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USGS Studies Wildfire Ecology in the Western United States Part 1
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: 310-206-9633 (through 9/18/99)

Catherine Haecker
Phone: 707-826-5645

Note to Reporters: Included are regional fire studies from the Sierra Nevada Forests; Interior and Coastal Shrublands; Mojave and Sonoran Deserts; and Southwest Forests


Dr. Craig Allen, a USGS research ecologist with the Midcontinent Ecological Science Center, is speaking of the New Mexico forest ecosystems he knows best, but his words apply equally well to most of western North America. "If you’re trying to understand past and present patterns on the landscape," Allen says, "first of all you need to know something about fire."

From the northern Rocky Mountains to the Southwest borderlands, wildland fires have burned and rejuvenated western forests over the course of millennia. And forests are not the only environments affected by fire; to a greater or lesser degree, fire influences the structure and dynamics of nearly all of the West’s terrestrial ecosystems. In some, such as the chaparral brushlands of California, fire has been a strong force guiding the evolution of local plant life, and a constant regulator of ecological communities. In many desert habitats, on the other hand, fires have been far less frequent, but represent a more severe disturbance when they do occur.

Scientists and managers increasingly recognize the importance of fire as a natural component of ecological systems. But while fire is often a beneficial process, it is always, in the short term, a destructive one. The presence of fire has usually been seen as incompatible with both human land-use practices and aesthetics, and for over a century fires have been actively suppressed throughout the West.

The negative consequences of forest fire suppression can now be clearly seen. In many areas, disruption of the natural fire regime has produced overcrowded forests with vast accumulations of dry fuel. Blazes that break out under these conditions may be far more destructive than the normal fires of centuries past and are often extremely difficult or impossible to control. The absence of a regular fire cycle has also harmed many plant and animal species whose life histories are tightly linked to fire disturbance.

Across regions and among different forest types, the historical role of fire and the effects of recent fire suppression vary. And while fire suppression has fundamentally altered many forest ecosystems, the opposite is often true in grassland, shrubland and desert habitats. In these systems, fire incidence has been increasing, often due to the spread of non-native vegetation, with negative consequences for native plants and animals. Thus no single prescription for fire management will work in all areas. Programs of prescribed burning, highly successful in some forests, may not succeed in other habitats.

The challenge for managers seeking to restore more normal fire dynamics to a particular region is indeed, as Allen observes, to know something about fire: how fire has historically affected the local system, and how it functions today. Across the West, USGS researchers, in collaboration with scientists from numerous other agencies and institutions, are providing this information through detailed studies of fire history and fire ecology in different environments. USGS Director Charles G. "Chip" Groat says that, "Knowledge from these studies is forming the basis for new policies aimed at restoring fire cycles that will present a lower risk to human life and property, and help safeguard the stability and diversity of western ecosystems."

Sierra Nevada Forests

Since the 1960s, pioneering studies on the effects of both forest fires and decades of fire suppression have been carried out in the Sierra Nevada mountains of California in Yosemite, Sequoia and Kings Canyon National Parks. As in the Southwest, fire suppression in the Sierra Nevada has now led to conditions in which catastrophic fires may threaten the forests themselves. Suppression of lightning-caused fires has resulted in denser forests, invasion of open areas by trees and shrubs and large accumulations of woody debris.

Scientists and managers in the Sierra Nevada parks have long recognized the essential nature of fire in these forests and have responded over the years with an increasingly sophisticated fire restoration program using both prescribed burns and natural fires. At Yosemite, USGS fire ecologist Dr. Jan van Wagtendonk has devoted over a quarter-century of research to understanding what controls the behavior of forest fires, and how natural and prescribed fires can best be managed to reduce understory fuel loads and restore normal ecosystem dynamics.

Simple in overall conception, the use of fire in ecological restoration is a highly complex undertaking. Van Wagtendonk says that to be successful, fire management programs require a clear set of goals based on a detailed understanding of the role fire has played in the local forest environment. Managers also need extensive information regarding fuel loads, weather, topography and other factors to make informed decisions on where, when, how often and how hot to burn. "To know whether or not to allow a lightning fire to burn, managers need to know where it might spread in the next three months -- or the next three hours," van Wagtendonk says.

His current work has centered on the development of a new, high-resolution fuels map for Yosemite National Park. The map is based on satellite images of vegetative cover broken down into 30 by 30 meter squares, each representing one of 30 unique fuel categories. Additional data are provided by geographic information system (GIS) maps, aerial photographs and field measurements from more than 1,000 sites. All of this information is coupled with a computer model for predicting exactly where and how fast a given fire may spread.

The final product is a highly versatile tool for understanding fire behavior. Because of its relatively fine scale, van Wagtendonk says, the map captures the mosaic-like nature of surface fuels over fairly small areas. Studies have demonstrated that fire spread is highly sensitive to this kind of local variability in fuel type, but previous fuel maps derived from remote sensing data have been unable to capture this level of detail. Moreover, the depth of information contained in the map allows researchers to conduct both long-term and real-time predictive modeling.

The map and model have already been used on several occasions to predict the behavior of natural fires. From each such application, further refinements are made. In these initial tests, such as during Yosemite’s Horizon Fire in 1994, the model performed well, said van Wagtendonk, providing managers with maps showing where fire perimeters would be at various future times, based on existing or changing weather conditions. The model has since been used to plan and execute prescribed burns in the park and to predict fire behavior on landscapes subjected to different techniques of understory fuel reduction, from mechanical thinning of trees to prescribed burning.

Van Wagtendonk says potential applications go beyond managing fires within the park. The mapping and data analysis techniques he has developed can in principle be extended to much larger areas, such as the entire Sierra Nevada. The fuels modeling package can also be used as a research tool. For example, scientists can approximate what the local landscape might look like without a history of fire suppression, by allowing past suppressed fires to "burn" and run their course on computers.

While advanced imaging and computer technology can help predict fire behavior in the future, tree ring analysis reveals fire patterns of centuries past. At Sequoia and Kings Canyon National Parks, USGS researchers and collaborators from the University of Arizona’s Laboratory of Tree-Ring Research have put together the longest and most detailed fire histories anywhere. The records, assembled from fire scars in the annual growth rings of giant sequoias, extend back over 2,000 years, and show that fire typically burned on the floor of sequoia groves every 3 to 8 years.

USGS ecologist Dr. Nate Stephenson, from the Western Ecological Research Center, says the record shows how sequoias have responded to what has been, on a scale of centuries, an ever-changing climate and fire regime. The historical record shows a shifting matrix of low to moderate-intensity fires, with occasional hot spots of severe fire that open gaps in the forest and clear the way for sequoia regeneration. "The hot spots reduce competition so that the sequoia seedlings have chance," Stephenson says. Sequoia seeds require contact with bare soil in order to germinate, and this is possible only when fire has cleared away the layers of leaf litter and debris.

The loss of fire in sequoia groves has greatly affected the population. "Fire exclusion by humans has done more than the last three millennia of climate and fire regime changes," Stephenson says. "Essentially, when you exclude fire, sequoia reproduction crashes to zero." That means that in sequoia groves today, even the youngest trees are over a century old. Most areas in most groves have not burned for 100-130 years.

The good news, says Stephenson, is that the research message is reaching managers. He has studied the effects of different forest restoration measures including prescribed burning and mechanical thinning of trees. Unlike some other forest systems, Stephenson says, sequoia groves respond extremely well to prescribed burning alone, with no other treatment needed. "Where we have had prescribed fires, there’s now a lot of sequoia reproduction -- enough that if it is maintained over the long term it will maintain the populations."

Is a burned forest a healthier forest? Certainly by some standards, but Stephenson prefers to say that fire restores stability and resiliency to forest ecosystems. "We’re restoring a forest structure that’s more stable, meaning if you give it a shove it’s less likely to be bent out of shape. And it’s more resilient, because if you do bend it out of shape it will bounce back quicker."

Fire management and restoration programs in the Sierra National Parks now reflect much of what researchers like van Wagtendonk and Stephenson have learned about the behavior and ecology of wildfires. The current prescribed burning program, says Stephenson, is highly successful. "It’s an excellent example of how research has fed into management and changed management direction."

Nevertheless, says van Wagtendonk, "so much needs to be done, it’s hard to get ahead of the game." One major constraint is smoke, which limits the amount of prescribed burning that can be done. Fire managers must work to stay within the bounds of clear air standards, and limit the amount of smoke descending on local communities. Stephenson says that while only a few prescribed fires create a smoke problem, these can erode public support for fire restoration. Continuing education is vital, he says, for people to understand that without some fire, both forests and human communities face the ever-growing danger of a major conflagration.

Interior and Coastal Shrublands

While the decline of old-growth forests has long been a high-profile issue in the West, the widespread loss of arid shrublands has gone practically unnoticed. But in the sagebrush ecosystems of the Great Basin and the Columbia River Basin, fire and a non-native plant species known as cheatgrass are together transforming ecological communities across a vast area. These changes may be irreversible, says USGS ecologist Dr. Steve Knick of the USGS Forest and Rangeland Ecosystem Science Center.

Knick studies these transformations at the Snake River Birds of Prey National Conservation Area in southwestern Idaho. Here, as in much of the Great Basin, the dominant vegetation -- sagebrush and other shrubs adapted to the harsh seasonal climate -- is disappearing. Of the roughly 100,000 hectares of shrubland present in the National Conservation Area in 1979, only 46,000 hectares remain.

To put it simply, Knick says, the shrubland is burning up. Wildfire incidence has increased by a factor of three since 1980, and fires are getting larger. But behind this increase -- and in turn capitalizing on it -- is the fast-spreading, exotic annual grass. "It’s a synergistic thing," says Knick. "Cheatgrass promotes fire spread, and the larger fires eliminate more shrubs."

Knick says that fire has always been a factor in sagebrush ecosystems, creating openings in the shrub canopy and constraining the density of woody plants much the same as in forests. But an understory of native bunch grasses, which grow in isolated patches, tends to limit the intensity of blazes in these systems and prevent them from spreading over a wide area.

Cheatgrass, which has been advancing since the early 1900s, in part due to overgrazing and drought, creates a continuous carpet of fuel. And cheatgrass thrives on recently burned land, thus perpetuating the altered fire regime. If the shrubs in an area don’t have time to recover before the next fire hits, they eventually disappear. "Fire has gone from maintaining a shrubland, to destroying a shrubland, to ultimately maintaining an exotic grassland," Knick says.

Using data from a number of sources including satellite imagery, historical records of fire frequency and behavior, and ground measurements of vegetation, Knick’s team has developed a computer model for predicting long-term changes resulting from different scenarios of burning and regeneration of vegetation. The model shows that in shrublands with a cheatgrass understory, fire can easily trigger a rapid transition to grassland. But once established, these grassland systems tend to be relatively stable, even when fire is suppressed.

"The daunting thing is that it’s going to take a long time to replace what has been destroyed in the last 20 years," Knick says. "We’re looking at centuries if we rely only on natural processes for recovery." Knick’s work suggests that preserving intact shrublands through more active fire suppression may be the only way to halt the losses. Restoration practices based on prescribed burning, as have been successfully carried out in forest ecosystems, may not work well in invaded shrublands.

"In forests you can use prescribed burning to remove a lot of the fine fuels, with the expectation that they are going to take several years to grow back," Knick says. "In shrublands dominated by cheatgrass, the cheatgrass will be back next year. It’s using a disturbance to try to eliminate a species that likes disturbance."

While interior shrubland ecosystems have only a limited tolerance for fire, a very different kind of fire dynamic exists in the chaparral shrublands of coastal California. Ecologists have long known that chaparral ecosystems burn extensively and often, and much of the dominant vegetation in these systems is highly adapted to a fire-prone environment. Many plants have seeds that require fire to germinate, or need the kind of disturbed habitat fires leave behind in order to grow.

Dr. Jon Keeley, a USGS research ecologist with the Western Ecological Research Center, has studied the physiological adaptations that link the life cycles of chaparral vegetation with the natural regime of frequent brushfires. Upon reproduction, many species drop seeds that remain dormant in the soil "seed bank" until fire creates favorable growth conditions. When the area burns, these seeds receive a number of cues that may cause them to germinate. While seed germination in some species is stimulated by heat, in many others the onset of plant growth requires chemical exposure to combustion products such as charred wood.

Recently Keeley and Dr. C. J. Fotheringham, of California State University, Los Angeles, published a study demonstrating that for many species, smoke can also trigger seed germination. In some species smoke alone is sufficient to induce growth, while in others a combination of factors is required. Because of the diverse cues through which vegetation may respond to fire, blazes of different intensities or degrees of smoke production may result in different plants dominating the post-fire recovery. Of particular interest is their discovery, detailed last year in the journal Science, that nitrogen oxides, which are also important components of air pollution, are the chemicals in smoke responsible for germination of some species.

Keeley and his collaborators have also examined historical patterns of California shrubland wildfires. Life and property losses from shrubland fires in California have been increasing in recent decades. It has long been thought that fire suppression has played the same role in chaparral shrubland as it has in forests, creating a build-up of fuels that eventually leads to more destructive fires.

Surprisingly however, a close analysis of state fire records revealed a different story. This June, in the journal Science, Keeley and his co-authors reported that since 1910, chaparral fire frequency has not changed and fire size has not increased. The researchers found that large, intense fires were equally common in the years before widespread fire suppression as today, and do not appear to be the result of fuels build-up. In this highly fire-prone ecosystem, suppression efforts appear not to have greatly altered normal patterns of fire incidence. Keeley says the greater financial cost of fires today is more likely the result of constant urban expansion into areas subject to frequent burning.

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