How wildfires happen

CONCLUSION:

Las Conchas New Mexico fire,

June-July 2011

People caused the Las Conchas fire in New Mexico's Santa Fe National Forest.

It scorched over 150,000 acres. It sparked fears of deadly radiation release from the nation's top nuclear weapons laboratory at Los Alamos. It overran centuries-old cultural and sacred landmarks. It destroyed about half the natural resources of the Santa Clara native American tribe, now threatened by potentially catastrophic floods from the burned areas. It struck lands owned by four other pueblos. Ashy runoff from the fire affected water supplies in Santa Fe and distant Albuquerque.

And it all started at about one o'clock on the afternoon of Sunday June 26th, 2011, when one aspen tree struck one power line at the boundary of real estate investor Roger Cox's 200-acre ranch in the Jemez Mountains. A power line, installed by people and for their exclusive benefit. The ranch's caretaker was away in Los Alamos on an errand.

Santa Fe National Forest is one of five national forests in New Mexico. "The National Forests are America's great outdoors," says the Forest Service, "here to serve the American people at work and play. Some of the finest mountain scenery in the Southwest is found in the 1.6 million acres covered by the Santa Fe National Forest. Elevations rise from 5,300 to 13,103 feet at the summit of Truchas Peak, located within the Pecos Wilderness. Our objective is to maintain that natural beauty."

HOW WILDFIRE WORKS

"The earth has known fire for over 400 million years," Stephen J. Pyne, professor in the Biology and Society Program at Arizona State, tells us in a NOVA Online feature. "Life made it possible. Marine life pumped the atmosphere full of oxygen; terrestrial life lathered the crust with fuels. When oxygen and fuel meet a spark under the right circumstances, a fire kindles. (Lightning is an ancient and ample ignitor.) The fundamental chemistry of combustion lies at the core of the living world. When it happens within a cell, it's called respiration. When it happens outside organisms, it's called fire. It's that basic."

Lightning, sparks from rockfalls, spontaneous combustion, and volcanic eruption ignite most natural wildfires. Though discarded cigarettes and arson usually occur to us as being the main human causes, sparks from equipment and arcing electricity from a stressed or downed power line, as in Las Conchas, are also strong manmade igniters.

Even a spark from a blown-out tire can start a fire. So can the slash-and-burn techniques which humans have used to clear land for agriculture since prehistory, military operations, mining, and logging, which leaves behind abandoned roads. Overgrown by flammable grasses and other vegetation, these logging corridors foster wildfire and enable it to spread quickly.

According to the New Mexico Forestry Division, 106 fires on state and private land (federal lands not included) in the past year were started when trees fell on power lines. Only equipment use, lightning, and burning debris caused more fires in the state than power arcs.

Surface fires, which consume mostly grasses, detritus (fallen leaves, needles, bark, sticks, branches, and live tree stems), downed trees, small living shrubs, and other combustibles at ground level, are dangerous but can usually be controlled. When burning enters subterranean roots and buried organic matter such as peat, it can smolder for days to months. Lower down, mine fires or coal seam fires can burn for decades or even centuries.
When a fire rises above ground level, its chances of going out of control also soar. An uncontrolled ground fire becomes a ladder fire in wooded areas. Fire in low-level vegetation, climbing ferns, vines, mosses, and the branching, leafy understory of trees make up a ladder of combustibles. As the flames burn continuously upward, fed by the ladder fuels, the danger increases. It reaches a critical level when the fire begins to ignite the canopy, atop the trees.
Once a fire crowns out, it can spread from tree top to tree top. Extreme heat, intensifying winds, convection currents and updrafts, and tornado-force fire whirls then take the fire to extremes. Crown fires are the most deadly and most damaging type of wildfire. Crews find them nearly impossible to control. Sometimes they rage on until the weather changes.

Las Conchas fire: Postscript

Halfway through the course of the Las Conchas fire, twice as much land burned at the Honey Prairie complex in Georgia's Okefenokee National Wildlife Refuge. In Texas, fires have ravaged almost three and a half million acres so far this year. Arizona and California have also been hard-hit. Colorado's Pike and San Isabel fires, ignited in May, are still burning.

A month after the New Mexico aspen tree struck the power line near Mr. Cox's ranch, 207 fire workers still remain in the forest. Thirty are surveying and planning reclamation of the burned area. Pockets of the largest fire in New Mexico's recorded history are still burning and under patrol, though the flames have been 95% contained.

In this area of the Jemez Mountains, thunderstorms can form and produce lightning within 30 minutes. Yesterday, lightning started a new fire within the Las Conchas perimeter. This one has been contained.

NEXT: Benefits of wildfire

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