Humans and nature turn American West into a tinderbox

Eleven new large fires raced across the American west last week, adding to a conflagration that has left more than 30,000 homeless and scorched nearly one million acres of forest. Besides the adverse conditions, earlier success at fighting smaller fires is also contributing to the disaster.

"This hot, dry, windy weather is killing us," says Jeff Jahnke, of the Colorado State Forest Service in Fort Collins, Colorado. "The wind spreads fire like you wouldn't imagine. And our equipment and crews don't work as well as they could when you have three-digit temperature and single-digit relative humidity."

Consequently, firefighters are stretched thin. For only the third time in the last two decades, the National Interagency Fire Center (NIFC), based in Boise, Idaho, has already mobilised more than half of its professional woodland firefighters. If the fires continue to rage, the NIFC would have to turn to volunteer firefighters for help.

Nonetheless, 2012 compares favourably with previous years. NIFC statistics show that more acres of forest had burned by June during five of the last 10 years, and there were a greater number of fires in nine of those years.

According to the US National Oceanic and Atmospheric Administration, June was one of the hottest on record in the US since data collection began in 1880. This has led to fires across the US, from Virginia to Alaska, although the worst hit areas are Colorado and other states in the south west. The high heat, combined with more than a decade-long drought in Colorado and ever-decreasing snowpack in the Rockies have turned many of the state's forests into a tinderbox.

"We have had a bad streak since 2000," says Wally Covington, of the Ecological Restoration Institute in Northern Arizona University in Flagstaff, Arizona. However, he adds that warmer weather is only part of the problem. "Since the beginning of the 20th century we have had this propensity to fight small natural forest fires that [otherwise] kill off dense, young growth," he says.

Historically, small ground fires would burn through old growth forests every 20 to 30 years, using up the fuel on the forest floor, but leaving the big trees mostly unaffected. Without these small, natural fires, the build-up of undergrowth on the forest floor acts as fuel for more massive fires.

"The ponderosa pine forests that are burning right now in Colorado have all kinds of understory species that historically wouldn't have been there," says Jahnke. "This fuel, combined with the summer's heat and wind, is providing the opportunity for these low-intensity ground fires to become high-intensity crown fires. And that is not good."

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