Burned Area Emergency Response (BAER) teams completed their Soil Burn Severity (SBS) map of the Pipeline Fire on Friday, providing crucial data for calculations of post-fire flood risk and estimations of long term recovery.
Overall, the BAER team assessing the Pipeline Fire determined that within the fire boundary, approximately 15,004 acres (56%) have low soil burn severity, 9,141 acres (34%) sustained moderate severity and 1,315 acres (5%) were identified as having high severity. Only 1,310 acres (1%) of the area within the fire boundary are unburned.
The distribution of the varying severities in the SBS maps showed that moderate and high severity burning was more concentrated at the “heel” of the fire in steep slope areas, wooded areas that had not been previously burned or thinned. SBS map data has been provided to city and county partners so that they can produce flood models that should be completed within the coming week.
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Public information officer Dick Fleishman led the Arizona Daily Sun behind the fire line of Friday to see the effects of the Pipeline Fire firsthand. Traveling up Schultz Pass road to the Sunset Trailhead, Fleishman guided us into Weatherford Canyon through a mix of moderately and severely burned areas.
Immediately, it was clear how effective firefighting efforts had been at keeping the fire north of Forest Road 743. Pinkish stains of fire retardant waited just beyond a burn scar that stopped dead it in its tracks at the clearing made for the underground pipeline.
“No fuel,” Fleishman said in explanation of the fire’s sudden stop. “Or at least not enough.”
Just north of the road, the effectiveness of preventive thinning was also displayed. To the west of the trail, low-density ponderosa stands thinned from the Orion timber sale stood with green needles in their crowns.
“You can see, the fire definitely stayed below the canopy,” Fleishman said. “These trees will probably make it.”
But just on other side of the trail, all that remained were tightly packed black sticks where the fire lethally burned through the dense forest.
“Treated,” Fleishman gestured to the green-needled ponderosas, then waved back to the completely burned stand. “And untreated.”
This trend was thematic across the Pipeline Fire, Fleishman said, where most severe were the “new burns” in areas that were both untreated and unburned from previous fires like the 2010 Schultz Fire. The Pipeline Fire burned into the Schultz Fire scar in places, and while this “re-burn” definitely set back recovery of the areas, “it's probably not as dire of a situation as it was immediately post-Schultz Fire,” he said.
Part of the reason for that is the Schultz Fire was “80-85% high severity,” Fleishman said, and caused significant soil damage because there was plenty of fuel to keep the fire burning long and hot. When the Pipeline Fire came along, it “flashed through” on fine fuels such as grasses that had regrown since 2010, resulting in a lesser soil burn severity. The full impact of re-burns is not completely understood, but from what Fleishman explained, an area that burns severely once is not as likely to burn severely again.
Ecosystem type also accounted for burn severity differences in the Pipeline Fire. Much of the more severe “stand replacing” burns occurred in mixed conifer areas characterized by the presence of fir and spruce trees. These burns might seem catastrophic, but Fleishman rejected the term.
“This is exactly how it’s supposed to burn,” he said. “For spruce and fir the natural fire return interval is every 200 to 400 years, and its natural fire regime is stand replacing.”
After a spruce/fir stand is burned down, aspen is the “primary successional species,” that takes over and rehabilitates the landscape so that spruce and fir can grow once more. It’s a natural cycle that has defined the area for millennia and produced iconic areas like the aspen groves of Inner Basin.
“One person’s catastrophic is another person’s beautiful,” Fleishman said.
Deeper into Weatherford Canyon, we got to see what moderate and severe burn really looks like. Fleishman warned the crew to “look up and down.” Overhead loomed the threat of falling, burned out “hazard trees,” sometimes called “widowmakers.” Underfoot, “stump holes” where the fire had burned through a tree stump and root system were the primary risk. Fleishman said they were notorious for hollowing the surrounding earth to the point that a single step could collapse the crust and swallow unsuspecting ankles in pockets of burning ash.
“Don’t stand anywhere near them,” he said.
Kneeling in the ash, BAER team lead Eric Schroder demonstrated the process by which BAER teams evaluate soil burn severity. It begins with satellite imagery, Schroder explained, which use before and after comparisons of the ground “reflectance” to estimate how badly a fire damaged an area. Then, crews go in and “ground-truth” these estimations observationally and through soil tests.
“My first observation here is for ground cover,” Schroder said, looking at an area deemed “moderate” severity. Where once would have been a forest floor covered in duff and pine needles, there was now a layer of black, shining char.
“Charred but not consumed,” Schroder said. Then he pointed up, where browned needles lingered, ready to fall from their black branches. They represented a “tremendous potential for ground cover replacement through needle cast,” he said.
The final observation that affirmed the area as “moderate” and not “severe” came from the soil. Schroder whipped out a rusted pocket knife and scraped at the coal-black soil.
“What I'm seeing is an eighth to a quarter-inch of heat impacted soil on the surface,” Schroder said, examining a palm full of soil. “We're seeing some of the organic content of the soil has been consumed — a loss of soil structure, fine roots have been burned out.”
He scraped a little deeper. The soil turned from a glossy black to a dull clay color. Little hair-like roots were visible in the granular structure.
“Moderate soil burn severity does mean that you can have some minimal fire impacts on soil,” Schroder said. “And that appears to be the case here.” Had the area been more severely burned, the soil impacts would have been deeper, going down a half-inch or more.
Taking out a water bottle with an angled spout, Schroder then demonstrated why soil damage is a problem. He delicately squirted water onto the burned soil and watched as it beaded up into a clear globe. This, he said, showed how burned soil creates a “hydrophobic” water repellent layer.
“What happens is with the consumption of materials at soil surface, there's organic gases that condense on the surface of the soil particles,” he said. “You might call it a waxy coating of sorts. So there's less infiltration of water into the soil. That’s just one factor that can increase the post-wildfire runoff.”
In Fleishman’s words, these waxy coatings in severely burned soils make the soil go from “a sponge to a roof.” Post-fire flooding remains a concern for the Flagstaff community, one that will be understood in greater detail thanks to SBS analysis.
When we continued up the trail, Fleishman pointed out dark, undulating patterns like creeping mold in the burned earth.
“That’s water,” he said. “It’s already starting to pool and move from the rain we got.” Fleishman estimated that recent storms dropped about 0.12 inches of rain on the area.
“That’s not even a hard rain,” he said. “This is evidence of what we can expect.”
Further on we came to a severely burned area. The signs of extreme heat were staggering. Rocks had spalled, cracked and shattered as if in a furnace. The fine, black and gray ash was crisscrossed with reddish lines. These were scars left where downed logs “burned to complete consumption,” Fleishman said, and the phrase left little to be desired. Complete was correct — there was no sign of a fallen tree beyond the red shadow of deeply burned earth.
In the middle of this severely burned area was an aspen grove across a gully from a decimated stand of mixed conifers. The aspens were charred around their bases, leaves long gone.
“When the fire came through, the winds had the flames laying over like this,” Fleishman flattened his hand. “These aspens were taking all the heat from this stand.” Aspens don't readily burn compared to spruce and fir, and are considered a natural fire barrier. This grove showed precisely why. A hundred yards back was a small oasis protected from the hellfire, green leaves visible in the heart of the grove.
“You can see ferns on the ground,” Fleishman said, pointing through the white trunks.
In some cases, aspen sprouts can be seen mere days after a fire passes through, Fleishman said. While that was not the case in Weatherford Canyon, Fleishman isn’t concerned.
“Every fire I’ve ever been on, I’ve seen aspen come back,” he said.