Introduction

Wildland fire (fig. 1) is a serious and

growing hazard over much of the United

States, posing a great threat to life and

property, particularly when it moves from

forest or rangeland into developed areas.

However, wildland fire is also a natural

process, and its suppression is now

recognized to have created a larger fire

hazard, as live and dead vegetation

accumulates in areas where fire has been

excluded. In addition, the absence of fire

has altered or disrupted the cycle of

natural plant succession and wildlife

habitat in many areas. Consequently, U.S.

land management agencies are committed

to finding ways, such as prescribed

burning to reintroduce fire into natural

ecosystems, while recognizing that fire

fighting and suppression are still

important. The U.S. Geological Survey

(USGS) conducts fire-related research to

meet the varied needs of the fire management community and to understand the

role of fire in the landscape; this research

includes fire management support, studies

of postfire effects, and a wide range of

studies on fire history and ecology.

Fire Management Support

Greenness Mapping

Land and fire managers rely upon

accurate and timely information to help

reduce wildland fire hazards. Since the

early 1990's, the EROS Data Center

(EDC) in Sioux Falls, S. Dak., has been

producing weekly and biweekly maps for

the 48 contiguous States and Alaska that

display plant growth and vigor, vegetation

cover, and biomass production, using

multispectral data from satellites of the

National Oceanic and Atmospheric

Administration (NOAA). The ability to

measure vegetation greenness over time

provides fire managers vital information

concerning vegetation conditions for any

2-week period (fig. 2). EDC also produces

maps that relate current vegetation conditions for the 2-week period to average

(normal) conditions for the same period

during the past 7 years. The two types of

images provide comprehensive growing

season profiles for forests, rangelands and

grasslands, and agricultural areas. They

are used by fire and land managers to

assess the condition of all vegetation

throughout the growing season, so they

provide a foundation for planning for fire

suppression, scheduling prescribed burns,

or studying long-term vegetation changes

resulting from human or natural factors.

For example, the U.S. Forest Service

(USFS) uses greenness maps to generate

national maps of selected fire weather and

fire danger components of their Wildland

Fire Assessment System (http://www.fs.

fed.us/land/wfas/welcome.html).

Fire Potential Index

The Fire Potential Index (FPI) is a

valuable fire management tool that has

been developed by USGS scientists in

collaboration with scientists at the USFS.

The FPI characterizes relative fire potential

for forests, rangelands, and grasslands,

both regionally and locally, so that land

managers can develop plans for minimizing the threat from fires. The FPI

USGS Wildland Fire Research

U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet 125-98

October 1998

Figure 2. Greenness map for Alaska, for the period of August 1998. Percent moisture is a measure of how much more water is

in live vegetation than in completely dry vegetation.

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Percent Moisture

Figure 1. Wildfire in the Pacific Northwest.
combines multispectral satellite data from

NOAA with geographic information system (GIS) technology to generate 1-km

resolution fire potential maps. Input data

include the total amount of burnable plant

material or fuel load (derived from

vegetation maps), plus the water content

of the dead vegetation, and the fraction of

the total fuel load that is live vegetation.

Water content of dead vegetation is

calculated from temperature, relative

humidity, cloud cover, and precipitation,

and the proportion of living plants is

derived from the greenness maps

described above. The FPI is updated daily

to reflect the changing weather conditions

and is posted by the USFS on their Web

site. Figure 3 is the FPI map for August

13, 1998.

The Bureau of Land Management (BLM),

Bureau of Indian Affairs, and USFS are

working with the USGS to validate the

model. Fire management staffs in Oregon,

Nevada, and California are using the FPI

in their decision-making process daily to

supplement their traditional information

sources. They use these data for purposes

such as establishing priorities across the

area for prevention activities to reduce the

risk of wildland fire ignition and spread,

and planning the allocation of suppression

forces to improve the probability that

initial attack will control fires occurring in

areas of high concern. The FPI is also

being tested in Argentina, Chile, Mexico,

and Spain with the support of the Pan

American Institute for Geography and

History.

Postfire Effects

Fire and Debris Flows: A Double Hazard

Wildfires leave problems behind them,

even when the last ember is extinguished.

In July 1994, a wildfire burned 2,000

acres of forest and scrub on the steep

slopes of Storm King Mountain, near

Glenwood Springs, Colo. In September

1994, torrential rains triggered debris

flows, which poured from this burned area

and inundated a 3-mile stretch of

Interstate 70 with tons of mud, rock, and

other debris (fig. 4). The flows engulfed

30 cars, sweeping 2 into the Colorado

River. Some travelers were seriously

injured, but fortunately there were no

deaths. 

Scientists from the Landslides Hazards

Program of the USGS are studying the

linkage between wildfires and debris

flows at Storm King Mountain and sites

in several other States. During an intense

wildfire, all vegetation may be destroyed;

also the organic material in the soil may

be burned away or may decompose into

water-repellent substances that prevent

water from percolating into the soil. As a

result, even normal rainfall may result in

unusual erosion or flooding from a burned

area; heavy rain can produce destructive

debris flows, as happened at Storm King

Mountain.

In 1997, wildfires charred many areas of

southern California, leaving them bare

before the 1997-98 winter’s heavy El

Niño rainfall. A map delineating

perimeters of 25 of the 1997 southern

California wildfires that burned more than 

300 acres can be found at

http://geohazards.cr.usgs.gov/html_files/

landslides/scfires/scfiresloc.html. Ten of

these burns produced debris flows after

the first major winter storm, and flooding

dominated in eight areas. Only four

showed little or no erosion or runoff. The

relative importance of topography,

vegetation conditions, and the amount and

pattern of rainfall are being investigated at

each of the sites develop a model for

debris-flow susceptibility from recently

burned areas.

Effects of Fire on Water Supplies

Water supplies are also affected by fire:

the loss of ground-surface cover, such as

needles and small branches, and the

chemical transformation of burned soils

make watersheds more susceptible to

erosion from rainstorms. In May 1996, an

11,900-acre (4,820 hectares) fire burned

most of the Buffalo Creek and Spring

Creek watersheds; these small watersheds

feed into the Strontia Springs Reservoir,

which supplies more than 75 percent of

the municipal water for the cities of

Denver and Aurora (fig. 5). Two months

after the fire, a severe thunderstorm

caused flooding from the burned area,

killing two people. In addition, the

Denver Water Department immediately

experienced a deterioration of water

quality from floating burned debris and

high levels of manganese. Two years after

the fire, phosphate levels in the water

remain high, and the Denver Water

Department is concerned about loss of

reservoir capacity and impaired water

quality.

Figure 3. Fire potential index map of the conterminous United States , for August 13, 1998. White areas are cloud cover, gray

are agricultural lands (not rated by the FPI).

Figure 4. Debris flows cover parts of Interstate 70 below

Storm King Mountain, Colo.
USGS researchers are studying the postfire hydrogeology of Buffalo and Spring

Creeks to understand the processes of

runoff generation, sediment production,

and the transport of sediment out of the

watersheds and into the reservoir.

Although the burned hillslopes are

showing signs of revegetation, 2 years

later sediment is still being eroded.

Between September 1996 and August

1998, about 260,000 cubic yards of coarse

sediment were trapped in a delta at the

upper end of the reservoir. The postfire

accumulation rate is about 10 times that

calculated by the engineers who designed

the reservoir. Furthermore, this was not an

unprecedented event: stratigraphic studies

in the Buffalo Creek watershed suggest

that there may have been about seven

similar fire-flood events in the last 2,000

years. The Buffalo Creek Fire has

produced disastrous consequences for

Denver and surrounding communities.

However, the postfire evaluation is

expected to help land managers along the

Colorado Front Range better anticipate

the effects of wildfire on other watersheds

and other reservoirs. 

Fire in the Landscape: History,

Ecology, and Regional Behavior

Fire Effects in Sequoia and Kings Canyon

National Parks

In the giant sequoia groves of the Sierra

Nevada, Calif., cooperative research by

the USGS and the University of Arizona

has used fire scars found in giant sequoia

tree rings (fig. 6) to develop the world's

longest and most detailed fire histories.

The annual- and seasonal-resolution fire

records span two millennia, and show that

during most of that period, low- to

moderate-intensity surface fires burned in

parts of individual sequoia groves, on

average, about every 3 to 8 years. With

the loss of fires set by Native Americans

and the suppression of lightning fires that

followed Euroamerican settlement, most

grove areas today have experienced a 100-

to 130-year period without fire—a firefree period that is unprecedented in the

past 2,000 years.

The consequences of fire exclusion for

sequoia groves are beginning to show:

almost no new giant sequoias have begun

to grow in the last 130 years, because

sequoia seedling establishment depends

on fire to expose bare mineral soil and to

create clearings, which with their extra

sunlight and soil moisture, are especially

favorable for the growth of new trees. Fire

exclusion has also allowed the growth of

dense thickets of flammable, shadetolerant tree species. Forest litter and

woody debris have accumulated, causing

a huge buildup of surface fuels, which has

led to an increased hazard of intense

wildfires sweeping through groves, severe

enough to damage or even kill full-grown

sequoias.

This fire history information is being used

by giant sequoia managers to guide the

restoration and preservation of sequoia

ecosystems for present and future

generations. Managers are using low- to

moderate-intensity prescribed fires (fig. 7)

to reduce hazardous fuel loads, to thin

shade-tolerant tree species, and to open

small holes in the forest canopy. The

results are sequoia groves that are less

vulnerable to high-intensity wildfires and

that are experiencing their first abundant

sequoia reproduction in more than a

century.

Fire Ecology Research in South Florida

Prescribed fire has a long history in the

southeastern United States, where it has

been used to improve forage quality,

reduce pest populations, and decrease the

likelihood of destructive wildfires. Fire

has been so pervasive that most of the

familiar habitats in the southeast require

periodic fire to perpetuate themselves: if

pine forests remain unburned for long

periods, they are replaced by hardwood

forests that do not support the flora or

fauna native to the area. If left unburned

for as little as 5 to 10 years, pine forests

accumulate large amounts of fuel, making

them susceptible to destructive wildfires

such as occurred this spring in northern

Florida. The understory vegetation may

Figure 5. Map showing location of Buffalo Creek and Spring Creek fires relative to Strontia Springs Reservoir, near Denver, Colo.

Figure 6. Section of a giant sequoia, showing fire scars. Figure 7. A prescribed burn smolders on the floor of a

sequoia grove.
recover quickly from such wildfires, but

the canopy of pine trees may be

completely killed. This spells disaster for

species such as the endangered redcockaded woodpecker that depends on

large, live pine trees for its nesting

cavities.

USGS scientists are studying the ecological effects of prescribed fire in south

Florida pinelands. The National Park

Service (NPS) has a vigorous prescribed

burning program in south Florida in Big

Cypress National Preserve and Everglades

National Park (fig. 8). The USGS is

researching the response of pine forest

vegetation, including trees, shrubs, and

herbs, to different seasons and frequencies

of prescribed fire. Results of the study

will allow NPS fire managers to better

meet the ecological objectives of their

prescribed burning program. 

In the Lower Florida Keys, much of the

habitat of the endangered Key deer is pine

forest managed by the Fish and Wildlife

Service (FWS). Relatively little burning

has occurred here in the last several years,

and the accumulation of fuel and succession of other vegetation are objects of

concern. To complicate matters, houses

are scattered throughout the pineland, and

residential neighborhoods adjoin some of

the larger pine forest areas. Therefore,

smoke management and protection of

private property are primary considerations, along with ecological objectives.

USGS scientists and collaborators from

Florida International University are

studying the ecological effects of burns

during the winter dry season and the

summer wet season. In developing a fire

management program for the National

Key Deer Refuge, the FWS will consider

how unique herbaceous plants, pine trees,

and food plants of the Key deer respond

to the different seasons of burns, along

with logistical fire management

considerations. 

Regional Effects of Fire on Vegetation

In cooperation with the USDA Forest

Service, USGS scientists are developing

tools for the accurate assessment of fire

effects at regional scales. Fire is the

dominant ecological disturbance in forests

of the Interior Columbia River Basin in

the Pacific Northwest. Historical firedisturbance regimes in this region have

varied widely with respect to fire

frequency and severity, but the number of

available fire-history studies is limited.

We know that fire-return intervals in the

Interior Columbia River Basin during the

past century are longer than in previous

centuries because of landscape

fragmentation (caused by timber

harvesting, agriculture, and roads), fire

suppression, and lack of deliberate

burning as formerly practiced by all

inhabitants of the area. The lack of

frequent fire means that forest fuels per

acre are greater in many locations than

they would have been during the past few

centuries. 

Resource managers are reintroducing fire

into these landscapes to reduce fuel

loadings and the intensity of future fires. 

Accurate estimates of historical fire-return

intervals are necessary to implement

reasonable forest management 

plans. USGS scientists, in cooperation

with Forest Service scientists, have

developed a model that uses existing firehistory data for the region and calculates

statistical relationships between fire

occurrence and latitude, altitude, forest

type, and other variables to create a

regional map for fire-return intervals 

(fig. 9). Rather than having a number of

widely scattered points of information on

fire history, we now have a complete map

for all forest types in the Interior

Columbia River Basin. This model is

informative and cost effective, and it can

be used by scientists and resource

managers as a basis for further studies 

or land management practices. This

approach should be applicable for other

regions of North America as well. 

Information

For information on these and other USGS

products and services, call 1-800-USAMAPS, use the EARTHFAX fax-ondemand system, which is available 24

hours a day at 703-648-4888, or visit the

general interest publications Web site at

http://mapping.usgs.gov/www/products/

mappubs.html.

Please visit the Wildfire home page at

http://www.usgs.gov/themes/wildfire.html

and the USGS home page at

http://www.usgs.gov/.

Figure 9. Fire history sites and model fire return intervals in the Columbia River Basin.

Figure 8. Prescribed fire in Big Cypress National Preserve, Fla.