The high incidence of fire during the Cretaceous Period, 120 to 65 million years ago, was responsible for the spread of the earliest flowering plants, scientists from Royal Holloway, University of London and the University of Cape Town claim in new research.
Flowering plants first evolved during the earliest part of the Cretaceous, at a time, that evidence suggests, oxygen levels in the atmosphere were higher than today and the incidence of fire was greater. For most of the Cretaceous, angiosperms were low shrubs, herbs or small trees. The invention of flowers speeded up their life cycle while innovations in the leaf and stem allowed them to grow faster than their competitors. In a sense, early angiosperms were the 'weeds' of the Cretaceous world. So the ecological mystery is how did weedy plants, fast growing and fast reproducing, take over this ancient world?
Now Professor William Bond of the Department of Botany at the University of Cape Town and Professor Andrew C. Scott of the Department of Earth Sciences at Royal Holloway have assembled evidence to argue that wildfires were a major factor in the early spread of flowering plants (angiosperms).
Conditions in the Cretaceous were conducive to fires. High atmospheric oxygen (>25% relative to today's 21%) would have promoted fires in wetter vegetation than today. A warmer world would favour convectional storms and the necessary lightning to ignite fires. Seasonal climates provided the necessary dry season when fires could flourish. With these pre-conditions for high fire activity, all that was needed were plants that could generate the fuel to promote frequent fires. Angiosperms, they argue, had the innovations necessary to make the most of conditions conducive to fires. Rapid vegetative growth in the open conditions after a fire, coupled with rapid reproductive rates to reach maturation before the next fire, set up a 'flower-fire' cycle of repeated fires.
"With fire on their side," Professor Bond said, "angiosperms would have been able to penetrate the gymnosperm-dominated forest of the Cretaceous creating their own preferred growing conditions of high light environments."
Professor Bond and Scott suggest that a fire-flower cycle promoted the spread of angiosperms at the expense of slower growing gymnosperms trees by rapid accumulation of fuels. In the 1970s, Bakker suggested that dinosaurs had a similar role in opening up ancient forests providing opportunities for light-demanding angiosperms by 'dinoturbation' and their feeding activity. The widespread occurrence of charcoal in the Cretaceous suggests that flower-fuelled fires were at least as important in opening up forests to the new fast-paced world of the angiosperms.
They also suggest that cycles of high fire activity may have triggered the evolution of novel growth forms in other periods of earth history. They draw parallels to the remarkable spread of tropical grasses creating the vast savannas of today. Here too, fire is implicated in their spread in a new green 'revolution' carving holes into tropical forests over the last 8 million years.
Professor Scott says: "fire has been a neglected process in earth history and now it is time to reassess it's role in shaping our world."
- William J. Bond, Andrew C. Scott. Fire and the spread of flowering plants in the Cretaceous. New Phytologist, 2010; DOI: 10.1111/j.1469-8137.2010.03418.x
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