The ancient trees that laugh in the face of climate change and dinosaur-killing asteroids
Charles Quest-Ritson meets the 'living fossil' trees that survived mass extinctions.
You can see ginkgos, araucarias and wollemias in many English gardens, but what do they have in common, apart from difficult names? The answer is that they are all living fossils — truly ancient conifer species that ruled our planet long before flowering plants evolved. They existed before most of our insects, too — which is just as well, because they are pollinated only by the wind. Like all conifers, mature trees release vast quantities of pollen for gusts of wind to carry away in search of a grateful reception by other trees. What's more, ginkgos and araucarias are either male or female — a rare phenomenon in the tree kingdom. So some trees release the pollen and others say thank you for it. Wollemia, however, has the best of both sexes, because it produces both male and female cones.
The amazing thing about fossil trees is that they have survived so long. Most of their relatives in the group that botanists call the Gymnospermae were squeezed out by climate change and the rise of flowering trees, the so-called Angiosperms. But these fossil trees are tough; some ginkgos even survived the atom bomb that fell on Hiroshima. And now it is widely planted as a street tree, because it is disease-free and utterly imperturbable in the face of air pollution, quite apart from its pretty leaves and amazing autumn colour.
Triassic fossils prove that ginkgos date back about 250 million years. Nevertheless, they no longer exist in the wild, but owe their survival entirely to human intervention. Individual trees live for 1,000 years — some are reputed to be 4,000 years old — and they were widely planted in Taoist temples in Ancient China, as symbols of immortality, rather like yew trees in our own churchyards.
The leaves on a Ginkgo biloba tree transition from a bright green in spring and summer to a uniform yellow in the autumn.
Young Ginkgo biloba trees are highly resilient to pollution, but you cannot determine their sex until they begin producing fruit at around 20 years of age.
Why then have they died out in the wild? The usefulness of their wood is one explanation — humans destroy diversity. And there is another reason, which is that gingkos ignored the maxim 'diversify or die'. The usual aftermath of a catastrophic event such as sudden climate change or an asteroid impact is to speed up evolution. Ginkgos should have evolved and developed new forms and species that could cope with change, but they didn't. Or perhaps they did, because it may be that Ginkgo biloba is the last survivor precisely because it was able to evolve constantly.
However, the trees that were preserved in Chinese temples are all identical, with little or no variation among them. And since they were first taken into cultivation in Europe in the 18th century, a host of new variations have turned up. No one knows why. There are more than 320 different cultivars in our National Collection in Worcestershire. Curious ginkgophiles should contact Plant Heritage to know more.
Ginkgos hold one more mystery. Their fruits have a disgusting smell of putrefaction, which combines all the most revolting odours you can think of. And they are thought to be carcinogenic. Then why are the nuts a valued food ingredient in several East Asian countries? Because they taste delicious.
Araucarias are the monkey-puzzle trees and just as prehistoric as ginkgos, dating back at least 175 million years, but they have diversified over the millennia so that there are still 20 species in the wild. Fourteen are unique to just one island, New Caledonia, famous as one of the world's diversity hotspots. Some botanists think that the proliferation of monkey-puzzle species, all on one tropical island smaller in area than Wales, could only be the result of a burst of radiation about 5 million years ago, but they are at a loss to explain how this might have occurred.
Best known to us, of course, is the original monkey puzzle Araucaria araucana from Chile, first introduced to England in 1795 by a botanist called Archibald Menzies, who travelled round the world with the explorer ship HMS Discovery. When the Spanish governor of Chile, an Irishman with the glorious name of Ambrosio Bernardo O'Higgins y O'Higgins, invited the ship's officers to dine with him, they were served a dish of large nuts for dessert. Menzies slipped a handful into his pocket, germinated them on the voyage home and presented five saplings to Royal Botanic Gardens, Kew.
The seeds of a monkey puzzle are not notably large — about the size of an almond — but the cone that carries them has more than 200 seeds and drops heavily off the tree when it is ripe. The largest of all monkey puzzle cones belongs to the bunya pine, Araucaria bidwellii from Queensland in Australia. It has the size and shape of a rugby ball or a large pineapple, weighs up to 20lbs and may fall from a height of 70ft. Best avoided.
The monkey puzzle tree is the national tree of Chile (pictured here in the country's Conguillio National Park). Fossils from the Jurassic period show that plants very similar to the monkey puzzle tree have been growing for around 200 million years.
There was great excitement in the 1940s when Chinese botanists discovered a prehistoric tree that was known to palaeobotanists as Metasequoia glyptostroboides. Palaeobotanists are students of extinct plants that existed millions of years ago — they hope, by working with fossils, to develop a better understanding of evolution.
The popular name for metasequoias is dawn redwood because they resemble the redwoods of California, but pre-date their evolution. In 1947, American botanists financed an expedition to collect metasequoia seed, which was shared between Chinese, American and European botanic gardens, including Kew and Edinburgh. Metasequoia glyptostroboides has proved very amenable to cultivation and is now available from garden centres all over the world. The city of Pizhou in the Chinese province of Jiangsu has planted 5 million metasequoias as street trees, including the longest avenue in the world — nearly 30 miles, thickly lined with metasequoias on both sides.
The wollemia pine — Wollemia nobilis — is the latest fossil tree to have been discovered, as recently as 1994. It exists as just one wild population of about 80 trees in a ravine less than 100 miles north-west of Sydney, Australia. Actually, no wollemia fossils have ever been found, but they are thought to have evolved about the same time as the monkey puzzles.
Like all fossil trees, however, wollemias have proved easy to grow in a great number of climates and soils all over the world. It seems that fossil trees are great survivors so that, once discovered and cosseted in cultivation, they fare very well. It has no close relations — there is only one species of wollemia — so how did it survive so long, and in just one very small area? No-one knows. Forty million years ago, Australia was warm, humid and thickly forested. Now, by contrast, it is hot, dry and slowly turning into a desert — but wollemia has adapted to the change. And, over the last million years or so, Australia has on several occasions suffered extensive glaciation, just as we did during the European ice ages. But wollemia has survived and now you see it as an ornamental tree in a wide range of climates all over the world.
'Bees did not evolve until about 50 million years ago — their ancestors were predatory wasps who hunted other insects'
One of the big changes in the world's flora followed from the evolution of flowering plants. The earliest flowering trees to evolve — the ancestors of magnolias — date back nearly 100 million years and could not rely on wind to disperse their pollen. Some other creature, usually an insect, had to be drawn to their flowers and transfer the pollen to another so that seeds could be set. Bees did not evolve until about 50 million years ago — their ancestors were predatory wasps who hunted other insects, whereas bees feed on nectar and pollen and have pollen baskets on their legs to carry supplies back to their young.
How then were magnolias and other flowering plants pollinated before bees evolved? The answer is that beetles are attracted to the protein-rich pollen and spread it from tree to tree. They still do so today. Bees do not bother with magnolias, but the flowers continue to attract beetles. Magnolias have large, thick petals (known as tepals) that are strong enough to stand up to large insects. And sometimes they close their flowers to trap the beetles inside for a fruitful overnight experience of pollen-shedding.
Many species of water lilies — another prehistoric plant — also seek beetles to pollinate them, drawing them deep into sweet, sticky nectar to relieve them of the pollen they carry. They, too, close their flowers at night and, once fertilised, plunge their flowers beneath the surface of the water, whether or not the insects have been able to escape. Other plants are pollinated by bats, and even lizards. And the common harvest mouse, searching for pollen and nectar, will climb into tulip flowers, gorge itself and fall asleep inside after feeding.
These ancient trees — some of the oldest plants in the world — survived so much climate change, as well as catastrophes such as the mass extinction that killed off the dinosaurs (as well as three quarters of all living species) 69 million years ago, because they produce a very large number of seeds. Plus, mass extinctions are surprisingly good for evolution. And, despite everything you read about mankind's destruction of the natural world, it is quite possible that further fossil trees remain to be discovered somewhere in almost every corner of the world.
