When the Caldor Fire tore through the Sierra Nevada in 2021, it left behind 221,000 acres of charred earth and skeletal trees. But beneath the ash, something survived: a network of microscopic fungal threads that would determine whether those forests could ever come back.
The Underground Economy That Outlasts Fire
Mycorrhizal fungi form partnerships with tree roots, wrapping around them in sheaths or penetrating between root cells. The arrangement works like this: trees pump about 30% of the sugars they make through photosynthesis down to the fungi. In exchange, the fungi extend far beyond where roots can reach, mining soil for phosphorus, nitrogen, and water that trees desperately need.
These fungal threads—called mycelium—connect multiple trees into what German forester Peter Wohlleben dubbed the "woodwide web." Older "mother trees" with the most connections can detect when neighboring saplings are struggling and route nutrients their way through fungal intermediaries. The whole system depends on those fungal partners staying alive.
When wildfire sweeps through, everything aboveground burns. The question is what happens below.
What Lives Through the Flames
After California's Rim Fire consumed over 1,000 square kilometers in 2013, researchers had a rare opportunity. They'd surveyed the fungal communities before the fire, so they could compare what survived.
The news was mixed. Fire reduced the overall diversity of fungal spores in the soil, wiping out rare species. But the core community structure stayed intact. Certain fungi, it turned out, had evolved specifically for this moment.
Rhizopogon olivaceotinctus, a truffle-like fungus that partners with pines, actually increased after the Rim Fire. Its spores can withstand intense heat. Other fungi produce resting structures—essentially going dormant until conditions improve. Some bacteria develop cyst-like cells that protect against heat, drought, and the oxidative damage that fire creates. In one burned Spanish oak forest, over 21% of soil microbes belonged to Arthrobacter, a genus known for extreme resilience.
These survivors aren't just hanging on. They're the advance guard of forest recovery.
The Charcoal Eaters
Fire leaves behind pyrogenic organic matter—charcoal and other combustion products that didn't exist before the flames. This material is chemically different from normal forest litter, and most organisms can't digest it.
Enter the pyrophilous fungi and bacteria, species naturally selected for their ability to break down fire-derived compounds. Fungi like Pyronema and bacteria like Massilia colonize the burned soil and begin metabolizing catechol and protocatechuate, byproducts of lignin combustion. No single microbe can complete this breakdown alone. Different bacterial species cooperate, passing partially digested compounds between them until they're reduced to succinyl-CoA and acetyl-CoA—molecules that feed into the citric acid cycle for energy.
This metabolic cooperation transforms toxic fire residue into nutrients that new plants can use. The process takes months to years, depending on fire severity.
When the Network Fails
Not all fires are equal. Low-intensity fires that burn through quickly leave much of the fungal network intact. High-severity fires that consume everything—the kind that have become common as climate change intensifies and fuel loads build up from decades of fire suppression—can sterilize the soil.
UC Davis researcher Dustin Lower ran experiments with Douglas-fir seedlings planted in soils from different burn severities. Seedlings in high-severity burn soil showed reduced root colonization by beneficial fungi and stunted growth. Those in low-severity burn soil thrived, nearly matching seedlings in unburned soil.
Pine regeneration often fails entirely after severe fires because the fungal inoculum is gone. Young trees can survive short periods without mycorrhizal partners, but they can't establish long-term without them. And fungi face dispersal limitations—they may not recolonize from sources more than a kilometer away.
This creates a grim scenario. Mega-fires, which have become routine in the western United States, can burn hundreds of thousands of acres in stand-replacing events. If surviving trees are too far apart and fungal networks are destroyed, the forest may not come back at all. Grassland or shrubland takes over instead.
Restoring What Fire Destroyed
The fire regime itself has changed. Before European colonization, many western forests burned frequently at low intensity. Indigenous peoples used cultural burning to manage landscapes. When Canada outlawed these practices in the late 1800s and early 1900s, and when the U.S. adopted aggressive fire suppression starting in the 1950s, fuel accumulated. Now fires burn hotter and larger than the ecosystems evolved to handle.
Researchers are testing whether they can short-circuit the recovery bottleneck by reintroducing beneficial fungi. Greenhouse bioassays can predict which fungal species will successfully colonize seedlings after specific fires. CAL FIRE-funded graduate research is developing soil inoculation treatments and nursery protocols to give outplanted seedlings a fungal head start.
The approach mimics what happens naturally after low-severity fires, when "ruderal" fungi—pioneer species that colonize disturbed ground—take advantage of open niche space and help new trees establish. But it requires knowing which fungi to use, when to apply them, and how to keep them alive in harsh post-fire conditions.
The Three-Part Test
Forest recovery after severe fire depends on three factors: proximity to surviving seed sources, microclimate extremes that seedlings must endure, and the presence of mycorrhizal networks. Remove any one and recovery stalls.
Climate change is making the microclimate factor worse—hotter, drier summers stress seedlings that already lack adequate fungal support. Mega-fires are pushing seed sources farther apart. That leaves the mycorrhizal network as the one factor humans might actually restore.
The irony is that we're only now understanding how essential these underground partnerships are because we've created fire conditions severe enough to destroy them. For millions of years, the fungal networks persisted through fire after fire. We changed the game. Now we're racing to put the pieces back together before the forests decide not to return at all.