- The Secret Life of Trees: How They Live and Why They Matter by Colin Tudge
Allen Lane, 452 pp, £20.00, November 2005, ISBN 0 7139 9698 6
They are pollarding the plane trees in our street. They do it every few years: left to themselves, branches would overtop the houses by many metres and form a summer tunnel of green. In other places and at other times the lopped branches would have been a resource. In Trees and Woodland in the British Landscape (1976), Oliver Rackham makes a distinction between wood and timber. Wood, the renewable crop, the source of staves, bean poles, hurdles, fodder and firewood, is what was coppiced from the same stools or pruned from the same trunks and branches over many years, in some cases many centuries. Timber is the builders’ merchant’s solid planks and beams, which are now, in Britain, mostly imported, or cut from trees grown in plantations.
London is not woodland. We don’t fell timber, and we prune to give nature a slap on the wrist, not to provide firewood or hurdles. The knobbly extremities which result are evidence that we are in control. I know what it’s like to be challenged by a tree: I’m told that the Irish yew I planted in the front garden thirty years ago is out of hand. I’ll have it lopped, but will feel about it as I have seen others feel about having a tomcat neutered.
We may trim trees, butcher them even, but we cherish them too. British species, native and exotic, include those appropriate to the scale of narrow streets – rowan, birch, cherry and hawthorn are more recent additions to ours – and a whole range of others, some larger, some smaller, to be found not just in parks, woods, hedgerows, arboreta and botanical gardens but on wasteland, canal banks and commons.
The distress we feel when a tree goes is not always rational. Woodland regenerates, town trees can be replanted, fallen trees are good for beetles. It’s different when a whole species goes, however, as elms did in the 1970s. You can still see them full grown in the centre of Brighton, one of the few places where Dutch elm disease has been kept at bay. In the years when the disease was spreading it was hard to look out of a train window without seeing a field margin punctuated by skeletal remains. It was a dreary sight, although a long view can be taken even of epidemics: this one wasn’t the first. Rackham argues that the sudden disappearance of elm pollen in records from the Neolithic period may be evidence of an earlier fungal wipe-out, and quotes passages showing that elms have always had a reputation for being sickly. He suggests that the pathogen may go through cycles of greater and lesser virulence. Vulnerability is in part a function of variability; many of the English elms which went were clones. Monocultures don’t occur only in fields: the London plane is another species which, were the fungal infestation which killed off planes in Italy to arrive here, would be short on the genetic variation that increases the chances of resistance.
We become attached to trees, even though they can get in the way and cause anxiety when they are diseased or when they suck moisture from clay and unsettle foundations. Holy men sit under them. According to Herodotus, ‘Xerxes found a plane tree, to which for its beauty he gave an adornment of gold, and appointed that someone should have charge of it always in undying succession.’ The burden of being responsible for their well-being is justified by the rewards their presence brings. Their longevity is a reminder of the brevity of human life: we can feel that looking after them ensures, in some sense, our own continuity.
And we collect them. Fewer than 40 species are native to Britain; the last Ice Age cleared most of the country and recolonisation was slow. But today you can find, in one place or another, almost any species of any distinction that will tolerate our climate. And as trees are long-lived, past fashions leave their mark on the landscape. Rackham, writing about elms, distinguishes ‘fashion trees’ – varieties chosen in part for their appearance, a development which can be traced back to the 17th century – from native elms. Californian redwoods planted about 150 years ago, when they first became available from British nurseries, are the tallest trees on many horizons. A relation of the monkey puzzle, Wollemia, was thought to be extinct until, in 1994, a park ranger discovered a clump growing in an inaccessible gorge in the Blue Mountains outside Sydney. It is now on the verge of becoming available to British gardeners via Kew and it, too, will doubtless become a marker for garden historians.
Despite the variety of introduced species, if you confine yourself to England’s managed, temperate landscape the picture you get is very limited. Alan Mitchell’s Trees of Britain and Northern Europe, a field guide which reckons to include ‘every species and large-growing cultivar to be seen in the countryside, parks and gardens of Europe north of the Mediterranean littoral’, is a book of modest proportions. The rainforest is a different matter, as is the foggy, rain-free Californian coast, as is the Cerrado of Brazil or the eucalyptus forests in Australia; and the central theme of Colin Tudge’s The Secret Life of Trees is variety and the evolution of variety: variety in form, in adaptation, in kinds of dependence – most trees need to cohabit with soil fungi, some need insects, birds or fruit bats to pollinate them and spread their seed, some are parasitic on other trees.
Tudge makes the British experience seem truly insular. He writes, for example, about ‘the wondrous Reserva Florestal Adolfo Ducke’, which covers a hundred square kilometres of Amazon rainforest. Two thousand times smaller than Britain, it has forty times as many native trees – 1300 species. That in itself raises a question: why are there so many more species in the tropics than in higher latitudes? Tudge goes through the theories (not all of them – there are in print, he says, about 120 recognisably distinct attempts at an explanation). He finds no clear winner but thinks two ideas are particularly cogent. First, that away from the tropics ice ages wiped the slate clean again and again, leaving tough terrain in tough climates where only the adept survive. Second, that in the tropics the pressures are biological rather than physical, and that pressure from parasites, which a species of fewer, widely spaced individuals is better able to counter, is a major reason for diversity.
Whatever its origin, variety makes the rainforest hard to read. The members of a given species are dispersed and in the seasonless climate tend to flower unpredictably. The leaves of many of the thousands of species which make up the forest canopy have responded to a regime of desiccating heat alternating with regular downpours by evolving leaves of similar shape and texture – leathery ovals with a projecting ‘drip tip’. Bare trunks, reaching upwards for the light, take what would be useful evidence of identity – leaves, fruit, flowers, young shoots – up into the inaccessible crowns.
Identification in these circumstances requires skills beyond those needed to identify trees in British woodland. It breeds two overlapping kinds of expertise. One is that of the botanist who gives a name to an unknown specimen with the help of published floras, their diagnostic keys, descriptions and illustrations, maybe even with knowledge gained from herbarium specimens. The other is the expertise of those who know trees through long familiarity, like the indigenous experts, the mateiros, in Brazil. Tudge quotes a scientist at a forest research centre who says mateiros may disagree with professional botanists, usually claiming similar trees were different when the botanists thought they were the same. He had never known mateiros to be proved wrong when the tree flowered or DNA analysis was done and a firm identification became possible.
But to know is not to name, or at least not to name in a way that allows a species to enter the literature and trickle down from journal articles and monographs to hand lists and field guides. Only professional botanists spread the word in that way. In Britain, where endemic species are few and new species very, very rare, amateur users of field guides can think of technical description as a task now complete. Our flora is as well mapped botanically as our countryside is geographically. But in the tropics and in higher, wilder, mountainous parts of the world, the task of description is far from complete.
The common names used by native speakers serve ordinary purposes well, as do the names used in the marketplace – where rosewood has nothing to do with roses, where almost any new-found evergreen is a pine and any tropical hardwood a mahogany. But even non-botanists need precise identifications. The science has its roots in medicine; misidentified simples and mushrooms can be fatal. Foresters who wish to sustain variety in tropical forests also need to be able to make distinctions – between a rare tree and a similar, common one. Species lists are necessary if environmental change is to be monitored.
At the species level, names change very little. The rule that the earliest published name prevails occasionally causes discrepancies between one plant list and another when an earlier publication is uncovered, but the names of genera and species are essentially stable. The different accounts of the evolution of plant life implied in higher levels of naming – changes in the assignment of species to the divisions, subdivision and sub-subdivisions (orders, families and so on) mirror discoveries and theories about where and when branching occurred. DNA analysis gives the history of these bifurcations a foundation in genetic fact, but those facts must be interpreted. Genes may diverge and converge; the fossil record is fragmentary. Revisions are argued for, and Tudge records many cases where re-sorting is going on: taxonomy makes scientific communication possible, but follow it beyond the species level and you can get, rather quickly, into confusing and controversial waters.
Knowledge about the ways in which growth, morphogenesis and reproduction are regulated has been advanced through studies of a few species. One, Arabidopsis thaliana, has become a model organism because it has a short life-cycle, produces abundant seed and is easy to manipulate genetically. The insight into plant life which has come from such single-species studies has begun to make sense of plant form and function at the level of the cell. This science is, to borrow the old terminology, ‘natural philosophy’. The term ‘natural history’ better fits descriptions and analyses which emphasise the variety of species, their adaptations and their interdependence in communities. The work of the ecologists, foresters, field botanists and taxonomists that Tudge draws on is of that kind. It’s not surprising that he loves reference books. The six titles in his ‘shortlist of books that I refer to constantly’ are dominated by multi-author works – the four-volume Encyclopedia of Forest Sciences and Plant Systematics. Elsewhere he describes the published guide-book to the Flora da Reserva Ducke as mouth-wateringly as he does the reserve itself: ‘a magnificent work that drips with colour photographs and diagrams of everything pertinent . . . The guide would be among my favourite reading if only it wasn’t in Portuguese.’
The structure of The Secret Life of Trees is encyclopedic. The book is thick with incidental information. Some acacias, when nibbled, produce chemicals that make their leaves unpalatable. More remarkably, a signal that browsers are at work reaches nearby members of the species, which start producing the same chemicals. Purpleheart, from South America, a species of Peltogyne, is figured like fine tweed and used to make gymnasium apparatus, skis and billiard-cue butts. Information of this kind acts mnemonically: through it, species take on a more concrete and distinct character.
Tudge ends his book with a secular sermon. The modern world, he says, is governed by the idea – call it a principle – that the only measure of right policy is an increase in the volume and money-value of goods and services; that this destroys the possibility of a sustainable relationship between us and our planet; that fertile land is taken over by mechanised agriculture, to the benefit of the few; that the many, forced off the land, gather in the slums of cities which already contain half the world’s population; that burning fossil fuel to drive our industries is leading to deleterious climate change; and that our only hope lies in a new kind of politics, a democracy in which people realise where their true advantage lies and are able to prevent governments and businesses standing in the way of its achievement. His text for the day is Wangari Maathai’s Green Belt movement in Kenya, which has led to the planting of 30 million trees by women, to provide food, fuel, shelter and income.
When it comes to thinking about the changes his refreshed democracy might bring about, examples in Tudge’s book of connections between plants, animals and the environment show how complicated decisions would be. To act locally would, on his evidence, often be the best option. Interactions, easy to understand with hindsight, are hard to predict. A vigorous new eucalyptus forest in an arid region may lower the water table. Not all clever guesses are good ones. Take the tambalacoque, a tree growing on Mauritius. In 1977, Stanley Temple of the University of Wisconsin reported that the only specimens alive on the island were more than 300 years old – no new candidates, that’s to say, since the last dodo was clobbered. Maybe, he surmised, the tambalacoque seeds had to pass through the dodo if they were to germinate. With no dodos available, he tried turkeys. Seeds planted after the turkey treatment did indeed germinate, and it looked as though a precise plant/ animal association had been uncovered. But then it was found that in areas of forest where introduced animal species (deer, pigs, monkeys) were fenced out, tambalacoque saplings began to appear unaided. In this case, the voracity of introduced animals might have been anticipated, but the complexity of a symbiosis which has evolved over a long period can be extraordinary and not obvious at all.
A special plant/animal dependency that Tudge gives more space to is the one between figs and wasps. Edward Allen Herre and his colleagues have been able to uncover its wonderful intricacy. For example, there are wasps which both fertilise and parasitise figs, lesser wasps which parasitise the greater, and nematode worms which parasitise them. It was established that different male/female ratios occurred among offspring depending on how many queens occupied a fig, and shown that this could be explained by the strict mathematics of evolutionary advantage. Thus observations of insects made almost in the spirit of the turn-of-the-century literary entomologists (Jean-Henri Fabre, Eugène Marais, Maurice Maeterlinck) were married to evolutionary theory.
Introduced plants can be as unpredictable in their effects as animals. The fact that many trees need fire – the seeds of some will not germinate without it – is accepted. But it was not easy to guess the effect of the introduction into Brazil of African grasses which provide better fodder for cattle. They have begun to colonise the dry forest of the Cerrado, and because they burn more slowly they are more destructive of trees which have evolved to survive the briefer, though hotter, flames of native grass fires. In 2002, a disastrous fire followed 14 years of successful if misguided fire prevention by the Brasilia fire brigade, when the country (firemen too) had a day off to vote in the general election. The Cerrado caught fire; accumulated dead vegetation and the new slow-burning grasses resulted in a blaze so fierce that it not only destroyed fire-adapted trees but spread into the gallery forest along the rivers.
On a more cheerful note, Tudge points to positive steps that can be taken: more timber building (it locks up carbon trees have drawn down from the atmosphere); farming trees instead of cereals (olives, chestnuts, fruit and any number of tropical and subtropical examples); growing crops under trees, feeding livestock from them, protecting free-range poultry with them (British raptors take more chickens than foxes do); planting trees for shade – in Costa Rica it was shown that dairy herds offered shade produced 30 per cent more milk. Look again at indigenous species; avoid sterile, disease-prone monocultures.
It sounds familiar if you’ve been reading Oliver Rackham. Coppicing, woodland management, wood pasture, trees in hedgerows, the problems of replanting clear-felled woodland: all these are subjects about which he says things relevant to the sustainable forest and tree culture Tudge looks to when he says that trees are essential to the future of humanity and could indeed ‘stand at the heart of all the world’s economics and politics’.