Petrified Wood Contains Oldest Fossilized Fire Scar

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Byers' petrified wood had the healing curls. When the piece was cut and polished, he could also see a light-colored band dividing the pre- and post-fire growth, a mark that is also found in modern trees, as well as the unique growth-ray pattern.

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With networking and cold calls, Byers put together a dream team that could help him polish his findings and publish the results in a scientific journal. The study will appear Oct. 1 in the journal Palaeogeography, Palaeoclimatology, Palaeoecology. His collaborators include the University of New Mexico's Sidney Ash, who may have looked at more petrified wood from the Southwest than anyone else on Earth; Dan Chaney, an expert on ancient plants at the Smithsonian National Museum of Natural History; and Lucía DeSoto, a professor at Portugal's University of Coimbra and a leader in analyzing tree growth, cell by cell.

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One of the team's key findings was that the petrified tree seemed to react to fire stress in the same way as modern trees. Stress from forest fires can leave behind bands of narrow tree rings in some modern trees, a result of the struggle to recover and grow. The ancient tree was a species called Agathoxylon arizonicum, a species that never forms growth rings. However, its microscopic tree cells, called tracheids — internal highways that transfer water and nutrients — show signs of fire stress in a manner similar to tree rings. Six to eight rows of tiny tracheids suggest suppressed growth immediately after the fire, the researchers report. The next rows are bigger than prefire tracheids, likely because the forest fire's survivors had less competition for water and nutrients, Byers said. This post-fire growth spurt is also seen in modern trees.

If more evidence for fire damage turns up in ancient wood, will ecologists reconsider the impacts of fire on plant evolution? In the Cretaceous Period, between 120 million and 65 million years ago, researchers now think wildfires helped trigger the development of the first flowering plants.

No one knows for sure what the Chinle Formation's trees looked like, but some researchers think they resembled monkey puzzle trees, the evergreen conifers from Chile that drop their lower branches until only an umbrella-shaped top remains. That's a classic fire adaptation (so ground fires can't climb up lower branches), Byers points out.

"I think fire has been a selective force in forests since way, way back," he said.

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