Wildfires That Leave Soils Cool


The California Springs fire ravaged more than 24,000 acres, damaged six buildings and injured 10 people before being brought under control.

NEWS: Calif. Wildfires Put Thousands of Homes at Risk

As the fire fighters clear out the area, another team of emergency responders will be marching in. They are members of the US Forest Service’s Burned Area Emergency Response (BAER) and their job is to map out the areas that have the most severely burnt soils that are vulnerable to erosion. That information is used to avoid deadly floods and mudslides.

Boots are needed on the ground because it is difficult to predict the effects of a fire on soil. And contrary to conventional wisdom, researchers have found that sometimes the fiercest, hottest flames can leave behind the least degraded soils. So, just because a fire is hot doesn’t mean it will completely destroy the soil, according to a paper in Geophysical Research Letters.

The study was carried out in 2009 in Portugal’s Valtorto catchment, a nine-hectare watershed of heather and heath shrubbery. Cathelijne Stoof, a graduate student at Holland’s Wageningen University, had chosen Portugal because wildfires are a major problem there during the dry summers.

Two years earlier, she had installed sensors to collect baseline data about the soil. She got information about soil moisture, rainfall, changes to the shrub canopy and the ability of the soil to repel water.

Once she had the measurements, the next step was to set the forest on fire. The experimental burn began in February 2009, once Portugal’s rainy season ended.

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Stoof wanted to simulate a wildfire, similar in intensity to the ones seen in nature. A group of 12 forest firemen fanned out in two opposing flanks on the north and south slopes of the watershed, carrying flaming torches. They set fire to the vegetation on the opposing slopes.

The two fire fronts moved quickly and converged at the heavily vegetated base of the valley in a smoky orange climax. That was the first time such a huge burn had been initiated at the watershed scale, Stoof said. It spanned an area of about 17 football fields.

The flames were about 740 degrees Centigrade (1,364 degrees Fahrenheit). In densely vegetated areas, such as the base of the valley where the flames had been most intense, the soil temperature was about 87 degrees Centigrade (188 degrees Fahrenheit). Areas with sparse vegetation had hotter soils of about 188 degrees Centigrade (370 degrees Fahrenheit).

That means the places where the fire burned most intensely had the coolest soils, and the places where the fire generated the least heat had the hottest soils.

The idea is counter-intuitive. Stoof explains her results in two ways: In order for the soil to become really hot, the heat needs to be trapped just above the ground for some time. This happens when there is litter on the forest floor, which keeps smoldering for a long time.

And the fire in Portugal quickly converged on the area that has the most vegetation. This also happened to be a place where the soil was laden with moisture before the controlled burn. Water typically absorbs a lot of energy before it heats up, as anyone who has waited for a pot of water to boil knows. So, even though the fire burned intensely, most of its energy was transferred to the water and not the soil.

Fire scientists know that it is difficult to predict where the soils will be most severely burned without getting on the ground and assessing the impact of the fire on ecology. That’s why BAER teams venture into wildfires to generate maps of the burned Earth. With that information, they will be able to devise plans for preventing erosion of the most scorched soils.

IMAGE: Wildfire in Portugal. (Cathelijne Stoof)

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