The Secret Life of Trees

by Colin Tudge

  The family of the box trees, Buxaceae, has not been easy to place. It has at times been linked to the rubber tree within the Euphorbiales order – although its members do not have latex, as the euphorbias do. Now it seems to have fetched up in the Proteales. Its 100 or so species (in four to six genera) stretch throughout all the tropics (apart from Australia) and through Europe as far as Scandinavia. Common box, Buxus sentpervirens, is one of only two evergreen hardwood trees that seem to be native to Britain. (The other is the holly.) But I say ‘seem’ because botanists have been arguing for 150 years about whether box really is a native, or was simply introduced a long time ago. The box tree is not big – up to 10 metres – but it is highly prized, partly for high-class hedges (the stuff of medieval knot gardens) but also for its close-grained, pale-yellow timber: favoured for rulers and also for wood engraving. Traditionally woodcuts were made with blocks of fruit wood (usually pear), cutting along the grain (as ’twere into a plank), while finer work – engraving – needed metal blocks. But then in the late eighteenth century the English artist Thomas Bewick showed that boxwood, cut across the grain, gave results comparable with metal: and boxwood is now the wood engraver’s staple fare (and extremely expensive).

  An Oddball from Japan and Taiwan: ORDER TROCHODENDRALES

  There is just one family (the Trochondraceae) in this order, which has just one species, Trochodendron aralioides. It comes from Japan and Taiwan, and is large, with a trunk up to 1.5 metres thick. I regret I have not seen a Trochodendron and can throw no further light on it (but it provides yet another reason for getting into the countryside in Japan and Taiwan and checking out the extraordinary flora there).

  Prickly Trees from Madagascar and ‘Tree’ Cacti: ORDER CARYOPHYLLALES

  A big grouping, with 8,600 species in eighteen families, two of them including trees (the others range from carnations to sugar beet). First we have the extraordinary family of the Didiereaceae. They are unique – endemic – to semi-desert in Madagascar: that long-isolated island, effectively a small continent, that has so many unique creatures including all the present-day lemurs (and other ‘prosimians’ like the aye-aye); the fossa, a civet that looks for all the world like a slinky cat; and, probably until about the fifth century AD, the heaviest bird of modern times, the elephant bird – which is probably the roc of Arabian legend. The elephant bird laid the biggest eggs of any known creature, big as an over-inflated rugby football (fragments of shell can still be found). For their part, the Didiereaceae offer yet another remarkable example of convergent evolution, for most of their eleven species (in four genera) might reasonably be compared to the swollen baobob tree, which is also at its most various in Madagascar, though also is wide-spread in Africa. But more strikingly the trees of the Didiereaceae resemble columnar cacti, with green, swollen, prickly trunks. One of them, however, Alluaudia procera, Heywood describes as ‘a bent and thorny telegraph pole up to 15 metres high’. It is possible, I suppose, that if the historical coin had flipped differently then Europe and North America might have finished up with Didiereaceae, and Madagascar with oaks and ashes; and then we would think that oaks were weird. Incidentally, Judd no longer recognizes Didiereaceae as a discrete family, but includes it within the purslane family, The Portulacaceae. More work has still to be done on the DNA, however. Watch this space.

  Also within the Caryophyllales order are the cacti themselves, the Cactaceae. Although they are close relatives of the Didiereaceae, they surely evolved the cactus-like form independently. Almost all cacti are American (though many – notably the prickly pears in the genus Opuntia – have been naturalized in warm dry countries everywhere, and indeed were once a serious pest in Australia). Of course, most of the 1,400 or so species of cacti (in ninety-three genera) are not tree-like. Many horticultural favourites like Mammillaria and Notocactus art for the most part prickly orbs. But some, including Opuntia and many of the columnar Cereus and Carnegia have woody trunks, and form veritable desert forests. Most tree-like of all are the extraordinary Pereskias, almost certainly the most primitive cacti of all, which have green, swollen branches but also have leaves. Within the cacti, indeed, we can see how a family that began as trees (similar to Pereskia, and resembling Didiereaceae) has evolved several times, independently, into forms that are very non-tree-like: including the spherical types and the various epiphytes.

  All in all, phylogeny never fails to intrigue. It is pleasant to contemplate that cacti and carnations, which seem such poles apart, are not too distant cousins; or that the supremely exotic Didiereaceae and the purslanes that grow as wayside weeds (and feature in traditional European salads), are of the same (extended) family. What does this say of their evolutionary history, and the forces and happenstance that took virtual siblings in such different directions?

  Sandalwood and the Mistletoes: ORDER SANTALALES

  The Santalales are a strange lot. Many are epiphytes (plants that grow on other plants – usually trees); and different groups have evidently evolved the epiphyte mode independently. Many are parasites or semi-parasites at least for part of their lives – not simply hitching a ride on trees but tapping into their stems or roots. In the parasitic kinds, the roots are replaced by penetrating organs known as ‘haustoria’. But some (including some of the parasites!) are valuable trees – and among them is sandalwood, one of the most treasured of all. Sandalwood shows another common quality of the Santalales: some are exquisitely scented.

  The order has proved hard to classify. Many traditional taxonomies recognize four families, although modern work on DNA suggests they should be split into seven – but at least two of those seven (says Judd) probably need regrouping. So the present classification is a little untidy but Santalales as a whole seems to be a good grouping, a true clade, which is what really matters.

  Two families, the Loranthaceae and the Viscaceae (traditionally often combined into one), are those of the mistletoes. They are green and do their own photosynthesizing, but their haustoria tap in to the xylem of the trees on which they grow, typically high on small branches, and because they exert more osmotic pressure than the tree itself they suck out the mineral-rich sap. The mistletoes are quaint. Some of the Loranthaceae have bright flowers and are pollinated by birds. The Viscaceae are the ‘Christmas mistletoes’: favoured as Christmas decorations and charms: Viscutn album in Europe and Phoradendron leucarpon in North America. Christmas mistletoes were sacred to the Druids, symbols of immortality – the obsession of Asterix’s chum and Mentor, Getafix; and (so legend has it) provided the spear that killed the Norse god Balder. But they also do immense damage to the trees that they favour – mainly members of the Rosaceae and Tiliaceae families, but others too. They create general water stress and ruin the timber by inducing big knots and causing their hosts to produce masses of twigs known as ‘witch’s brooms’ (though there are other causes of witch’s broom as well, including the bacteria which commonly affect birches). Arceuthobium of the western United States is the dwarf mistletoe: a major pest of conifers.

  The Misodendraceae family contains just one genus with about eleven species of shrubs that live as parasites on the trunks and branches of the southern beech, Nothofagus. As we will see shortly, Nothofagus is a great Gondwanan genus, related to the northern oaks and beeches (although now given its own family), which extends all through Australasia and South America; but the particular parasite that plagues it is confined to southern Chile and Tierra del Fuego. I never cease to be amazed by the number of sub-plots in nature. To the Chilean Nothofagus and the parasites of the Misodendraceae their contest is a huge drama – of which virtually all other species, including most of our own, are completely unaware.

  Among the twenty-five genera of the loosely defined tropical family Olacaceae (including the Opiliaceae and Schoepficaceae) are shrubs and climbers. There are also some bona fide trees, including some that are locally valuable. Scorodocarpus of Asia smells strongly of garlic (its name means ‘garlic fruit’) and its timber, though smelly, is
very strong and used in heavy construction. Tallow wood or hog plum (Ximenia americana) is used as a substitute for sandalwood in South America: its timber is hard and yellowy-pink (and its fruits, laden with prussic acid, are extremely bitter). The African walnut, Coula edulis is another very strong timber for building.

  But the family that makes the Santalales order so important in human affairs is the Santalaceae, whose 400 or so species in thirty-five or so genera include Santalum album, the sandalwood tree (from the Hindi, sandal). Its heartwood is the stuff of incense and is beautiful: smooth and pale sandy-coloured, cutting like wax, carved throughout Asia into a million artefacts (endlessly intricate boxes, elephants inside other elephants, and so on). Entire trunks are smoothed and used as scented pillars in many an eastern temple. In Mysore it was a royal tree. Apparently sandalwood is endemic to Timor and its neighbouring islands, but was brought to India more than 2,ooo years ago: there are texts that apparently refer to it in the Pali Milanda-panha (150 BC) and the Mahabharata. Its small, purple-black fruits are dispersed by birds, who presumably helped it to spread throughout India. All in all it is immensely valuable commercially and culturally.

  Yet the sandalwood tree is a parasite too: at least when young it taps into the roots of a variety of trees. Among its many favoured hosts are the strychnine tree, Strychnos nux-vomica (a relative of the buddleia). Another host is the pestilential Lantana – a shrub imported into India by the British from South America as an ornamental, which now seems to grow along every wayside and in every wood throughout the tropics. Dr Sas Biswas, of the Forestry Research Institute in Dehra Dun, north India, tells a charming story of sandalwood trees he once found growing in a dead straight row in the middle of nowhere. Why were they there? Who had planted them so carefully and then abandoned them? No one, is the answer. But in the past there had been a garden; and around the garden was a fence; and along the fence grew the inevitable Lantana; and sandalwood had grown as parasites from its roots. Now the garden and the fence are long gone and the Lantana with it, but the sandalwood remains. Dr Biswas is adept at reading the history of landscapes from the trees that are left in it: the ambitions of the people who lived there, and the time and chance that in the end reduced their ambitions to nonsense. But nature is opportunist, and lives on.

  Finally, members of this whole order are particular pests of some species of acacia trees and other leguminous trees, of which more later. On the other hand, opportunist foresters seeking to grow sandalwood trees in plantations, often use acacias as hosts to start them off. It’s an ill wind.

  9

  From Oaks to Mangoes: The Glorious Inventory of Rose-like Eudicots

  Big, hollow baobabs may double up as funeral parlours and cafés

  Fifteen of the eudicot orders are grouped together to form what might be called a sub-class, known informally as ‘the rosids’: the rose-like eudicots. In truth, most of them are not literally rose-like. What really links them all together is not any obvious feature of their flowers, but details of their DNA. But it seems to be enough. At least in the present state of knowledge, the rosids do seem to form a coherent group. If future scholars decide that they are not so closely related as it now seems – well: that’s the way science goes. Nothing is ever absolutely certain.

  Of the fifteen rosid orders, four do not contain significant trees. The Vitales are vines, including the families of grapes and of Virginia creeper. The Geraniales include the cranesbills and the pelargoniums, parched and sooty-potted on their suburban windowsills. The Cucurbitales includes the family of the melons, cucumbers, squashes, marrows, gherkins, gourds and pumpkins. They are singularly untree-like – and yet the Cucurbitales seems to be closely related to the Fagales, which contains only trees, including some of the mightiest, like the oaks and beeches. The Brassicales includes the family of cabbages and wallflowers. Fine plants all. But the rest of this chapter focuses on the eleven rosid orders that do contain trees – including most of the world’s finest and most valued.

  Witch Hazels, Katsura and Sweet-gums: ORDER SAXIFRAGALES

  The Saxifragales are a difficult group, still being sorted out. One reason they are tricky is that within the Rosid subclass are many plants that are clearly somewhat primitive – close to the presumed ancestral state. The Saxifragales may be the most primitive of all (the ‘sister’ group to all the other Rosid orders). Plants with lots of primitive features and few truly distinctive features are always hard to classify – and modern studies, based on cladistic analysis of DNA, often differ markedly from more traditional treatments based solely on anatomy.1 Judd now includes thirteen families within the Saxifragales, totalling about 2,470 species. Among them are many pleasant and well known herbs and shrubs, including the families of the saxifrages, the stonecrops and houseleeks, and the peonies.

  But there are also a few families with some most interesting trees. The Hamamelidaceae includes about eighty species in twenty-five genera of which the most pertinent here is Hamamelis, the witch hazels. Hamamelis virginiana from the eastern United States has hazel-like leaves and yields a lotion widely used as an astringent and for soothing cuts and bruises; and diviners, at least in the US, favour its twigs for detecting subterranean water. Other Hamamelis, like the winter-flowering H. mollis from China, are fine ornamentals.

  The Cercidiphyllaceae has just one species, Cerciphyllum japonicum2 – the much-valued katsura tree from the northern temperate forests of Japan, China and Korea. The katsura is big – up to 30 metres high with a trunk up to 1.2 metres in diameter – and sometimes it has several trunks, deeply furrowed and often spirally twisted. The timber of katsura is excellent: not too heavy, straight-grained, lustrous, much prized for wood-carving, delicate mouldings, high-class furniture and veneers. It is also used for pencils and cigar boxes, and to make traditional Japanese shoes.

  Finally, the Altingiaceae family includes rasamala (Altingia excelsa) from Assam through South-East Asia, valued both for its heavy timber and for its fragrant gum, used in perfumery; and also Liquidambar, which grows both in Asia and North America. Superficially, Liquidambar species look like maple. Best known is Liquidambar styraciflua: the American sweet gum which grows from New England through Mexico and into Central America. The trees are often huge – up to 46 metres tall and 1 metre in diameter. Sweet-gum heartwood is brownish and pinkish, often darkly streaked, and is sold as ‘red gum’; while its sapwood is creamy white and is sold separately as ‘sap gum’. Both are favoured for furniture and veneers (and also more mundanely for packing cases and pallets). But also, when sweet-gum bark is wounded it exudes a vanilla-scented resin known as storax or styrax which is used in perfumery and also as a medicine (as an expectorant and inhalant, and to treat skin diseases). Like maple, Liquidambar is famed for its autumn colours and several kinds are grown as ornamentals.

  Creosote Bushes and the Wood of Life: ORDER ZYGOPHYLLALES

  The plants of the Zygophyllaceae family, the only one in the order, are herbs, shrubs and trees with showy flowers pollinated by insects, that tend to favour dry and salty places and often predominate in scrub. They are also rich biochemically and so are oily and aromatic, and are favoured as medicines. Among them is the creosote bush, Larrea to be found out in semi-deserts and in gardens as ornamentals. Among them too is the genus Guaiacum from the Caribbean and Central and South America: small trees (up to around 9 metres tall and 40 centimetres thick), known collectively as ‘lignum vitae’.

  Lignum vitae of course means ‘wood of life’, for in the sixteenth century the tree was believed to offer a cure for syphilis. But in addition, the timber, sometimes greeny-brown and sometimes almost black, is one of the heaviest of all timbers, and enormously strong, with prodigious crushing strength. So it finds favour for sculpting and turning (although it is fearsomely hard on the tools), and for example for mallet heads: ideal for croquet. Because the wood is innately oily, too, it is self-lubricating, and so is favoured for rollers and wheels in pulleys and various machines – especially f
or parts that are hard to get at and lubricate. The related American genus Bulsnia is also sold as lignum vitae, but is not so good. But Guiacum is becoming rare, and indeed is now covered by CITES (the Convention on International Trade in Endangered Species), which at least gives some legal control over harvesting.

  Spindle trees and Khat: ORDER CELASTRALES

  The Celastraceae family is named after Celastra: a climber known as bittersweet. C. scandens is the native North American species, much admired. But it is now largely replaced by C. orbiculatus, introduced from East Asia – an aggressive weed that grows over everything, often smothering other plants, and is pertinent here not as a tree but as a spoiler of trees (at least when out of its native lands).

  But the fifty-five genera of Celastraceae also include the genus Euonymous. Among these is Euonymous europaeus, the spindle tree, whose fine-grained wood is favoured for carving and turnery, and whose seeds yield an oil for soap and a yellow dye for colouring butter. Euonymous hians from Japan is also used in turning, and for making printing blocks. Other Euonymous, including some that are shrubby rather than arboreal, yield a rubber-like latex and some feature in Native American medicine. The pharmacological propensities of the Celastraceae family are also evident in the khat tree, Catha edulis, whose leaves are chewed in the Middle East as a stimulant. The kokoon tree from Sri Lanka (Kokoona zeylanica) yields a useful oil.

  Rubber Trees, Mangroves, Willows, Poplars, and Some Truly Prodigious Hardwoods: ORDER MALPIGHIALES