Introduction
Dr. Craig Allen, a U.S. Geological Survey 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 — 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, shrub land 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. 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 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 what is believed to be 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 three to eight 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 (a) 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.”
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