Sunday, 7 November 2010

Trees of Bristol: The Ginkgo

 At this time of year, among the fallen leaves can be seen the distinctive fan-shaped leaves of the zoo’s Ginkgo trees. Ginkgos can be seen in many parts of the world today, as they make good street trees. Ginkgos adapt well to the urban environment, tolerating pollution and confined soil spaces. They rarely suffer disease problems, even in urban conditions, and are attacked by few insects. That they are to be seen anywhere however is a major triumph of survival, as Ginkgos are among the oldest living plants.

'Autumn Gold'
For centuries it was thought to be extinct in the wild, but is now known to grow in at least two small areas in Zhejiang province in Eastern China, in the Tian Mu Shan Reserve. These populations may have received at least some assistance by local people in their survival, as ginkgos are often grown in the grounds of Buddhist monasteries.

Prior to their discovery by western explorers of China, Ginkgos were only known to western science as fossils, mostly of leaves. The fossil record of the group reaches back to the Permian period, 270 million years ago, and at their height there were at least 16 genera with a variety of forms. Some were like the trees of today, others were more shrubby and with divided leaves that must have made them look rather like Japanese maples. Their origins are unclear (this is the case with most plant groups), as in some respects they resemble cycads, in others conifers. The mostly likely origin is a group generally referred to as the seed ferns, as they had fern like leaf structures but produced seeds like more advances plants..

The genus Ginkgo itself dates back to the Jurassic about 190 mya, but by the end of the Cretaceous at 65 mya there appears to have been only a few species, and by 30 mya only a single species, Ginkgo adiantoides, with a global distribution seems to have survived. This was the ancestor of the modern G. biloba. Ginkgos disappeared from the fossil record in North America by 7 mya, but held on in Europe until 2.5 mya, probably finally dying out as the onset of the ice ages made Europe too cold for them.

Modern-day G. biloba grows best in environments that are well-watered and drained, and the extremely similar fossil Ginkgo favored similar environments: the sediment record at the majority of fossil Ginkgo localities indicates it grew primarily in disturbed environments along streams and levees.

The decline of the group is probably related to the rise of flowering plants. When they evolved, ferns dominated disturbed streamside environments, forming a low, open, shrubby canopy. Ginkgo's large seeds and habit of "bolting" - growing to a height of 10 m before elongating its side branches - may be adaptations to such an environment. The fact that diversity in the genus Ginkgo drops through the Cretaceous (along with that of ferns, cycads, and conifers) at the same time that flowering plants were on the rise, supports the notion that flowering plants with better adaptations to disturbance displaced Ginkgo and its associates over time.
Ginkgo is a relatively shade-intolerant species that (at least in cultivation) grows best in environments that are well-watered and well-drained. The species shows a preference for disturbed sites; in the "semi-wild" stands at Tian Mu Shan, many specimens are found along stream banks, rocky slopes, and cliff edges. Accordingly, Ginkgo retains a prodigious capacity for vegetative growth. It is capable of sprouting from embedded buds near the base of the trunk in response to disturbances, such as soil erosion. Old individuals are also capable of producing aerial roots on the undersides of large branches in response to disturbances such as crown damage; these roots can lead to successful clonal reproduction upon contacting the soil.

Modern G.biloba is dioecius (has separate male and female trees). Unfortunately, the seeds are enclosed in a foul smelling fleshy seed coat, which means that anyone growing one has to make sure they plant a male tree, as otherwise they run the risk of their lawns being covered with extremely smelly fruit. Fortunately, there are a large variety of cultivars now available, mostly of male plants, with different growth habits and leaf shapes, including variegated forms.

The seeds are eaten in Asia, and the leaves are also used in herbal medicine, although the actual medicinal value is uncertain to say the least. Nonetheless, nearly 2 million kg of leaves are harvested each year worldwide for the herbal medicine trade.

Part of the reason for the Ginkgo’s success in cultivation is a result of its longevity. It has outlasted all the pests and diseases that once attacked its ancestors – there is simply nothing left alive that wants to eat it. Of course, it has also outlasted most of the animals that once distributed its seeds, which is why its wild range is practically nonexistent. However, having made itself useful to people, there is a slight irony in the fact that owing to human gardeners preference for variety, there are probably more ginkgo trees alive today, in more phenotypes (one nurseryalone lists 11 different varieties), and over a vastly larger geographical range, than at any time in the last 65 million years.
Images from wikipedia, Big Plant Nursery

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