1,500 Years of Tree Ring Data Suggests Today's Southwestern U.S. Megafires Atypical

Posted on May 21, 2012

A new study suggests that today's mega forest fires of the southwestern U.S. are truly unusual and exceptional in the long-term record. Researchers examined hundreds of years of ancient tree ring data for Ponderosa Pine forests in the southwest United States and fire-scar records. The findings implicate as the cause not only modern climate change, but also human activity over the last century.

The study was conducted by fire anthropologist Christopher I. Roos, Southern Methodist University, Dallas, and Thomas W. Swetnam, director of the University of Arizona Laboratory of Tree-Ring Research. The researchers constructed and analyzed a statistical model that encompassed 1,500 years of climate and fire patterns. The research was published here in The Holocene.

Roos says, "The U.S. would not be experiencing massive large-canopy-killing crown fires today if human activities had not begun to suppress the low-severity surface fires that were so common more than a century ago."

Roos and Swetnam calibrated a statistical model that combined 200 years of Little Ice Age fire-scar data and nearly 1,500 years of climate data derived from existing tree rings. They were able to predict what the annual fire activity would have been almost 1,500 years ago. They discovered that the Medieval Warm Period was no different from the Little Ice Age in terms of what drives frequent low-severity surface fires: year-to-year moisture patterns.

Roos says, "It's true that global warming is increasing the magnitude of the droughts we're facing, but droughts were even more severe during the Medieval Warm Period. It turns out that what's driving the frequency of surface fires is having a couple wet years that allow grasses to grow continuously across the forest floor and then a dry year in which they can burn. We found a really strong statistical relationship between two or more wet years followed by a dry year, which produced lots of fires."

The researchers say today's huge canopy fires are the cumulative result of human activity. The researchers say ancient forest had frequent fires, but these fires didn't burn the forest canopy like today's megafires. Roos says, "The fires cleaned up the understory, kept it very open, and made it resilient to climate changes because even if there was a really severe drought, there weren't the big explosive fires that burn through the canopy because there were no fuels to take it up there."

Dr. Roos explains how the tree rings contain a history of forest fires. He also explains how modern fires are different than fires hundreds of years ago. Take a look:


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