Tree

• 1. Introduction

  ›

  A tree on this planet can outlive empires. Some living trees have reached several thousand years old, and a Great Basin bristlecone pine in California's White Mountains has been dated by counting its annual rings, ring by ring, into the deep past. There are roughly three trillion mature trees in the world, by a 2015 estimate, and the first of them evolved around 400 million years ago. Yet for all their familiarity, no one can give a precise definition of what a tree actually is. The word is common. The category is slippery. A tree is not a single branch on the family tree of plants. It is a shape that countless unrelated species reached for, again and again, by separate roads. So what counts as a tree, and what does not? How does a stem of wood haul water dozens of meters into the air without a pump? Why have human cultures buried their weapons and their dead beneath these plants, and why are a third of all tree species now sliding toward extinction? The answers run from the chemistry inside a trunk to the myths beneath a sacred grove.

• 2. The Slippery Definition

  ›

  "Tree" has no universally recognised precise definition, botanically or in common language. In its broadest sense, a tree is any plant with an elongated stem, or trunk, holding leaves or branches some distance above the ground. Under that wide umbrella, large herbaceous plants like papaya and bananas qualify, and so do taller palms, tree ferns and bamboos. Height alone fails to settle the matter, since smaller plants from 0.5 to 10 m are called shrubs, leaving the minimum height of a tree only loosely defined. A narrower and commonly applied definition demands a woody trunk formed by secondary growth, meaning the trunk thickens outward year by year rather than only growing up from its tip. By that rule, palms, bananas and papayas are not trees, whatever their height or girth. The line stays blurry even there. The Joshua tree, bamboos and palms never produce true wood with growth rings, yet they may make "pseudo-wood" by lignifying cells from primary growth. Trees in the genus Dracaena are monocots that do have secondary growth, driven by meristem in the trunk, though it differs from the thickening found in dicot trees. Some people skip structure entirely and define a tree by use, simply as a plant that yields lumber. With so many competing tests, estimates of how many tree species exist swing widely, from 60,000 to 100,000.

• 3. Parallel Roads To Wood

  ›

  Towering above the neighbours is the whole point. Trees evolved height as a way to compete for sunlight, and that same growth habit appeared independently across unrelated classes of plants facing similar pressures, a classic example of parallel evolution. The majority of tree species are angiosperms, or hardwoods. Of the rest, many are gymnosperms or softwoods, a group that includes conifers, cycads, ginkgophytes and gnetales, which carry seeds in open structures such as pine cones rather than inside fruits. Most angiosperm trees are eudicots, the "true dicotyledons," whose seeds hold two seed leaves. Among the old lineages called basal angiosperms sit Amborella, Magnolia, nutmeg and avocado, while bamboo, palms and bananas are monocots. The deep past shows the same theme of separate origins. The earliest trees were tree ferns, horsetails and lycophytes growing in the forests of the Carboniferous. The first tree may have been Wattieza, whose fossils were found in New York state in 2007, dating to the Middle Devonian about 385 million years ago. Before that find, Archaeopteris was the earliest known tree, and both reproduced by spores rather than seeds. The maidenhair tree, Ginkgo biloba, is the only survivor of the once-widespread Ginkgophyta. It is called a living fossil, virtually unchanged from specimens preserved in Triassic deposits.

• 4. Engineering Of A Trunk

  ›

  Water has to climb. A tree draws it up the stem through the xylem from the roots by capillary action, because water keeps evaporating from the leaves in transpiration, and if too little is available the leaves die. The trunk's job is to lift the leaves above competitors for light while ferrying water up and distributing food back down. Its anatomy is layered like a cross-section of an argument. The outermost layer is bark, mostly dead cork cells, a waterproof shield against weather, disease, animals and fire, pierced by fine breathing pores called lenticels. Just inside lies the phloem, a soft spongy layer of living cells that carries sugary sap. Wrapped between them is the vascular cambium, a sheet of dividing cells just one cell thick, making phloem on the outside and xylem, or wood, on the inside. Newly made xylem is the pale sapwood, conducting water and minerals upward; the oldest inner sapwood converts into darker heartwood, the dense rigid core. Three quarters of the dry mass of xylem is cellulose, with most of the rest lignin. Cut across the trunk and concentric tree rings appear, the annual growth rings, sometimes crossed by thin vascular rays. This machinery is fragile to invaders. Dutch elm disease, caused by an Ophiostoma fungus and carried between elms by beetles, makes the tree block its own xylem until it starves; in Britain in the 1990s, 25 million elm trees died of it.

• 5. What The Roots Do

  ›

  The first thing to break out of a seed is the radicle, the embryonic root, which dives down as a taproot before lateral roots branch sideways through the upper soil. Roots anchor the tree and pull in water and nutrients, and near the tips, single-cell root hairs press against soil particles to absorb potassium and moisture in solution. They need oxygen to respire, so only a few species, such as mangroves and the pond cypress, survive permanently waterlogged ground. Underground, the roots strike up one of biology's great partnerships. Fungal hyphae form mycorrhiza, a mutualistic bond in which the tree gives the fungus sugars and receives minerals like phosphorus in return. Some fungi are specialists tied to a single tree species that cannot flourish without them. The hyphae can link different trees into a network, shuttling nutrients and signals between them, and they have done so since the early Paleozoic, four hundred million years ago, when the first vascular plants colonised land. Alders go further, hosting Frankia bacteria in actinorhizal root nodules that fix nitrogen from the air into ammonia, letting the trees thrive in poor soil. Some trees even fuse together. Through inosculation, a natural grafting of tissues, neighbours interconnect into a colony, a link proven by injecting chemicals, sometimes radioactive, into one tree and finding them in another. Above the mud, the red mangrove throws down looping prop roots, while many rainforest giants flare out buttress roots that brace them like angle brackets against the wind.

• 6. Seeds On The Move

  ›

  A seedling cannot grow up in its parent's shadow. Light is what it needs, and if seeds only dropped straight down, the crowded saplings and the shade above would likely smother them, so trees have evolved an extravagant range of ways to send their offspring elsewhere. Birch seeds are tiny with papery wings for the wind. Ash and maple seeds carry blade-shaped wings that spiral down as they fall. The kapok tree spins cottony threads to catch the breeze, while the flame tree Delonix regia fires its seeds through the air as its long pods crack apart on drying. Alder catkins release seeds bearing oil droplets that help them ride the surface of water. Animals do much of the rest of the work. Apple pips, plum stones and hawthorn seeds travel inside edible flesh, eaten by mammals and birds that drop or pass the seeds far from the parent, sometimes with germination improved by the journey through a gut. Squirrels cache nuts and forget many of them, and grizzly bears raid red squirrel hoards of pine cones, scattering the seed. Fire is a trigger of its own. It stimulates release and germination in the jack pine while clearing competitors and enriching the floor with ash, and seeds of Acacia cyclops and Acacia mangium germinate better after high heat. There is a strange asymmetry hiding in all this: the largest tree, Sequoiadendron giganteum, produces one of the smallest tree seeds.

• 7. A Working Tree

  ›

  Quinine once was the remedy of choice against malaria, drawn from the bark of the cinchona tree. At least 120 drugs come from plant sources, many from tree bark. Aspirin replaced the sodium salicylate taken from willow bark, which had unpleasant side effects, and the anti-cancer drug Paclitaxel comes from taxol in the bark of the Pacific yew. Bark feeds far more than medicine. Cork, from the thick bark of the cork oak, is harvested about once a decade in a sustainable industry, with more than half the world's supply from Portugal, mostly turned into wine-bottle stoppers. Oak bark has long supplied tannin to make hides supple and water-resistant. Food trees stretch from the temperate orchard to the tropics, giving apples, pears, plums, cherries, citrus, dates, figs and olives, plus cocoa for chocolate and the berries of Coffea arabica and Coffea canephora for coffee. In North America the sap of the sugar maple becomes maple syrup, about 90% of it water; in Alaska the sap of the sweet birch is boiled into a syrup, though a hundred litres of sap yield just one litre. Wood itself remains the workhorse, cut into lumber for buildings, bridges, masts, furniture, musical instruments and a long catalogue more, then pulped for paper and bound into engineered products like plywood. Living trees become art too. Bonsai began in China as penjing and spread to Japan more than a thousand years ago, while the oldest known tree shaping survives as the living root bridges built by the Khasi people of Meghalaya, India, from the roots of the rubber tree Ficus elastica.

• 8. Sacred And Superlative

  ›

  Dryads, in Greek mythology, were shy nymphs believed to live inside trees. Trees have been venerated since time immemorial, and the reverence takes startling forms across cultures. The Oubangui people of west Africa plant a tree at a child's birth; as the tree flourishes so does the child, its flowering signals the time for marriage, and when the person dies their spirit is believed to live on in it. The image of a tree linking underworld, earth and sky recurs across religions, from Yggdrasil in Norse mythology to the wish-fulfilling Kalpavriksha of India and the Maya ceiba tree connecting the sky and the underworld of Xibalba, now a national symbol in Guatemala. Democracy in North America is tied to a tree as well: the Great Peacemaker formed the Iroquois Confederacy, and the warriors of the original five nations buried their weapons under the Tree of Peace, an eastern white pine. The record-holders are no less astonishing. The tallest known tree is a coast redwood at Redwood National Park, California, named Hyperion, standing 115.85 m. The largest by volume is a giant sequoia called the General Sherman Tree in Sequoia National Park, its trunk estimated at 1487 m3. The broadest trunk belongs to a Montezuma cypress at Santa Maria del Tule, Oaxaca, the Árbol del Tule, with a girth of 36.2 m, much of it empty space between buttress roots. Yet about a third of all tree species, some twenty thousand, sit on the IUCN Red List of Threatened Species, with at least 1400 classed as critically endangered.