Wood Burns

Weather: After several short rains, finally got the real thing Friday; 14:29 hours of daylight today.

In the dry areas of my yard yesterday, water had seeped down about three inches. It was gone from the top inch, the day after we had some real rain.

What’s blooming in the area: Hybrid roses, daylilies, silver lace vine, bouncing Bess, blue flax, alfalfa, brome grass.

Beyond the walls and fences: Trumpet creeper, tamarix, scurf, scarlet and sweet peas, showy milkweed, buffalo gourd, purple mat flower, stickleaf, leather-leafed globe mallow, bindweed, greenleaf five-eyes, scarlet bee blossom, velvetweed, Queen Anne’s lace, goat’s beard, Hopi tea, Tahoka daisy, strap-leaf and golden hairy asters, horsetail, rice grass.

In my yard, looking east: Baby’s breath, coral bells, pink evening primrose, winecup mallow.

Looking south: Rugosa, floribunda and miniature roses.

Looking west: Lilies, Johnson Blue geranium, catmints, sea lavender, ladybells, white mullein, white spurge.

Looking north: Golden spur columbine, coral beardtongue, Hartweig primrose, butterfly weed, chocolate flowers, anthemis, yellow yarrow.

In the open, along the drive: Dutch clover, hollyhock, Shirley and California poppies, larkspur, white yarrow, blanket flower, black-eyed Susan, plains and lance-leaf coreopsis, yellow, red and mixed Mexican hats.

Bedding plants: Wax begonias, pansies, snapdragons, French marigolds, gazanias.

What’s blooming inside: Zonal geraniums, aptenia.

Animal sightings: Rabbit, snake, hummingbird, chickadees, goldfinches, bumble and small bees, hornets, flying grasshoppers, large and smaller black ants.

Weekly update: Sometimes, someone says something so absurd, your mind freezes.

In his History of the English Countryside, Oliver Rackham wrote:

"Our only native tree which can be burnt standing is pine, whose early disappearance from England has been ascribed to prehistoric men setting fire to it."

He adds, most British species "will not burn."

I read that again, and again, with eyes gummed by smoke from the local fires.

I think he means, Britain does not produce conditions that burn trees. But, he implied they are singularly virtuous, as "compared with the world’s trees as a whole," and that all the island’s deciduous species underwent the same genetic mutation in Mesolithic times.

It’s not the thought, but the wording that makes you wonder, where was he the day they were teaching elementary science in school?

After years of sitting through fire extinguisher training in Los Alamos, I know fire requires three things: fuel, oxygen and heat. If you have enough heat, you’ll have a fire.

Fire fighting involves removing one of the components. They contain forest fires by creating areas where there is so little fuel, it would take a great deal more heat to ignite it. Dozers and back fires often are used, the one to clear an area, the other to destroy fuel at a manageable temperature.

They lower temperatures by dropping water. Suppressants may lower the temperature or may smother an area by decreasing the oxygen. Still, it takes monsoon rains to lower temperatures enough for smoldering embers to finally stop igniting. A good winter snow cover helps.

The Cerro Grande fire near Los Alamos officially was contained on June 6 of 2000 and declared controlled on July 20. The word extinguished was not used.

Bureaucrats and scientists are careful with their words. They prefer the term "combustion" to "fire." They use it to refer to a chemical reaction that occurs when a carbon molecule is heated in an oxygen environment until the two recombine to create carbon dioxide and water. The point of conversion is the explosion we call fire.

Not all carbon molecules burn easily. Some have internal structures that make combustion difficult. British trees are not one of those. All wood cells contain cellulose fibers suspended in lignin. Both are organic compounds, both contain carbon.

Some tars and resins may alter fire behavior slightly by species.  Moisture in the sapwood can slow the process. Relative amounts of lignin and cellulose may differ by tree. But, the chemistry is absolute. When the temperature of wood reaches the boiling point of water, 212 degrees (100 Celsius), chemical bonds begin breaking.

By the time temperatures reach 392 (200C), wood begins losing moisture and releases water vapor.

Between 392 and 572 degrees (300C), lignin loses its liquid chemicals and turns to char. Cellulose does not. Technically it has entered the pyrolysis phase. Charcoal is made by this kind of controlled heating.

Cellulose begins to disintegrate around 660 degrees (350C). Carbon bonds begin breaking around 700 to 750 degrees (370 to 400C).

The ignition point is reached around 750 degrees F (400C). After that, a fire is self-sustaining. The heat from the explosion of molecules warms the air so others can ignite. The more explosions, the greater the generated heat and the greater the number of subsequent explosions.

Before that magic temperature, you can only keep something burning by maintaining contact with something hotter. You use kindling to start a camp fire. When people use flame throwers to burn weeds, they provide a constant heat source to organic matter too widely dispersed to spread heat. One company advertises the propane-fueled temperature of its thrower is 2050 degrees F.

At 842 degrees (450C) wood cells no longer emit the volatile chemicals that color the smoke and poison the air. What remains is char that is converted to carbon dioxide and water, until only ashes remain.

High intensity fires spread when they burn in tree canopies or crowns where heat transfers easily from dried leaf to instantly dried leaf.

Low intensity ones spread along the ground through plants and organic debris. It’s difficult for them to jump from trunk to trunk, because there’s too much oxygen between the two poles of carbon for temperatures to sustain them. Instead, they creep up the trunks, charring as they rise.

At moderate temperatures, ground fires may occasionally flare high enough to torch lower leaves. Usually, the trees are too widely spaced for one canopy to ignite another.

After the Cerro Grande fire, people who surveyed the grounds saw areas of ash, iron oxides, and clays where once there had been trees. The reddened soils were ones where the water-containing iron compounds (iron hydroxides) had become so hot they lost their water and some form of hematite emerged. To produce those results, soil temperatures were at least 482 degrees (250C). The severely burned surface may be sterile for some time.

More common were areas where ground litter burned and only charred logs remain. Here soil temperatures were above the boiling point of water and could have been lethal down 2". Even if only ash were present, there were nutrients left to support seeds that blew in from adjoining areas.

Surrounding them were the scorched trees that hadn’t ignited. Over the nearly 6,000 acres reviewed by Raymond Kokaly’s team, nearly half were ash, char or bare ground. More than 15% were dried conifer. Almost 4% were grasses dried into straw. The rest, 28.9%, were still green.

Randy Balice’s group found, outside the lands managed by LANL, 48.6% of the acres burned at high intensity and 42.6% at moderate temperatures. The remaining 8.8% suffered low intensity heat. Fire intensity and fire severity refer to two different phenomena, the one to chemistry and air temperatures, the other to biological consequences and soil temperatures.

The reality of forest fires for most of us in the valley isn’t heat and flames. It’s the formation of clouds every morning that fill the sky by late afternoon. We don’t know if they are simple water vapor or smoke or if there’s a difference. The mechanics of combustion are too abstract.

It’s easy to abandon scientific thinking for the mythic. To think, the smoke people are frightened by fire, and run south along the Rio Grande to escape. When they reach the Caribbean, the refuges confront the water people jealous of their lands. The angry nativists chase the invaders back up the river, and we get rain that dowses the fire.

Or, maybe one believes burning bushes that flare but do not burn are sacred.

Or, perhaps one believes British trees represent the survival of the fittest, the ones that survived Mesolithic man and won’t burn.

Addendum: A friend reminded me that during the worst of the Cerro Grande fire he made deliveries to the firefighters in Los Alamos. As he drove up the truck route, he saw isolated trees take off like bottle rockets, then burst into flame.

Notes:
Balice, Randy G., Kathryn D. Bennett, and Marjorie A. Wright. Burn Severities, Fire Intensities, and Impacts to Major Vegetation Types from the Cerro Grande Fire, 2004; good maps and pictures

Kokaly, Raymond F., Barnaby W. Rockwell, Sandra L. Haire, and Trude V.V. King. "Characterization of Post-fire Surface Cover, Soils, and Burn Severity at the Cerro Grande Fire, New Mexico, Using Hyperspectral and Multispectral Remote Sensing," Remote Sensing of Environment 106:305-325:2007

Rackham, Oliver. The History of the English Countryside (1986); quotations from pages 71-72 of the 1987 paperback reprinted in 1993 by J. M. Dent, London. In Trees and Woodland in the British Landscape (1990 edition), he more correctly said, "Except for the pine, no British wood can be destroyed by fire" [p33]. Woodland would have been clearer to an American.

White, Robert H. and Mark A. Dietenberger. "Fire Safety of Wood Construction" in USDA, Forest Products Laboratory, Wood Handbook, 2010; defines the stages of wood burning

Photographs: Unless noted, all were taken 4 July 2013 along Route 4 from the entrance to Bandelier National Monument to the base of Cerro Grande. The altitudes are from the camera’s GPS interface.

1. Ponderosa pine on flat land, unburned, with grasses and litter on the ground. Altitude: 7529'.

2. Ponderosa pine on steep hill, burned so severely, little has come back in 13 years. Altitude: 8049'.

3. Cottonwood near village, 3 May 2008. After it blew down, the owners tried to start a fire in the trunk’s interior. It would be difficult to create a high enough temperature inside and provide it with enough oxygen to get it to smolder, let alone burn. Forrest fires work from the outside in, and first have to dry the sapwood. The log is still there.

4. Junipers often grow in gullies or areas where buried rocks collect water. Trees in the gully have not come back, probably because there’s no grass yet to catch water. The sky is typical for noon time above the Jémez. The top white clouds are truncated by a featureless band of blue. In front are puffs of rising white. Altitude: 7223'.

5. Charred wood; it could be from the nearby short-needled pine.

6. Area of #5 charred wood. It looks like a dozer knocked down trees like the one laying in back with short needles. The grasses have not revived in the drought, but shrubs have regrown and the trees in the distance weren’t killed. Altitude: 7054'.

7. Close-up of gambel oak in #6. The trunks were scorched enough to kill the buds buried in the sapwood, but the roots have been able to sucker.

8. Gully was not destroyed and juniper have come back. Other sloping ground in the area is still infertile, but the ponderosa pine on the far bank survived. Trees in the distance, closer to Cerro Grande, are still black specks. Altitude: 7211'.

9. Storm over the Jémez, 3 July 2013, from my back porch. Three layers of clouds are visible, with the ones nearest the horizon backlit by the lowering sun at 6:15 pm. The blocked sun is responsible for the monochromic effects.

10. Russian olive protected when a field was burned near the village, 28 February 2009.

11. Ponderosa pine that survived in area where understory burned. Grasses now browned by the drought came back, as has the blue stem in front near the spillway for the road. Altitude: 7377'.

12. Juniper gully near White Rock, outside the range of the fire, with four-winged saltbush in front. Ground is kept cleared by water. Altitude: 6576'.