SANTA FE, N.M. -- The Southwest's forests are under greater stress today than they have been in centuries, thanks to the vagaries of climate change, researchers said at a fire ecology conference held here this week.

As average temperatures in the region have risen, forests across the Southwest have become drier, said Craig Allen, a research ecologist with the U.S. Geological Survey who is based at the Jemez Mountain Field Station near Los Alamos, N.M. Drought conditions, along with fuel buildups from a century of fire suppression beginning around 1900, have led to an increase in severe fires in recent years.

These ponderosa pines in northern New Mexico's Jemez Mountains died from a combination of drought stress and bark beetles, which often attack weakened trees. Photo courtesy of Craig Allen/U.S. Geological Survey.

Some of the largest and hottest-burning of those conflagrations include the 469,000-acre Rodeo-Chediski fire in Arizona in 2002 and, more recently, the 538,000-acre Wallow fire on the Arizona-New Mexico border and the Las Conchas fire, which blackened 156,000 acres last summer in the Jemez Mountains near Los Alamos, making it the largest wildfire in New Mexico history.

"Surprise, surprise -- dry years are driving fire activity," Allen told attendees of the Southwest Fire Ecology Conference during a presentation on Tuesday. The likelihood of fire is especially great in dry years that follow a wet year or two, when an area greens up and then dries out, he added.

During the 100 years or so when forest managers snuffed out almost all fires on public lands -- before forest scientists understood the importance of fire to many forest types -- some of the Southwest's forests saw fewer fires than they had at any time in the past 9,000 years, according to Allen's research, which uses tree rings to reconstruct historic climate conditions.

The lack of fire during the 20th century has greatly affected the vegetation on Southwestern lands, creating "homogenized" forests that were once far more diverse and allowing trees to encroach into grasslands, Allen added. For instance, in the Jemez Mountains in northern New Mexico, where Allen conducts most of his research, grasslands declined by 55 percent between 1935 and 1981 due to tree encroachment.

At the same time, warming temperatures are contributing to diebacks of large swaths of forest and appear to be slowing down tree growth, he added, which could hinder the ability of forests to regenerate after a fire.

"The trees here aren't growing very well," Allen said. "The Southwest is an area where the trees seem to be more sensitive."

The region's trees -- piñon pines and juniper at lower elevations; ponderosa pines at mid-elevations; and aspen, spruce and fir higher upslope -- haven't seen such tremendous drought stress since the 13th century, he added. "The drought stress in the 2000s matches the 1200s drought," he said.

But no one really knows exactly how extensive the damage will be, he said.

"How much heat stress can trees tolerate?" he asked, adding that it's difficult to predict how long a tree withering in the heat can survive. "It may take years for a tree to break down."

Another implication of the unusally challenging conditions in Southwestern forests is a higher risk to water supplies in some areas.

If a severe fire burned through Santa Fe National Forest, for example, the resulting erosion of sediment could clog two reservoirs above the city that supply 40 percent of its drinking water, said Deborah Finch, a researcher with the Forest Service's Rocky Mountain Research Station in Albuquerque, N.M.

A more accepting public?

Meanwhile, as conditions in the Southwest become drier and severe fires become more frequent, the region's residents are increasingly accepting of fuel treatments designed to reduce the risk of such fires, said Sarah McCaffrey, a social scientist with the Forest Service's Northern Research Station in Evanston, Ill., who studies attitudes toward fire.

Examining several studies from around the country on public attitudes toward fuel treatments, McCaffrey found that about 80 percent of all respondents said they see forest thinning and prescribed burning as appropriate tools to manage fire risk.

"People actually have a pretty good understanding of fire ecology," she said during her presentation at the conference.

Once people understand the importance of fuel treatments for restoring ecosystem health and reducing the risk of severe fires, they're more willing to support those treatments, she said.

"Treatments are generally acceptable, as long as they're done by people who know what they're doing," she said.

Reese writes from Santa Fe, N.M.