Low-Severity Fires Like Prescribed Burns Might Not Be So Harmless To Soil

Returning fire to the landscape in a managed way has been touted as an approach to better prevent large wildfires in California. But some researchers are finding prescribed burns may not be so harmless.

The harm they're talking about is to soil.

“Our work shows that low-severity fires are not as harmless as they may appear,” said UC Merced’s Teamrat Ghezzehei. He’s the principal investigator of two studies with UC Merced and the Desert Research Institute in Reno on low severity burns and carbon lost during them.

The research shows that soil health deteriorated in the weeks and months after a low-severity fire, like a prescribed burn where forest managers light a designated patch of land on fire on purpose to decrease the likelihood of it burning at severe rate in the future.

“There is no visible destruction right away,” Ghezzehei said. “But the burning weakens the soil structure, and unless you come back at a later time and carefully look at the soil, you wouldn’t notice the damage.”

The researchers found the heating of soils led to the release of carbon dioxide — which contributes to climate change — instead of storing it in soils. They found damage is worse if soils are wet.

"Even under low-intensity burn conditions the soil at the surface gets hot enough that the structure of the soil can change,” said Markus Berli with the Desert Research Institute, who authored one of the reports.

Berli found these conditions, which happen during planned burns, reduce organic matter — like microbes and decomposing plant tissue — in the soils, which holds it together. This has negative effects, such as  erosion from wind and water.

"Adding nutrients to the soil by burning off material is a good thing, but if at the same time you're losing the soil you might miss the point with your prescribed burns,” Berli adds.

The authors admit fires like this reduce overgrown vegetation and recycle nutrients into the soil, but they're unsure if the negatives outweigh the positives.

“This is important information for resource managers because it implies that prescribed burns and other fires that occur during wetter times of year may be more harmful to soils than fires that occur during dry times,” Berli said.