by Colin Tudge
There is only one species of Sequoiadendron left to us, too: the giant sequoia (S. giganteum), which 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. 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 broad-leaved 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 broad-leaved trees. Again, a once-various genus is now reduced to two species: T. distichum, of the southeastern 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, also reduced to a single species that grows along streams and in 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 the 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; only a few pines can take extreme aridity. Firs, larches, spruces, and pines are among the most extreme northerners among trees; they are found along the northernmost tree line in Eurasia and North America and climb mountains as high as any tree will go. Sometimes they 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, and Russia. Yet these boreal forests contain very few different species. As with most groups of creatures, the greatest diversity is toward the equator, where the growing season is longer and there is abundant seasonal rain. There are also mountains toward 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 centers of diversity (although we might with justice add a fifth). Two of these centers 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 centers—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 center, with species from ten of the eleven genera. The one lacking is Cedrus, the genus of the true cedars.
Of the North American centers, California has five genera: Abies, Picea, Tsuga, Pseudotsuga, and Pinus. Mexico, a separate center, 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, they are dependent on fire: they cannot release or germinate their seeds without it. Finally, the Atlantic plain of the southeast United States really forms yet another center, 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. Most genera of the Pinaceae date only from the early Tertiary—60 million years ago or less. But fossils of Pinus are known from well before the Tertiary. The earliest date from the Cretaceous, the last great age of the dinosaurs. 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 borne 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 multistemmed shrubs.
Pinus is also the most versatile of the Pinaceae. Its various species extend from the tundra tree line in Eurasia up to the alpine tree line 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 shrubland; and many are particularly well adapted to fire and grow on savannahs and in northern forests that are prone to fire. Indeed, Pinus dominates most where fires are frequent. In the lowland tropics of Central America and Southeast Asia, too, species of Pinus flourish largely because they recover from hurricanes more quickly than most. Most Pinus species are adapted to poor soils, their roots extended by mycorrhizae. In fact, the vast pine (and birch, spruce, and alder) forests of Latvia seem to be largely rooted in sand. The first Europeans to settle on Cape Cod in the wake of the Pilgrims cut down the pine forest to expose what they assumed would be rich soil beneath. They found only sand dunes, which are most unsuitable for wheat—and, since they didn’t have a fishhook 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 includes the firs—all forty-eight species of them: mostly denizens of uplands up to subalpine 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 on Vancouver Island approach 90 meters. Needles from the balsam fir (A. balsamea) of Canada and the states around the Great Lakes, yield the scent of “pine” soap, while resin from its trunk becomes Canada balsam, the finest cement for optical instruments, much favored 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 centimeters 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.” 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 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 to grow alongside other conifers in cool mountains 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 favored 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 southwest 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; they shed 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. decidua of Europe provides all-purpose timber: for pit props in mines, for ships (still used for trawlers in Scotland), and much favored on housing estates for gates, posts, and fences. The northernmost tree in eastern 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 single-species forests. Farther 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 steeplelike 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 “Pruce,” which derives from “Prussia,” which is where Christmas trees originally came from. After the First World War Britain planted vast plantations of Sitka spruce, P. sitchensis. It is a lovely tree that in its native northwest America may grow to 80 meters. But in the fashion of those postwar, 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 meters 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. canadensis 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 also 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 meters 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 broad-leaved trees in deciduous mountain forests.
RIMU, TOTARA, KAHIKATEA—THE MOST VARIOUS CONIFERS OF ALL: FAMILY PODOCARPACEAE
The Podocarpaceae family currently has eighteen genera, but these are underresearched and difficult to classify, not least because their cones are much reduced and hard to decipher (in most conifers it is the cones that are most informative). With more study (particularly of DNA), the Podocarpaceae might be split further. Already, however, they are the second-largest family of conifers (after Pinaceae), with 185 species.
All of the genera are found in the Southern Hemisphere, but only half of them venture into the north. In the Northern Hemisphere podocarps extend from the Andes into Central America and the Venezuelan highlands, but they are most widespread in Malaysia, Indonesia, Indochina, and subtropical China north into southern Japan. It is not clear that the Podocarpaceae family as a whole originated in Gondwana (though this is the best bet), but its biggest genus, Podocarpus, almost certainly did. Fossils of this family are known from the Jurassic onward (about 140 million years ago), all from the south.
In both form and ecology the podocarps are impressively various. Most are trees, scattered in moist tropical or subtropical forest. Some help to form the understory, others join the canopy, and some tower above the rest as “emergents.” Some grow in mossy forests in the highest tropical mountains. Some—especially in the far south—form low shrubs well above the tree line. Many grow in poor soils, including swampy peat, but others compete with angiosperms where nutrients are more abundant. Many, including Podocarpus, have broad evergreen leaves that are unlike those of most living conifers, and fleshy cones of various shapes and colors that again are much underresearched but are evidently dispersed by mammals and birds. Thus Podocarpus plays the angiosperms at their own game. The extraordinary island of New Caledonia, as usual, has the greatest oddities. There, Retrophyllum minus grows in running water—which is a most unusual way for a tree to behave. Parasitaxus usta, from New Caledonia, is the only parasitic conifer: it taps into the roots of another podocarp, Falcatifolium taxoides.
Podocarpus includes 107 living species, though more are likely to be found in remote tropical forests. On the other hand, says Dr. Farjon, “revision of the genus is long overdue,” which means that it could well be divided (and Podocarpus itself will be reduced). Podocarpus is the only genus of podocarps to occur on most of the southern continents and major islands, especially Borneo, with thirteen species, and New Guinea, with fifteen. Six are known from the Venezuelan highlands—but there is much exploration yet to be done.
Podocarps are key players in New Zealand’s ecology and were vital to the religion and economy of the Maoris, and later prized by the Europeans. The tallest of all New Zealand’s trees is the kahikatea, Dacrycarpus dacrydioides, at nearly 60 meters. In the early nineteenth century, soon after they decided that God had obviously made New Zealand for their personal use, the British coveted the fluted, flaky-barked trunks of kahikatea as masts for their swelling fleet: warships to cou
nter Napoleon and cutters for fast, reliable trade overseas. But the timber of kahikatea is weak. It was a great disappointment. Nowadays, relationships between the Maoris and the Europeans, though far from disastrous, continue to be edgy. Recently air-traffic control demanded that a lofty row of kahikateas near Rotorua airfield should be trimmed. The Maoris said no way. Rotorua is definitely Maori country. The last I heard, the airfield may have to be moved.
Though it disappointed the British navy, the kahikatea is a fine tree. As in many plants, the juvenile and mature leaves are different: the former superficially yew-like in two rows, the latter fleshy and roughly cypress-like. When the female cones mature the receptacles grow red and berry-like, each with a purple seed at its tip, so the two together are like Russian dolls.
The matai is better for timber—known to the timber trade as “black pine” and to botanists as Prumnopitys taxifolia. P. ferruginea is the miro, alias “brown pine.” The “mountain pine” and the “yellow pine” belong in yet another genus, Halocarpus, while the “silver pine” is Lagarostrobus (and the “yellow silver pine” is Lepidothamnus). But perhaps the greatest of these misnamed “pines” is the rimu, a.k.a. “red pine,” Dacrydium cupressinum. It is no longer cut, but existing planks and joists are rescued for furniture and household goods of all kinds, like bowls and salad servers.
To the Maoris, however, the greatest of all trees was the totara, Podocarpus totara (with the smaller upland version, P. cunninghamii). P. totara grows on both islands, up to 40 meters tall, with trunks 2 meters across. The Maoris revered it not simply for its general magnificence but for the red of its timber, the color of royalty. They also hollowed out the trunks, to make from a single piece of timber a canoe to be paddled by a hundred men.
THE CELERY PINES: FAMILY PHYLLOCLADACEAE
The single genus Phyllocladus, with just four species, is commonly included within the Podocarpaceae. These are the “celery pines”; they grow in Malaysia, Indonesia, New Zealand, and Tasmania, usually as large canopy trees, though stunted up toward the tree line in mountain cloud forests. They are called celery pines because their leaves are celery-like, though fleshy—not what you would associate with a conifer. Botanically speaking, however, these are not leaves at all. They are “phyllodes”: flattened green stems that do the job of leaves. The true leaves have gone missing. But such vegetative features are not usually considered of huge taxonomic significance. True evolutionary relationships are more reliably revealed by sexual characters, which with conifers means cones; and these, and the fruitlike bodies that develop from them after fertilization, are very like those of podocarps. But Phyllocladus was first placed in its own family in the 1960s; and although many taxonomists do not favor this separation (including Judd), Aljos Farjon feels that the phyllodes are such a striking feature that they justify separation. He also points out that the celery pines have a different number of chromosomes from the Podocarpaceae, and their mechanism for pollination is also distinct. The discussions will doubtless continue. More data are clearly needed, not least from DNA.