Fires fuelled spread of grasslands on Earth: Study

Ancient wildfires played a crucial role in the formation and spread of grasslands like those that now cover large parts of the Earth, according to a study.

The study, published in the journal PNAS, links a large rise in wildfires nearly 10 million years ago with a major shift in vegetation on land.

This is indicated by carbon isotopes of plant biomarkers found in the fossil record, said researchers from the Pennsylvania State University in the US.

Frequent, seasonal fires helped turn forested areas into open landscapes, and drove the expansion of grasslands, they said.

The researchers developed an innovative approach to test the role of fire in the rise of early grasslands.

They analysed tracers of ancient leaves and of burned organic matter left behind in paleosols, or fossil soils, in northern Pakistan.

"The tools we use are molecules and biomarkers produced by organisms in Earth history and preserved in rocks," said Allison Karp, a graduate student at Pennsylvania State University.

"We can use these as clues to figure out what was happening with climate and ecology in the past," said Karp.

The new technique has broad implications as a tool for scientists seeking to answer questions about past vegetation and climate change, the researcher said.

This shows that the tool can pinpoint the location of a fire where it occurred, according to Karp.

"In a paleosol record you are really capturing an integrated picture of what was happening when the soil was forming," she said.

"This is one of the biggest ecological changes in the last 66 million years," said Karp.

"None of the open grassland systems we have today existed before this transition. It was a very different looking world, especially in sub-tropical places like Pakistan," she said.

Scientists have long studied the rise of C4 grasslands, named after plants that evolved a new way to handle photosynthesis that allows them to thrive in dry, tropical conditions and with lower amounts of carbon dioxide.

These plants include modern crops like corn and sugarcane.

A drop in global carbon dioxide levels was once believed to be behind the rise of C4 grasslands.

More recent research has shown that the grasses spread at different rates on different continents, indicating that regional factors, like rain patterns -- and potentially fire -- played important roles.

However, there had been little direct evidence that linked a rise in wildfires to this transition.

"We were interested in reconstructing fire and the expansion of grasslands in the same geologic record to see if we could find proxy evidence of the role fire played," Karp said.