Ginkgo in Australia

42 Australian Age of Dinosaurs in Australia Story by Dr Stephen McLoughlin controversial since these trees have been widely redistributed by humans over thousands of years. Some stands of the tree such as at West Tianmu Mountain, Zhejiang Province that were once thought to be ‘natural’ have since been found to have very low genetic variability, suggesting that they were all planted from cuttings by Buddhist monks hundreds of years ago. Nevertheless, both DNA variability analysis and historical records do favour a source for the species in the forests of Zhejiang province in southwest China. The trees appear to have become more widespread since the early 11th century, as they are extensively depicted in Chinese art and poetry from that time onwards. Because of the longevity of individual trees (many hundreds of years), its unusual fan-shaped leaves that turn a picturesque golden yellow in the A cultural icon autumn, and because of its traditional medicinal applications and the use of The single living species, Ginkgo its seeds as a food item, Ginkgo biloba biloba, had its last natural refuge in has become a cultural icon in China. southwest China, but it has now been Many modern boulevards and parks widely planted around the world as in Chinese cities are lined by rows of an ornamental tree. Its precise origginkgos; their use favoured due to the inal distribution remains somewhat tree’s resistance to pollution and insect attack. Ginkgo trees are also commonly found in prominent positions within the grounds of ancient temples throughout China. Indeed, the plant was often held in such reverence that some Buddhist and Confucian temples appear to have been specifically built around particularly grand trees. By the late 17th century, Ginkgo The seeds of Ginkgo biloba (above) are used in an trees had been extensive range of Chinese and Japanese dishes transported to and are often served on special occasions such as Japan where they weddings or new year feasts. The Chinese Maidenhair Tree (Ginkgo biloba) is a large tree averaging 20m to 35m in height that is dioecious (having separate male and female plants) with attractive fan-shaped leaves. It is often described as a ‘living fossil’ – meaning that it appears to be the same as a species otherwise known only from fossils, and it has no close living relatives. In fact, there are many Ginkgo species in the fossil record and their leaf shape has not remained constant through time. Nevertheless, Ginkgo biloba is held in special esteem by botanists, since it is the last survivor of a once important group of plants known as the Ginkgophyta, with a world-wide distribution and origins extending back to at least the Permian period over 250 million years ago. are held in equally high regard. The first European record of the tree dates to a report from 1690 by the German botanist Engelbert Kaemfer who encountered the plant in Japanese temple gardens. Ginkgo seeds were subsequently brought to Europe with the burgeoning of medicinal and botanical interest during the late renaissance. The plant was given its formal scientific name in 1771 by the Swedish taxonomist Carl Linnaeus. The oldest surviving Ginkgo trees in Europe can be found in Geetbets (Belgium), Utrecht (The Netherlands), and in Kew Gardens (UK). These individual trees date back to the early to mid-1700s. Today, Ginkgo biloba can be found in almost every significant botanical garden around the world. A living fossil as a food and a medicine Much is claimed about the medicinal value of Ginkgo biloba extracts, but their true effectiveness is less certain. Traditional Chinese medicinal uses for Ginkgo extracts include applications to combat bronchitis, eye degeneration, tuberculosis, tinnitus, vertigo, gonorrhoea, asthma, stomach pain, high blood pressure, angina, skin diseases, depression, anxiety, dysentery and general headaches – in essence, a cureall! Buoyed by a burgeoning natural medicines industry, Ginkgo biloba supplements have become among the best selling herbal medications in the western world. Ginkgo leaves indeed carry two groups of chemicals – terpenoids and flavenoids – that are considered to have antioxidant properties (scavenging free radicals that damage cell membranes). Some laboratory studies have also reported that Ginkgo biloba extract improves blood circulation by dilating blood vessels and lowering the stickiness of blood platelets. On this basis, Ginkgo extracts have particularly been prescribed as a supplement to combat Alzheimer’s disease. Issue 8 43 1cm The Early Triassic was a time of aridity – a condition not favourable to ginkgos. However, the return of wetter conditions in the late Middle Triassic saw the development of coal-forming swamp forests with ginkgos, such as Ginkgoites semirotunda (above) from near Dubbo, New South Wales, becoming much more common components of the vegetation. 1cm 44 Australian Age of Dinosaurs Ginkgo regnellii from the Middle Jurassic of Sweden with original organic tissue preserved (far left). The preparation of this tissue with acids reveals the fine cell structure and gasexchange pores (stomata) on the leaf surface (left) – the white arrow indicating the position of veins (long cells) and red arrows indicating stomata. Each cell is about 10–20 microns long. However, the most recent and most comprehensive (3,000 person) longterm study of the effects of Ginkgo biloba extract on Alzheimer’s disease (Snitz et al 2009) found that patients given Ginkgo supplements fared no better than those given placebo treatments. Ginkgo extracts have also found their way into soft drinks, smoothies and nutrition bars as a ‘brain tonic’, but the minute quantities of active Ginkgo-derived chemicals in these products would seem unlikely to be effective. The fleshy Ginkgo seeds or ‘nuts’ have found their way into an extensive range of Chinese and Japanese dishes and they are often served on special occasions such as weddings or new year feasts. The seeds are typically steamed until the shell cracks and the kernel can be removed and eaten (like pistachio nuts) or added to stirfries, soups, porridgeske and pot-steamed egg dishes. Grilled nuts have also become a common accompaniment to the drinking of saké in Japan, although caution is necessary with children as consumption of the raw pulp of the seed in large quantities (over five seeds per day) or over a long period can cause poisoning by MPN (4-methoxypyridoxine). Early Aussie ginkgos million years ago, Ginkgo-like leaves became more common (though never dominant) components of the vegetation. Several species of Ginkgo (or related genera, such as Ginkgophytopsis, Baiera and Sphenobaiera) have been described from deposits of this age in the Nymboida Basin and equivalent strata of northern New South Wales and the Esk Trough and Ipswich Basin of south-east Queensland. Some of these leaves reached considerable size (over 15cm long) suggesting lush, well watered conditions. For those who live in Queensland or South Australia it must be remembered that much of the electricity generated in these States derives from the burning of Triassic coal from mines at Callide, Tarong, Leigh Creek, and formerly at Ipswich. Ginkgo leaves were minor but consistent constituents of these Triassic coals, occurring together with the much more common remains of seed-ferns, such as Dicroidium. So for those of you who are reading this magazine by a desk lamp in Queensland or South Australia, it is humbling to remember that some of the energy shining a light on your page was derived from the wood and leaves of long-lost Ginkgo trees that lived in an Aussie swamp forest some 220 million years ago. Fossil Ginkgo-like leaves first appear in rocks of Permian age and are present in scattered deposits throughout the world. Ginkgo has similarities in some aspects of its wood and reproductive structures to conifers and the extinct ‘seed-ferns’, but it also shares some characters with cycads, including possession of motile sperm cells. This combination of features suggests that ginkgos diverged relatively early in the evolution of the major seed-plant groups. The oldest fossil potentially attributable to the Ginkgophyta in Australia is a leaf recorded from the Late Permian (ca 255 million years ago) coal measures of New South Wales by Mary White in 1986. Although there are no diagnostic reproductive structures associated with it, the leaf has a long petiole (stalk) and fan-shaped blade with regularly forking veins, typical of ginkgos. Few and scattered examples of ginkgophytes from Lower to midTriassic rocks have been reported from New South Wales, but this appears to have been a time of significant aridity – climatic conditions not favoured by ginkgos. With the return of wetter conditions and the development of coal-forming swamp forests in parts of eastern Australia in the late Middle and Late Triassic around 230 to 200 Autumnal fossil leaf accumulation of Ginkgo cordilobata (above), a Jurassic Ginkgo from Ishpushta, Afghanistan. Although ginkgos are unknown from the Jurassic and earliest Cretaceous in Australia, the group was thriving in other parts of the world during this time. Photo Yvonne Arremo The Jurassic vacuum Entering the Jurassic, deposits of plant fossils continue to be common across Australia but convincing examples of Ginkgo are nowhere to be found. The disappearance of ginkgos in the Jurassic remains something of a mystery in Australian palaeobotany as the group was generally thriving in other parts of the world at this time. Australia remained in middle to high latitudes during the Jurassic (200145 million years) as it had been in the Triassic, but this was also a time of strong ‘greenhouse’ conditions where average global temperatures were significantly higher than those of today, allowing plants to survive even at polar latitudes. It may be that ginkgos, being deciduous plants favouring cool-temperate conditions, simply found the climate too warm in Australia during the Jurassic. Fossil Ginkgo leaves potentially have an important role to play in unravelling Earth’s past climatic swings. Not only does the presence or absence of Ginkgo suggest cooler/moister versus warmer/drier conditions, but the distribution of tiny stomata (pores for the exchange of gases) on the surface of the leaves may be even more informative. Experimental studies in glasshouses have shown that the density of pores on a leaf can be influenced by Issue 8 45 then the plant needs fewer pores per unit area on its leaves to extract carbon from the air to build its tissues. Since CO2 is a known greenhouse gas, and since the leaf shape and ecological tolerances of Ginkgo species have remained much the same over the past 250 million years, variations in the density of pores on the surface of Ginkgo leaves through time can potentially provide us with a palaeothermometer for Earth spanning the past quarter of a billion years. dimentary features significant cooling. lude evidence of icensported boulders in marine deposits, glendonites (mineral growths typical of cold-water condiions), and palaeosoil rizons distorted by frost. Hence the ginkgophytes seems facilitated by cooling of the ginkgos The dearth of Australian Ginkgo fossils persisted into the earliest stages of the Cretaceous. None are found in the well studied earliest Cretaceous floras of Victoria or Western Australia. However, by the Aptian (ca 120 million years ago), a new and distinctive species; Ginkgoites australis makes a prominent appearance in the Victorian fossil record. The Aptian and parts of the succeeding Albian stage are of the climate. Ginkgos persisted in Australia through the middle part of the Cretaceous and would no doubt have provided a tasty snack for any large herbivorous dinosaur of the time. They are represented at this time by Ginkgo wintonensis in the well known Cenomanian (ca 98 million year old) Winton Formation flora of western Queensland. Moreover, the group persisted at least until the Turonian (ca 92 million years ago) in Victoria where Ginkgoites waarrensis has been described. Gone but not forgotten What happened to the ginkgophytes in Australia through the latest part of the Cretaceous (90-65 million years ago) is unclear. No deposits of fossil leaves are known from the continent in this interval. After 150 years of fossil research, it remains a black hole in palaeobotanical knowledge. Even the fossil record of dispersed pollen grains is of no help to us, since the pollen of Ginkgo is indistinguishable from that of cycads and some seed-ferns. However, it is generally assumed that either by climate change or competition from more adaptable, fast-reproducing, flowering plants, Ginkgo was pushed to extinction on this continent by the end of the Cretaceous. After a long Australian absence, ginkgos reappeared in the Aptian Age; persisting through the Middle Cretaceous into the Turonian Age (ca 92 million years ago). The early Late Cretaceous period is represented by Ginkgo wintonensis (left) which was prevalent in the Winton Formation, western Queensland about 98 million years ago. Although there is some evidence that the Ginkgoites lingered on into the Cenozoic era in Tasmania, the only species found in Australia today is the imported Ginkgo biloba which, like these young trees at the Beijing Zoo (right) are a native of China. 46 Australian Age of Dinosaurs That, at least, was the assumption followed for many decades. However, in 1999, palaeobotanists Bob Hill and Ray Carpenter (then at the University of Tasmania) discovered a single complete leaf and some additional fragments of a Ginkgo in probable Paleogene (65-35 million year old) sediments near Richmond in southeast Tasmania. There in the farthest south-east corner of Australia, it seems, Ginkgo lingered on well into the Cenozoic era. Although a surprising discovery at the time, other relict taxa including the youngest known seed-fern fossils have since been discovered in Tasmania, suggesting that this region provided a cool moist refugium for several typical Mesozoic plants. Even today, several unique conifer, flowering plant and animal groups (for example, anaspid crustaceans) with links to the Mesozoic retain a relictual presence in Tasmania. Ginkgo also appears to have persisted on the Antarctic Peninsula and near the southern tip of South America through the final stages of the Cretaceous and into the Paleogene. The final disappearance of Ginkgo in the Southern Hemisphere may have been related to a combination of factors including dramatic cooling and expansion of the Antarctic ice sheet in the mid-Cenozoic, Australia’s northward movement into the mid-latitude dry-climate belt and continuing competitive pressures from flowering plants. In assessing the parallel decline of Ginkgo in the Northern Hemisphere through the Late Cretaceous and Cenozoic, a study by Royer and others in 2003 concluded that ginkgos had been long adapted to disturbed streamside and levee habitats but were progressively displaced by flowering plants with better adaptations to frequent disturbance. Despite their survival of previous icehouse and greenhouse climate swings and such global catastrophes as The Author Steve McLoughlin completed his PhD in the palaeobotany of Permian floras at The University of Queensland in 1990. He has subsequently undertaken research and teaching at the University of Western Australia, University of Melbourne and the Queensland University of Technology. Steve is now a senior curator in the Department of Paleobotany at the Swedish Museum of Natural History, Stockholm. the end-Permian and end-Cretaceous mass-extinction events, ginkgos ultimately met their demise in Australia some time in the Cenozoic. Only their fossil leaves have left a signature in the rocks of their rich contribution to the development of life on this continent. Postscript: Survive that! Not all Ginkgo trees suffer extinction meekly. On August 6, 1945, the US Air Force B29 Super-fortress bomber Enola Gay y released an atomic bomb over the Japanese city of Hiroshima. The resulting blast devastated the city, killing upwards of 100,000 people and ushering in the age of atomic warfare. Amidst the incinerated ruins of the city stood the rubble-strewn foundations of a temple at Housenbou just 1.1km from ground zero. In front of the ruins stood the charred stump of a Ginkgo tree – its summer leaves and branches swept away by the nuclear inferno. Despite the approaching autumn, by September that year, the apparently lifeless stump sprouted buds, issued forth new leaves, and since then it has prospered without ill effects. The temple was eventually rebuilt – its twin front staircases wrapping around and protecting the tree. Engraved on it are the words, “No more Hiroshimas”. The tree remains a symb for the endurance of nature and the follies of humanity. Issue 8 47