How do we protect archaeological sites, especially rock-art from the impact of wildland fire?

In the 2009 fire season (as of July 27, 2009), more than  5,591,424 acres of public and private lands in the United States have burned in over 70,944 wildland fires.  Within this charred acreage are 1,000's of archaeological sites both prehistoric and historic.  Many historic sites [pioneer buildings, etc.] are gone but most prehistoric sites are intact [bedrock mortars, petroglyphs, hidden pictograph panels, and living sites that have melted into the earth].

Before we can talk about protection and preservation of rock-art sites from wildland fire we must understand the dynamics of fire and wildland fire terminology.  First I will share some terminology then build upon the fire language.  If you are a trained fireman or fire behavior scientist please bear with me as I am trying to bring understanding to all who are interested in rock-art site protection.  Within the wildland fire business we have fuel loading, vectoring, upslope winds, crowning, fuel bedding, backfires, back burning, fire head, flanks, ground clearance, and so on, and so on. I will list those terms that are important to site protection.

TERMS:

Wildland Fire : Any uncontrolled fire burning vegetation. Grass, timber, brush fires. A fire burning on a vacant lot within a city is a wildland fire.

Fuel :  Anything that will burn; a combustible material.  Wood, grass, trees, paper, houses, gasoline.  If the ignition temperature is high enough almost everything is a fuel.

Heat Transfer :  There are three types of heat transfer; convection, conduction, and radiation.  In wildland fire conduction is not a factor in fire spread.
 

Fire Triangle : The fire triangle is the model used to explain the physics of fire.  There are three parts to fire and without any one part the fire is extinguished or there is the absence of the phenomenon of fire.  The triangle is made up of oxygen [air], fuel, and heat.  You must have something to burn, a heat source, and air is always there in the form of weather.

Ignition Temperature :  The temperature at that a fuel will combust [burn].  Dry grasses will burn at 300 to 400 degrees F. Density of fuel matter raises the ignition temperature. The lighter bark on a tree will burn at a lower ignition temperature than the center of the tree bole that is usually more compact.

Fuel Moisture :  Fuel Moisture is the amount of water held by fuel at any given time.  In dead vegetation the amount water held is dependent on the yearly season, time of day, weather conditions, and location of the fuel.  Water causes cooling...the removal of heat.

Relative Humidity :  The amount of water held by the air flow at any given period in time.  Low RH = dry air. 100 per cent RH and it is raining or foggy.

Fuel Loading :  The amount of fuel at a given location. Usually measured in tons per acre.

Fuel Bed : The depth of ground fuel; litter [leaves, pine needles, etc.], duff [fine fuels, leaf mold, composted material, etc.]

Upslope & down slope :  Actually the topography.  This impacts heat transfer: basically fire burns up hill faster than down hill.  The Chinook and Santa Ana down slope gradient winds cause the exception to this rule.

Now if you mix or combine the above listed terms you will be able to create an scenario for a wildland fire for any given location.  Remember that heat rises.

I will give you a possible scenario:  We are north of Flagstaff, Arizona and the land is covered with dry grass, very short sage, and a few pinyon and juniper trees.  It is late August, no rain for a few months [low fuel moisture], late afternoon temperatures in the high 90's with RH's in the low 20's.  The monsoon clouds build over the katsina mountains with dry lightning on the flats where we stand.  Lightning strikes [ignition source] a nearby knoll.  The lighter fuels [grasses] ignite and the flames rise [heat rises] into the air, now the prevailing wind bends [vectoring] the flames [heat] and this heat preheats the neighboring fuel [fuel temperature] and that creates an increase in fuel temperature that accelerates the spread of the wildland fire.  As the fire spread backs down slope it is a slow spread [heat rises] but when it gets to the bottom of the next slope; watch out!

I have tried to be very simplistic without getting into a Fire Science 1A discussion. If this is needed contact me via email.

Rock-art site protection from Wildland Fire:

Within your area or region identify all rock-art sites that are upslope of a possible ignition source [heavily traveled roads and trails, or places used by land users].  Be especially concerned about sites at the head of or in small narrow canyons because this type of topography will act like a chimney carrying high temperatures because of rapid fire spread.  At the site, remove any fuel bed that lies on the rock-art or is near or beneath panels on walls or ceilings of small caves and overhangs. Think about local weather phenomenon such as wind eddies and up drafts.

Most wildland fire spread will pass over bedrock mortars and petroglyphs without damage but if there is a deep fuel bed this will hold the heat because of the compacting of fuel and the resulting heat transfer into the rock bed will cause spalling [exfoliating of the rock].  If the flame lengths are long enough to reach wall panels and ceilings there will be soot and ash build up on paintings. DO NOT WASH OR TRY TO REMOVE ANY SOOT OR ASH.  LET THE WINDS TAKE CARE OF REMOVAL.  THERE HAVE BEEN WILDLAND FIRES AT THE SITE MANY TIMES BEFORE.  [Soot and ash are very oily and will enter the painted panel if rubbed.  Water will fade some paintings.]

The best approach to protecting rock-art sites is the selective removal of fuel, usually out from the site for approximately 30 feet.  More distance [down slope from the site] may be needed if the slope is steeper or in  a very narrow canyon with a headwall at the top.  On horizontal surfaces remove any fuel bed that covers the rock-art.  At heavily used sites build trails that surround the rock-art as this will remove fuels near the rock-art and give protective space between the adjacent fuels and the site.  Radiant heat may be a problem so more air space is needed.

No, we can not protect all sites from wildland fire but you can protect your favorite sites with a little pre-planning and labor.  There are many "volunteers" and County and State inmate work crews that are available for this type of venture.  Involve your local community, educate and have fun protecting "your" rock-art.

Selective Removal of Fuel:  Remove only the fuel that will accomplish the goal to protect the rock-art site from wildland fire.  This may require only removing the fuel bed or a few tree branches.  A clear cut or major clean-up at a site will normally expose the site to the curious and climate and this may cause more harm than any fire damage.  Most rock-art sites are archaeologically sensitive and have not been examined so consider if it is worth the effort to make changes at a site. Fire is only one impact and a very minor one at most sites.  For most folks, those whom are experiencing major wildland fire for the first time, wildland fires are awesome and very emotional.  Remember fire is part of the ecology, not the rock-art.  In the western United States wildland fire is the norm and the rock-art sites have survived without human intervention for many 100's of years.

... I would emphasize the 'better to remove vegetation for fire protection than leave it for security' philosophy.  I am encouraging my stewards to carry a pair of pruning shears to remove branches and leather gloves to pull cheat grass and red brome.  I would also emphasize the "new normal" perspective.  The native Mojave vegetation is not fire adapted.  The historical condition in the Mojave is not the condition today with non-native imports (Bromus sp).  These fine fuels carry fire in the Mohave and in the Great Basin Desert unlike before.  A single tree lightening fire is a thing of the past, the fires as you know spread rapidly through the fine fuels to at least several acres, and any brush build up in front of rock art is likely to permanently damage it with the firestorms that we see more often.  I think this topic would make a great workshop/roundtable discussion at a Site Steward Committee meeting or at the NAA Annual Meeting.  (Nevada Archaeological Association)


Mark Henderson, BLM Archaeologist, Ely District Field Office
Ely, Nevada

 

Carl A. Bjork, retired forest ranger - California Department of Forestry and Fire Protection (CDF/CALFIRE)

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