When Tom Swetnam joined the U. S. Forest Service in the 1970s, his mandate was to "put everything out," he recalled. But when Swetnam enrolled in graduate school at the University of Arizona's Laboratory of Tree-Ring Research, he was surprised to find a record of repeated blazes dating back hundreds of years before European colonists arrived on the continent. Some of the trees he analyzed bore more than 20 fire scars spaced among their rings.

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