Hotter Fires Are Transforming California's Forests

A team of UC Davis researchers finds that plant diversity drops where the hottest fires burn, turning forests into shrublands. (Photo: Clark Richter, UC Davis)

Hotter-burning wildfires are transforming California’s forests, and not for the better. A new study from UC Davis finds high-intensity fires leave fewer trees and a less diverse population of plants behind.

“We're finding that high intensity, really dangerous fires ... are becoming more frequent,” said Clark Richter, a graduate student in ecology at UC Davis. “They're burning a larger area than was typical historically.”

Richter and a team sampled plants across patches of land in the Sierra Nevada that were burned in at least eight fires over the last 20 years. Their goal was to get a better picture of what happened to plants in the forest understory.

Richter calls the current pattern of fire in California, one where fires burn intensely hot, “novel.” He and a research team saw that “when fire climbs up into the canopy, it often kills the adult trees,” he said. “And that causes a dramatic change to the system.”

Those intense fires transform forest into shrubland. And according to Richter, the more frequent and the larger the area burned at these high-severity sites, the larger the shrub fields left behind.

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Less Diversity

Field sampling by the researchers found less plant diversity on the landscape after high-intensity fires, especially in shrublands. And when fire returns to that landscape, it’s more likely “that system is going to continue to be a high-severity site, that's going to continue to be a shrub field potentially indefinitely,” Richter said.

It wasn’t always this way in California’s yellow pine mixed-conifer forests, Richter says. Historically, the fires that burned these forests at elevation were low- and moderate-intensity. “These weren't really fierce, fierce fires with really tall flame lengths in the canopies of trees,” Richter said.

Richter thinks of these less-intense fires as recurring events that “chew up” but don't destroy understory fuels on the ground, thus preserving an array of plants after the fire is out.

The study supports the idea that active management of forests, including reducing tree density and managing understory growth with prescribed fire and mechanical means, promotes ecosystem health. The study’s authors acknowledged the challenges to managing forests this way, including increased costs and legal opposition.

But, Richter said, “Our concern is that if these shrub fields continue to burn, that there will be an overall loss of understory plant diversity in these sites at a landscape level."

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