The Tree (26 page)

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-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—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.

T
ERMINALIA
, M
YRTLES
,
AND
E
UCALYPTS
: O
RDER
M
YRTALES

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 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. 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.

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 vaporized—they are in many ways fire-adapted. Like the banksias (of the Proteaceae), they typically hold their seeds in little wooden capsules, and the seeds are able to germinate only after they’ve been cooked. After fires the seeds are shed in vast numbers, onto 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 that spring to life when the bark is burned off, so the charred eucalypt reemerges as a coppice. Many, too, have “lignotubers”—tuber-like swellings of the lower trunk that 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 help 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 rain forest. There are vast areas that don’t suit them—including the tropical and subtropical rain forests 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 13 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 meter 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 meters tall; and some are huge forest trees—notably the mountain ash,
E. regnans
(“ruling eucalypt”), which can grow to nearly 100 meters. 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 nonbotanist 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 off line, another group coming into play. Sometimes when a eucalypt regenerates from buds and lignotubers, the new stems again produce sessile juvenile leaves.