Fires fueled the spread of grasslands on ancient land

A new study links a sharp increase in forest fires, there are nearly 10 million years, in the late Miocene, to a major change in terrestrial vegetation, as indicated by carbon isotopes of plant biomarkers found in the fossil record. Frequent and seasonal fires have helped transform forest areas into open landscapes and have led to the expansion of grasslands, the researchers said.

The team has developed an innovative approach to test the role of fire in climbing ancient grasslands. They analyzed tracers of old leaves and burnt organic matter left in paleosols or fossil soils in northern Pakistan.

"The tools we use are molecules and biomarkers produced by Earth's history and preserved in rocks," said Allison Karp, a geoscience graduate student at Penn State and lead author of the journal. "We can use them as clues to understand what was happening in the past with climate and ecology."

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

This shows that the tool can locate the place of a fire, according to Karp. "In a paleosol record, you actually capture an integrated picture of what was happening when the ground was forming," she said.

The researchers recently reported their findings in the Proceedings of the National Academy of Sciences. Katherine Freeman, Professor of Geoscience at Penn State University of Evan Pugh University and Karp Consultant, is co-author of the paper.

"This is one of the greatest ecological changes of the last 66 million years," said Karp. "None of the open grassland systems that we have today existed before this transition.It was a world of very different appearance, especially in subtropical areas like Pakistan."

Scientists have been studying the rise of C4 grasslands for a long time, in reference to plants that have developed a new way to manage photosynthesis, allowing them to thrive in dry tropical conditions with less carbon dioxide. These plants include modern crops like corn and sugar cane.

It was once believed that a decline in global levels of carbon dioxide was driving the rise of C4 grasslands. More recent research has shown that grasses spread at different rates on different continents, indicating that regional factors, such as rainfall patterns – and potentially fires – have played an important role. But there was little direct evidence linking an increase in forest fires to this transition.

"We were interested in reconstructing the fire and expanding grasslands in the same geological record to see if we could find indirect evidence of the role played by the fire," Karp said. "We now have a nice series of proofs of observation to compare with what models have said."

Karp and his collaborators used polycyclic aromatic hydrocarbons (PAHs), present in paleosols, as indirect indicators of fire. PAHs are chemicals created by burning organic materials such as wood and plants. They are also naturally found in coal and crude oil.

PAHs increased five-fold throughout the study area, while conifer indices declined and eventually disappeared. The heavily wooded landscape opened in two stages. About 10 million years ago, forests were replaced by forests or grasslands open and more exposed to fire, and between six and eight million years ago, C4 grasslands became dominant, then that the number of fire signatures has greatly increased.

The ecology of modern fire can explain the process. Grasses grow faster than trees after a fire and also help create conditions for future fires, thus promoting open landscapes. At the end of the Miocene, the rainy seasons caused by the monsoon have favored the growth of plants, which has created more fuel for fires during the hot and dry seasons in Pakistan.

"The role of fires in the expansion and evolution of deep-sea grassland systems is important because understanding how fires have preserved systems in the past can help us predict what could happen in the past. these important systems in the future, as the climate evolves "he said.

The new approach to fire markers could be used to examine scale – scale interactions between fire and vegetation in other geographic regions and climatic transitions, such as glacial – interglacial transitions or changes. catastrophic weather, researchers said.