D. Andrew White 30/09/04
Laurasia's Ancient Circumpolar Forest
The hills are shadows, and they flow
From form to form, and nothing stands;
They melt like mist, the solid lands,
Like clouds they shape themselves and go.
Lord Tennyson - In Memoriam
Some facts of biogeography are a cause for wonder. North America and Eurasian have much native flora and fauna in common. In the arctic regions quite a few organisms are the same at the species level. In the central parts of Eurasia and North America, organisms tend to be similar at the taxonomic level of genus. While Africa, Australia and South America's flora and fauna resemble each other mostly above the generic level. Indeed, often even the taxonomic families are not the same.
One of the central assumptions of biogeography has been that the more recently regions have been connected, the more closely their flora and fauna resemble each other. North America and Eurasia have very strong taxonomic resemblances. Australia, Africa and South America have resemblances also. Fossil evidence shows that India also had typical 'southern' flora. While they are not as close as the northern ones, southern flora are quite distinct, and often very different from the north. This suggests that there was some ancient connection between the southern continents, and a less ancient connection in the north.
As early as 1885 naturalists, such as Eduard Seuss, hypothesised that the peculiar features of biogeography could be explained by 'land-bridges', ancient dry-land connections between now separate continents. In the 1970s Continental Drift theory gained favour. This theory explained almost exactly the same set of taxonomic facts. Indeed both theories referred to the once connected southern landmass by Seuss's term Gondwana Land. The old trans-Indian Ocean land-bridge called 'Lemuria', has been dropped as a term. Other old terms remain. The hypothesised northern continental connection is still in favour, but not as a 'bridge'. This continent, Laurasia, is now believed to have been the actual fusion of North America, Europe and Asia. Where once landmass bridges were hypothesised, now it is suspected that the continents themselves were in close contact.
The Continental Drift theory was presented in 1912 by Alfred Wegener. He proposed that the continents had once been joined together into one landmass called 'Pangaea'. In 1937 A.L. Du Toit argued that there were actually two original continents, Laurasia and Gondwana. Both theorists based their ideas on not only bio-geographic evidence, but also fossils, geologic data, continental outlines, and other facts. At the time the Drift hypothesis was received sceptically. There was no known mechanism that could move continents about. In the 1960s 'plate tectonic' motions were detected. Vast convective motions in the Earth's magma push the continents about. Nowadays it is generally agreed that the continents do indeed drift.
A few theoretical land-bridges have survived in the world of ideas. The main examples being the on-and-off-again connection between the British Isles and Europe proper, and several land-bridges in the East Indies. These land-bridge are all still shallow water continental shelves. They were connected sometimes by low sea level, at other times by tectonic activity. Beringia is one of the most famous land bridges. It is the probable route that human beings took to the Americas. Beringia is the land-bridge that seems to have once connected Alaska to Siberia. There are many lines of evidence suggesting that the relatively shallow Bering Strait has been bridged by dry-land, and then submerged again, off-and-on several times. It was a land-bridge long before there were humans to cross it. Indeed, the cats, bears, bison, saiga, mammoth, the moose and other deer, all seemed to have crossed over to the Americas long before humans did. Horses, wolves and camelids, seem to have went the other way.
Ash, maples and oak are well known genera that occur in both the Americas and Eurasia. Some obscure and rather uncommon temperate genera also occur in both the Old World and New World. They are genera without broad ranges, and with very few species in each taxon. The buckeyes, the buckthorns, the Castanopsis golden-chinkapins, the Catalpa trees, the Gymnocladus coffeetrees, the hackberries, the hickories, the hollies, the hop-hornbeams, the hornbeams, the mulberries, the Liquidambers, the Lirodendron tuliptrees, the ebony-persimmons, the planetrees, the Sassafras, the Lithocarpus tanoaks, the yew-like Torreyas, the walnuts, and the Cladrastis yellowwoods occur on both continents. Even the Osage orange is very similar to the Cudrania spp. of Asia. All of these genera are comprised of temperate species that cannot live in the cold tiaga nor in the tundra regions nowadays.
How did the shared temperate genera manage to become common to both continents, without being able to live on the cold sub-actic land-bridge between them? It turns out that Beringia was not always as cold as it is now. Fossil evidence indicates that the Canadian and Siberian arctic regions were once temperate in climate. On Axel Heiberg Island there are even dehydrated remains - sub-fossils - of temperate trees such as birch, tupelo, walnut, hickory and oak. Conifers, such as the Metasequoia and the pines, were common. Other fossil finds suggest that in the far northern parts of the region conifers were more numerous than in the south. Turtles and crocodilians lived on Axel Heiberg also, suggesting a warm climate. Some of these fossils are over 45 million years old. From then until about 2.5 million years ago there were virtually no continental glaciers in the region now called 'the Arctic'. Winters were very mild, and broadleaved trees could grow even in the Arctic Archipelago. Temperate forests stretched from Central Asia through to Beringia to North America. Although, tree-ring evidence indicates that there was a distinct winter season. The winters were mild compared to those of today's arctic regions. Some of the tree species in the north were winter deciduous, as they are today. In other words, it is possible that the trees simply passively dispersed in both directions across Beringia. Genetic tests tend to confirm this scenario. A cladistic analysis of the chloroplast DNA of the thirteen Gleditsia species of the Old and New World suggest the 'locust trees' diverged only four million years ago. In other words, before the last set of ice ages the locust trees were a single species. Many Old and New World plants, such as the planetrees, are so close that they can easily hybridise. The same is true for the white oaks and some of the junipers. At one time, clearly, there was a temperate forest stretching from what is now Europe, through Asia to North America. Many genera and species became common to both hemispheres. Later as Beringia cooled the now isolated species evolved into separate species.
In general the ice ages in that last few million years have not exterminated a great number of tree species. This suggests that, for the most part, ice ages came and went fairly slowly. Most species were able to adapt. Non-boreal gymnosperms, as a whole, seem to have been more sensitive to the climatic swings of the current ice epoch. American ginkgo species seem to have disappeared early in this ice epoch. The redwoods have slowly declined since the last batch of ice ages. The coast redwood (Sequoia sempervirens) and the giant sequoia (Sequoiadendron giganteum) have become confined to the Americas. The Metasequoia species grows wild, nowadays, only in China. All three 'redwoods' were once natives of both the Old and New Worlds. Likewise the Torreya conifers have shrank in natural ranges in both Asia and the Americas. These trees have all declined drastically when the Cenozoic Ice Epoch started. However, during this glacial epoch the conifers Pinus and Picea have expanded their range. Possibly these conifers were natives of the small boreal forest zone during the Mesozoic and early Cenozoic. The ice ages apparently have been good for them. Most angiosperms did not decline. However, the Osage orange has declined since the very last ice age. Perhaps, although, it was originally dependent on megafauna for its dispersal. With the decline of the megafauna, some species way have lost their primary dispersal agents.
It is probable that the glacial advances and retreats have stimulated increased speciation in plants. During the height of the ice ages the tundra, boreal and deciduous vegetation zones have moved south, and then north again. During the cold periods the vegetation zones were smaller and pressed closer to the sub-tropical zones. (The tropics and sub-tropics were very similar in size and range during both periods,) During some of these ice ages populations may have been marooned on either side of barriers such as mountain ranges. Probably these populations were isolated enough to evolve into different species. This may explain such east-west parallels as: jack pine and Virginia pine, eastern white oak and burloak , sugar and black maple, and scarlet and pin oak. After the ranges expanded north, and the barriers were breached, the related species were different enough not to fuse back into single species.
References
Basinger, J. F., 1989. Early Tertiary floristics and paleoclimate in the very high latitudes. American Journal of Botany. 76 (6) Supplement: 158.
Corfield, Richard. 2001. Architects of Eternity - the new science of fossils. Headline Book Publishing. London.
Croizat, Leon. 1952. Manual of Phytogeography. Uitgeverij, Dr. W. Junk. The Hague.
Flannery, Tim. 2001. The Eternal Frontier - an ecological history of North America and its peoples. Atlantic Monthly Press. New York.
Greenwood, David R., and Basinger, James F., 1994. The paleoecology of high-latitude Eocene swamp forests from Axel Heiberg Island, Canadian High Arctic. Review of Palaeobotany and Palynology. 81 (1): 83-97.
Hoyle, Fred. 1981. Ice - how the next ice age will come - and how we can prevent it. Hutchinson. London.
Merrill, Elmer D. 1981. Plant Life of the Pacific World. Charles E. Tuttle Co.: Publishers. Tokyo, Japan.
Purcell, Jessica., Brelsford, Alan and Kessler, Michael. 2004. The World's Highest Forest. American Scientist. 92(5): 454-461.
Quammen, D. 1996. The Song of the Dodo - island biogeography in an age of extinctions. Touchstone. New York.
Roberts, David C. 1996. A Field Guide to Geology - eastern North America. Houghton Mifflin Company. New York.
Winchetser, S. 2003. Krakatoa - the day the world exploded: August 27, 1888. Harper Collins Publishers. New York.
Metasequoia or Dawn Redwood
Meatsequoia, dawn redwood or shui shan (Metasequoia glyptostroboides) is a conifer in the baldcypress family (Taxodiaceae). It is a forest tree with a broad tapering crown. Its needles are arranged in a plane like those of the coast redwood or the baldcypresses. Twig are arranged oppositely in flat splays. Like baldcypress it is deciduous, dropping needles and small twigs in the autumn. Its cone is round and globular, like a redwood's. The bark is orange-reddish. Like both baldcypresses and the redwoods it is a fast growing tree. In less than fifty years it can reach 30 metres in height.
Metasequoia has been known to the people of the Hupeh region of China for centuries. Surprisingly, the genus was not catalogued by scientists until 1941, when a Japanese paleobotanist, Shigeru Miki, distinguished it from other redwood fossils. Before that time, the Metasequoia genus was only known from fossils! Then, in 1944-45, a Chinese forester, Gan Duo, noticed living specimens in the Hupeh region!
Metasequoia was much more common before the current set of ice ages. The genus was once native to both Asia and North America. Metasequoia during the Eocene lived as far north as Axel Heiberg Island. The ranges of both the redwoods and metasequoias have shrunk back drastically since that time. Now all redwoods are native to the Americas, and wild metasequoia occur only in Asia. Perhaps they could not adjust well to the climatic fluctuations during the last set of ice ages.
In the late 1940s specimens of this so-called living fossil dawn redwood were shipped to gardens all over the world. In many temperate regions of Europe and North America plantings have grown to over 30 metres in less than fifty years. In its growth rate it is much like the redwoods, which grow rapidly in the first few decades. Redwoods and baldcypresses can live for a millennium or more if they are not disturbed. No one knows how long the metasequoia can live. The specimens in China are not particularly ancient.
Metasequoia is fairly tolerant of cooler temperate climates such as occur near Toronto. It is a rather peculiar looking tree. Like baldcypress and tamarack it is bald in the winter, and looks like a 'dead pine'. Yet it is a curious conifer of medium hardiness. If one appreciates conifers it is a good arboretum choice.
The ISA Species Rating: 87%.
Redcedar & Junipers
Eastern redcedar, redstick cedar, pencil cedar, Virginia juniper or baton rouge (Juniperus virginiana) is a conifer in the Cupressaceae family. Redcedar can be up to 16 metres tall, with a very pointed dense crown. The foliage often has a dark reddish cast. The evergreen has short pointed needles tightly appressed to the three to four-sided twig. The leaves are prickly, unlike arbor-vitae or cypress leaves. Like most junipers it is usually dioecious, but monoecious (hermaphrodite) trees do occur. Juniper cones have fleshy cone scales, which make them superficially berry-like. When mature the 'berries' turn bluish or purplish and are eaten by birds, which can pass and disperse the seeds. The wood is soft, reddish and excellent for carving and woodcraft. The bark is fibrous and reddish. The redcedar is native to the Carolinian forest, and it is most common in the drier upland regions.
Eastern redcedar has two main 'races'. J. virginiana var. crebra has a pointed crown, and grows mostly in the north-east. The variety south-west of the Mississippi and Great Lakes has a broader sprawling crown. They are definitely the same species, as they hybridise where the ranges overlap.
Juniperus is a genus common to North America, Eurasia and Africa. Junipers generally grow in dry well-drained soils in the uplands. Often they grow where other trees cannot survive. There are at least 70 juniper species in the world. The small Utah juniper (J. osteosperma) graces the dry hilltops in the Great Basin and the Sierras. The larger Rocky Mountain juniper (J. scopulorum) grows mostly in the southern Rockies. One-seed juniper (J. monosperma) is a juniper of the semi-arid American south-west and Mexico. The strangest juniper is the most common species. The common juniper (J. communis) grows from the boreal forest, to the northern Rockies, across to north-temperate Eurasia, and in montane North Africa. Common Juniper is circumpolar in distrubution, with racial variations from flat-shrub to full tree size. This genévrier is the species from which gin gains its flavour. Without the 'berries' gin would be a bland whisky.
Redcedar is less pointed and tidy than the ornamental cypresses, but it does have an almost Mediterranean charm. It is a carrier of a rust fungi that has the apple tree as an alternate host. Apples and redcedar should not be grown in close proximity. Redcedar can survive fairly well in Toronto. It is not as suitable for hedges as is white cedar. This juniper is in its northern range in Ontario, hence boreal conifers tend to be more hardy, especially north of Toronto.
The ISA Species Rating: 55-68%.
Ginkgo
Ginkgo biloba, also called ginkgo or gingko, is a gymnosperm in the Ginkgoaceae family. It is a deciduous gymnosperm, which is not totally unusual, several other conifer species loose their leaves in the winter. However, the ginkgo leaf is broad and fan-shaped, with bifurcating veins that radiate out from the leaf petiole. Superficially, the leaf form looks like a maidenhair fern leaflet. Only a few 'conifers' have broad leaves nowadays, but in the Mesozoic era many gymnosperms had broad leaves.
Ginkgo biloba is the last species of the Ginkgoaceae to survive to modern times. Apparently, the number of ginkgo species declined over the millennia until only one remained. Seven million years ago ginkgoes of various species grew in the Americas and Eurasia. Today ginkgo apparently grows wild only in the Tian Mu Shan reserve in eastern China. In the 1800s ginkgo survived predominantly as a cultivated tree. Chinese and Japanese gardeners planted ginkgo as an urban tree and garden ornamental. Most of the wild ginkgo stands in China have long ago been cleared for farmland or logged over. If not for human cultivation the plant would have become extinct. The scarcity of wild ginkgo is remarkable, considering how very hardy the tree is. Today the ginkgo is planted all over the temperate world as an stress tolerant urban tree.
Ginkgo has a pyramidal crown, like many other gymnosperms. It can grow to at least 30 metres in height. In the autumn its leaves turn a beautiful bright yellow, before they fall. Ginkgo is dioecious, male and female sporophylls are on separate trees. The male strobilus is catkin-like and produce windborne pollen. The sperm cell in each pollen grain is flagellated, like the sperm of ferns. Female sporophylls contain only a few ova each. After pollination these grow into plum-like seeds. Usually only one or two seeds develop fully on each raceme. A fleshy husk covers each seed. The husk becomes fetid and has a rancid odour when the seed matures. Wild foxes, racoon-dogs and sometimes squirrels, eat the 'fruit', even though it is slightly toxic. These animals do not seem to be effective seed dispersers.
Ginkgo diaspores may originally have utilised a dispersal strategy similar to the walnuts. The rather poor tasting husk may discourage most casual fruit eaters, but does not deter animals that horde nuts, like squirrels. Such creatures leave a portion of the surplus nuts or diaspores that they bury. Indeed squirrels do plant ginkgo seeds, given the chance. This raises and interesting question: what horded the ginkgo diaspores during the Mesozoic? (That is, if the berries were similar in taste back then.) During the Mesozoic the closest analogues of the squirrels were the multituberculates. Did these rodent-like mammals horde seeds?
Nowadays ginkgo trees tend to reproduce by cloning. Coppice sprouts swell up from the lignotubers of old trees. Often aerial 'roots' extend downward from old trees. If these touch the ground they take root, and send up shoots. The Japanese named these adventitious sprouts chichi, or 'breasts'. Because under older trees the chichi swell up into pointed masses, and the chichi that hang down can thicken into masses like sagging breasts. If the parent tree dies, or a chichi touches the soil, the chichi sprout into new clones. The Tian Mu Shan ginkgo trees do not reproduce with great frequency. Seeds seldom form new trees, and chichi coppice out only when a clearing forms in the forest canopy. Possibly the slow reproductive rate of ginkgo disadvantaged them relative to angiosperms. One would think that there must be some ecological weakness that accounts for the decline of the wild ginkgo. Although, the exact explanation is a not known.
Ginkgo makes a very hardy urban tree. Ginkgo is surprisingly tolerant of air pollution and water stress. Adventitious sprouts form from stressed and nearly dead branches, and if crown die-back occurs, coppice shoots almost always sprout from the tree base. Ginkgo is very drought resistant. A ginkgo can loose all of its leaves in a dry spell, and re-grow a new set after a rain! Since the 'fruit' is smelly, slightly toxic and an allergen, the planting of female ginkgo trees is generally avoided by urban foresters. In the last few decades of the twentieth century urban forester have planted only the male trees as ornamentals. Although, in many cities the older planting still have many female trees among them. Because of its hardiness, the ginkgo is fast becoming a preferred ornamental. As long as a region is temperate, but not too dry, ginkgo trees will prosper. They are therefore planted all across most of temperate North America. Boulevards all over New York City are lined with male ginkgo trees. And significant number of ginkgo trees have been planted in Toronto, including those outside the Art Gallery of Ontario, and on the grounds of the University of Toronto.
The ISA Species Rating: 76-87%.
Ginkgo as a Herb
Ginkgo biloba is also known for its herbal qualities. Much has been written about this herb, both good and bad. To make a long story short, the ginkgo extract can be too strong if used improperly, it can also interfere with other medications. In the past the medicinal efficacy of ginkgo has been greatly exaggerated. See comments on - herbalism.
Yews
The yew or if trees (Taxus spp.) are evergreen conifers in the family Taxaceae. There are many species which vary from shrubs to full-sized trees. They have waxy flat needles (leaves), like a fir, but with pointed tips. The needles are actually arranged spirally, but they are twisted at the petioles such as to form flat splays. The dark green sides face upward, the paler lower sides face downward. The needles are retained for several years. The ‘cone’ is so modified from the conifer norm that it looks like a single seeded red berry. (Some cultivars have yellow ‘berries’.) It is not a normal ‘berry’ as is manifest by the seed which is exposed at the tip of the red diaspore. Like the stem, twigs and leaves, the yew seed is laced with a toxic alkaloid called taxine. Some of the other toxins in the yew’s tissues originate from endophytic fungi. Yews often have strands of fungal mycelium throughout their tissues. These fungi are symbiotic in the sense that they aid in the plant's defences. The flesh of the ‘berry’ itself is non-toxic, it is edible and tastes rather good. Birds relish these ‘berries’, and learn to avoid breaking the seed when they eat them. Yew diaspores are apparently adapted to dispersal by fruit-eating birds. Yews are shade tolerant trees, or shrubs, in the moist woodlands of Eurasia and North America. Their crowns are generally rounded, and thickly foliated. (Yews can be excellent nest sites for birds.) Some species of yew trees can grow slowly to in excess of 20 metres. Full-grown ornamental yews are still uncommon in Canada.
The old Gallic name for yew ‘eburo’ or 'yvuro' is still evident in European place names such as: Ivry in France, Ypres in Belgium, Yverdon in Switzerland, and even York in England. Yew was one of the sacred trees of the western Celtic tribes.
Originally the most common ornamental yew planted in eastern Canada was the English yew (Taxus buccata). Now the Japanese yew (T. cuspidata) is the favoured choice. Japanese yew is the hardier of the two species. Both yews are rather similar, but they each have a different set of commercial cultivars. The Canada yew (T. canadensis) of the eastern boreal forest is a ground-hugging shrub. It is not suitable as an ornamental. The Pacific yew (T. brevifolia) is tall along the coastal ranges, but shrubby in the interior Rockies. This west coast species is sometimes planted as an ornamental.
Yews are fairly popular as ornamentals. Often gardeners try to trim them into hedges. Yews do co-operate with attempted hedging, to some limited extent. They sometimes have the ability to re-sprout foliage after having been trimmed bare. However, since this recuperation is not guaranteed, it is best not to trim any conifer such that it is bald of greenery. Yews are very densely foliated, and they often become too dense for the gardener’s liking. It is highly recommended that one consider carefully the appearance of the mature tree before choosing to plant one.
The ISA Species Rating of the Japanese yew: 72%.
References
Mitchell, Allen. 1979. Trees of North America. Prospero Books. Toronto.
Peattie, Donald Culross. 1991. Trees of Eastern and Central North America. Houghton Mifflin Company. Boston.
Polunin, Oleg and Huxley, Anthony. 1987. Flowers of the Mediterranean. Chatto & Windus. London.
Walter, Henriette, 1994. L'Aventure des langues en Occident. Robert Laffont. Paris.