by Colin Tudge
Three more species of Thuja live in north-east China, Korea, Japan and Taiwan. This is a common pattern of distribution: many trees of many kinds – including oaks – are distributed both in North America and East Asia, notably China. They seem for some reason to have found it easy to straddle the Pacific. In any case, trees do not respect political boundaries and indeed reveal how arbitrary the lines that we draw on maps really are. Thuja in general like it cool and moist, and grow from the coast to the hills. Though most are tall there is one, T. koraiensis of north-east China and Korea, that grows on exposed mountain ridges as a twisted shrub; showing again that any one group of organisms may essay a great variety of body forms. T. sutchuensis is one of those conifers that was presumed to be extinct but then turned up. To be sure, it didn’t go missing for quite as long as Metasequoia or Wollemia, but it was thought to be long deceased until the late twentieth century when some were found alive and well in the Daban Shan mountains of northern China.
The genus Thujopsis has only one species, from Japan, which can look curiously like a plastic imitation of a cypress. It again favours cool moist places from the coast to the mountains, and is another conifer that grows slowly at first in shade but eventually overtops its neighbours. Thujopsis comes in various cultivated varieties and is much favoured in gardens.
The four species of Widdringtonia are among the few conifers that put in an appearance in sub-Saharan Africa: in South Africa, Malawi, Mozambique and Zimbabwe. Widdringtonia are fire-adapted: they do well when the surrounding shrubs are cleared by fire, as happens regularly in the African summer. The genus has been sadly depleted by felling, however. The forests of ‘Mulanje cedar’ on the steep slopes of Malawi’s Mount Mulanje are the only substantial stands of Widdringtonia left to us. They are on my wish list of trees to be seen in the wild.
Two other genera of the old-style Cupressaceae are worth particular mention. Both have only one species each, both of which can grow into very large trees. They are Platycladus of east and north-east China, Korea and the Russian Far East; and Taiwania, which of course occurs in Taiwan and also in China (Yunnan), Myanmar (Burma), and Vietnam – where it was discovered only in this century. It grows up to 70 metres. The businesspeople and diplomats who flock to Taiwan probably feel they have little time for trees. A pity. Perhaps they should make time.
Of the eight genera that once formed the Taxodiaceae family, five have only one species each: Cryptomeria, Glyptostrobus, Meta-sequoia, Sequoia and Sequoiadendron. Of the other three genera, Athrotaxis has three species, while Cunninghamia and Taxodium have two apiece. This gives a grand total of twelve species. Here is a bunch of relicts indeed – magnificent, most of them – but not much left from a group that once bestrode the northern hemisphere.
Cryptomeria japonica, the ‘Japanese cedar’, known in the trade as ‘sugi’, accounts for much of the remaining forest in Japan. Yet those forests are probably not natural: rather, they represent the remains of some of the oldest forestry plantations in the world. The timber turns dark green when buried in the ground to give jindai-sugi, which serves as a semi-precious ‘stone’.
Three genera of redwoods are left to us: the coastal redwood (Sequoia); the giant sequoia (Sequoiadendron); and the dawn redwood (Metasequoia). Truly the remaining redwoods are ‘relicts’, for 100 million years ago, when the climate was much milder and flowering plants were first coming into their own, there were a dozen more species of redwood throughout western North America, Europe and Asia. There was even one in Australia. Various species of Metasequoia were widespread in the Tertiary. In the Eocene, around 45 million years ago, when all the world was wonderfully warm, they grew far up in what is now Arctic Canada, only 10 to 15 degrees from the Pole. But then the world began to cool in what has been called an ‘icebox effect’ – prompted by a steady diminution of atmospheric carbon dioxide – and by the time of the Pleistocene (around 2 million years ago), the genus almost went extinct. Now, only the dawn redwood is left to us: Metasequoia glyptostroboides. Even this was presumed to be extinct, until a few turned up in central China in the 1940s. It’s hard to say exactly how the last dawn redwoods in the wild have been scratching a living since the time they went missing, because the area around their present habitat has been so cultivated this past few hundred years; but in general they seem to prefer the same kind of niche (wet) as the swamp cypress, Taxodium.
The genus Sequoia is also reduced to a single species: S. sempervirens – the coastal redwood of western, lowland California and Oregon. Yet the genus was once present in three continents (of which one was Australia). S. sempervirens needs the coast. It gets about a third of its water from the fogs that rise almost daily from the cold currents of the north Pacific and condense against its dark green leathery feathery leaves. Some coastal redwoods are managed for timber on a 100-year cycle: their rich reddish brown wood provides everything from telegraph poles to coffins and organ pipes while its fireproof bark, up to 20 centimetres thick, supplies fibre for fibreboard.
There is only one species of Sequoiadendron left to us, too: the giant sequoia (S. giganteum) that survives in groves on the western slopes of the Sierra Nevada in California, sometimes in pure stands and sometimes with other conifers. Like Sequoia, Sequoiadendron once included several species, widespread over North America. Like Sequoia too, it is adapted to fire, and in general regenerates only after fires, so that there are successive waves of new recruits, each following a fire. It grows quickly when young, soon overtops its rivals, and may live for thousands of years – not as tall as the coastal redwood, but sturdier. On the face of things Sequoiadendron seems a fine survivor. Yet it is down to one species, in only one small restricted area. Nature is unpredictable.
The old-style Taxodiaceae family takes its name from Taxodium, the genus of the swamp cypresses. They generally live as their name suggests in swamps and on the waterlogged fringes of lakes; and their roots have vertical outgrowths that project above the ground or the surface of the water which apparently act like the ventilator pipes on the decks of ocean liners, bringing air with its much-needed oxygen down to the roots. The broadleaved trees of the mangrove swamps have a comparable arrangement. In Mexico, however, swamp cypresses sometimes grow well above the water-table, sometimes in the company of broadleaved trees. Again, a once-various genus is now reduced to two species: T. disticbum of the south-eastern United States, which is deciduous; and T. mucronatum, of Guatemala, Mexico and southern Texas, which is reported to be evergreen.
Finally, closely related to Taxodium, is its Asian equivalent Glyptostrobus: this time again reduced to a single species along streams and other damp places, including river deltas, in southern China and Indochina. Again we see the North American–Chinese connection.
Pines, Firs, Spruces, True Cedars, Larches and Hemlocks: FAMILY PINACEAE
Pinaceae is not the biggest family when measured in genera – a mere eleven – but it does have the most known species, with 225. This may simply be because of all the conifers, the Pinaceae are the best described. This is partly because the three biggest genera include the most economically valuable trees of all – the pines Pinus, the firs (Abies) and the spruces (Picea) – and partly because all but one species of Pinaceae are native to the northern hemisphere, where most scientists do their work. The only one of the family that has strayed south of the equator is Pinus merkusii, from northern Sumatra (note the similarity to junipers, which also have just one southern species). Even the fossils of the Pinaceae are exclusively northern. It is unclear, as we have seen, where most of the conifer families originated, but the Pinaceae family seems emphatically Laurasian.
The Pinaceae as a whole live in many habitats, but when it’s very dry they tend to be replaced by Cupressaceae – all except some pines, which can take extreme aridity. Firs, larches, spruces and pines are among the most extreme northerners among trees – found along the northernmost treeline in Eurasia and North America, and climbing mountains as high as any tree will go. Sometimes th
ey grow alongside broadleaves – especially pines, which are pioneers, and firs and hemlocks (Tsuga), which tend to creep in later.
The biggest natural forests of Pinaceae are in the extreme north – North America, Scandinavia, Russia. Yet these boreal forests contain very few different species. As with most groups of creatures, the greatest diversity is towards the equator, where the growing season is longer and there is abundant seasonal rain. There are also mountains towards the equator, which create boundaries between populations which, when thus isolated, evolve into new species.
In practice the various Pinaceae are focused mainly in four great centres of diversity (although we might with justice add a fifth). Two of these centres are in Asia, which has the greatest variety of genera; and two (or three) are in North America, which has fewer genera but has the greater number of species, though mainly from the genus Pinus.
The greatest of the four centres – with species from all eleven genera – runs from China through to the Himalayas (Sichuan and Yunnan to Nepal). Most various among them are the firs and the spruces. Several genera are endemic to China: Cathaya, Nothotsuga and Pseudolarix. Japan and Taiwan form another, separate centre, with species from ten of the eleven genera. The one lacking is Cedrus, the genus of the true cedars.
Of the North American centres, California has five genera: Abies, Picea, Tsuga, Pseudotsuga and Pinus. Mexico, a separate centre, has the lion’s share of Pinus, with forty-three species; and also has Abies, Picea and Pseudotsuga. Perhaps Mexico has so many species of Pinus because it has so many natural fires, and pines are good fire-resisters – indeed are fire dependent: they cannot release or germinate their seeds without it. Finally, the Atlantic plain of the south-east United States really forms yet another centre, with an array of pines that is largely different from those of California and Mexico.
With 109 known species, Pinus is the largest genus of all the living conifers (with Podocarpus running a close second). It also seems to be the oldest known genus of Pinaceae, known from the Cretaceous of Europe and North America: all the other genera first appear early in the Tertiary, about 60 million years ago. Many Pinus species were more widespread in former times than they are today, yet they have not retreated as the sequoias or the araucarias have done. Features that distinguish Pinus include the papery sheath that surrounds the base of their needle leaves – leaves that sometimes are born singly, but more often in clusters of up to five. Yet their overall form is highly various. Most grow tall with a central trunk; some are more spreading like cedars; and some are multi-stemmed shrubs.
Pinus is also the most versatile of the Pinaceae. Its various species extend from the tundra treeline in Eurasia, up to the alpine treeline in Europe and the western United States, through the salt-spray Pacific coast of North America, and down to the tropical coastal savannahs of North America. Some form big, open forests with just one species; others grow on mountains with other conifers; many populate desert shrub land; and many are particularly fire-adapted and grow in savannahs and northern forests that are prone to fire. Indeed, Pinus dominates most where fires are frequent. In the lowland tropics of Central America and South-East Asia, too, species of Pinus flourish largely because they recover from hurricanes more quickly than most. Most Pinus are adapted to poor soils, their roots extended by mycorrhizae. Indeed, the vast pine (and birch, spruce and alder) forests of Latvia seem largely to be rooted in sand. The first Europeans to settle on Cape Cod in the wake of the Pilgrim Fathers cut down the pine forest to expose what they assumed would be rich soil beneath and found only sand dunes that are most unsuitable for wheat – and, since they didn’t have a fish-hook between them to pull the teeming cod from the sea, they nearly starved. In Spain pines grow half buried by the dunes.
Economically, Pinus is the most important of all the genera of trees. There are vast plantations worldwide of several species, in the southern hemisphere as well as the northern, like those of Caribbean pine (Pinus caribaea) near Brasilia and Monterey pine (Pinus radiata) just about everywhere.
The genus Abies are the firs – all forty-eight species of them: mostly denizens of uplands up to sub-alpine altitudes, from temperate to extreme northern cold. They grow in North Africa, throughout Europe and Asia south to North Vietnam, and in North and Central America (Honduras): sometimes in pure fir forests, sometimes with other conifers or broadleaves. Unlike most of the Pinaceae, they prefer rich soils. Most firs grow as spires like Christmas trees and can be very tall: the tallest are the grand firs (A. grandis) which in Vancouver Island approach 90 metres. Needles from the balsam fir (A. balsamea) of Canada and the lake states of the US, yield the scent of ‘pine’ soap, while resin from its trunk becomes ‘Canada balsam’, the finest cement for optical instruments, much favoured in microscopy for sticking cover slips to slides. The female cones of firs stand tall and upright along the upper branches: those of the noble fir (A. procera) of Washington and Oregon are magnificent, up to 25 centimetres long.
Cathaya is worth a passing note – it consists of a single species (C. argyrophylla) indigenous to central China, scattered on limestone or up on mountain slopes among the deciduous broadleaves. It was not described until 1958. The Chinese guard it jealously. No part may be collected, and it may not be cultivated from seed outside China. But, says Aljos Farjon, ‘it is neither particularly rare nor threatened, nor does it appear to be of high economic value, either in forestry or horticulture’. But perhaps, thus guarded, it is one for the connoisseur to seek out; like some rare icon in some remote Greek monastery.
The four species of Cedrus might be called the true cedars (although of course since common names are largely arbitrary it’s a moot point what is ‘true’ and what isn’t). They are scattered along the Mediterranean from the Atlas Mountains of North Africa to Cyprus, Lebanon, Syria and Turkey, with one species – somewhat different from the rest – in the western Himalayas. They love cool mountains, alongside other conifers, where there is plenty of snow in winter. As befits a conifer of low latitudes, they hold their branches and their leaves horizontally, layered like a cake stand. C. atlantica is the Atlantic cedar, with the lovely blue variety C. atlantica var. glauca, much favoured in gardens. C. indica alias C. deodara is the deodar from the western Himalayas. C. libani is the cedar of Lebanon, which indeed is from Lebanon and also from south-west Turkey. It provided the timber for Solomon’s temple at Jerusalem. The timber is much too scarce now for everyday use; when large logs become available they are cut radially to provide veneers with a sinuous grain.
Larches (Larix), eleven species of them, are among that minority of conifers that are deciduous: shedding their leaves in winter to stand characteristically skeletal with a straight central trunk and thin simple branches held more or less horizontally. They grow widely through the boreal forest of Eurasia and North America, while in lower latitudes, as in the Himalayas, they tend to prefer mountainsides. L. deciduas of Europe provides all-purpose timber: for pit-props, ships (still used for trawlers in Scotland), and much favoured on housing estates for gates, posts and fences. The northernmost tree in east Siberia – at latitude 73 degrees – is L. gmelinii. The most northerly are extremely stunted.
But in North America the northernmost conifer is a spruce, Picea glauca. In all, there are thirty-four species of Picea. In the extreme north, in the boreal zone of North America and Eurasia, they often form vast sifigle-species forests. Further south they prefer mountains and often grow in mixed forests with other conifers. Western China and the eastern Himalayas again have the greatest diversity. Spruces are beautiful trees – tall and steeple-like like firs; Europe’s tallest native tree is a spruce – Norway spruce (P. abies) from Scandinavia, Eastern Europe and the Alps. The virgin cones of some species, still to be pollinated, are beautiful reds and yellows. Spruces are useful too. They provide light timber (known as ‘deal’) and are the archetypal Christmas tree: the name ‘spruce’ is related to Truce’, which derives from ‘Prussia’, which is where Christmas trees originally cam
e from. After the First World War Britain planted vast plantations of sitka spruce, P. sitchensis. It is a lovely tree that in its native north-west America may grow to 80 metres. But in the fashion of those post-war, no-nonsense days it was planted in military lines and largely at the expense of native species and familiar landscape, and got itself a bad name. Now, at least sometimes, commercial planting is more sensitive.
Tsuga is the genus of the hemlocks: nine species, native to North America (two in the east and one in the west) and Asia – spreading up to 3,000 metres in the Himalayas and through China to Japan and Taiwan. Again, trees reflect the historical link between North America and Northern Asia – the Pacific athletically bridged. Several of the hemlocks are most people’s idea of what a conifer should look like: tall, dark and needle-leaved like spruces, although some like T. cartadensis of the eastern United States are smaller, slow growing, and often cut into hedges. Hemlocks are also among the minority of conifers that tolerate shade, and in Asia in particular grow in forests alongside broadleaves, when again they tend to have more rounded crowns.
The four living species of Pseudotsuga again show the historical link of North America to Eastern Asia: there are two in America (one endemic to California) and two more through China, Japan and Taiwan. The Douglas fir, P. menziesii, grows along California’s Pacific coast right up to British Columbia, and also in the Rockies from Canada to Mexico, and was first brought to Europe by the great Scottish naturalist-explorer David Douglas. It is the biggest of the Pinaceae – indeed among the biggest trees of all: there is a flagpole in Kew Gardens more than 60 metres tall, sawn straight as a die from a single trunk. Douglas fir timber is among the strongest and most rigid of softwoods, and is used for everything from heavy construction to carvings; it also supplies more veneer and plywood than any other species. The Asian Pseudotsuga are in general more modest, mostly growing among broadleaved trees in deciduous mountain forests.
