, and the pigments of a changing ozone layer. It is harder to imagine the complexity of a tree below ground, where trees are blind explorers, guided by fungi and bacteria but vulnerable to them, carrying an internal compass of gravity-sensing proteins, their only lodestar the centre of the earth.
This book is the story of the agency of trees, a story which – amazingly – no one has told before. How trees communicate and co-operate with one another, what trees do for humans: all this we have been told. But what have trees done simply through the accidents of their evolution, and why? This is the story of how tree-ish trees are, and how, by being tree-ish, they have woven the world into a place of great beauty and extraordinary variety. It is also the story of how we both under- and overestimate trees, and a plea to distinguish rootedness from inactivity and subtlety from simplicity. But while this is a story of science, it’s also about taking the science and imagining beyond it, for early in its evolution our very imagination was itself shaped by trees.
It’s easy to take our senses and technological genius for granted, and forget how ignorant we might be of the very different, embodied genius of these vast organisms. For a start we can try using those senses better than we usually do. Clothing the physical form of a tree in dazzling, virtuosic polyphony is its chemical impact, and here our senses of smell and taste are our way in. Have you ever wondered why bay leaves smell so fragrant and spicy? Why monkey-puzzle bark smells of cucumber? Why a cedar-wood coffer smells warm and soft? Scientists can provide names – polyphenols, sesquiterpenes, esters – but they’ll be hesitant to give reasons for the chemical behind each scent. To say that a particular scent is to repel herbivores or prevent rot is to provide a tiny sliver of the reason that a tree would create its own chemical cocktails.*
In the same way, a nutritionist can give you percentages and
* In her book Good Nature (Bloomsbury 2024), Kathy Willis points out that human noses – although not as acute as dogs’, can detect one trillion different smells.
chemical formulae for the vitamin C in an orange, or the health benefits of protein in açai, or a picture of improved skin from the fats in an avocado, but the complete story of their construction is that of 20 million years or more of tweaks and biochemical cycles and patience deep within a tree. After all, trees start with the basics: air, water, salts and sunlight. Even AI is unlikely to find enough information to tell the story fully.* Humans have a tendency to underestimate trees: to assume that if we plant one it will grow, that if we cut one down we can simply plant another. Because trees don’t move it is easy to forget that chemically they can run rings round us while we sleep, that they are moving the earth under our feet and shifting the colours of the sky over our head, not just by shading out the sun, but by taking out carbon dioxide and releasing oxygen. Easy to forget, too, that tiny amounts of nitrogen we can’t see can unbalance and destroy an oak tree with a trunk thicker than a car, which was planted when the Welsh and English were still at war.
We share one world with trees and one need for survival. Over the years of their evolution, trees have become not less but more complicated. When the atmosphere became inhospitable or the earth became too dry, their response was to diversify – to change form to an astounding extent – in order to continue to fit their ecosystem like a glove. The flexibility that we assume in animals, because proteins are inherently more mutable than carbohydrate, is better seen in the micrometre interactions between trees and their environment. Where one tree species burnt or starved or was blown away, hundreds more came to fill their place.
What is a tree? And how and why did trees appear on earth? Most of us can look and know: leaf, branch, trunk, tall, rough or smooth bark. The technical definition is a woody plant with one erect perennial stem, its trunk at least 7 cm in diameter at a point 1.3 metres above the ground, a formed crown of foliage, and a mature height of
* We have made some progress in understanding the molecules that trees have created: the mass spectrometer can be used to tease out the weight and structure of molecules, x-ray crystallography can reveal complex protein structures, and fluorescent proteins can show some chemical movements happening in real-time.
at least 4 metres, although as with all definitions this is just a starting point.
Bamboo is excluded, no matter how tall, on the grounds of its stem structure – it’s a grass. A bonsai is included, however tiny, because it exhibits the chemistry common to all trees, and its miniature leaves and flowers conform to a tree’s outward shape.
Why do trees share the attributes they do? Mostly it’s a result of the life they lead. Trees are successful because they use sunlight and carbon dioxide to make carbon backbones, in the form of sugar, which provide energy for the rest of life on earth. This is photosynthesis, a process that makes plants both eaten and indispensable. Sunlight and carbon dioxide have both been plentiful for billions of years, and trees inherited the fundamental chemical components to deal with them from their single-celled ancestors. DNA was one essential component. Its stable double helix – the founding structure of life – evolved 4 billion years ago, but its combinations are still typing out the replicating code that allows for life’s continuity and development. Chlorophyll evolved in bacteria living in the sea 3.5 million years ago, enabling sunlight to be captured by trees in the form of chemical electrical energy. Rubisco, the most common enzyme on earth, comprising almost 50 per cent of the protein in leaves, evolved 2.4 billion years ago, and enabled photosynthesising bacteria (called cyanobacteria because they appeared blue) to store the energy they were capturing from the sun by forcing carbon dioxide molecules to form a carbon-carbon bond and enter the stable chain of life as glucose – which can in turn be consumed for energy by other life forms.*
And about two billion years ago, cyanobacteria found a way of spinning many glucose chains together to make cellulose, which allowed them to create a structure and build cell walls. When the first plant cells found themselves beached on land they were burnt up over and over again, until finally a free-radical reaction produced a molecule that offered at least some sun-protection, and soon became lignin, the woody part of trees.
* An enzyme is a biological catalyst: a protein that acts to push two chemicals together in a correct configuration which enables them to react.
So far, so chemical; but as more and more green cells came together, structure became all important. The combination of lignin and cellulose was indispensable in the development of trunks, wicking water upwards and providing a casing of Kevlar strength against the decay induced by bacteria and fungi.*
Structure was necessary for efficiency and protection. How this resulted in a type of organism that can grow 116 metres tall, more than twice the height of the Leaning Tower of Pisa, is the truly extraordinary question. To understand how these superstructures formed and diversified we must follow tree evolution down uncertain roads of which only faint traces remain, in fossil sites in French mines and Chinese valleys and German quarries.
Going back so far in time leads us to a strange, apparently accelerated world, in which continents drift around like rubber ducks, bumping into one another. Geological periods are usually defined by a big extinction event in which the world abruptly changes, and only a small portion of life on earth carries on. In the Silurian period, for example, 440 million years ago, fungi rather than trees towered over the swamps of the great supercontinent of Gondwana. Draped among them were lichens, strange organisms that kept co-operation between their single cells of algae and fungi loosely structured in order to exploit the violent new environment of the land. But the first land plants with structure and a vascular system were already developing; they grew upwards as well as horizontally.
In Lindlar in Germany, fossils indicate plants growing upwards in a leafy spike, tufty shrubs about a metre long that swirled like millipedes over the forest floor. By the Devonian period about 383 million years ago, trees had developed woody trunks, strengthened by lignin, and co-operated and fitted together as forest much as they do today, while fungi had sunk back into the ground.
There is a recent school of thought that says trees endured and
* In a book of 80,000 words there is an enormous amount of trapped energy: a library of borrowed tree stability. If a tree is twenty when it is cut and pulped it would have taken about four months’ continuous work for that tree to produce the biomass for one book.
flourished in the Devonian because they had started to shape the abiotic environment – water, trees, air – around themselves. Dry land was still a relatively new environment for life, and a destructively chaotic one. Typhoons, electrical storms, shifting sediment, bare rock: it was not the gentle, fertile land we know today until trees got to work on it. The best example we have of a fossil forest that can be reconstructed was found at Gilboa in upstate New York in the 1920s. There, rooted fossil tree trunks almost a metre across have been matched up with fossil trunks more than 8 metres tall which end in a fan arrangement of fronds.
The fossil site shows that leaf litter was thick and decayed slowly. The forerunners of centipedes and spiders flourished. Shrubs grew underneath the trees and entwined with their roots. Connections, root grafts and inosculations – literally ‘kissings’, where trees grow into and merge with one another – were common. The picture of the swampy forest floor we can see from these fossils couldn’t be less like a modern plantation, where trees stand upright in rows, but it was productive at a level modern foresters would envy: photosynthesising fast and locking down lots of carbon dioxide out of the atmosphere they forced water into clearly defined channels – the beginnings of what we now call rivers.
In 2019, a new discovery added another dimension to our understanding of the impact of trees on the environment 386 million years ago. Excavations in the Cairo quarry near Gilboa in Albany, New York, revealed an enormous 11-metre-wide root system. It belonged to a tree called Archaeopteris which was weathering rocks at a rate no one had expected, in unexpectedly dry conditions. The process was already under way, but trees industrialised it, breaking up rock to make earth and sending sand particles down into the sea, where they reacted with acidic water, locking down dramatic levels of carbon dioxide.
These trees, giant clubmosses and tree ferns, spread widely during the Devonian period and had some of the impact we would lump together today under ecosystem services or natural capital. Flood plains were stabilised by tree-like plants and their roots, and meandering rivers appeared, which were blocked by log jams of clubmoss trunks that looked almost like beaver dams. Trees acted as shelter belts, stabilising air movements as winds roaring in from the ocean
met with resistance in a form that absorbed and diffused the energy. Rain that fell on these coastal forests re-evaporated and fell again as rain further inland, reducing the dry desert area of the vast continents, and allowing trees to penetrate further into the interior.
While we have only a few rare complete fossils of the trees themselves, the distinctive spores of each species, produced in their millions, are possible to follow across the continents in layers of rock. The spore record reveals huge changes in the species distribution and spread of trees across the world in the first 50 million years of their development. By the Carboniferous period 300 million years ago, trees had spread over so much of the globe and locked down so much carbon that they altered the atmosphere completely. Levels of oxygen went rocketing up from 15 per cent to 35 per cent, with a correspondingly massive impact on life on earth. Animals became gigantic, plants struggled to adapt and oxygen became so available that fire destroyed huge swathes of trees. A particular type of fossil carbon called fusain is everywhere in Carboniferous fossil deposits, evidence of widespread forest fires.
But through their structure and chemistry, trees learnt to survive and even to master fire, as well as the other elements, and out of the ashes of the Carboniferous forests a thousand new types of tree evolved. Trees had found a way of life and a timescale to operate on that proved immensely successful, and from their position as primary producers and earth’s architects, even enormous herbivores like the dinosaurs could be tamed.
In 2018 I went back to Iraq with a clearance team to a field in Anbar province near the Euphrates River, where ISIS had laid a minefield of big improvised explosives. We started at 4 a.m. and by midday the sun was blisteringly hot, so I sheltered under a grove of date palms and talked to a man who, having lost two goats to mines, was understandably keen to make sure they were all safely cleared. Perhaps it was because I had already been jerked out of my comfort zone by the desert, and by seeing the city of Ramadi flattened, the floors of the
houses concertinaed on each other, but when he said that times were bad, and that the palm trees were dying one by one, I felt I understood for the first time what environmental destruction was – the removal, piece by piece, tree by tree, of the chain of life. Everything – lack of water, dust storms, blazing heat, war – was acting together in concert to destroy these trees one by one, and without them everything else – soil, birds, plants – would go, too, and we would be left battling typhoons and dust storms, starving and thirsty, back in the world before trees, 385 million years ago.
What could possibly halt this? I realised the answer had been there four years earlier in Amedi, as we’d walked down the stone steps from the citadel, through the carved gate and into the woodland surrounding the town. The oaks I had looked at – multi-stemmed and gnarled, with leaves and trunks eerily different from and yet so similar to the oak woodlands of Devon over 3,000 km away – had lived for a hundred, two hundred, maybe a thousand years. They had adapted and shaped the humans, animals and plants around them; their roots were breaking rocks up into earth, sharing useful chemicals; their interlaced twigs sensitive to a thousand chemical signals; their leaves, exquisitely adapted to their environment, releasing water vapour that formed into clouds and precipitated rain. Besides communicating with the trees nearest to them, they were part of a meta-organism, a treescape that had evolved and spread over thousands of years all around the world. We know the bigger picture, that in the early days on this storm-swept planet, trees created places of greater safety, areas of deep-rooted stability – and that, interlaced together, they are actively generating that stability still.
One of the more specific superpowers of the oak trees found near Amedi is that as they get to a certain age they can change sex. Most of the young trees are male and produce pollen, while the older ones are female and produce acorns. This allows the oaks to feel their way forward into the future, with the huge old female trees being pollinated by multiple young males. The established oak tree, successful at surviving, provides lots of fatty acorns with energy for the next generation, an environment the young trees can grow into and DNA with a proven track record of success over many seasons, even as the
wind and climate have gradually altered around them. The sapling males, on the other hand, have only recently grown up, and their pollen contains DNA and modifying proteins that have responded successfully to new diseases and sudden transformations in water or climate. The resulting offspring, then, should have the best of both worlds: an innate ability to harness the elements tempered by recent vicissitudes, honed by the ruthless hand of annihilation year after year after year.
It is too easy to think of trees as passive because we cannot see what they are doing. Victims, because we can take a chainsaw and clear a hillside of them. Does our aesthetic appreciation of trees incline us to preserve them? Have trees in some way, indeed, cultivated that aesthetic sense? The greatest wisdom is understanding that appreciation and conservation are two sides of the same coin. What we call the environmental benefits of trees – air cleaning, prevention of flooding, even sequestration of carbon dioxide – are side effects of trees’ abilities to shape the environment. Thankfully, we evolved into a tree-shaped environment, and our interests are aligned. Our existence, just like theirs, depends on it.
The agency of trees gives us hope for the future – at least if we can rein in our good intentions and let trees know best. Tree planting programmes are one of the major ways that tree diseases spread. Planting eucalyptus, most pyrogenic of all trees, will not capture carbon long-term, and neither will pine trees. Rewilding too can allow trees to grow up which are more likely to be short-termist and burn or self-destruct. The spectre of many unsuccessful forestry projects must be a lesson that one size can never fit all, and that dogma destroys nature. Like all environmentalism this must be a global story of adaptation and exchange. Trees can’t be everywhere, but they don’t have to be – their moderating effect can stetch far beyond them, connecting Russia to America, China to Chile.
But our story begins off the coast of Africa, in damp forest on the tiny island of La Gomera, with the wild smell of laurel in our nostrils . . .
CHAPTER 1
Trees shaping water
How trees brought water down and sent it back up
there is a sky behind the forest, there are seas unbounded, seething, waves made from the foam of dreams and churned by hands of light.
Nāzik al-Malā’ikah, Revolt Against the Sun trans. Emily Drumsta
The cloud tasted of almonds. The tree I was looking at grew up greybarked and straight for almost 30 metres until its trunk dissolved into the thick mist, and everything about it – slim emerald leaves, dead flaking twigs, mossy junctions between the leaves – was dripping with great drops of water that bent a thousand reflections into smooth curves. It was a tented world of cloud and moss and deep green. I knew, but couldn’t see, that on the ridge to either side the laurel trees and the clouds continued, but if I walked just a little way down the mountain the sky would clear, there’d be a hot sun in a blue sky, I would see the sea, be surrounded by cactus and spurge and feel the baking heat. That alternate reality seemed a million miles away. It was February and I was in the Canary Islands, in the cloud forests of La Gomera.*
Trees are cloud chasers, and cloud forests – known for their constant, lingering mists – don’t exist just in the Canaries; they are in Brazil and Costa Rica, China and Borneo, Australia and the Philippines. Cloud forests tend to occur in places where the landscape
* The Canary Islands sit off the coast of Africa, bathed by the desert winds off the Sahara and the trade winds coming northeast down from Europe. The island of La Gomera is one of the smallest, a little volcano that rises to 1500 metres above sea level.
gathers water from the air – typically mountains next to the sea. But trees don’t affect water only in cloud forests; all three trillion trees across the world have an effect on rainfall and waterflows above and below them.
In a sense, trees developed into trees to gain power over water. During photosynthesis, trees use packets of solar energy to split water into hydrogen and oxygen, and transfer the electrons onto carbon dioxide so it can start to make sugars.1 This means they need large quantities of both air and water – mutually exclusive unless you can operate vertically. In the earliest stages of their evolution as trees in water-logged environments, upward growth raised green parts of the plant above the water and into the air where it could photosynthesise, while in dry areas vertical growth downwards allowed access to deeper water tables.2
Having started successfully, trees continued to evolve a tightly engineered anatomy to chase this advantage.* Above the ground, trees are rainmakers, growing tall to interrupt air flow with their leaves and trunks and branches, emitting volatile organic compounds like scents and alcohols to seed clouds, and releasing water vapour out of their stomata to cycle a gentle, consistent flow of moisture from the air. Below ground, their roots collect and redistribute water, ushering water down to the water table, lowering or raising the level of the water table to ensure they have just the right amount for their roots to be on stable ground. And in between, the tree can control and use the water within itself. Just as humans can reach up to pluck an apple, crunch hard to eat it and bend down to plant the core, so trees use all these three capabilities to direct water across the earth.
Cumulatively, the earth’s trees sweepingly adjust global water flow. Trees of all 73,000 species are constantly making minute adjustments, but normally the resultant changes are subtle, deniable and easy for humans to ignore, or, as in the Amazon rainforest, on a scale
* The anatomy trees evolved is more tightly engineered than a human body is engineered, because rather than moving to avoid drought and flood they must control their own environment in dynamic equilibrium.
too enormous to be easily comprehended. I had gone to La Gomera because the dramatic change from desert to cloud forest is heightened by the extreme lengths to which the trees have gone, and continue to go, to maintain their clouds. You can see the water pouring off their branches, smell the terpenes seeding the cloud, and in the tangle of dark-green leaf shapes above your head it is obvious that you are looking at cloud catchers, branches designed to scoop out the belly of a cloud. What you can’t see is the effect of transpiration – water molecules sucked up by the tree’s roots hustling minerals through the trunk, up to the furthest leaves 30 metres above, and then with a final puff of energy evaporating off and out into the air. You can, however, feel it in the cool under the trees as heat departs with the water molecules that are heading up to swell the clouds.
But does water enable the trees, or did the trees enable the water? A little bit of both, but trees are good at clinging on where they are given even a hint of water to work with. When the climate changes around them, trees tend to evolve, so that, where possible, they outflank the change by getting even better at shaping water. Most of the trees that grow in the monteverde forests, for example, the mostly evergreen forests that flourish in mountainous areas, are rare laurels, surviving forest that diversified out of the chaos after the last great extinction 66 million years ago when an asteroid hit the earth at Chicxulub in Mexico, causing darkness, chaos, a cloak of iridium over the earth and the extinction of the (non-avian) dinosaurs.
Before the extinction, forests were open canopied and dominated by gymnosperms (literally, naked seeds), which grow tall and straight and stiff and live long, and alter the abiotic environment – water, air, earth and fire – dramatically. For the first 300 million years of tree existence, these were the trees that were shaping the world, and they still grow on six out of seven continents, and thrive in some of the most inhospitable parts of the world. They include the pines and the firs and the larch, the monkey-puzzle, the yew and the Wollemi pine, the giant redwoods and the Podocarpus, the towering alerce and the mighty kauri, and also some of the most endangered trees on the planet. The asteroid strike marked the beginning of the end for many of them, and, because the trees that replaced them differ
fundamentally, I am, dear reader, going to crave your indulgence as we make a quick diversion into tree history. Stay with me, because it’s only a page, and essential to understanding not just why trees have had an effect on water, but why they look and grow as they do, and the effect that this has had on the world.
There is a split down the middle of the tree world, separating the conifers and the broadleaved trees. With a few exceptions (including the broadleaved deciduous alder trees, lover of riverbanks, which produce little cones which float downriver, the Casuarina of India, the Philippines and Australia with its tubular, weeping leaves like a green plume of horsehair, and its spiky green cones, and Platycarya strobilacea, found in fossils in the London clay and now happily embedded in forests in China and Korea and Vietnam, walnut-like until you see its bristling brown cones), all trees with cones are gymnosperms.
On the other side of the split are the angiosperms. Two hundred million years ago, a flowering plant developed under the gymnosperm canopy that later benefitted from the revolutionary impact of the Chicxulub asteroid and became the diverse and abundant group of plants we see today as the flowering plants. Some of these angiosperms (literally ‘contained seeds’) grew up to be trees, and they are mostly broadleaved and often deciduous, losing their wide leaves once a year in colder or darker seasons. The angiosperms include the oaks and the ash, the Parrotia and the baobab, the eucalyptus and the laurels, the palm trees and the rhododendrons. The angio-, or container part of their name, denotes the fact that they have a carpel: a fleshy, adapted leaf that folds around the ovaries, meaning that pollen has to penetrate through some of the plant before it can produce seed. This was an extraordinarily powerful mechanism, because it gave the female plant – which here simply means the plant that will produce seeds and is therefore investing most nutrients in the potential offspring – the power of selection, a dogma-defying ability to determine its offspring’s DNA and therefore influence evolution. The result was diversity and flexibility, chemical and physical. It’s
not just imagination that makes angiosperms look more youthful and less staid than gymnosperms. Giant sequoias and other gymnosperms often have burls, huge shoots waiting to spring up if they hit the ground, and it is supposed that these are an adaptation to the trees being knocked over by dinosaurs. By contrast, an angiosperm will root in a thousand places – even a 100-year-old beech tree can send up shoots from its trunk if it falls over, and becomes a phoenix tree. Genetically too, angiosperms tend to be more flexible, happily duplicating their DNA and experimenting with new chemical compounds. The ability to produce flowers and fruit, as well as shorter timescales of reproduction, meant that angiosperms shaped biotic factors – bacteria, fungi, plants, animals and probably also humans – more than gymnosperms.
In the dark of the impact winter that followed the asteroid, the gymnosperms, those old evergreen trees which smothered the earth in the Carboniferous period, suffered.*3 Their leaves, big enough when carbon dioxide levels were high and the earth peaceful, were suddenly too small, the veins bringing water and nutrients to the leaves too rudimentary for a dramatically changed climate, and their long lifespans a disadvantage in a world of chaos. They were supplanted by the angiosperms, which could cycle water quickly and drop their leaves when necessary, conserving energy until the next opportunity arose.
As a result of the rise of the angiosperms, tree productivity increased hugely. Whereas gymnosperms grew (and continue to grow) in elegantly spaced forests with one imposing species dominant, the new angiosperm rainforests were crowded and multi-layered, fast moving and packed with diverse species. The laurel forest that arose in a humid period 40 to 15 million years ago is just one way tree ecosystems co-evolved to bring down water together.4
The period laurels developed into is broadly known as the Tertiary, and the trees that survived the droughts and subsequent glaciation are, like the spouses of a dead climate, sometimes known by the Dickensian term the Tertiary Relicts.† About 15 million years
* Gymnosperms are still struggling – 37 per cent are in danger of extinction. † This term is technically no longer in use, but I think it’s expressive so continue to use it throughout.
ago, monteverde forests, the green, tropical or subtropical montane cloud forests, were widespread across Europe and North Africa, the vegetation of the Mediterranean before the ice age and subsequent onset of the hot Mediterranean summers. But as ice spread across the northern hemisphere and locked up water, the climate became drier and most of the laurel forest died off, leaving tiny reminders of itself around the shores of the Mediterranean: the fragrant bay laurel of Italy used in osso bucco, the warty Zelkova of the rain-catching White Mountains of Crete, and the maple-leaved liquidambars of humid coastal Lycia, perfume-bottles of the ancient world.
Close to the sea, trees have moisture in the air to work with, and in the Canary Islands you can see, with dramatic clarity, how the cloud forest adapted the climate to itself. On La Gomera, only 322 km from the Sahara, and 1,000 metres above the sea, this fragment of the earth’s cloud forest precipitates a world better known to high mountain dwellers: a world of thick mist, high rainfall and cool, changeable weather. It’s also an area of massive diversity and high carbon capture. On the Garajonay peak of La Gomera I was in a place where you could see and feel trees manipulating water.5 I stood under a holly and looked down over the northern side of the island. There were mackerelled clouds blowing towards me, and as they approached they seemed to rise up. Three minutes later I was drenched.
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I had arrived in La Gomera by boat on the island’s south side, where dragon-trees sit in formal gardens and houses bask in the heat. In the hotel there was a spectacular dragon’s blood tree, Dracaena draco, a spiky, canopied tree with dramatic grey leaves like rapiers, sap as red as carmine and fragrant white flowers.* That night I went and furtively lay underneath the tree, looking up at the stars through its strange fractal branches. Just like the laurels on the hill, it is a survivor from one of the oldest ecosystems extant today, the warm, wet
* Which is technically an overgrown lily.
woodlands of the Tertiary, but unlike the laurels it has adapted to survive on small amounts of water, and to transpire as little as possible. Its relatives cling on across Africa, further south on the Cape Verde islands and on the island of Socotra in Yemen, where they form ravishingly beautiful woods across island slopes in the blazing heat of the Indian Ocean. Its more distant relatives have survived in China, Vietnam and Thailand, stretched thinly around the world by the movement of tectonic plates, and there are even two species in Central America.* When Columbus set sail in 1492 from San Sebastian for his first Atlantic crossing, massively understating the distances to his timorous crew and shying at every crab, bird and sign of life he came across, Dracaena ’s spears had already been influencing both sides of the Atlantic for millions of years.
How do different trees adapt to their distinct niches and still shape water? The fastest route is by adaptation of leaves, and particularly changes in the stomata – microscopic pores within the leaves which allow water, gases and other chemicals in and out. I had looked closely at the grey rapier leaves of Dracaena under a scanning electron microscope in a laboratory at Oxford.6 You can see scurfy layers of wax flaking off the cuticle surface, a neat tilework of rectangular cells and navette-shaped pores deeply embedded in the surface of the leaf. If the leaf’s cuticle is a defensive, Byzantine wall of cells between a tree and the outside air with all its perils – bacteria and fungi and their spores waiting to loot leaves of sugar – then the pores are conduits that can be opened to allow air in. But the cuticle is also designed to keep water in, and under drought conditions pores can become a liability, allowing more loss of water than a plant can stand.
The pores or stomata (from the Greek for ‘mouth’) are gateways for carbon, and because they are so fundamental to tree survival they can’t evolve quickly – it would be as risky as our lungs suddenly changing structure. This means that stomata of Dracaena all over the globe look very similar, but the dragon’s blood tree uses the wax that protected those mouths from drowning in the fine, damp days of
* One spread across Mexico and Belize, Panama and Columbia by howler monkeys, and one confined to eastern Cuba.
the Miocene to prevent water loss now. And its blade-like bunches of leaves7 use the same smooth waxiness in the winter months, as they whisk around in the wind, to capture any thin wisps of fog that blow over it, absorbing droplets of water along the blades and into the leaf bases and stems to act as a war chest for the summer.* It then oozes vibrant blood-sap over the water-swollen leaf bases; a resin full of ring compounds – garnet-red flavonoids, steroidal saponins and anti-microbial polyphenols – that both seal in water and repel any scrounging animals desperate to drink. Restorative, coagulant and antifungal, these compounds soon attracted the notice of humans. Who would not bleed a tree for compounds like these?
Not all the water dragon’s blood trees scythe out of the air is stored. Much of it drops onto the soil, helping other plants to establish themselves. After high rainfall the navette-shaped stomata open, dramatically increasing the amount of water vapour they release through transpiration, and seeding the air for the next wave of cloud. Even with these adaptations they can struggle for survival. As humans chase them to still drier, higher, steeper places – in some areas literally off cliffs – their wild range is shrinking rapidly. In gardens, however, like in this one in La Gomera, they are flourishing, fed by water taken from springs and groundwater, with half their adaptations redundant.
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Early the next morning, with dried fruit and water, I walked out of town, inland up the bare concrete-channelled river into the valley of the volcano. At first, little grew apart from Canary Island palms, Phoenix canariensis, lining the river, and I luxuriated like a lizard in the baking sun. Soon there were signs of irrigation; fruit trees and lush vines by the brooks and streams. Then, as I left the stream and turned right onto the zig-zagging path that led steeply up the edge of the volcano itself, everything started to change. It was like walking
* Abrasion by sand and other wear and tear of the cuticle can help the water to settle and be engulfed.
into an Instagram filter of green. Mosses, hollies, everything turned from grass-green to emerald-green; the vegetation thickened, trees thickened and appeared in every crack in the rock, epiphytes started growing on them, leaves became spine-like or like slim dark ovals, branches and trunks started to occupy every available piece of space and, finally, as I walked over the ridge of the mountain and looked down at the sea on the other side of the island, I was in the cloud; deep, dripping shade.
Laurels were all around me, 30 metres tall with smooth brown trunks and spear-shaped leaves. They smelt aromatic and woody, almond-bitter. Another cloud came over us. I looked at the laurel nearest me, an Ocotea foetens, with smooth bark and branches supplicating the fog I could see settling on its leaves and coalescing, and pearling on the bryophytes that grew all over it, until after a little while rain was dripping all around me, water flowing from the leaves of the trees like a fountain. I wasn’t the first person to notice this; a tree of the same species called Garoé was a totem to the first peoples of El Hierro, the Bimbache.* A Spanish observer, marvelling at the tree, wrote, ‘There is always a little cloud over the tree . . . all the leaves and branches drip, all night and day but more in the mornings and afternoons.’8
It was like watching a tree gently milk a cloud. But beautiful as it was, I knew that the mechanical, tangible process of a tree physically condensing water in front of me like an alchemist with his still was unimportant compared to what was happening above my head, above the canopy of the trees. The invisible chemicals I could smell
* The Bimbache were relatives of the Berbers, and were flourishing when the French explorer Jean de Bethencourt landed on the island in 1403. He described the trees which drip always with clean and beautiful water, collected by a trench next to the trees. One tree in particular – called Garoé – was sacred to the Bimbache and was considered the ‘fountain tree’. It was not very tall, but it had long horizontal branches directed away from the trade winds, which caught the moisture in the air, and allowed it to act as a water source all year round. It was a totem to the Bimbache, and in the first uneasy days of Spanish rule, Spaniards incorporated it into the Hierro coat of arms as a green tree on water with its head in the clouds pouring water from its leaves, and called it the ‘saint-tree’.
coming off the trees – those tantalising hints of almond and camphor and cinnamon toying with my nose – were acting high up to bring down the clouds that the trees could clutch at. Small, ringed molecules of carbon were acting like grit in an oyster, seeding water droplets and creating clouds. Just as for centuries humans have sent incense and burnt offerings up to heaven as a sacrifice, so some of the most finely wrought chemicals that trees make were evanescing into the air, a sacrifice of energy to ensure reciprocal gains. The difference is that when trees do it there is a scientifically measurable response.
In the tropics an estimated 30–50 per cent of trees send up these chemicals. In the Canary Island monteverde forests no one has done the research yet, but my hunch is here the number is more like 80 per cent. In the little ridge of forest I was among, not only the Ocotea foetens I was looking at but also the tree next to it, Rhamnus glandulosa,* and the tree beyond that, Apollonius barbujana subsp. ceballosi, smelt strongly, a sure sign they were sacrificing some interesting molecules. The molecules trees use to cloud seed are called volatile organic compounds or VOC s: volatile, because they evaporate easily; organic, because they’re made of carbon; and compounds, because their chemical makeup is a fascinating, tangled complex of many bonded atoms. Trees have tweaked these chemicals for thousands of years, but we still know few specifics about how they operate. All we know is that they can have different effects.
Let’s follow a phenylpropanoid molecule as it seeds a cloud. A sacrifice of Apollonius, a tree related to avocado, 2-(3-methoxy-4hydroxyphenyl)-1,3-propanediol’s structure was defined in 1995 and (considering the name!) it looks simple: a hexagonal ring of carbon, with one carbon arm sticking out, and various prickles of hydrogen and oxygen attached. It travels up and down the tree trunk surrounded by water, but when shoved into the free air of the stomata it releases itself from the surrounding molecules and heads up into the atmosphere. After minutes or hours, it will normally have reacted, often with ozone or oxygen, sometimes with nitrogen oxides or a
* Rhamnus glandulosa is called this because it has glands in the axils of the veins – the stubs where they join onto the twigs of the tree.
human pollutant, sometimes with other compounds, to make a tiny solid or liquid particle. And then, inexorably, water molecules start to cluster to it, until like a small planet it plunges into a liquid state and falls as rain.
I could smell it, I was sure, that night, as I lay freezing in my bivvy bag on a flattish patch of moss under a Canary Island holly and soaked to the skin, I could certainly feel its effects. Pine and cedar, oak and laurel: those smells floating on the mist are one of the unappreciated superpowers of trees. Our noses are unable to detect all the hundreds to thousands of volatile organic compounds9 trees produce that are a fundamental part of their immune systems, their communication systems and their ability to influence water and pull down rain from the clouds. They are the thread running through all parts of this book, the mercurial quicksilver that runs through the veins of trees and allows the immobile solidity of a tree to act at a distance and cast a web up into the air or under the ground. Most of the laurels in the forest had been isolated on La Gomera for so long that they are distinct, very rare subspecies, but in this they are behaving like trees all over the world.
So far, few scientists have researched volatile organic compounds in the real world, so in 2014 the Amazon Tall Tower Observatory project set out to discover more about these chemicals that trees release. Brazilian scientists built a spindly tower reaching high above the canopy of the Amazon rainforest, and then captured air samples and measured concentrations of different organic compounds, their effects on the clouds and how they interact with human pollution. Flying a small plane over the city of Manaus,10 the only big city for thousands of kilometres, researchers could detect the plume of anthropogenic pollution, and established that human pollution interacts with VOC s produced by trees to produce strange and dramatic results: deluges and floods. They started to define some of the staggering variations in the ways different trees release VOC s, and found that almost everything had an effect: species, temperature, light, leaf age and carbon dioxide concentration near the tree. Some trees were absorbing VOC s as well as releasing them. The research also revealed mechanisms of trees shaping water that had
been overlooked by satellites.* One massive discovery was that isoprene, the most common VOC released by all tree species, was three times more abundant than thought. The measurements also showed that trees released more isoprene under conditions of drought or stress, a crucial discovery because it suggested that trees were acting to encourage rain when there was a shortage, thereby stabilising the ecosystem.†
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If trees can call down rain, does deforestation lead to drought? The Russian author Anton Chekhov thought so. In his short story Pan Pipes, an old shepherd bemoans the way that after the forests were cut down the rain dried up:
‘And what became of all them little streams?’ he said. ‘In this very wood there used to be a stream with so much water in it the peasants only had to dip their creels in it to catch pike, and wild duck used to winter there. But even at spring flood there’s no decent water in it now.’
Most Victorian scientists also believed this to be the case. The drought and famine of 1877–9 in Bengal prompted a lot of work to investigate the links between deforestation and rainfall, and the Yanomami people of the Amazon and the Bishnoi of India are among many others who believe that drought comes if trees are removed.
Curiously, however, it has never been proved, and some have even suggested the opposite.11 The world’s forests are a collection of
* The Amazon Tall Tower Observatory project discovered that the biggest mistakes found in models were the result of them ignoring the massive variations between tree species.
† Volatile organic compounds vary from tree to tree, environment to environment, and they are one of the ways that trees can speed up their own evolution. The gases can affect trees epigenetically, which means using proteins to amp up genes, and because they work in a modular way, doubling the genes for them can alter their size, or the amount that is created. Most of all, they are very dependent on temperature and light, so they are often moderated by and moderators of circadian rhythms. They make good feedback loops.
cycles, confusing, stochastic, made up of the interactions of a million different organisms living on a complex, time-warped surface. There is no model widely accepted to show how forests promote rainfall. In the absence of such a theory it somehow became a given that rainfall decreased exponentially across continents, according to the prevalence not of vegetation but of mountains or lakes. Although the modelling did not fit actual rainfall data, the huge number of factors involved meant something – hills, lakes, temperature – could always be used to explain the numbers.
One idea which did fit that has consequently generated a lot of traction is the biotic pump theory: that trees use transpiration to cycle water inland from the coast – as long as tree cover remains unbroken. When trees first spread their leaves, they encountered a problem. Absorbing energy from the sun was necessary for photosynthesis, but the heat the sun generated could destroy the proteins in the leaf. Trees survived by releasing water from their leaves; as the water molecules evaporated, they took the sun’s energy with them, allowing the leaf to cool down. This means that trees release a lot – almost 97 per cent – of the water they take up just to cool themselves down. A single tree can give off a thousand litres of water a day, and as this water rises and cools it condenses, lowering the air pressure. If this happens next to the sea, moisture-laden air is sucked in over the forest, setting up a cycle that can continue for miles inland.
The theory was formulated during fieldwork carried out by Anastassia Makarieva, a theoretical physicist working in the Petersburg Nuclear Physics Institute. During her holidays she would travel to the north coast of Russia to spend time in the wet larch and pine forests near the Kara Sea, and became intrigued by the physics of how quite so much rain and snow made its way south along the Yenisey River basin and inland as far as Mongolia. After over sixty months of field research in the remote north, she formulated a theory which she published in 2007 with her colleague Viktor Gorshkov. Water cannot make it far inland in deforested continents, but rain penetrates far into river basins like the Yenisey and Amazon; ‘this points to the existence of an active biotic pump’, they wrote, ‘transporting atmospheric moisture inland from the ocean’. 12