The Wisdom of Trees

by Max Adams

  Apples in blossom are a buzz of social competition among bees, beetles, wasps and flies; they light up an orchard or a hedgerow, a back garden or avenue. In their late summer pomp, boughs bent low, laden with red-green globes full of the promise of juice and sweetness, they are natural symbols of the fertility of nature. They ought to remind us that we should not take the cycle of flower, pollination, fruit and seed for granted; that if we interfere in the complex ecosystems that support such apparently simple treasures we will come a cropper.

  4

  Trees at war

  This means war—Conventional weapons—Plan B—Battle of the trees—The wood of life—

  TREE TALE: THE YEW

  We have nothing to fear and a great deal to learn from trees, that vigorous and pacific tribe which without stint produces strengthening essences for us...

  MARCEL PROUST

  This means war

  TREES HAVE BEEN around long enough to attract plenty of unwanted attention. For the rest of nature, a tree is like a Whole Earth catalogue and store in one. Leaves are food. Fresh buds are food. Leaves make good platforms to lay eggs on and perfect wrappings for insect larvae. The cracks in bark are great places to live, breed, hide, hunt and shelter. The juicy sap lying just beneath the bark (in the cambium) is succulent and sweet. And it is not just healthy trees that are manna for the rest of nature: rotting wood provides opportunities for vast ranges of insects, bacteria, algae and fungi to recycle nutrients, which are then made available to another generation of trees. Wooden structures, from roof beams to fine furniture, are a meal waiting to be digested as far as whole battalions of worms, beetles and woodlice are concerned. But they cannot be accused of malice—unless it is your favourite piece of furniture, or your roof. Animals munching leaves, insects boring holes, birds stealing twigs, and vines using trees as climbing frames are innocent of the crime of premeditation; they are just doing their thing. Biologists often portray woodland ecosystems, especially tropical jungles, as war zones. In truth, there are no friends or enemies, just organisms adapting to opportunities. Some relationships develop as partnerships with mutual benefits; others are destructive. But break any part of the cycle of life and death and the law of unintended consequences wields its uncompromising gavel.

  Trees are vulnerable to predation and so, like the rest of us, eventually fall to one or other force of nature. So far as native British trees are concerned, the big killers are wind, drought, ice and flood—all geographically localized—and fungal and insect attacks, sometimes combined as in Dutch elm disease (see ‘Tree tale: the Elm’), which may be localized but are often specific to a species or group of species. In coniferous woods and in dry climates fire supplants wind as the main pathogenic force. One might also argue that the greatest threat posed to trees is human: carelessness, negligence, greed or expedience—and these seem not to be localized. Tree biologists have long realized that the biology, chemistry and mechanics of tree defences are complex—not surprising, considering how long the ‘battle’ has been raging. Our insight into the arts of this long-running war is merely in its infancy.

  The central design compromise faced by trees is that, aspiring to the most advantageous position vis-à-vis sunlight, they are tall with large areas of solar collectors. Like sailing ships and windmills they are top-heavy and can occasionally be caught out carrying too much canvas. Wind-throw is accordingly the greatest risk, especially in temperate and northern regions where equinoctial storms frequently produce damaging winter gusts. Trees that are deciduous and shed their leaves in autumn are more likely to survive such meteorological onslaught; it’s a rather neat thought that conifers, which tend to be very tall and pointy to get the most out of the low sun of the extreme latitudes, also present less surface area to the wind than broadleaves. As any sailor knows, the pressure applied by wind acts as the square of its velocity. To work out how a tree responds to that, you have to take into consideration its surface area, its drag coefficient (leaves or no leaves, density of crown, etc.), the air density, the wind force, the terrain and the presence or absence of other trees. Trees have such complex structures and are so commercially important that there are actually international conferences devoted entirely to their interaction with wind. Suffice it to say that the load on a tree increases on a sharply upward trajectory as wind increases: the energy and resources put into staying upright are formidable.

  Young trees are able to obviate the effects of wind by bending over (there’s a proverb there, somewhere), but in order to reach their mature height trees must become stiff—and stiffness is a disadvantage. Sailors take in a reef in a storm to reduce the area of sail and therefore the pressure on the vessel; but they also know that the vessel’s keel and the negative force of water acting against it absorbs much of the energy of the wind. Trees are forced to root themselves immovably in the earth, so they must tough it out.

  It is amazing that trees are able to take root as effectively as they do in such a shallow depth of soil (often less than a couple of feet). Lone trees adapt to the wind as they grow. One problem faced by conifers is the domino effect, which one regularly sees in densely stocked plantations on moorlands and mountains: when the edge trees succumb to a violent storm, the whole lot can then go down in one fell swoop. Conifers tend to be very shallow-rooted and their root systems, like those of broadleaves, usually cover the same surface area below ground that the canopy spreads above. When you see a wind-thrown tree, the amount of soil that was keeping it attached to the earth seems pathetically small. There is no perfect solution to the problem of wind, as every winter storm testifies. In recent memory, the Great Storm of 15–16 October 1987, which topped out at a scary 106 miles per hour, felled an estimated 15 million trees. I was lucky: prevented from getting to work across South London by the swathes of dead plane trees littering the roads, I made hay with some friends, a van and a chainsaw. As I write, Wales and the Southwest of England are taking a terrible battering from the wettest, stormiest winter on record.

  Trees have adapted well, in general, to the problem of drought. Waxy cuticles on leaves prevent excess evaporation, while stomata on the underside of leaves will close to slow down the photosynthetic processes, which demand large volumes of water. The whole metabolism goes into a sort of hibernation. Roots can spread huge distances from a tree in the search for water sources, and some trees will allow die-back of branches to protect the whole. Very rarely does drought reach such proportions in Britain that large numbers of trees die—unless the trees have only recently been established and are yet to develop effective root systems. Floods can devastate woodlands by landslip and waterlogging. Conversely, tree cover is generally regarded as an insurance against flood, because it slows the passage of water between canopy and ground, thereby allowing more time for groundwater soaking to relieve the overloaded natural and man-made drainage systems. Deforestation—that is to say, clear-felling of large areas of forest without replanting—is ecologically disastrous.

  Conventional weapons

  Even the simplest-looking tree defence is more complex than it seems at first sight. Take thorns: a thorn is a thorn, right? Not so. Thorns are formed in different ways, an example of convergent evolution—when the same solution derives from different starting points. The hawthorn has spines made from modified branches that grow just above the new bud to protect it from prospective diners; the blackthorn (Prunus spinosa) only has spines at the end of normal branches so that birds, but not cows, can get at ripened sloes (the blackthorn doesn’t do this deliberately, you understand; it just seems like it). Some thorns are modified leaves, and there are leaves, such as those of the holly, that have their own built-in prickles. One thing these plants have in common is that they have evolved their defences at herbivore level, in the understorey of a wood where they are going to be subject to browsing by deer, cattle and horses. Above that height, prickles are a waste of energy to produce, which is why the top branches of holly trees generally do not have prickly leaves. Brit
ish trees would consequently be ill-prepared for an invasion of African megafauna.

  It is not hard to see how humans might have learned lessons in self-defence from the menacing armour of trees like the African thorny acacia. A row of sharpened stakes around an encampment serves the same function; many an ancient writer, from Homer onward, has described an army bristling with spears in imitation of a thorny thicket. We do not know when the first hedge was planted deliberately to keep out unwanted intruders (sometime in the late Bronze Age is my guess); but a well-maintained quickset hedge—‘quickset’ being the old country name for hawthorn—will keep out all but the most determined burglar, and while a barbed-wire fence will fail in ten or so years, when the cheap posts holding it up rot in the ground, a well-laid hedge will last for hundreds. A much-debated theory suggests that hedges can be dated by counting the number of species in a thirty-metre length. One per hundred years may be about the right number.

  If thorns, spines and prickles form the outer defences of a tree subject to browsing, the next line of defence, the bark, is even more important and more complex. Remove the bark from even a small section of a trunk all the way round—‘ring-barking’ it’s called, and squirrels and deer are experts at it—and you will kill most trees. Bark is remarkable stuff. It protects its tree through a combination of thick corky cells and the impregnation of the epidermis with waxes and oils to keep them watertight. As with skin, it must continually expand; it must keep out moisture, pollutants, greedy insects and invasive fungi; it must breathe too. When you next look at a birch or cherry tree you will see very obvious horizontal striations, which look almost like leaf scars; but on closer inspection you’ll see that they are tiny ruptures in the shiny white or purple-brown surface of the trunk. These are lenticels: most trees have them in some form or another, but in the Prunus (cherry) species and in the silver birch they are very obvious. These lenticels allow oxygen to pass through the bark, providing an energy source that supplements the oxygen produced in leaves by photosynthesis. Some apples develop lenticels on their skins as they ripen. Naturally enough, insects find the ruptures produced in expanding bark a useful place to shelter, to hunt, to reproduce, and as a means of ingress. Some trees shed their bark to slough off a generation of irritating hangers-on; others, like the London plane (Platanus hispanica) and the silver birch, shed layers of bark containing atmospheric pollutants.

  An exception to the ring-barking rule is the cork oak (Quercus suber), which can stand regular de-barking and is a distinct feature of managed Mediterranean forests. The trunks of the trees, red in their nakedness, stand out in striking contrast to the dark green of their permanent foliage against the deep blue of the summer skies of Spain and Portugal, where they are tightly protected by law, and the vivid green of the vineyards with which they form a symbiotic economic partnership. The cork seems to be an adaptation to fire, after which cork oaks just keep on growing.

  Plan B

  The conventional weaponry of barb, thorn and thick bark is supplemented in many trees by a suite of more subtle defences. Oak trees attacked by armies of insects in June can shed an entire complement of leaves and grow another the same summer. In energy terms that is expensive, but the tree stores enough reserves to afford it. It’s like burning bridges behind a retreating army. After that, oaks deploy their chemical trickery. Tannins stored in the bark are released as vapour when the tree is under attack from a wave of insects bent on eating its leaves. Tannin is a poison which can deter the invader; but the effect is delayed—it requires time to deploy. And so oaks releasing vapour send an aerial, gaseous message to others of their kind in the vicinity to start producing their own tannins. The message can be passed on throughout the oak population of a forest. It is almost like a herd response; and, what is more marvellous, oaks thus warned can produce those tannins two years running, as if there is a form of chemical memory, which demands first a reactive then a prophylactic treatment.

  Oaks are not the only trees to produce poisons. Walnuts make juglone, another gaseous chemical, which suppresses competing vegetation and so reduces competition. The Australian eucalyptus produces volatile oils, which, aside from their human use as antiseptics and solvents, also repel insects. Laburnums produce cytosine which makes all parts of the tree highly poisonous: it seems odd that the wood is used to make chanters for bagpipes.

  WALNUT

  There are not enough walnut trees about. I suspect people think it a waste of time planting them, because they take a long time to grow and get a little large for the average garden. Give one as a present to a grandchild. One way or another, they won’t forget you.

  Other trees, notably the acacias, take the idea of self-defence even further by attracting mercenaries to their side. Ants are most often recruited to deter offensive plants and browsing creatures. But how does a tree recruit a whole army to its cause? There are suggestions that, through their sub-soil mychorrizal connections, acacias can warn each other of an aphid attack, triggering the secretion of nectar, which attracts ants to the trees in their thousands. The ants then devour the aphids, and as an extra reward get not just the nectar too but also the run of the tree house. The Bullhorn acacia (Acacia cornigera) gets its ant friends not only to eat other insects, like crickets, but also to eat the leaves of competing epiphytic vines trying to use the acacia as a climbing frame. One East African acacia (Acacia drepanolobium) deploys its ant-army recruits in immense swarms to keep voracious elephants and giraffes at bay. The acacia also provides reinforced niches in its bark for ants to nest in. But all is not quite as simple as it seems. The nectar that the trees produce to pay their guards is also produced in their flowers, and the trees would rather that their pollinating insect-partners got this juicy bribe. So, they secrete a volatile compound from their pollen that is irresistible to the pollinator and disgusting to the ant. Once that pollen has been successfully removed and taken to another tree, the ants can come and guard the developing seeds against all-comers. Brilliant... but still not intelligent.

  Battle of the trees

  A medieval Welsh poem, the Cad Goddeu, supposedly composed by the Arthurian bard Taliesin, tells of a war mounted by the druidic Gwydyon (the name means ‘tree knowledge’), who is advised by God to call on the spirits of all the trees to fight for him. The vanguard is led by the alder—not perhaps the first metaphor that springs to mind, but as a tough, durable light wood, and as a favoured tree for protecting earth banks against floods, it has its defensive uses. Birch is late to the fray, slow to put on its shining armour; spiky blackthorn marshals its thorns to effect, while cherry trees ‘raise the alarm’ and oak, ‘the best’, is ‘exalted before other rulers’. The poem has little in the way of narrative structure, and there is much to suggest that it derives from a shamanic transformation chant. Its fascination lies in the way that it taps into animist beliefs in trees as living spirits with personality and character.

  It is easy to forget how deeply tribal peoples identified with trees and wood; easy too to forget how heavily kings and their armies relied on wood and its properties in conflict. Swords and spear tips may have been fashioned from steel, but shafts and handles, clubs and staves were wooden. Defences were wooden: sharpened stakes, palisades, banks and walls laced with timber. There is some evidence, too, that areas of woodland were deliberately maintained as border defences. The Old English and Irish words for freeborn warriors—gesith and gaesedach respectively—come from the word gar (the ash spear) and the ancient word for shield: sciath in Old Irish, and scield in Old English. The shield was such a strong metaphor for the defensive battle line that armies were said to form a linden wall—linden or lime being the light, strong wood used to make the shield.

  One of the most enigmatic locations for a Dark Age battle was Cat Coit Celidon (the Battle of the Great Wood of Caledonia), listed as one of those in which Arthur was victorious. Unfortunately, we do not know where it was—perhaps in what is now Dumfries and Galloway.

  The wood of life

>   A good heavy stick will see off a predator, or the fellow whose pig you just stole. Many variations on the stick have experimented with length, weight, modifications and materials. Police truncheons were traditionally made from one of the heaviest woods: Lignum vitae, the ‘wood of life’. The tree from which this wood is cut is the guaiacum of Central America. It is so dense and heavy that it will sink in water and is therefore not much use as a life-raft. The famous Longitude clockmaker John Harrison, who began life as a carpenter, used both oak and Lignum vitae for bearings in some of his early clocks; and he was not the only one to recognize the value of this self-lubricating wood, which behaves like a metal. The propeller-shaft bearings on the first nuclear-powered submarine, USS Nautilus, were made of the same stuff: fifteen-hundred pounds of it, cut and shaped to fit around the steel shaft. It only had to be replaced every few years, when the boat came into dry dock for a major re-fitting. The same wood is familiar to listeners of the BBC’s Test Match Special cricket commentary: when conditions are sufficiently windy to knock the bails off a set of stumps, out come the ‘heavy bails’ made of Lignum vitae, leading to a certain amount of juvenile hilarity in the commentary box.

  TREE TALE

  The Yew

  Yew is a tree, rough on the outside,

  Hard and firm in the earth, guardian of fires,

  Supported by roots, a joy on the estate.

  So runs an Old English rune poem. The common yew (Taxus baccata) is unique in more ways than one. It is Britain’s longest-lived native tree by far. No-one knows how old the oldest is—the stunted remnant of a former monster at Fortingall, in Perth and Kinross, may be two-thousand-and-more years old—because all old yews are hollow and therefore lacking in a complete set of rings. In any case, confusingly, they do not produce a ring every year. Nor do they produce a single trunk but, like the hawthorn, tend to form multiple stems. Yews are one of three conifers native to Britain—Scots pine and juniper (Juniperus communis) being the others—and it is the only one of our native trees to retain its pre-English, Brythonic name (yw forms the prefix to the place that later became Eboracum, then Eoforwic, then Jorvik and in its final evolution—York). The name is also preserved in the Irish Maigh Eo, County Mayo, the ‘fertile plain of the yew tree’.