Rise in Super Hot Fires are Killing Trees and Transforming Forests Across the West

Year after year, more wildfires are burning more forests across California and the West.

A new study from a research team with the U.S. Forest Service and UC Merced shows that an increasing number of these fires are highly severe — hot enough to kill nearly all of the trees, even in forests that are adapted to fire.

“Between 1985 and 2017, we documented an eightfold increase in area burned at high severity,” said research ecologist Sean Parks, lead author of the study. “When high-severity fires kill all or most trees — and under a warming climate — the forest has a hard time recovering.”

The increasing prevalence of high-severity fire could mean that the wildfires burning across the West are converting forests once thick with towering trees into shrubland, or they might transform coastal mountains blanketed in oak and madrone trees into scrubby grassland.

Parks says these super-hot fires are another sign of a warming world. He’s presenting his wildfire research at the American Geophysical Union’s annual fall meeting, normally held at the Moscone Center in San Francisco, but now in session online due to the pandemic.

Parks spoke recently with KQED’s Brian Watt about this new research. The excerpts below have been edited for length and clarity.

What happens when the trees can’t survive?

Historically, many California ecosystems experienced frequent, low-severity fire, but the trees had mechanisms such as thick bark or resprouting behavior that allowed them to survive. But now we’re seeing an increased prevalence of high-severity fire, which overwhelms the defenses of trees and oftentimes will kill them. And when the trees are gone, the seed source is gone. And when the seed source is gone, it becomes more difficult for the forest to recover.

Where did you see this effect happening?

We did not document a statistically significant increase in fire severity or area burned with high severity on the coast, but we did see this increase in the Sierra Nevada. It is important to recognize, however, that our study did not include the years 2018 and 2020, both of which were fairly active fire years in California. We may have actually seen an increase in area burned on the coast had we incorporated those years.

Will the problem get worse with rising temperatures?

We documented a fairly strong relationship between fire season, climate, and area burned at high severity. During warmer and drier fire seasons — typically in the summer — we see elevated fire severity and elevated area burned at high severity. So if those relationships continue to hold in the future, there’s no reason to expect they wouldn’t, we’d expect to see more high severity fires because we’re expecting warmer temperatures and drier conditions.

What would a helpful government response look like?

Completely excluding fire from the landscapes in California is not really possible or even advisable. As a society, we need to think about living with and coexisting with fire. One response could be to increase the scale and magnitude that prescribed fire is implemented across your state. But that would need to be a fairly large increase.