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Earth's biggest tree rings tell fiery tales

Fifty-two giant fallen giant sequoias reveal a 3,000-year-old history of fire and drought after giant chainsaws expose their rings.
The tree rings of 52 fallen giant sequoias, Earth's biggest trees, have revealed a 3,000-year-old history of droughts and fires.
The tree rings of 52 fallen giant sequoias, Earth's biggest trees, have revealed a 3,000-year-old history of droughts and fires.Hemera/ThinkStock
/ Source: Discovery Channel

Using huge chainsaws and strong backs, the largest trees in the world are finally giving up their 3,000-year record of fires and droughts. No trees, however, were harmed in the making of this fire history.

"We only used dead trees," emphasized tree ring researcher Thomas Swetnam of the University of Arizona. Swetnam led the study that was reported in a recent issue of the journal Fire Ecology. "We spent multiple years collecting the wood and hauling it back to Tucson."

The giant sequoias in California's Sequoia National Park are far too thick to be cored for the extraction of the pencil-thin cores typically used by tree ring researchers. So the authors of a new report on tree ring evidence of past droughts and fires used all sorts of other tools to slice and dice 52 giant dead and fallen sequoias, lug the pieces back to roads by hand. Then they spent years piecing together the valuable history in their laboratories.

Among the things they found in the ring record was a very dry and fiery period from 800 to 1300 A.D. That corresponds to a controversial climate interval called the Medieval Warm Period.

That period was very dry," said Swetnam. "But we're not so clear how warm it was."

Modern temperatures already exceed those of the Medieval Warm Period, said Swetnam. So if heat has anything to do with fire frequency, we could expect more fires.

"What makes this work unique is that it goes so far back in time," said U.S. Geological Survey research ecologist Nathan Stephenson, who has spent a lot of years studying sequoias. Usually if you are working with pines you get centuries. With these you get multi-millennial annual resolution records."

But unlike a tree ring history that's based on just rings, this one is based cross dating rings between various trees the dating of fire scars.

These scars happen during natural fires when debris close to the tree bakes and burns the trunk, which is otherwise fire resistant. The trees can grow over a lot of these scars, but in cross sections, that can be easily spotted and dated.

"That way were able to establish a fire chronology," Swetnam told Discovery News. Of course, there have been other fire chronologies. Some are based, for instance, on charcoal layers found in mountain lakes. But nothing has quite the resolution of tree rings.

"The punch line from all of this," said Stephenson, "Is that over at least 2,000 years the most severe (sequoia) reproduction reduction has been in the last 100 years. Human land use changes have had greater effect than the preceding 2,000 years of changing fire regimes."

The problem, said Stephenson, is fire suppression. Excluding fires from the sequoia groves, makes it very difficult for sequoia seeds to germinate or have enough space for saplings to get started.