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USGS Studies Wildfire Ecology in the Western United States Part 2

Released: 9/17/1999

Contact Information:
U.S. Department of the Interior, U.S. Geological Survey
Office of Communication
119 National Center
Reston, VA 20192
At SEJ: Gloria Maender 1-click interview
Phone: 301-206-9633 (through 9/18/99)

Catherine Haecker
Phone: 707-826-5645

The Mojave and Sonoran Deserts

In the Mojave Desert of California and Nevada, and the Sonoran Desert of Arizona, researchers are grappling with a fire and invasive species problem similar to that affecting Great Basin shrublands. Fire has not traditionally played a large role in organizing biological communities in these environments, where extremely arid conditions limit the density of vegetation. But in the deserts, too, alien grasses are now spreading, bringing rapid fire cycles with them.

Todd Esque, an ecologist with the USGS Western Ecological Research Center, says that although cheatgrass is not a big problem in the Mojave, other exotic species such as red brome are spreading fire through native communities that often have few evolved defenses against such disturbances. Esque and others are conducting detailed studies to better understand how increased fire size and frequency can affect desert ecosystems, and how native plants and animals can be protected. "We’re trying to take a holistic view of the fire-weed cycle," says Esque. "We’re looking at how fire changes nutrients in the soil, which changes the plants that are there, and in turn how animals respond to this dramatic change in habitat."

One of Esque’s USGS collaborators, Dr. Matt Brooks, has studied recent historical changes in fire incidence and fire effects in the Mojave. Brooks says that while it is difficult to reconstruct long-term fire histories in desert systems, records from federal land management agencies do show an increase in Mojave Desert fires over the past two decades. Expanding human use of desert lands may be behind some of the increase, but Brooks says the pattern holds even in remote areas where fires are almost all lightning-caused.

"The increase in fires seems to be due to the alien annual grasses," Brooks says. These grasses often build up during years of heavy rainfall and, unlike many native annuals, their dry stalks may remain rooted in the ground for many years after they die, providing a lasting fuel source.

Brooks and others have also found that native Mojave Desert plants are often particularly vulnerable to fire. Although some species do resprout after burning if the fire intensity is not too high, few can tolerate successive burns. "If a second fire occurs before fire-damaged individuals have a chance to build back their above-ground, photosynthesizing biomass, they often die," says Brooks. "The grass-fire cycle reduces the return interval between fires to the point where most native desert shrubs and bunchgrasses cannot survive."

Much of Esque’s own work is focused on understanding the mechanisms of invasion. He is carrying out a set of manipulative experiments comparing deliberately burned and unburned plots. Previous researchers have found that in desert environments, seed-eating rodents and ants often play a large role in determining the structure and composition of the vegetation. By removing ants and rodents from some of his plots, Esque can assess how the presence or absence of seed-eaters, along with changes in the surface vegetation and soil nutrients caused by fire, interact to determine the course of weed invasion and habitat transformation.

Esque also takes advantage of the "natural experiments" provided by desert wildfires. In 1994, for example, he began a collaborative studies with USGS research ecologist Dr. Cecil Schwalbe, of the Western Ecological Research Center, on the effects through time of a large fire in Saguaro National Park in Arizona. In the aftermath of the fire, the researchers quickly assembled a field team and began a detailed census and monitoring effort both inside the burn area and on adjacent unburned lands.

In documenting the ensuing changes to the area’s plant and animal life, the study has focused on two of the most representative species of the Sonoran Desert: the saguaro cactus and the desert tortoise. Both suffered high mortality, and damaged saguaros continued to die several years after the fire -- which Schwalbe notes was of only moderate intensity. "Both tortoises and saguaros are long-lived species, which need very low annual mortality rates in order to maintain stable populations," Schwalbe says. "This fire resulted in a catastrophic loss for both of those species."

As in the Mojave, the fire problem in the Sonoran Desert is worsening. The 1994 fire in Saguaro National Park was spread by red brome. And Esque says his team’s surveys in remote, unburned areas of the park have revealed that penetration by exotic grasses -- including a perennial, drought-adapted species from Africa known as buffelgrass -- is far worse than was previously known. "There wasn’t a fire problem in this area before the exotic species came in," says Schwalbe. "Now we’re seeing a biome conversion, from palo verde and saguaro habitat to a mesquite-acacia savannah with a Mediterranean exotic grass understory. That’s the future of the Sonoran Desert -- especially near roads."

Southwest Forests

Some of the most extensive and detailed records of past fire activity come from the southwestern United States. Over thousands of years, this region’s widespread ponderosa pine forests have been shaped and structured by fire. Historically, frequent low-intensity ground fires maintained open, park-like forests with grassy understories. Although such fires are often very local in nature, a broad historical perspective reveals regional-scale patterns of fire incidence and intensity, driven by climatic variability.

Dr. Craig Allen has studied the history and effects of fire in the Jemez Mountains of northern New Mexico since 1986. He and his collaborators employ several different methods for reconstructing the fire history of the Jemez and neighboring Sangre de Cristo mountains. Fires that do not kill a tree often leave a scar, which is recorded in the tree’s annual growth ring. By carefully examining the tree rings, researchers can determine the year and often even the season in which the fire occurred.

Allen’s team has put together over 4,500 fire dates, from over 600 trees, logs and stumps. "The Jemez is one of the better-sampled landscapes of its size anywhere," says Allen. "Very few areas have as much fire history." Analysis of tree rings is carried out in cooperation with Dr. Thomas Swetnam of the Laboratory of Tree Ring Research at the University of Arizona. The Jemez data form part of the lab’s regional tree ring network for the entire Southwest, which contains regional fire history and climatological records for over 1,000 years.

Now Allen is using a different method to extend the Jemez fire history record back even further. In collaboration with Dr. Scott Anderson of Northern Arizona University, he is reviewing pollen and charcoal deposits in soil cores extracted from several northern New Mexico bogs. The cores contain a record of sediment deposition going back over 10,000 years. In one Jemez sample that has already been analyzed, Allen says, the contrast between the current century of fire suppression and the millennia that preceded it are clearly visible. "There is abundant charcoal throughout the core, except for the last couple of centimeters, corresponding to the past 125 years," he says. "It shows that fire has been an important ecological process here in the Jemez for at least 8,000 years."

Although humans have long shaped their landscapes through deliberate use of fire, Allen says fire patterns in the Southwest have largely been driven by the region’s weather patterns. "Human ignitions were probably less important here than in most places on the planet," he says. Recent data show that the Jemez Mountains average about 16,000 lightning strikes per year, and Allen’s analysis of fire suppression records for roughly 5,000 fires since 1909 indicate about 75 percent were of lightning origin.

Fire scars indicate that historically, blazes were most frequent in the dry spring and early summer period, before the arrival of the late-summer monsoon rains. Most burned only along the ground, clearing away debris and maintaining open, montane grasslands over large areas. Some of the trees Allen has sampled experienced more than 30 fires over the course of a few hundred years, without being killed.

Swetnam and climate change scientist Dr. Julio Betancourt, of the USGS Desert Laboratory, have shown that patterns of fire incidence in Allen’s Jemez data are often mirrored across the broader Southwest region. The episodic occurrence of "regional fire years" appears to be associated with El Niño and La Niña events. Much of the Southwest is strongly affected by the weather patterns that characteristically follow these shifts in equatorial Pacific Ocean currents. El Niño years bring above-normal precipitation to the region, while La Niña years -- which often follow on the heels of El Niños -- are dry.

As might be expected, fire activity historically is greatest during La Niña events and droughts. But wet El Niño episodes play a role as well. In an environment in which water is often a limiting resource, wet years result in a rapid build-up of herbaceous understory vegetation. Years of intense regional fire activity often occur at the end of an El Niño-La Niña cycle, when this extra plant growth becomes a blanket of dry fuel across southwestern mountain ranges.

In the late 19th century, however, other factors came to dominate the region’s fire regime. Allen says that in the arid Southwest, grazing has played at least as big a role as fire suppression in altering the natural pattern of frequent, low-intensity burns. Beginning in the 1880s, large numbers of cattle and sheep were introduced into southwestern forests. As grazers consumed the grasses and other herbaceous vegetation fires need in order to spread, fire activity dropped off. In addition, said Allen, the trails created by livestock over time probably constrained the spread of fire as well by breaking up the continuity of the surface fuels.

"The initial cessation of fires preceded active fire suppression by several decades," Allen says. As grasses were reduced and fires ceased to spread across the landscape, more trees were able to get established. The effects of grazing were then multiplied when fire suppression became the norm early this century. Eventually, open areas were replaced by dense tree stands. Allen says that historically, a typical density of ponderosa pines in the Southwest was around 100 stems per acre. Today, densities at many sites exceed 2,000 stems per acre.

In these dense forests, destructive insect outbreaks are common. And fires, when they do get established, now often leave the ground and climb "ladder fuels" into the treetops. "We did not start to see extensive crown fires in ponderosa pine forests until around the 1950s," Allen says. "It took that long for the forests to get dense enough and for the fuel conditions to change."

Tree ring records show that, in addition to the El Niño-La Niña cycles, periodic droughts and wet periods of much longer duration are also part of the normal climatic variability in the Southwest. A prolonged drought during the 1950s contributed to outbreaks of large, destructive fires at that time. However since then -- particularly over the last 20 years -- precipitation totals across most of the Southwest have been abnormally high -- a fact that Allen says should be cause for concern.

"It’s been extremely good for tree growth, and a lot of extra forest biomass has accumulated on the landscape," he says. "The next time we have a significant drought of any sort, we can expect some very severe fire behavior. It’s the scale and associated ecological effects of potential crown fires that we worry about. The forests across whole mountainsides can just go up, burning entire watersheds and resulting in severe post-fire erosion and flooding. Once such crown fires are in progress, we can’t stop them through direct suppression methods."

The solution, says Allen, is to thin the forests, but how this should be done in overcrowded ponderosa pine and mixed conifer systems is the subject of much debate. Many fire ecologists feel that prescribed burning is the best way to restore a more normal ecosystem structure; others argue that in at least some areas, trees are so dense that some mechanical thinning should be done first. Allen says current fuel conditions are so hazardous that prescribed burning needs to be done with caution. "You’ve got to get it hot enough to change the structure of the forest," he says. "But the difference between burning it hot enough to kill some trees and burning it so it kills all the trees . . . that’s a challenging prescription."

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