Megafire kills Joshua trees, but not fungi

When the Dome Fire tore through the Mojave Desert in 2020, it reduced one million Eastern Joshua trees to blackened skeletons. Scientists expected the underground ecosystem to be equally devastated. Instead, they found it thriving.

Joshua trees on fire. (National Park Service)

That unexpected outcome is detailed in the journal Fire Ecology, and it suggests that the loss of helpful soil fungi is likely not what is preventing Joshua tree recovery after the wildfire.

Joshua trees, like other trees, depend on mycorrhizal fungi that help their roots absorb water and nutrients from the soil. Earlier research shows Eastern Joshua trees in the Mojave's Cima Dome area are hosts to diverse communities of these fungi, raising questions about whether the fire erased them from the soil, and whether that erasure is hindering the trees' recovery, or just one part of a larger story.

"We thought the microbes would all be dead when we got there," said UC Riverside fungal ecologist Sydney Glassman, senior author of the study. "The trees were devastated aboveground, and usually the soil story matches that kind of destruction."

Aftermath of the Dome Fire. Despite the burnt trees above ground, there is an unseen world below ground that still thrives. (Arik Joukhajian/UCR)

At first, there were hopes for a recovery. In the days after the lightning-generated, 43,000-acre fire, a majority of the Joshua trees still had some green leaves. After a year, survivorship dropped to 50%. At the three-year mark, only 20% of trees in the burn plots were still alive.

And the appearance of the burn scar changed during the post-fire years in ways that made for a strange landscape. The dead Joshua trees became covered in a fire-loving, bright colored fungus called Neurospora discreta.

The researchers believe much of the delayed reaction to the fire was likely due to compound stresses, including drought.

"The trees were already mortally wounded, then drought and rodents helped finish them off," said UCR research ecologist and paper co-author Lynn Sweet.

Wondering if the fire might have caused a similar delayed reaction below ground, the researchers repeatedly sampled both burned and unburned soils from just over two weeks after the fire through three years later. They found no detectable declines in fungal biomass, microbial richness, or the overall abundance of bacteria and fungi. In some cases, mycorrhizal fungal and bacterial diversity increased slightly after the fire.

Fire-loving orange Neurospora on a burned Joshua tree. (Arik Joukhajian/UCR)

"The existing community of microbes stayed, and some fire specialists even joined the party," Glassman said.

While in some cases fires can devastate underground microbial life for years or even decades, the researchers think that in this case it was different because the Joshua trees and the other herbs and shrubs in this Mojave area were spread relatively far apart. This likely limited heat penetration into the soil, sparing much of the underground ecosystem even as the visible landscape transformed.

The study findings carry important implications for restoration efforts, which will likely be arduous. Joshua trees grow slowly, and small desert herbivores often eat seedlings in landscapes where other plant life has burned away.

However, because fungi remained in place, the study suggests that conservation efforts should not require costly soil amendments to replace missing mycorrhizal partners.

"There is no evidence the fungi are limiting regeneration because they didn't disappear," Glassman said. 'If the trees can figure out how to survive, the microbes are there for them."

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