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A new study by Penn State and the Smithsonian Institute associates a sharp increase in forest fires, there are nearly 10 million years (Miocene times), to a major change in terrestrial vegetation, as the ## 147 ## 39 indicate the carbon isotopes of plant biomarkers found in the fossil record.
Agricultural fires in Pakistan and India today. Image Credit: NASA Goddard Space Flight Center / Lynn Jenner / Jeff Schmaltz, MODIS Rapid Response Team.
Penn State researcher Allison Karp and her co-authors have developed an innovative approach to test the role of fire in climbing ancient grasslands.
They analyzed tracers of old leaves and burnt organic material left in paleosols in northern Pakistan.
"This is one of the most important ecological changes of the 66 million years," said Karp.
"None of the open grassland systems we have today existed before this transition. The world was very different, especially in subtropical regions such as 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 fire reconstruction and grassland expansion in the same geological record to see if we could find indirect evidence of the role played by fire. We now have a nice set of proofs of observation to compare with what models have said, "said Karp.
The researchers used polycyclic aromatic hydrocarbons (PAHs), which are present in paleosols, as indicators of fire substitution. These chemicals are 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 6 and 10 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 fire marker approach 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 inter – glacial transitions. catastrophic climate change, "said the authors of the study.
The research is published in the Proceedings of the National Academy of Sciences.
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Allison T. Karp et al. 2018. The ecology of fires in the grasslands has its roots in the late Miocene. PNAS 115 (48): 12130-12135; doi: 10.1073 / pnas.1809758115
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