Protecting Archaeological Sites with Prescribed Fire

Jim Smith
California Department of Forestry and Fire Protection
Fresno, California 93727

CAL FIRE Editorial Note: This excellent paper was written by CAL FIRE Forester/Battalion Chief Jim Smith, currently assigned to CAL FIRE's Fresno-Kings Unit. Jim presented this paper at the 33rd Annual Meeting of the Society for California Archaeology held in Sacramento in April 1999, and his paper, along with 14 others presented at that symposium were subsequently published in: The Effects of Fire and Heat on Obsidian, Janine M. Loyd, Thomas M. Origer, and David A. Fredrickson volume editors, U.S. Department of Interior, Bureau of Land Management, Sacramento. CAL FIRE received permission from the publisher and the author to post this article on the CAL FIRE Web Site. Prescribed Fire Managers and others interested in this topic are encouraged to obtain the complete volume from BLM.


Past fire studies have shown that fire has a measurable effect on the hydration rind that forms on obsidian artifacts. Ecosystem management requires the reintroduction of fire through either prescribed fires or to allow wildfires to burn unabated. Wildfires are happenstance and when occurring in areas where significant archaeological resources are located, damage to sites can occur not only through suppression actions but from the unnatural fire intensities generated from accumulated fuel loading attributed to successful fire management practices. Wildfires therefore, do not afford the opportunity for archaeologists to successfully protect known and newly discovered sites. Prescribed fire, through proper planning and site surveys can protect archaeological resources and allow the reintroduction of fires as a natural process in fire dependent ecosystems.

Prescribed Burning is a Multi-Disciplinary Process

A single prescribed burn can achieve multiple benefits. Fire in resource management can achieve specific results for hazardous fuel reduction, prepare sites for seeding and planting, dispose of logging debris, improve wildfire habitat, manage competing vegetation, control disease, improve forage, enhance appearance, improve access, perpetuate fire dependent species, cycle nutrients, manage endangered species, and improve air quality. Since the early 1960's resource managers from all disciplines have been taking a constructive view of fire in North America. Prescribed fire is a reasonable way to reinforce fire into many ecosystems. Prescribed fire is defined as fire applied in a skillful manner, under exacting weather conditions, in a definite place to achieve specific objectives

In the hands of confident prescribed burners, fire is a versatile process that can achieve many objectives simultaneously in many plant communities. It is natural and frequently the only management method available to achieve particular objectives. Resource managers often speak of "tools" that can be used to manipulate the ecosystem. Tools can consist of hand clearing, ball and chaining, use of herbicides, logging, disking, chipping and mulching, fertilizing, and other mechanical methods. These are tools, but fire is not a tool, fire is a process that cannot be replaced or mimicked by any other means. Fire has been and is an essential component of our ecosystems.

Historic and Prehistoric Fire Severity

We often overlook the importance of aboriginal burning (Lewis 1973). Often we think of fire and fire ecology as originating from natural causes like lightning. Equally important and even more profoundly dominate; fire is a cultural phenomenon. It is among man's oldest tools, one of the first products of the natural world that was domesticated. Anthropological burning is the primary source of ignition in the world and mankind is the most significant modifier of the fire environment, most notably its fuels. Today, it is nearly impossible to discriminate between the influences of climatic change, biotic migrations, natural fire, and aboriginal firing of the landscape. Fire has been applied and reapplied for new as well as old purposes resulting in profound cultural and environmental changes. One would think then, that if man alone can create fire, he alone can extinguish it, and he can alter the landscape as much by excluding fire as by introducing it.

Prior to the 1930's large-scale fire suppression actions were not common. Fires continued to burn across the landscape and often were quite large in size. When the limited resources of personnel would gather to extinguish a fire threatening a town or a ranch, they would use burning out techniques by lighting backfires from existing roads, creeks, rivers, or other natural barriers. Even while such suppression actions were being taken on one fire, many more fires continued to burn until the fall rains or natural barriers extinguished them. Visualize the type of fire that was being experienced prior to 1930. The fires were often large in size covering thousands of acres. The fires were variable in intensity, but were most often of low intensity. The fires would spread with the changing wind conditions sometimes moving rapidly through the fuels and sometimes moving slowly. The fires would move rapidly upslope and back slowly downslope and into creeks and drainages. Fires burning in forested landscapes would often be ground fires preserving the canopy. In chaparral fires were often moderate intensities leaving a mosaic of uneven age classes. For the most part we can visualize that fire burned known and unknown archaeological sites, historical sites, and in many cases several times.

Following the introduction of large scale and effective fire suppression methods in the 1930's, large-scale fires would become less common. Agencies trusted with the protection of our natural resources found decades of success in their fire suppression efforts. Unknowingly to most of our public land managers, each successful year began the eventual buildup of fuels far beyond what would ever have been seen in previous centuries. Finally, in the early 1960's the catastrophic results of this unnatural fuel buildup began and continue today. Think now of the fires of today, where in the past, fires that burned unknowingly through archaeological and historic sites were of low intensities resulting in insignificant changes to artifacts, fires of recent history have far exceeded these intensities and result in significant and dynamic changes to artifacts.

Prescribed Fire Project Planning

How then, can we use prescribed fire to protect prehistoric and historic sites? Balancing all the environmental concerns for re-applying fire to the landscape is the responsibility of the Prescribed Fire Project Manager. Project development often has to balance protection of watercourses, rare and endangered plant habitats, wildlife corridors, smoke management considerations, aesthetics, and prehistoric and historic sites. What often is first proposed as a five hundred acres project can become less than a fifty acre project if all the resource managers consider their specialties to be mutual exclusive. Integrated resource management is the key to returning the project to its maximum intended size and yet achieves the results desired for all resource managers.

CAL FIRE uses a Programmatic Environmental Checklist with its Vegetation Management Program (VMP) to evaluate the protection measures needed to minimize impacts to archaeological sites.

In order to complete this checklist, the project manager and the associate archaeologist have the opportunity to review the project on the ground and search for potential sites. The use of CAL FIRE's VMP has allowed the legal access to thousands of acres of private lands and has led to the locations of numerous new archaeological sites. This opportunity to locate these sites has yielded significant information to add to the ethnology of California.

Once located, archaeological sites need to be protected from undesirable fire and site preparation effects. The prescribed fire project manager and the cultural resource manager should then evaluate three parameters. First, has the site experienced a wildfire in the past 75 years? If so, most of the perishable artifacts have already been destroyed. Second, the placement of fire lines must be considered to avoid impacts to the site. Third, what effects will the fire have on the remaining artifacts and what significance are they to future scientific study.

The easiest protection to provide is from mechanized equipment by excluding the area from encroachment of the equipment. In most cases this can easily be developed into the burn plan.

The difficult decision then needs to be given to excluding the site from fire. This decision may sound simple but can lead to disastrous results. Leaving the site untouched by fire during the prescribed burn may seem like the correct choice, but what will be the long-term implications?

Consider the results of a prescribed fire project where it was required to leave vegetation buffer along all ephemeral streams. Five years later a wildfire burned through the area, resulting in high intensity runs through the buffers left along the streams. All of the previously burned areas of the project received only light fire intensity levels and adequate surface material was left to protect the soil. Winter rainfall resulted in intense scouring of the stream courses where the vegetation was completely removed. Had the biologist understood fire behavior and allowed the prescribed fire project manager to let the fire slowly back into the stream course, the resulting wildfire and rainfall event would have been less catastrophic.

So now lets revisit the simple recommendation to exclude an archaeological site from a prescribed fire. Considerations must be given to:


1. Will the site stand out so as to be readily identifiable to unauthorized persons? and


2. Would a resultant wildfire cause extreme spalling, cause artifacts to be exploded, exposed to disintegration, or prevent further research and dating of artifacts?


Rather than a simple statement to exclude from the burn project a better course of action may be to evaluate the fire effect to the site and incorporate appropriate mitigation methods.

The first step is to consider the significance of the site. If the site doesn't appear to contain any artifacts out of the ordinary or would not make a dramatic change to ethnology then allowing fire onto the site may be appropriate. The next step would then be to mitigate the intensity of the fire. The project manager can provide an analysis of the intensity of the fire that would be applied, the duration of the fire, and the firing technique used on the site.

For example, if we chose to allow fire to burn over a site that contained a grass and woody debris ground cover we would look first at allowing a heading fire (burning with the wind) to cross the site. The fire behavior prediction run shows that fire line intensity would be 394 Btu/Ft/S. This is the amount of heat that would be generated at the flaming front. We must also keep in mind that 70 percent of the heat is transferred to convective lift. Note that flame lengths would be nearly seven feet and this would appear to the untrained observer as a very intense fire!


Rate of Spread, CH/H---------------------- 44.0

Heat per Unit Area, Btu/SqFt ----------- 490.0

Fireline Intensity, Btu/Ft/S--------------- 384.0

Flame Length, Ft---------------------------- 7.0

Effective Wind Speed, MI/H --------------- 5.0


The same project would then consider the use of a backing fire (burning against the wind). The fire behavior analysis shows us that by using a backing fire on 22 Btu/Ft/S would be generated at the flaming front almost 70% would be transferred through convective lift.


Rate of Spread, CH/H ------------------ 2.0

Heat per Unit Area, Btu/SqFt ------- 490.0

Fire Line Intensity, Btu/Ft/S --------- 22.0

Flame Length, Ft ----------------------- 1.9

Effective Wind Speed, MI/H ---------- 0.0

The simple solution would appear to use a backing fire on the site, however we must now consider the duration the fire would remain on the site. Head-firing shows that the fire would spread across the site at 44 chains per hour, or travel at about 48 feet per minute, thus the exposure of heat to the site would be short-lived. The backing fire would spread across the site in two chains per hour, which is approximately two feet per minute. Exposure of the fire to the site is significantly longer is we use a backing fire. This implication would lead us to consider heat penetration through the duff and it's effect on artifacts below the surface. Laboratory tests on stone and ceramic artifacts found that the threshold temperature below which most objects are not changed sufficiently to alter their diagnostic values to be 800 degrees fahrenheit. Above this temperature water loss, increased friability, discoloration, and change in form could occur. In both firing methods above the fire intensity is well below this threshold. To protect this site a firing technique using a head fire would be the best course of action for long-term protection.

In planning such a course of action, other mitigation could be incorporated. These could include, hand-clearing of excessive fuel buildup prior to burning, application of water to following the fire passage to immediately cool the site, and pre-burning the site prior to the main prescribed fire under even cooler prescriptions than called for in the burn plan. Most prescribed fires burned at a temperature under 80°F and relative humidity above 20 percent will keep the surface temperature below 800°F and soil temperature under 100°F up to two inches below the surface. Large logs and woody debris should be hand cleared form the site, if possible, to avoid long-term residency.

If the site contains significant features like rock art, special measures can be taken. Use of aluminum fire blankets to shield the art from the affects of smoke and heat may be needed.

If absolutely required to exclude the site from prescribed fire activities, then the archeologist should work with the prescribed fire project manager to design a burn pattern that blends the site into the surrounding unburned vegetation. It is never preferred to leave the site as an unburned island within the project perimeter. Constructing handlines around the site to exclude is not as preferred as the use of Class A foams. The foams can be applied prior to burning and are very effective by preventing ignitions yet leaving vegetation undisturbed.


There are few if any sites within California that have not been burned at one time or another. Most cultural sites have been subject to low-intensity fires many times in the past and whatever damage is possible under these conditions has already occurred. The least impact to sites has been to those that have been burned prior to 1930. Archaeological sites that have not been exposed to fire in the last 60 to 100 years are in peril. Significant changes to surface and below ground artifacts will occur if they are burned by high intensity wildfires.

Prescribed fire projects afford the opportunity for the archeologist and the prescribed fire manager to work together and provide long term protection to our cultural resources. The archeologist should welcome the opportunity for prescribed fire projects as a chance to document sites on private lands, to evaluate the effects of low intensity fires on cultural sites, and to mitigate the long term impacts that could be caused by high intensity wildfires.

References Cited

Allen, Craig (editor)
1996 Fire Effects in Southwest Forest: Proceedings of the Second La Mesa Fire Symposium, 1994. General Technical Report RM-GTR-286. USDA Forest Service, Rocky Mountain Forest and Range Experiment Station, Fort Collins, Colorado.

Biswell, Harold
1989 Prescribed Burning in California Wildlands Vegetation Management. University of California Press, Berkeley, California.

Clark, Bob, and Melanie Miller
1994 Fire Effects Guide. USDA National Wildfire Coordination Group.

Lewis, Henry
1973 Patterns of Indian Burning in California: Ecology and Ethnohistory. Anthropological Papers No. 1. Ballena Press, Ramona, California.