The Secret Life of Trees

by Colin Tudge

  The eastern black walnut of eastern North America (Juglans nigra) is famed for the fine furniture made from its timber. J. regia is sometimes known as the Persian walnut and sometimes as the English walnut. But although it grows naturally in a wide variety of habitats from cool steppe to moist subtropical forest, it does not grow wild in England at all. It’s just that the British in general, and the English in particular, have an unsurpassed talent for expropriation. Thus by the same token, the Scots pine flourishes throughout Europe and into Asia Minor and could just as well be called the Russian pine (though it does at least grow naturally in Scotland as well). But walnuts are widely cultivated. The ancient Greeks and Romans had walnut orchards. Northern Europeans began to cultivate the walnut in the 1500s and its timber became the favourite for high-grade furniture until, from the 1600s onwards, it was ousted by mahogany from the Americas – although walnut is still used for gunstocks and the insides of prestige cars. The English have long cultivated walnuts with optimism if not always with success, and breeders at Oxford University are now seeking to create varieties that really can tolerate our decreasingly harsh but increasingly fickle climate. Already there are at least 400 varieties of cultivated walnut. Turkey is a great walnut producer, but the largest producer of all is now California.

  Judd claims sixteen species of hickory (Carya) – thirteen in North America (one restricted to Mexico) and three in Asia. Hickories too are known both for their nuts (including pecan) and for their timber. Hickory is wonderfully shock-resistant: much favoured for the handles of hammers and axes and, in the good old days, certainly well into the twentieth century, for the shafts of golf clubs. Once, too, it was the thing for barrel hoops. The intricate knowledge that our forebears had of each kind of plant and its caprices and possibilities never ceases to astonish me: knowledge now largely lost, or at least confined to academic tracts or whimsical accounts like this one. Maybe when the fossil fuels run out and heavy industry has run its course, such wonders may be rediscovered. The tropical Engelhardtia and the wingnuts (Pterocarya) also provide fine timber.

  Walnuts and hickories in various forms are also valued as ornamentals. So too are wingnuts, which, like walnuts, have big, sweeping, feather-like leaves. One wingnut tree that I know, in the Botanic Garden at Cambridge, has about a dozen trunks that clearly arose as suckers around a central trunk in the manner of giant sequoias; but as often happens with sequoias, the central trunk has gone, leaving the outriders in a circle, like standing stones in some ancient place of worship. Some of the world’s most startling plants are in gardens. Protected over decades or centuries from predators and competitors, they can grow more extravagantly than ever they are allowed to do in the wild.

  Finally, the Fagales order contains the Myricaceae family, aromatic trees or shrubs, rich in tannins and aromatic essential oils, widespread through the tropics and temperate countries. The genus Myrica includes the bayberry, wax myrtle and candleberry, which provide aromatic waxes. Some Myrica also have edible fruits; some are ornamental shrubs. They have small flowers pollinated by wind and fruits which are mostly dispersed by birds, although the small fruits of Myrica gale are fitted with bracts that act as floats, and are dispersed by water. Myricaceae in general are water lovers. Like alders – also in general water-lovers – they have nodules of nitrogen-fixing Frankia in their roots.

  So that completes the Fagales. To be sure, no order compares with the Fabales. But in the second league, to which all others belong, the Fagales are certainly among the greatest.

  Terminalia, Myrtles and Eucalypts: ORDER MYRTALES

  The Myrtales order is huge: 9,000 species in fourteen families. Some of those families have no significant trees. Among those that have a few intriguing kinds are the Lythraceae, which includes purple loosestrife (Lythrum salicaria), which is a pleasant wayside flower in Britain but, introduced into North America, is a major weed of wetlands; and also Punica, shrubs and small trees which include the pomegranate (P. granatum). The Onagraceae family contains the evening primrose, the willowherbs and the fuchsias, which include the kotukutuku, Fuchsia excorticata, the unique fuschia tree which grows throughout New Zealand to a height of around 14 metres. The Vochysiaceae family of Central and South America (with a small presence in West Africa) includes several trees that I have encountered in the dry forest of Brazil, the Cerrado, some of which are used for boats and furniture.

  Outstanding, however, is the Combretaceae family. It includes some important trees of the mangroves: the Pacific Lumnitzera of Asia, Australia and East Africa; and Languncularia, straddling the Atlantic in West Africa and America. The family also includes Terminalia, with many big tropical trees valued both for their physical beauty (they are much favoured in the grounds of gracious houses for their vast festoons of red and yellowish flowers) and for their timber. Several species of Terminalia are called ‘Indian laurel’. T. bialata is known in the trade as Indian silver-grey wood; West Africa’s idigbo is T. ivorensis; and afara, or limba, also from West Africa, is T. superba.

  Closely related to the Combretaceae, and even more outstanding, is the Myrtaceae family. Its members are wonderfully aromatic, stuffed with essential oils. The family is named for the myrtle, Myrtus, which seems to be the only European genus. It also includes the clove tree, Syzygium aromaticum. Pimenta is the genus of allspice (P. dioica or P. offtcinalis) and of bay rum (P. racemosa). Melaleuca with its bottle brush flowers is a favourite ornamental shrub, while M. leucadendron provides medicinal cajuput oil. The Myrtaceae family provides some fine fruits: notably the guava, Psidium guajava, from tropical America and the West Indies. Most of all, though, the Myrtaceae include the extraordinary genus Eucalyptus, also known as gum trees. As now defined, Eucalyptus is a huge genus with around 700 species – so big and various that it should probably be split into several smaller genera (as with Acacia).

  Eucalyptus is one of those fortunate organisms – you meet them in all walks of life – that found itself in the right place at the right time, ready-equipped with a bag of adaptive tricks that helped it to survive and flourish where others languished. To judge from present distribution, the genus seems to have arisen in Australia some time in the early Tertiary, around 60 to 50 million years ago. Australia was a lot wetter then than it is now, and at first the eucalypts had to compete with conifers such as cypress pine (Callitris) and the araucarias, and with broadleaves such as she-oak (Casuarina) and southern beech (Nothofagus). But the continent soon became a lot drier; and with aridity, comes fire. Furthermore, the land is exceedingly ancient, heavily eroded, with little volcanic action to stir the geological pot, and it has lost much of the fertility it might once have had – notably the rock-bound nitrogen and phosphorus, the principal nutrients of plants; but also potassium, sulphur and some essential trace minerals.

  Present-day eucalypts live in a wide variety of habitats but overall they specialize in dryness, sucking water from great depths; and although they may burn spectacularly when things get out of hand –the crowns exploding as their essential oils are vapourized – they are in many ways fire-adapted. Like the Banksias (of the Proteaceae) they typically hold their seeds in little wooden capsules, and they are able to germinate only after they’ve been cooked. After fires the seeds are shed in vast numbers, on to ground burned bare of litter and competitors, and transiently rich in nutrients from all the ash. Like the redwoods (another striking piece of convergence), eucalypts often have buds just beneath their bark which spring to life when the bark is burnt off, so the charred eucalypt re-emerges as a coppice. Many, too, have ‘lignotubers’ – tuber-like swellings of the lower trunk which generally become buried, and are packed with buds and regenerative tissue, which spring up like a phoenix when the main stem is destroyed. Eucalypts cope with low fertility through close associations with mycorrhizal fungi – often more than one kind of fungus at once – which greatly extend the range and efficacy of their roots. As we saw in Chapter 5, pines in northern continents also succeed in poor soils with the h
elp of mycorrhizae. Many trees have mycorrhizae, but pines and eucalypts seem particularly adept. Yet another example of nature’s reinvention.

  Thus in present-day Australia eucalypts are absent only from extreme desert, the highest mountains (not that Australia has very high mountains) and rainforest. There are vast areas that don’t suit them – including the tropical and subtropical rainforests of Queensland and northern New South Wales – but there are even vaster areas that suit them very well; much better than they suit the Callitris, araucarians, she-oaks and southern beeches, which over the past few tens of millions of years have been largely sidelined. Over most of the continent eucalypts reign supreme, typically forming open, evergreen woodland. They have diversified wonderfully – with more than 700 species at the latest official count, arranged in thirteen different ‘series’ (sometimes aggrandized as ‘subgenera’). All but a handful are exclusive to Australia. There are just five in some islands to the north, and a few in New Guinea.

  Eucalypts vary enormously in form: some like the yellow gum, E. vernicosa, of mountainsides are shrubs less than 1 metre tall; some, known as mallees, have many stems springing from the ground, like bamboo or seriously coppiced chestnut; many are woodland trees, 10 to 25 metres tall; and some are huge forest trees – notably the mountain ash, E. regnans (‘ruling eucalypt’), which can grow to nearly 100 metres. Mountain ash is the tallest of all flowering plants, almost matching California’s coastal redwoods. (You will be pleased to know that I have seen the biggest of the ones in New South Wales, and been properly awed.) The variety of eucalypts is further increased by a strong tendency to hybridize; and the non-botanist may think there are even more than there really are because the young plants typically have ‘sessile’ leaves (without stalks) and hold them horizontally, while leaves on older plants do have stalks and are held with the blade vertical. The plant’s chemistry may change at the same time, and so too its susceptibility to pests. It is a veritable metamorphosis, like caterpillar into moth: one group of genes going offline, another group coming into play. Sometimes when a eucalypt regenerates from buds and lignotubers, the new stems again produce sessile juvenile leaves.

  Nowadays, of course, eucalypts grow throughout the world: Californians, Indians, Africans, even Mediterraneans may be surprised to learn that they are, in origin, so emphatically Australian. The first specimen (in fact of E. obliqua) was sent to Europe in 1777. Charles-Louis L’Heritier de Brutelle coined the name Eucalyptus in 1788. But already by about 1790 eucalypts had been taken to India. By 1804 they were growing in France. Then to South America: Chile in 1823; Brazil in 1825. South Africa acquired them in 1828; Portugal in 1829.

  At first they were grown in botanic gardens, and their transition into commerce wasn’t always smooth. The first imports had little genetic variety, and did not necessarily lend themselves to much improvement. Some of those transferred from botanic gardens to plantations turned out to be hybrids. The first generation of hybrids – ‘F1’ – often grow very well. But in subsequent generations the genes are mixed up (‘recombined’) and the F2S, F3S, and so on are completely inconsistent. It took time, therefore, to create colonies of trees that performed well, bred true, and yet were not too inbred.

  Yet it was clear from the early nineteenth century that the right eucalypts in the right places could grow at an astonishing rate. Plantations were soon established in Brazil and South Africa, to help build railways and as fuel for locomotives; in Brazil they were also used for charcoal, for smelting iron and making steel; and in Chile and South Africa for pit props. Eucalypts were also deployed widely as windbreaks, and for land reclamation. The oil from their leaves was a bonus. The aroma was thought to discourage the mosquitoes that carry malaria and they became known as ‘fever gums’, and were planted even more enthusiastically than ever. Their flowers also provide bees with honey.

  But in the twentieth century, eucalypts were cultivated more and more for the great mass of short and uniform fibres in their wood, valued for paper for all purposes, from decrees nisi to disposable nappies. But still the wagon rolls on and eucalyptus is being used more and more in construction – for sawn wood, MDF (medium-density fibreboard), even veneers, and in plastics. More humbly, eucalypts have become hugely important for fuel in rural communities in India, China, Vietnam, Peru and Ethiopia. Indeed, eucalypts are now grown in more than ninety countries. The total area of plantation seems hard to estimate: some say around 9.5 million hectares in 1999, rising to 11.6 million by 2010; some say 16 million in 1999, rising to 20 million by 2010. In any case, it’s a lot – though much less than the 130 or so million hectares reckoned to grow wild in one form or another in Australia (although Australia also has plantations). The growth rate of eucalyptus becomes more and more fabulous. The average world-wide is probably about 20 cubic metres of timber per hectare per year but some plantations claim to achieve 60 cubic metres. The big and popular Eucalyptus grandis typically achieves 40 cubic metres per hectare per year and is harvested, as sizeable trees, in six to eight years.

  Beyond doubt, eucalypts are immensely valuable. Beyond doubt, too, they are a bandwagon: the more that is invested in them, the more their genetics and biology are studied, the more they are ‘improved’, and the more the gap grows between them and other species that should be given more of a chance. In much of the world they have become weeds, ousting the native flora. In some places, because of their great thirst and their supreme ability to drag water from the depths, they dry the land too much for native species to cope – and this among other things encourages fire that tips the ecological balance in their favour even more. So eucalypts overall are a mixed blessing. Their safe and advantageous deployment requires aesthetics, restraint and good husbandry, and not just an eye for the expedient. That of course is true of life as a whole.

  Lindens, Cocoa and Baobabs: ORDER MALVALES

  The Malvales are named for the Malvaceae family — which has always been a most intriguing family and, as modern taxonomists get to work, is becoming more so. Traditionally the Malvaceae included the homely mallows, Malva, and the English garden hollyhock, Althaea, but also the more exotic Hibiscus, including H. esculentus whose fruits manifest as okra, alias lady’s fingers, alias bindhi; and the cotton plant, Gossypium, whose hairy seeds, even more than the latex of the rubber tree, have changed the world. Yet modern DNA studies suggest that Malvaceae should be defined even more broadly. According to Judd, several families that have traditionally enjoyed independence – including several of very significant and sometimes extraordinary trees – should now be subsumed within the Malvaceae. These include the Tiliaceae, Sterculiaceae and Bombacaceae. Thus the newly-styled Malvaceae is rich and various indeed. For the purposes of this book, Malvaceae in the old sense would hardly deserve a mention at all, but in its new form it certainly does. In the following account, however, both for general ease and to facilitate cross-reference to traditional texts, I shall stick to the old-style family names.

  Tiliaceae is the family of the limes, alias lindens, and the American basswood (where ‘bass’ is pronounced with a short ‘a’ as in the fish, not as in the large guitar). Limes put up with heavy pruning and thus are often butchered as street trees (though they get their own back by attracting aphids, which secrete gum, which in turn attracts fungi, and so coat the cars parked beneath in what seems like tacky soot), but when allowed to grow to full magnificence they are unsurpassed in avenues, seen wondrously in English estates and in Berlin’s Unter den Linden, the lovely road that once ran from the Brandenburg Gate to the palace of the Kaiser Wilhelm. Lime timber is excellent. Europe’s Tilia cordata makes musical instruments and fine furniture (including the carved fronts of many a pulpit). America’s basswood, T. americana, is used for turnery (and even before America had lathes it was used for bowls, if the wedding sequence from Henry Wadsworth Longfellow’s Hiawatha is to be believed: ‘Sumptuous was the feast Nokomis/ Made at Hiawatha’s wedding./All the bowls were made of basswood/ White and polished very smoo
thly’). Limes are pollinated by bees, and provide excellent honey. Several other members of the Tiliaceae family provide fibres for ropes, notably of the genus Corchorus, grown in India and to some extent in Africa for jute. The West African danta, Nesogordonia papaverifera, provides fine flexible timber used for everything from telegraph poles to gunstocks, carriages, boats and veneers.

  The name of the Sterculiaceae family comes from the Latin stercus meaning ‘dung’. The huge ‘wild almond’ (up to 36 metres) of India is called Sterculia foetida (‘stinking’), as if to emphasize the point. As D. V. Cowen writes in her classic Flowering Trees and Shrubs in India (Thacker and Co Ltd, Bombay. 6th revision 1984): ‘Coming across a wild almond in bloom one’s first thoughts would be that one was near an open sewer and many parts of the tree when bruised or cut emit this rank, unpleasant odour. It is unfortunate as the tree is extremely handsome: tall and straight, its well shaped crown swathed in deep coral, often without a single touch of green, it stands out among the surrounding verdure in great beauty and dignity.’ Flowers of ordurous odour (there are many such) are invariably pollinated by flies. As always in the genus Sterculia, the flowers come out before the leaves. The fruits develop in April soon after the leaves appear and, says Mrs Cowen, they are ‘large as a man’s fist, woody and purse-shaped … like odd, dark objects casually thrown into the tree’. Yet the wild almond has many good points. The leaves and bark are medicinal, a useful gum comes from its trunk and branches, the bark yields fibres for cord, and the seeds can be eaten when cooked. For good measure the wood does not split and is used for spars. (D. V. Cowen was a true memsahib: a fine hostess who also painted and wrote beautifully about plants, was a competent birder, and, for good measure, a champion golfer. Her book is a grand piece of publishing. Everyone should own a copy. I got mine in Delhi.)