May 18, 2012

Tree Rings Reveal That ‘Megafires’ Mostly Caused By Human Interference

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Brett Smith for

A study led by anthropologist Christopher I. Roos from Southern Methodist University in Dallas shows that modern “mega forest fires” in the southwestern U.S. are the result of dense canopies that likely grew as a result of human interference.

“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 said.

Researchers theorized that a hundred years of livestock grazing and firefighting efforts have combined to create denser forests that are now more vulnerable than ever to extreme droughts. They said smaller surface fires used to clear grasses and keep saplings from reaching middling heights could extend later fires into the forest canopy.

“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,” Roos said. “The trees had adapted to frequent surface fires, and adult trees didn´t die from massive fire events because the fires burned on the surface and not in the canopy.”

The study, which was published in the scientific journal The Holocene, looked at data from tree rings to understand historical climates and events on a year-to-year basis. The growth rings can tell scientists if a particular year was cold and wet or dry and hot by their size and appearance. The rings also record nearby fires that appear as scars within the cross-section of the tree.

Two particular periods of time were of interest to Roos and his team: the Medieval Warm Period and the Little Ice Age. The Medieval Warm Period, from around 800 to 1300 A.D., was thought to have a warmer, drier climate similar to our modern climate. The climate of the Little Ice Age, from about 1600 to 1800, is thought to have been cooler and wetter than today.

Using a statistical model based on Little Ice Age fire-scar data and nearly 1,500 years of climate data derived from existing tree rings, the researchers were able to predict forest fire activity during the Medieval Warm Period. Their model showed no discernible difference in low level surface-clearing fire activity based on year-to-year moisture patterns between the two historical periods.

“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,” Roos said. “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.”

As settlers claimed the land in the 19th century, their livestock grazed on grasses that used to fuel these ℠preventative fires.´ The establishment of the U.S. Forest Service in the 20th century also meant that these easily extinguished fires were prevented from taking down middle-sized, canopy-igniting trees.

“Many of our modern forests in central Arizona and New Mexico haven´t had a fire of any kind on them in 130 or 140 years,” Roos said. “That´s very different from the records of the ancient forests. The longest they would have gone without fires was 40 or 50 years, and even that length of time would have been exceptional.”