When the Dome Fire swept through California's Mojave Desert in 2020, it reduced 1 million Eastern Joshua trees to blackened skeletons across 43,000 acres. The devastation above ground was unmistakable. But beneath the scorched landscape, something unexpected was thriving: the underground fungal communities that these ancient trees depend on for survival had remained nearly untouched.
This finding, detailed in the journal Fire Ecology, reveals a silver lining in an otherwise catastrophic fire — and it upends assumptions about what happens to ecosystems when wildfires roar through them. Joshua trees, like all trees, rely on mycorrhizal fungi to help their roots absorb water and nutrients from the soil. Scientists initially feared that the intense heat had obliterated these crucial underground partners, which could have made recovery impossible. Instead, researchers who repeatedly sampled both burned and unburned soils from just after the fire through three years later discovered something remarkable: there were no detectable declines in fungal biomass, microbial richness, or the overall abundance of bacteria and fungi. In some cases, mycorrhizal fungal and bacterial diversity actually increased slightly after the fire.
"We thought the microbes would all be dead when we got there," said UC Riverside fungal ecologist Sydney Glassman, the study's senior author. "The trees were devastated aboveground, and usually the soil story matches that kind of destruction." But this time, the soil story was entirely different. Some fire-specialist microbes even arrived at the scene, as if responding to an opportunity.
The aboveground picture told a grimmer tale. In the days after the lightning-generated fire, most Joshua trees still bore green leaves, offering false hope for recovery. Within a year, survivorship had dropped to 50%. By the three-year mark, only 20% of the trees in the burn plots remained alive. Adding injury to insult, the dead Joshua trees became covered in a fire-loving, bright-colored fungus called Neurospora discreta, creating an eerie, otherworldly landscape.
UC Riverside research ecologist Lynn Sweet, a co-author of the study, attributes this delayed collapse to compound stresses. "The trees were already mortally wounded, then drought and rodents helped finish them off," he explained. The wildfire left the Joshua trees vulnerable, and subsequent pressures in an already harsh desert environment sealed their fate.
The key to understanding why the soil survived lies in the Mojave's scattered plant communities. The Joshua trees and other herbs and shrubs in the Cima Dome area were spread relatively far apart, which likely limited heat penetration into the soil. This spacing spared much of the underground ecosystem even as the visible landscape transformed above.
For restoration efforts, the findings offer both a challenge and an opportunity. Joshua trees grow slowly, and seedlings face predation from small desert herbivores in landscapes denuded of competing plant life. Conservation will be arduous. Yet because the fungal partners remained in place, restoration efforts will not require costly soil amendments to replace missing mycorrhizal communities. As Glassman put it: "There is no evidence the fungi are limiting regeneration because they didn't disappear. If the trees can figure out how to survive, the microbes are there for them." The challenge now is helping the Joshua trees themselves return, with their underground allies already standing by.