The scientists used state and federal data to identify areas that had undergone mechanical thinning and burn treatments prior to the Dixie Fire to see how past treatments and burn history affected fire severity. They reported their findings June 21 in the journal Environmental Research Letters.

The researchers found that areas that had burned at low to moderate severity in the past burned at low to moderate severity during the Dixie Fire. Areas that burned at high severity in the past, on the other hand, burned at high severity again. Tellingly, areas that burned at high severity within the past four decades were more likely to burn at high severity during the Dixie Fire than areas that had not experienced a fire in the last 120 years, according to the researchers. They attributed these findings to the landscape’s ecological memory, or the legacy effects of past fires.

“Ecological memory is the idea that a particular landscape essentially has a memory of past events, whether that be a fire, logging, grazing or another type of disturbance,” said Harris. “Those events shape the characteristics of a landscape in a way that has lasting impacts. These impacts can include changing tree species composition, the structure of the forest, understory plants and their composition and quantity, or, in the case of fire, the arrangement of fuels on the forest floor and the vertical structure of fuels. Basically, the forest has a memory of past events that manifest in the present day, and we saw this when examining the data from the Dixie Fire.”

In a previous study in California’s Klamath Mountains, the team found that they could predict the severity of future fires by looking at one variable: how did an area burn during the last fire? The current study provides insights into what will happen to more than nearly 1 million acres should another fire break out. The goal now is to prevent another severe wildfire like the Dixie Fire from occurring by providing officials with an assessment of the preventative tools available, and the best tool happens to be fire, according to Taylor.

The research team found that low-severity fire treatments in the form of prescribed and managed fires were more effective than mechanical thinning at limiting the severity of the Dixie Fire. Likewise, the combination of mechanical thinning and prescribed fire, which helps to clear flammable trimmings and debris left after thinning, was more effective than mechanical treatments alone.