A train journey and some quiet time in the run up to Christmas meant I was supposed to have had time to think a bit more about ginkgos. However the train journey wasn’t as peaceful as expected (think drunk couple engaged in a domestic dispute in the quiet coach) so it’s had to wait until now. However the advantage of that is that I now have my own fossil Ginkgo leaves from Scalby Ness in N. Yorkshire to admire, thanks to my lovely friend Gill.
Fossil Ginkgo leaves from Scalby Ness
Our own trip there in October yielded plenty of plant remains but mostly fragments of root and stem which couldn’t be identified as anything in particular.
Fragments of plant material in rocks at Scalby Ness – stem above, rootlets below
One thing I’ve been giving some thought to is how a genus once so diverse and widespread became restricted a single species found growing wild in only small areas of S and E China today.
Having first appeared in the fossil record some 250 milllion years ago, ginkgos flourished and spread during the warm period in Earth’s history between 65 and 35 Ma, just as India was starting to collide with Eurasia in earnest. Accelerating continental drift around 35 Ma caused Earth’s climate to become cooler and drier, eventually leading to a period of glaciation during which steppe replaced forest, flowering plants diversified rapidly and ginkgos eventually became restricted to a few refuges.
Ginkgos don’t cope well with extremes of either heat or cold and are restricted to temperate regions today. As deciduous trees, they avoid the problem of drought during the winter and next season’s leaves are generally well protected from the cold inside buds. At this time of year it’s easy to see the difference between the new growth of long shoots and the closely-packed leaf scars below the buds of the stumpy side branches which bear most of the trees’ leaves (see Ginkgo – A living fossil).
Side branches with buds, showing leaf scars from previous years
New growth of a long shoot from an older side branch
The length of the growing season is often the real reason why tree growth is restricted. Trees need to be able to harvest sufficient energy over the summer for winter maintenance and annual growth. Young plants tend to be much more cold sensitive than older ones – a large tree losing a few of its new leaves to a late frost won’t be harmed, whereas a small tree with just a few leaves can’t easily replace them.
This effect is exacerbated in ginkgos by the remarkably slow development of the embryo inside developing seeds. Fertilization occurs 130-140 days after pollination but the embryo takes a further 100 days to reach maturity. Maturation slows down even further at low temperatures – a necessary safeguard to avoid the risk of a seed germinating prematurely during the winter for Gingko seeds which can germinate as soon as they are mature. However it means that germination will be delayed when winter temperatures are low (as the embryo is still developing) so the potential growing season will be even shorter. It may not be long enough for seedlings to establish themselves successfully before the following winter.
To start with, ginkgos seemed to adapt to the cooling climate around 35 Ma by altering their distribution, with fossils found further south and east in Europe than previously, rather as plant distribution is changing in response to climate change today. Eventually, however, wild Ginkgo became extinct in many areas, despite growing successfully in the same places today after human re-introduction. Why should this be? One theory is that, in the wild, Ginkgo seeds were dispersed by large animals of some sort (the fruits are relatively large). When these animals became extinct, the plant lost its ability to recolonize here the climate would otherwise have been suitable.
Anyway, ginkgos eventually became limited during the glaciations of the last two or three million years to small areas of China – the places where they are thought to grow wild today. Genetic diversity studies point the finger at the Jinfo Mountain area of Chonqing Municipality as being the oldest such relic population – doubly interesting to me, as Chonqing is where our son is teaching English this year. Jinfo Mountain is home to some of the largest ginkgos in the world, with trunks up to four metres in diameter, which dwarf the 250 year old Ginkgo in Kew gardens with a trunk diameter of less than two metres. Around 100 ginkgos in China are believed to be more than 1000 years old.
The rarity and isolation of ginkgos changed, though, when they were first noticed by people. In China, tenth and eleventh century reports describe Ginkgo nuts being given and received as gifts between friends and root material being taken from one city to another to cultivate new trees – the fruits don’t seem to have been widely harvested from wild trees. I’m sure the beauty of the trees must also have encouraged their cultivation in cities and at temples around China and, eventually, Korea and Japan. By the 18th Century, images of Ginkgo leaves appear on Japanese porcelain, some of which found its way to the west and must have sparked interest in these beautiful trees here.
Ginkgo fan with a Ginkgo fan, Edinburgh Botanic Gardens
The story of how ginkgos came to be such widespread ornamental trees will need to wait until next year….