Fires have scarred forests for millions of years

Fires have scarred forests for millions of years
21 September 2014

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USA — Prehistoric life on Earth can seem so strange, with the plants and animals resembling something out of a science-fiction novel about an alien planet.

Sometimes, however, a fossil is found that shows how similar ancient life was.

A recent paper in the journal Palaeogeography, Palaeoclimatology, Palaeoecology reported on the first known fossil fire scar on a tree from the Triassic Period, 211 million years ago.

The fossil comes from the Chinle Formation, the same rock unit that holds the famous Petrified Forest of Arizona (which is not really a petrified forest so much as a petrified log jam). It was found in a place called Bears Ears, Utah, just north of the Utah-Arizona border.

The Chinle was deposited by a large meandering river that flowed northwest from highlands in the panhandle region of Texas — across what is now New Mexico, Arizona and Utah — to the sea in Nevada. At that time, the area was in the tropics, 5 to 10 degrees north of the equator.

The specimen was an oval section of petrified wood 5.5 by 8 inches across and 4 to 6 inches long before being cut for study. It likely came from the most-common species of tree in the Chinle, which for more than 100 years had been called Araucarioxylon arizonicum but now is called Agathoxylon arizonicum. Its closest living relative is the Norfolk Island pine.

The 3- to 4-inch-wide scar extends vertically along one side of the trunk. On each side of it are curls of wood produced by the tree as it tried to close the wound. At the same radius as the scar is a ring of smaller tracheids — elongated cells for conducting water and nutrients — that shows the tree’s growth was slowed by the traumatic event.

Tracheids outside that ring are significantly larger than those produced earlier. Modern conifers that are subjected to fire often exhibit what is called “growth release,” a burst in growth produced by reduced competition for light, water and nutrients caused by the death of other vegetation and the release of nutrients by the fire.

After the fire, the Agathoxylon arizonicum added about 2 inches of wood to the diameter of its trunk.

The ancient scar perfectly matches modern scars caused by fast-moving ground fires that burn one side of a trunk but don’t kill the tree. There are even some cracks or gaps similar to traumatic resin ducts that form in modern conifers that have been burned.

Trees have had to contend with fire for a long time; evidence of forest blazes in the form of fossil charcoal dates to the Late Devonian, soon after the first forests arose 380 million years ago.

During the Pennsylvanian Period, 300 million years ago, fires were even more common, probably because oxygen levels were much higher — about 31 percent compared with 21 percent today. With that much oxygen, those fires must have been incredibly intense.

(As an aside, the increased oxygen also allowed insects and other arthropods to get much bigger — there were dragonflies with 2-foot wingspans and millipedes that were 5 feet long.)

Another increase in forest fires took place during the Cretaceous Period. It is fun thinking of a dinosaur in a Smokey Bear hat saying, “Only YOU can prevent forest fires.”

Dale Gnidovec is curator of the Orton Geological Museum at Ohio State University.

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