The Tree

by Colin Tudge

  There are 35 or so known species of southern beeches, although in reality there are probably many more. Most are evergreen, although a couple are deciduous. Nine live in South America, not least in the bands of forest along either side of the Andes in Patagonia, which are dominated largely by the lenga (N. dombeyi) and the nire (N. procera) and are home to pumas, guanacos (small relatives of the llama), southern river otters, geese, Andean condors, and deer. Here is another grand place to see and wander through, now sadly threatened by too much logging, although in part protected now by Patagonia’s Perito Moreno National Park. Three more southern beeches live in Australia. They were far more widespread there when Australia was wetter, but in these dry and fire-prone times they are largely supplanted by eucalyptus. They flourish still in New Zealand (four species); and there are eighteen more in New Guinea, New Britain, and New Caledonia (inevitably!) and a few more on other islands. There are none in Africa, but before Antarctica drifted to the South Pole and was buried in ice it clearly had great forests of southern beech. Often, within their range, they are the dominant broad-leaved trees, but in New Zealand and South America, in particular, they tend to share their forests with the great southern conifers, the podocarps and the various araucarias. Though we can reasonably suggest that southern beeches are the southern equivalent of the oaks, beeches, and chestnuts of the north, in truth the southern temperate forests tend to be very different in character: typically damper and, in New Zealand, having a wondrous understory of giant ferns, each worthy to stand in some stately conservatory in a grand ceramic pot.

  Challenging the Fagaceae family in diversity and ecological range (although nothing can quite challenge the Fagaceae) is the family of the birches, alders, hazels, and hornbeams, the Betulaceae. Again, all of them are trees and shrubs, widespread in the Northern Hemisphere—both in temperate regions and in the most extreme north. Just a few, notably some alders (Alnus), drift into the Southern Hemisphere. In general, trees of the Betulaceae are early on the scene when there is new ground to be colonized—and so they rapidly spread north after the last ice age, in the wake of the retreating glaciers. In the case of alders, they are helped in this by nitrogen-fixing Frankia bacteria in nodules in their roots. Alders in particular, too, largely because of their nitrogen-fixing bacteria, do well in waterlogged soils, in the same way as mangroves. Sometimes, however—as with the endless birch forests in Siberia and Canada—Betulaceae are the dominant forest trees. Their flowers are catkins, pollinated by wind; and their seeds are mostly distributed by wind (or water, in the case of alders and hop hornbeams); but the seeds of hazels are spread by rodents, with squirrels as key players, although they exact a huge fee for their services.

  Judd recognizes 6 genera, with a total of 157 species: 60 birches (Betula); 35 alders (Alnus); 35 hornbeams (Carpinus); 15 hazels (Corylus); ten hop hornbeams (Ostrya), also sometimes called ironwood; and a couple of Ostryopsis, which resemble hop hornbeams. Early botanists placed alders, birches, and willows together in a subfamily they called Amentiferae: all, after all, are trees, and have catkins. Walnuts, figs, and elms were bundled in too, for good measure. But the catkins that they all shared evolved independently, not from a single common ancestor. Alders, birches, and willows do share a propensity as pioneer species, however, and are used for similar things—not least to provide the charcoal needed to make gunpowder.

  Helped by their nitrogen-fixing bacteria, alders grow rapidly, sometimes reaching 30 meters in a decade—and so aggressively that they are often rated as weeds. They definitely have their upside, however. Because they fix nitrogen, they are able to improve the soil significantly, and so benefit the whole forest. They are excellent pioneers; in general they are widely planted, and in particular are pressed into service for soil reclamation.

  Though Judd recognizes 35 species of alder, this should not be taken as gospel. Stephen Harris, curator of the herbarium in Oxford University’s Department of Plant Sciences, puts the figure at nearer 25. But as outlined in Chapter 1, Alnus is one of those genera in which the concept of “species” is hard to pin down. They have a great tendency to form polyploid hybrids, and given the enormous geographical range of Alnus, and the remoteness and hostile nature of many of the places it tends to live, we can also be fairly sure that many species remain to be identified.

  For alders are indeed extremely widespread. A. acuminata lives in Central America and extends into the highlands of South America, and is planted extensively for timber and fuel. The black alder, A. glutinosa, is widely distributed in Europe. In workaday mode, it serves to stabilize riverbanks and roadsides, and is grown for fuel. More grandiosely, it once supplied much-valued timber for violins. In recent years, however, at least in Britain, black alder has suffered hugely from attack by the fungus-like Phytophthora cambivora, related to potato blight. The Nepal alder of the Himalayas, A. nepalensis, is also planted widely for timber and firewood, and as forage for cattle and sheep. The red alder of the American northwestern floodplain, A. rubra, is a huge tree, up to 40 meters, favored for building and furniture, as well as for fuel and pulp (though used mainly in mixtures). Red alder can also be a significant weed in plantations of pines. Alders do well in waterlogged places. Alders are among the many trees that tend to concentrate minerals within their cells: they pick up gold, for instance. Whether it is worth trying to extract gold from them, I do not know. Alder bark is also astringent and is traditionally used to treat burns and infections.

  The oldest known fossils of alder date from the Miocene, around eighteen million years ago. The genus Betula is much older: birch fossils date from the Upper Cretaceous, still in dinosaur times, and perhaps birches were most diverse in the Eocene, around forty-five million years ago. The 60 or so extant shrubs and trees also live in even more diverse habitats than alders do, from temperate lands to the extreme northern limit of trees. They are present and may be dominant in peat lands: along the banks of streams and the shores of lakes; in damp woods; on the margins of roads and railways; in alpine settings; and on tundra. Birches are pollinated by wind—and produce a great deal of pollen. Like alder pollen, this is bad for hay fever, though speaking as a sufferer, I find it a small price to pay for all that beauty. Again, birches tend to be polyploid, and prone to hybridizing.

  Fey, melancholic, and wonderfully hardy: the birch.

  Birches are good biochemists—and so are used for many things besides their attractive white timber. Their leaves are often rich in resins, and their bark (particularly from the white-barked kinds) is rich in phenolics. Some species produce betulin in their bark, which makes them waterproof. (These agents are also said to be effective “antifeedants,” repelling hungry browsing animals in winter; yet many insects feed on birch, and fungi may rot the heartwood.) Their twigs were traditionally used for punishing: the generic name Betula derives from the Latin for “beat.” Birch bark (for example, of the paper birch, B. papyrifera) is used for roofs and canoes, and is the stuff of the oldest known Hindu manuscripts, dating from around 1800 B.C. It is also rich in oil and starch and serves as food in times of famine. Like alder and willow, birch wood burns to make good charcoal, excellent for gunpowder (how many did the Russians fell in seeing off Napoleon?). The sap of the Appalachian B. lenta is tapped in the spring and fermented to make birch beer. Oil of wintergreen, containing methyl salicylate (related to aspirin), can be obtained from B. lenta and the yellow birch, B. alleghaniensis. The leaves of the European species downy birch (B. pubescens) and silver birch (B. pendula) produce a green dye. Birch, like alder, accumulates heavy metals in its leaves and can be used to reveal their presence in the soil beneath. Birches in general yield valuable timber, and also pulp. Many birches are grown as ornamentals, and quite right too.

  The hornbeam, Carpinus, is also known as ironwood; it is so hard that it was the traditional stuff of axles and cartwheel spokes before iron became cheap enough to take over. A brewery in my village that has somehow escaped the corporate ravages is powered by a nineteenth-ce
ntury steam engine, and the moving parts that extend from it have hornbeam cogs. They are better than iron, I’m told, because they don’t shear. Hornbeam is often overlooked. It looks somewhat like beech, but its leaves are generally smaller and more deeply furrowed, its trunk is fluted, and its nuts are winged as a beech’s are not. Like beech it is good for hedging, and for pleaching—a process in which branches from adjacent trees closely planted in a row are run together and trained to form what looks like a hedge on stilts. I first met the hop hornbeam (Ostrya), from Asia, Europe, and America, in the Cambridge University Botanic Garden one July: its bark shaggy, its cinnamon-colored dangly male catkins and its pale green female catkins hanging side by side like courting couples at the tips of the twigs. A delightful tree—again, with very hard wood. Most hazels and filberts (Corylus) are more shrub than tree, but C. colurna, from Turkey, grows to nearly 25 meters.

  The Casuarinaceae family are the she-oaks. There is only one genus, Casuarina, which is basically Australian but also grows widely through Asia and the Pacific islands, including Fiji and New Caledonia. Like alder, it has nitrogen-fixing Frankia in its roots; but unlike alder, it generally favors dry land and is grown for timber, fuel, and to provide shelter belts in China and shade on tropical and subtropical beaches in Africa and America. Three species are naturalized in Florida, where they flourish as significant weeds. Casuarina trees have grooved, green, jointed twigs; some are grown as ornamentals.

  The Juglandaceae family are the walnuts, hickories, and wingnuts, which have either big, fleshy fruits containing nutritious, aromatic nuts that are dispersed by animals (mainly rodents) or winged seeds that are dispersed by wind. In all of the Juglandaceae the pollen is wind-dispersed, and most have catkins. Most are bona fide trees, though a few are shrubs—mostly resinous, mostly aromatic, and mostly rich in tannins.

  The Juglandaceae family as a whole seems, like the genus Quercus, to have arisen in warm latitudes at a time when the world as a whole was warm—around forty to fifty million years ago, when there were palm trees in the Dakotas and temperate forests in Siberia; and in the present, cooler world the family spreads from tropical to temperate lands. Now there are species in North and Central America, in South America along the Andes, in Europe, and in Asia—India and Southeast Asia.

  Most widespread (in the Americas, Europe, and Asia) are the 20 or so species of walnuts (Juglans). They hate shade; when they are in forests, they need to be the dominant species, or at least codominant, able to shade out the rest. They are helped in this, it seems, because they produce juglone in their leaves, bark, husks, and roots; this is believed to be noxious to other trees, which give them a wide berth. Paper birch, apple trees, and various pines are said to be particularly sensitive. Fishermen have also used bruised walnut branches, leaves, and fruit to stun fish: unscrupulous if practiced by sportsmen, but perhaps more excusable among people whose lives may depend on the catch.

  The 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. By the same token, the Scotch 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). 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 favorite for high-grade furniture until, from the 1600s onward, 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 walnuts. 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 favored 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. It is 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 will 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 are wingnuts, which, like walnuts, have big, sweeping, feather-like leaves. One wingnut tree that I know, in the Cambridge University Botanic Garden, 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 that 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.

  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 14 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, including the pomegranate (P. granatum). The Onagraceae family contains the evening primrose, the willow herbs, and the fuchsias, which include the kotukutuku, Fuchsia excorticata, the unique fuchsia tree that grows throughout New Zealand to a height of around 14 meters. 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 western Africa and America. The family also includes Terminalia, with many big tropical trees valued both for their physical beauty (they are much favored on 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-gray wood; western 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 outstandin
g, 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. officinalis) and of bay rum (P. racemosa). Species of Melaleuca, with their bottlebrush flowers, are favorite ornamental shrubs, while M. leucadendron provides medicinal cajeput oil. The Myrtaceae family offers some fine fruits: notably the guava, Psidium guajava, from tropical America and the West Indies. Most of all, though, the Myrtaceae includes 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, already equipped with a bag of adaptive tricks that helped it survive and flourish where others languished. To judge from present distribution, the genus seems to have arisen in Australia sometime in the early Tertiary, around sixty to fifty 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, sulfur, and some essential trace minerals.