by Colin Tudge
The genus Prunus abounds with good things. P. dulcis is the almond. P. armeniaca is apricot. P. avium is the sweet cherry and P. cerasus the sour cherry. P. persica is the peach and P. domestica is the plum. Many Prunus too are grown as ornamentals – notably the Japanese flowering cherries – and several are noted for their fine reddish timber. In the wild, Prunus and Crataegus, the hawthorn, are early to put in an appearance as new forests establish themselves, although P. serotina, the black cherry, grows on in mature deciduous forest. P. avium, the bird cherry, grows widely in Europe including Britain, where it is apparently native.
Other ornamental trees (or near trees) include Amelanchier, which the Americans call the shadbush; the flowering quince (Chaenomeles); Cotoneaster, always known as cotoneaster; the hawthorn, Crataegus, otherwise known as the quickthorn or may tree; the firethorn (Pyracantha); the roses of course, in the genus Rosa – many thousands of cultivars hybridized from about nine wild ancestors; Sorbus, which includes the mountain ash, or rowan, and also the whitebeam; and the florists’ favourite, Spiraea. Some have other uses, too. Hawthorn in particular is Britain’s favourite hedging plant, layered – the branches half cut across then laid sideways – to form an impenetrable thorny barricade; and sometimes left to grow into a big mature tree in the hedge, as elms often were.
Five closely related genera within the Rosaceae (though none of the important ones mentioned above) have developed symbiotic relationships with nitrogen-fixing bacteria that live in nodules in their roots. The bacteria are not Rhizobium, as in the Fabaceae; but Frankia. In all, plants from about ten families harbour nitrogen-fixing Frankia in nodules in their roots (one of the chief of which is the alder, Alnus). It would be tempting to suggest that the Fabaceae also started out with Frankia in their roots, but that these were later displaced by Rhizobium. But if this were so, we would expect the most primitive Fabaceae to harbour one or other of the two bacteria. In fact the most primitive Fabaceae do not have nitrogen-fixing bacteria at all. Thus it seems that nodules to harbour Frankia, and nodules to harbour Rhizobia, are independent, parallel inventions – yet another, stunning case of convergent evolution. We also see, yet again, the propensity of organisms – one might almost say, their eagerness – to cooperate.
Then there are the Rhamnaceae, the family of the buckthorns: 850 species in forty-five genera: often thorny; some trees, some shrubs, some climbers – and again, sometimes, with nitrogen-fixing bacteria in their roots. We are familiar with a few Rhamnaceae in the temperate north: buckthorn is Rhamnus; Ceanothus is a highly fashionable ornamental. But the Rhamnaceae come mainly from the tropics, where many are useful. Hovenia dulcis is the raisin tree. Ziziphus jujube is the Indian jujube, alias flame-of-the-forest. The jujube grows fast on dry, poor land to form red-flowered trees that are often scrubby but can reach 24 metres. Its timber is good, and it burns well; its prickly branches make serviceable fences; its leaves and twigs are fodder for camels and goats; its wild green fruits make sherbet, sold in the markets and (it’s said) much loved by students; and it is cultivated for its fruit, used for seasoning, cooked with sugar, or stored in oil or sugar syrup. Perhaps most of all, though, the jujube is a fine host for the lac insect, which sucks its sap and exudes a reddish resin over the whole surface of the twigs – which yields a dye and also becomes shellac, once used for gramophone records and still favoured for polishes and for lacquer. The jujube illustrates a general principle: how much use is made of plants that outsiders would scarcely notice, by people who know about them; how entire economies and cultures can flourish under our noses without us noticing, and how easily and often those ways of life are swept aside – for what developer would care about the wild jujube trees? The lac insect also feeds on the peepul, the rain tree and the mango.
Both the Rosaceae and Rhamnaceae families are at the edge of the Rosales order, however. The remaining families all seem to group roughly together in one great clade. And what families they are.
First come the Ulmaceae, the family of the elms (Ulmus) and the favoured park tree Zelkova. There are six genera, and about forty species, all trees, mostly in the temperate north. Elms until recent times were so common in England they largely defined the lowland landscape: they dominate John Constable’s Suffolk landscapes in the east, and in the west were known as the ‘Wiltshire weed’. They commonly grew in hedges and formed fine trees whose timber was often used in great slabs, for example to make the buttock-moulded seats of rural wooden armchairs, and the sides of wheelbarrows. Then in the 1970s Britain’s elms were struck down by Dutch elm disease: caused by the fungus Ophiostoma novo-ulmi, and carried and introduced beneath the bark by bark beetles of the genus Scolytus. Within a few years, despite the best efforts of foresters and biologists, mature elms had all but disappeared. The original types hang on as hedgerow bushes – but as soon as they reach a critical height, within the beetle’s flying zone, they are attacked again, and die off. New resistant strains are being developed, but England’s lowlands will never be the same again (although, of course, the transformations wrought by urbanization and agribusiness are far more dramatic). Dutch elm disease occurs on mainland Europe and in America as well, where the fungal pathogen is carried both by Scolytus and Hylurgopinus.
Then there are the Celtidaceae, which include the hackberry or sugarberry (Celtis), whose colourful fruits are for the birds that disperse them, and are rarely eaten by humans. But hackberry is used for timber and grown as an ornamental.
Then comes a huge and supremely important family – the Moraceae. Its fifty-three genera (1,500 species) include shrubs, climbers and herbs – but also some intriguing and supremely important trees that grow throughout the tropics as key players in rainforest. The jackfruit or breadfruit (Artocarpus) has a pale grey-green warty-skinned fruit which is really a fused mass of fruits (an ‘infructescence’) and may be huge: as big as a sack of coal and up to 40 kilograms in weight. Brosimum is the breadnut. There are some fine timber trees too: the iroko (Chlorophora excelsa) from Central and West Africa is often used as a substitute for teak. The snakewood (Piratinera guianensis) from tropical America is extremely heavy (much heavier than water when dried) with a black-brown tortoiseshell pattern favoured for everything fancy, from the backs of brushes and umbrella handles to violin bows – and for native bows for shooting arrows. More temperate is the mulberry (Morus): the white kind grown to raise silk moths (4,000 kilograms of leaves for one silk blouse); the black kind favoured for its glorious edible blackberry-like fruits, popular from the seventeenth century and now featuring, gnarled, in many an old walled garden.
Then there is Ficus, the genus of the figs. Ficus seems to go out of its way to be extraordinary. First, it is enormously various, with about 750 known species: a huge presence throughout the tropics, primarily in tropical rainforests, but reaching too into the subtropics and Mediterranean. Then there is the way it grows. About half the species simply take root in the ground like most other trees. The other half begin their lives as epiphytes – plants that grow in other plants. The seed lodges in the fork of a branch of some forest tree; or, often, in the severed leaf-base of some palm; or sometimes in a crevice in a wall. Then as it grows, as an epiphytic cactus or orchid might, it sends down roots towards the ground – which eventually take root on their own account. Then the once-dangling roots function as stems: where previously they had carried nutrient and water downwards, from the epiphytic roots above, now they carry minerals and water upwards, from the new roots in the ground below. Generally the dangling roots/ stems fuse with each other to form what looks like an exercise in macramé: you finish up with what may be a massive trunk that you can see right through. In the epiphytic figs known as ‘stranglers’ the probing roots twine around the host trunk. In others, like the Indian banyan, F. benghalensis, there is less obvious entwining. The roots merely drop to the ground eventually to form multiple trunks. The throttling of the stranglers, the monstrous weight of the epiphytic fig as a whole as it grows (whether it strangles o
r not), and the blotting out of light, eventually kills the host – although the obliteration may sometimes take a century or more, until which they live in grisly union.
Many trees are sacred to the Hindus and Buddhists of India and two of the epiphytic figs are among the most sacred of all. The British in India were often cavalier, but on the whole I am told (by Indians), they respected the sacred trees and many ancient ones remain even in the heart of British enclaves – including one magnificent specimen (albeit split into two by a storm in 1947) in the Forestry Research Institute at Dehra Dun, at the foot of the Himalayas. The FRI was established at the turn of the nineteenth century on the site of what had been twenty-two villages (although many of the village people stayed to work at the institute, and many of their descendants still do). The indigenous vegetation was largely cleared – but this great banyan, dating at least from the eighteenth century and probably before, was spared. The greatest of all, in Calcutta, is reputed to be a quarter of a mile in circumference, with hundreds of trunks connected overhead to form a colonnade, able to provide shelter for 20,000 people. The roots are guided to the ground along sticks.
Banyans form new trunks by sending down roots from above
Most sacred of all the figs, however – most sacred of all trees indeed – is the peepul, F. religiosa, also known as the bo. For it was under a bo tree that the young Prince Siddhartha sat and meditated, some time in the sixth century BC, and received enlightenment, and thenceforth was known as the Buddha. Peepul trees are unmistakable. The leaves of many tropical trees have ‘drip tips’: extensions at the end that act like gargoyles, quickly getting shot of surplus rain. The bo leaf is heart-shaped, typically about the size of a man’s hand, and its drip tip is enormous, about a third of the total length. This form, so characteristic, has become a Buddhist icon; bo leaves often provide the background to pictures of the Buddha. Yet they grow like the humblest nursery clone by the side of Indian roads – or sometimes right in the road, with ox carts and bright-painted, black-smoking lorries milling in the dust around them. In India, sacred means sacred. The traffic gives way.
Most extraordinary of all, however, is the manner of the figs’ reproduction. The flower, or rather inflorescence, is a fleshy cup formed from the flower stem; and within that cup hundreds of flowers open inwards. The whole apparatus is pollinated by minute wasps: extreme specialists, for there is one dedicated species of wasp for each of the 750 species of fig. At least, one fig: one wasp is the general rule. The reality is turning out to be more complicated and even more fantastical, and is discussed at length in Chapter 13.
Finally, comes a pair of closely related families – the Urticaceae and the Cecropiaceae. The Urticaceae is the family of the nettles – which northerners know as herbs with a serious line in stinging (and which are also fine sources of fibre) – but it also includes some tropical forest trees, some of which also sting. In Queensland I was told the tale of well-known British botanist much given to the waving of arms who waved them a little too much, right through the branches of a nettle tree, and finished up in hospital.
Cecropiaceae is a most intriguing family, which includes the tropical genus Cecropia. Cecropias are pioneer trees par excellence, with hollow stems like bamboo for extra rapid growth, branching at the top to produce an umbrella of silvery-grey compound leaves roughly like a horse chestnut’s. In the hollow stems dwell ants which, as in acacias, are the tree’s housekeepers. In newly exposed land the cecropia provides almost instant shade, while its leaves, roots and latex are pharmacologically potent and are used to treat a range of conditions from hypertension and depression to gastric ulcers. In tropical America, three-toed sloths are fond of cecropias: sloths seem remarkably common, and all the ones I have seen (all as it happens in Panama) were in cecropias. But cecropias can bring bad news, too. Their silver foliage stands out in the forest, and since they grow fast in open space they show where gaps have recently formed. Some of those gaps are legitimate: old trees die naturally, and in some areas at least, favoured forest trees are selectively and carefully harvested. But often you see hillsides where no logging is allowed awash with cecropias. Then they reveal where the illegal loggers have been at work. In Brazil and elsewhere, such sights are all too frequent.
The Cecropiaceae family, as now defined by Judd, also includes Cannabis, provider both of hemp fibres and of marijuana, and Humulus (hops). This is how the molecular studies suggest they should be classified. In the past, however, Cannabis and Humulus have commonly been placed together in a separate family, Cannabinaceae, which in turn has been grouped together with the Urticaceae within their own order, the Urticales. Urticales in turn has sometimes been associated with the witch-hazels in the hamamelid group (now disbanded), and sometimes placed in the Malvales order, of which more later. Cecropia has been shuffled uncertainly between the Urticaceae and the Moraceae. For the time being things are as described here, with hemp and hops grouped with Cecropia in their own family, Cecropiaceae, among the Rosales. Let us hope it stays that way. Cannabis and nettles both produce fine fibres, good for ropes, though nettle ropes are not common these days.
Oaks, Beeches, Birches, Hazelnuts and Walnuts: ORDER FAGALES
Within the eight families of the Fagales order are some of the most beautiful, most iconic, most treasured, and most ecologically and economically significant of temperate broadleaved trees: the oaks, beeches, chestnuts, southern beeches, birches, alders, hornbeams, hazels, she-oaks, bayberries, walnuts, pecans, and hickories. Estimates of the total species vary wildly: Judd suggests 1,115, but there could be many more, not least because many of the Chinese kinds, where the order abounds, are as yet largely unstudied. All Fagales are trees or shrubs: there isn’t a herb in the entire order. The oldest fossils (of pollen and other parts) date from around 100 million years ago, when the dinosaurs were still in full pomp. Where they arose is a bit of a mystery. The oldest family of all seems to be that of the southern beeches (Nothofagaceae) – but they are the only Fagales family that lives in the southern hemisphere. All the rest are based in the northern hemisphere, with just a few venturing south of the equator here and there. But then, the evolutionary history of all groups is full of loose ends.
The Fagaceae family includes the best known trees of the Fagales order, and the most important economically and ecologically. The family probably first arose around 90 million years ago in tropical mountains – near the equator but nonetheless a relatively cool habitat – and it extends through the northern hemisphere from temperate lands to the tropics. All are trees or shrubs, rich in tannins. All have fruits in the form of a nut, with a spiny or scaly capsule round the outside – sometimes fully enclosing the nut, and sometimes holding it decorously like an egg in an egg cup.
Botanists tend to define and include around nine genera within the Fagaceae. The oaks, Quercus, are the biggest genus with 300 to 600 species, depending on who’s counting (Judd opts for 450). It seems, though, that many of the Asian oaks should be placed in the tanoak genus, Lithocarpus, which at present contains around 100 to 200 known species from North America, with only one officially recognized from Asia.4 Fagus is the genus of the beeches, with about ten species. Castanea, the chestnuts, also have about ten species. Castartopsis, which in some classifications embraces Chrysolepis, includes the 150 or so species of chinkapin or chinquapin, from North America, China, India and the Malay archipelago. There is just one species of Colombobalanus, from Columbia in South America; and two of Trigonobalanus, from China and Malaysia. About one in eight of all the species of Fagaceae –12 per cent – are included in the 2003 Red List of Threatened Plants.
North Europeans and Americans think of the genus Quercus as mighty trees of temperate lands that shed their leaves in winter. This is true of Britain’s two native oaks, the English or common oak, Q. robur, and the sessile oak, Q. petraea: and those two, plus ash and Scots pine, would now extend from the tundra of northern Scotland to the tip of mild and rainy Cornwall were it not for our forebears,
who stripped the post-Ice Age forest to make farmland and took the oaks in particular to build the medieval and Tudor cities, and the navy that defeated the Armada. (‘Heart of oak are our ships’ we used to sing at school in moments of patriotism. But later, the British navy made much use of teak, from India.)
The oaks are the most widely distributed of forest trees – and not particularly northern. Indeed, common and sessile oak are the only species that venture more than 50° north of the equator (and those two extend up to 6o° N). Evolutionary studies suggest that Quercus first appeared in South-East Asia, around 60 million years ago5 and most still live between 150 N and 300 N, especially in Mexico and Central America, and in the Yunnan province of China. In the south the genus peters out in Colombia (where oaks live in the highlands) and Indonesia. North America has the most species; Europe and Asia have many; and a few live in North Africa. Between them the different kinds are adapted to every habitat from desert to swamp and from sea level to the highlands – up to 4,000 metres in Yunnan: as high as the highest Rockies. In the wild, the most widespread of all the individual species are red oak (Q. rubra) and white oak (Q. alba) in North America; Q. acutissima and Q. mongolica in Asia; and common and sessile oak in Europe. Many are shrubs. Many – like live oak (Q. agrifolia) and Q. wislizenii of California; or Q. coccifera, holm oak (Q. ilex) and cork oak (Q. suber) of southern Europe – are evergreen.
