The Secret Life of Trees

by Colin Tudge

  Oaks are so tremendously successful partly because, like many other successful trees, they can reproduce in so many ways. Unlike most Fagaceae, but like beeches and southern beeches, oaks are pollinated by wind, and generally bear both male and female single-sex flowers on the same tree (that is, they are monoecious). In good years the acorn crop is prodigious; they are distributed largely by mammals (especially rodents). But oaks also sprout from damaged trunks, to form natural coppices; and coppicing once supported entire industries of foresters (broadly defined) throughout Europe. Oaks also send forth shoots after forest fires, again producing entire stands that are clones of the parent individual.

  Nowadays, of course, many have been transplanted from their native lands and have become ubiquitous: red oaks, from the eastern United States, are planted throughout Europe; common oak, from Europe, is now seen just about everywhere; and so on. The timber of many species is of legendary strength and beauty, and is used for everything from pit props to the finest veneers. Half of all hardwood in the United States comes from oaks. Their wood burns well: oak chips are essential for smoking kippers. Oaks are burned for charcoal, too. They make some of the finest barrels, most favoured for sherry and whisky. Their tannins are used for tanning leather. Cork oak has a thick fire-proof layer of cork beneath its bark, a key player in the traditional economies of Portugal and Andalucia – the plantations not only providing cork but also a home for the black pigs that produce such magnificent hams: a fine exercise in agroforestry. Entire streets in Andalucia are lined with shops that seem to sell hams exclusively. The young piglets are ginger-brown, and trot about in tightly coordinated squads as baby warthogs do. You meet them on Iberian hikes – torn, as wild animals so often are, between curiosity and nervousness: charming, fleeting companions. The cork forests, too, are home to some of the last of the Spanish lynx. Natural cork is now threatened by plastic: yet another degradation of the world’s ecological and cultural diversity.

  The tanoaks, Lithocarpus, have fruits like acorns but leaves like a chestnut’s. Tanoaks are less legendary than oaks; but they, too, produce hard, strong timber – though mostly used for pulp and firewood – and their tannins again are used to tan leather.

  Seven of the ten or so species of beech in the genus Fagus live in temperate east Asia. There is also one in North America, one in Europe, and one in the Caucasus. Normally they are found with other deciduous species in mixed forests, in temperate climes with soils that hold moisture well but are not waterlogged, for they like neither flooding nor drought. But they don’t mind shade and will grow for decades in the shadow of other trees. So it is that the European beech, F. sylvatica, grows happily alongside sessile oak, common oak and the European hornbeam, Carpinus betulus. In plantations, hornbeams are grown with beech to ‘train’ them – encourage them to grow straight and tall, and to shed their lower branches as they reach up. The American beech, F. grandiflora, features in about twenty different forest types in the eastern United States, sometimes dominant but by no means always. Its companions typically include hickories and oaks, yellow birch, sugar maple, red maple, American basswood, black cherry, plus eastern white pine and red spruce; and in the south, it may grow alongside Magnolia grandiflora (another combination I would very much like to see in the wild, and haven’t yet). All in all, beeches seem to be sociable trees. They are also long-lived. Among American broadleaves only the white oak and the sugar maple are said to live longer.

  One of the temperate world’s most valued hardwoods: the beech

  Beech, like oak, is highly prized, its silky, pale timber excellent for floors, furniture, veneers, turning and steam-bending. Beech that has been infected with fungi may be particularly valued for turnery: wiggly black lines etched by the fungus in the creamy wood. There is a parallel here with the ‘noble rots’, the yeasts and fungi that produce the world’s great wines and blue cheeses: one of the great leitmotifs of Western art is that beauty and decay are never far apart. People as well as wildlife eat the nuts or ‘mast’ of beech. I have waited in traffic in Holland while pigs snuffled beechmast off the road (and rightly so). Beeches are beautiful, too. Their trunks are smooth grey columns. Their diaphanous leaves filter the light like pale-green glass. Some garden varieties are stunning, like the copper beech (in truth deep red) and the weeping kinds that grow like spheres, their branches sweeping the ground if browsing animals are kept at bay. They also make fine hedges – their leaves turning red-brown in winter but nonetheless remaining on the hedge (although beeches that are allowed to grow into forest trees shed their leaves). Altogether, Fagus is a fabulous genus.

  The ten or so species of the genus Castanea are the sweet chestnuts. They are also known in some parts as chinkapins (but so are Castanopsis and Chrysolepis). Sweet chestnuts hail naturally from southern Europe, North Africa, south-west and east Asia, and the eastern USA. They are prized for their nuts, of course: traditionally roasted, and also the source of a fine stuffing, excellent with wintry goose. Before the 1930s, too, the American chestnut, C. dentata, was prized for its timber. But American chestnut is one of all too many trees that have been devastated by disease: in this case the chestnut blight, Endothia parasitica. As with Dutch elm disease, the fungus attacks larger trees, so American chestnuts are at least able to grow to the size of the shrub before the stems are killed. The roots survive, the stems regrow until they are big enough to be zapped again, and so on ad infinitum. It is all immensely sad.

  The southern beech, the genus Nothofagus, first attracted the attention of Joseph Banks in the late eighteenth century, during his trip south with James Cook. Then in the 1830s the young Joseph Hooker was struck by the similarity between different southern beeches on different continents. In particular, New Zealand’s tawhai or silver beech, N. menziesii, much favoured for its timber, is remarkably similar to Australia’s N. cunninghamii, another fine timber tree, confusingly if typically known as the Tasmanian myrtle; and also to N. betuloides of South America. Such observations fed the growing suspicion that different species in different places must have evolved from some common ancestor. (Hooker later became great friends with Charles Darwin, another pioneer naturalist in southern climes, and succeeded his father William Hooker as director of Kew.) Botanists for many years placed southern beech among the Fagaceae, alongside beech. But the cups that enclose the nuts of beeches, oaks and chestnuts are formed from the flower stalk, while those of southern beech are compacted from bracts. So the relationship is not as close as it may seem, and southern beech now has its own family, Nothofagaceae, although still within the order Fagales. Nothofagaceae is the only Fagales family that belongs to the southern hemisphere.

  There are thirty-five or so known species of southern beech, 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 ilama), 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 beech 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 Pole and was buried in ice it clearly had great forests of southern beech. Often, within their range, they are the dominant broadleaved 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 beech is 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 understorey 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 water-logged soils, in the same way as mangrove. Sometimes, however – as with the endless forests of birch 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 six genera, with a total of 157 species: sixty birches (Betula); thirty-five alders (Alnus); thirty-five hornbeams (Carpinus); fifteen 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 sub-family which 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, aiders grow rapidly, sometimes reaching 30 metres 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 thirty-five 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 twenty-five. 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 north-western floodplain, A. rubra, is a huge tree, up to 40 metres, favoured 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 used traditionally to treat burns and infections.

  The oldest known fossils of alder date from the Miocene, around 18 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 45 million years ago. The sixty or so living 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 in 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 hybridize.

  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) are 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-bark birch, B. papyrifera) is used for roofs and canoes, and is the stuff of the oldest known Hindu manuscripts, dating from around 1800 BC.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 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; 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-century 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 – when 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 Botanic Garden at Cambridge one July: its bark shaggy, its cinnamon-coloured 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 metres.

  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 favours drylands 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 naturali
zed in Florida, where they flourish as significant weeds. Casuarina have grooved, green, jointed twigs; and 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; 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 40 to 50 million years ago, when there were palm trees in the Dakotas and temperate forest 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 South-East Asia.

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