13 September 2010, by Tamera Jones
The beautiful flowers that have managed to colonise almost every habitat around the world owe their existence to frequent wildfires during the Cretaceous period 120 to 65 million years ago, claim researchers.
They say that fires would have created ideal conditions for flowering plants to spread rapidly, allowing them to go from a few lowly shrubs to become widespread in a relatively short space of time.
Back in the Cretaceous, oxygen made up more than 25 per cent of the atmosphere – compared with today's 21 per cent – temperatures were high, and there were exceptionally dry seasons. Add the high likelihood of lightning and you create the perfect conditions for regular wildfires, allowing even wet vegetation to burn.
Fossil evidence shows that flowering plants first appeared on Earth around 135 million years ago. At that time, the most successful plants around were slow-growing conifer-like trees called gymnosperms.
But scientists have struggled since Darwin's time to explain how and why flowering plants gained a foothold and spread around the world so quickly.
'Ancient fires preserved the fine detail of charcoalified flowers, fruits, seeds and other plant organs,'
Professor Andrew Scott, Royal Holloway, University of London
Professor Andrew Scott from Royal Holloway, University of London and co-author of the study published in New Phytologist suspected that fire could have played a role in the plants' success.
He and Professor William Bond from the University of Cape Town realised that the high oxygen levels during the Cretaceous predicted by models coincided exactly with when flowering plants started to spread.
So the researchers looked through all published data on fossilised charcoal – 'which preserves the anatomy of plants perfectly,' – coal and a rock called inertinite. Inertinite is the charcoal part of coal. 'You just have to look at charcoal under the microscope and you can see the remains of plants,' says Scott.
'Ancient fires preserved the fine detail of charcoalified flowers, fruits, seeds and other plant organs,' the authors report in the study.
The researchers found that a key to the flowering plants' success was their ability to grow very quickly compared with other plants. Flowers helped these emerging plants reproduce quickly, while leaves and stems allowed them to grow faster than the gymnosperms.
The sheer volume of flowering plants meant that they would have fuelled fires during the Cretaceous, creating wide open spaces, which would have been quickly colonised by more flowering plants before gymnosperms could get a foothold.
'This is exactly what happens in today's savannahs,' explains Scott. 'They're wide open because fires keep them open and slow-growing conifer-like trees don't get a look in before grasses spread.'
'Flowering plants, or angiosperms, basically outcompeted the gymnosperms, because they have more efficient metabolisms. Fire definitely would have helped them,' he adds.
Gymnosperms are still around today in the shape of trees like pines, cypresses and firs, but they're not nearly as successful as the flowering plants, or angiosperms.