Spread of early flowering plants 'aided by fire'

Fire has always played a key role in shaping landscapes around the world

A study has highlighted the importance of wildfires in allowing flowering plants to spread during the Cretaceous period (145-65 million years ago).

Researchers suggested that conditions at the time, like higher temperatures and atmospheric oxygen levels, established "angiosperm fire cycles".

The "small, weedy" plants were able to regenerate after a fire, giving them an advantage over conifers, they said.

The findings have been published on the New Phytologist journal's website.

"The Cretaceous is the period when flowering plants (known as angiosperms) first evolved," explained co-author Andrew Scott from the Department of Earth Sciences at Royal Holloway, University of London.

"It was also a time when there was a large number of fires, partly because there were probably higher atmospheric oxygen levels than there are today."

Research suggested that oxygen made up about 25% or higher during this period, compared with 21% today.

"Angiosperms, particularly weedy angiosperms, can do particularly well in these sorts of circumstances; they can regenerate very fast and take advantage of such a regime," Professor Scott told BBC News.

"They can also quickly provide fuel ready to be burnt; this means that they have quite a good competitive advantage over other plants."

He said that many of the early angiosperms were small, herbaceous or shrubby "weedy plants", so when wildfires - especially surface fires - frequently swept through an area, it created a positive feedback cycle.

"It prevented other plants getting into an area - most of the trees at this time were conifers. If you have relatively regular fires then the trees can never grown above the height of the flames.

This meant that gymnosperm plants, such as conifer trees, which had previously dominated the landscape, were unable to regenerate as quickly as the angiosperms and lost the opportunity to shape the surrounding habitat.

"It was rather like the savannahs of today - if you did not have fires in Africa, then you would not have savannahs."

'Novel fire regime'

Explaining the link between the spread of angiosperm plants and fire patterns at the time, Professor Scott said the level of oxygen in the atmosphere played a key role in allowing a "novel fire regime" to become established.

"In the geological past, there have been fluctuations in atmospheric oxygen, in particular if you get below 15%, then you cease to get the spread of fire," he said.

"If you get to 25% and above, you can burn wetter and wetter plants. The impact of this is that as you increase atmospheric oxygen, then you can increase the places where fires can start and how often they occur.

"So you have the coincidence of the evolution of flowering plants, which have an advantage over a number of other plant systems, linked together with a period of increase fire that clears out all competitors.

"This would have allowed angiosperms to really diversify, quicker than they would have done otherwise."

Evidence of the relatively rapid emergence and spread of the flowering plants was found in charcoal deposits that dated back to the Cretaceous period.

"The first charcoalified flower was discovered in the early 1980s," the Royal Holloway professor explained.

"One thing that people often don't realise is that charcoal beautifully preserves the anatomy of plants, so you can not only get charred wood, but charred leaves and flowers too.

"Over the past 30 years, there have been large numbers of deposits found containing charcoalified flowers, so much of our understanding of the early flowering plants comes from these flowers.

"As people found more and more charcoal deposits, they began to realise that fire could have been significant."

He said one of the aims of the paper, which he co-authored with Professor Will Bond of the University of Cape Town, was to point out "that fire had not really been considered as an element in the success of flowering plants during this period".

Professor Scott continued: "We now know that fire had a much more important impact in the Earth system than previously thought - not just in the Earth's atmosphere system, but as we are now suggesting, it also affected biological evolution and diversification."