Trees of Sanctuary

“When I said these trees were around at the time of the dinosaurs, I’m afraid I wasn’t meaning this exact tree.” I was trying to explain myself to a rather deflated Girl Guide. She had gone back home after a tree identification session I had run at our university field campus, to tell her parents she had seen a 200 million year old tree. I was trying to convince her that this tree was still amazing regardless. But she was pretty disappointed. “It’s the only species of ginkgo left; all the others are extinct. It’s the equivalent of the reptile in plant terms… It has seeds…” No, I had lost her. I’ll try and do a better job here.

Any living fossil interests me – I guess that’s why I am an evolutionary biologist. It’s something to do with the link back in time, creatures and plants that have remained unchanged over a geological time frame, allowing a tiny fragment of insight into those times millions of years ago when our planet with its flora and fauna were still a long way back in terms of evolution. One such plant is Ginkgo biloba, a tree also known as the Maidenhair tree. For years Europeans only knew of the existence of ginkgos from the fossil record and believed all ginkgo species to be extinct. But then specimens of Ginkgo biloba were found in the grounds of Oriental temples – the Chinese had been using them in herbal medicine for 5,000 years – and in 1946, trees were found growing in the wild in China.

People talk about ginkgos as primitive, but for me, I find Ginkgo biloba exciting for exactly the opposite reason. I find it so extraordinary to see and touch a plant that was once at the cutting edge of plant evolution. The ferns had made the move from water to land, but still needed water for reproduction. The ginkgo represents the next stage – a major step forward in terms of evolution; for the first time plants were free to move away from an external water supply. If ferns are the equivalent to amphibians in the animal kingdom, species that have moved from water to land but are still dependent on water for reproduction, the ginkgo is the equivalent to the reptile, species that have made the final break from water.  Ginkgos were among the first land plants able to reproduce without abundant standing water and able to withstand both cold and drought.  And with the massive changes that occurred on the Earth towards the end of the Paelozic, (about 225 million years ago), there was suddenly a whole new space ready and waiting to be colonised by anything that could adapt to it.

Ferns were abundant in the Paelozoic, when the Earth was covered in warm, shallow seas.  But the climate was changing.  The temperature dropped, mountains were formed.  A new invention in the plant world – the seed – suddenly became invaluable. Just at the end of the palaezoic, the first seed plants begin to appear in the fossil record. They have been named gymnosperms, meaning naked seed.  Fruit was still far in the future, but the gymnosperms had a massive advantage over the other plants around at the time – they had pollen to transport the male genetic information to the female so a new individual could be formed.  No more marathons for the sperm, no more reliance on watery highways.  And the ginkgo is right at the start of this evolutionary path as it still actually retained the sperm in the pollen.  The airborne pollen germinates very close to the ovule of the female tree and the sperm swims the tiny distance to the egg cell in fluid produced by the female plant.  I enjoy the way this plant shows us evolution in action.  We can look straight at it, see a stage of evolution which occurred so long ago, but was such a monumental step forward in the development of land plants.

The ginkgo is fun too.  It has beautiful fan shaped leaves with veins that spread from the leaf stem outwards, and in the autumn, leaves seem to glow with a luminous yellow colour.  Trees are either male or female (dioecious), though mostly we find the male trees planted in the UK as the female trees have seeds with a fleshy covering that when overripe, smells truly horrible.  The branching pattern is erratic, leaves born from the branches on stout short shoots.  The trees have character, there is something unusual about the way they look. Ginkgos covered huge parts of the Earth in the early Mezozoic; the golden age of the gymnosperms coincided with the rise and fall of the dinosaurs.  It is not hard to look at the odd form of these trees and imagine a dinosaur pushing its way through and stopping to browse on the incongruously shaped leaves.

So, I know these trees are not actually 200 million years old, but they are still pretty amazing… aren’t they?