California policymakers are in the midst of grappling with how to craft regulations that mitigate future risk to communities as a changing climate alters the threats from fires. Specifically, as the California Board of Forestry looks to pass rules about vegetation clearance around people’s homes, questions remain about the best path forward, especially as it relates to the role vegetation plays—or doesn’t—in the effectiveness of Zone 0, the five-foot area immediately around people’s homes.
On Friday, December 12, the Sustainable LA Grand Challenge (SLAGC) convened leading experts to discuss what the science says, and crucially, what it doesn’t about vegetation and fire spread. This conversation was the first in a series of webinar discussions on defensible space that SLAGC is organizing.
The December 12 webinar, Policymaking Under Uncertainty: Zone 0 and Vegetation Clearance, was moderated by Nurit Katz, UCLA’s Chief Sustainability Officer. It featured expert insights from:
- Travis Longcore, Senior Associate Director/Adjunct Professor, UCLA Institute of the Environment and Sustainability
- Max Moritz, Statewide Wildfire Specialist, UC Cooperative Extension
- Alessandro Ossola, Associate Professor, UC Davis Department of Plant Sciences
- Edith de Guzman, Water Equity and Adaptation Specialist, UC Cooperative Extension
And it included opening remarks from SLAGC faculty director and UCLA climate scientist Alex Hall, who launched the Climate & Wildfire Research Initiative (CWRI) through SLAGC in 2024. Hall’s work with CWRI and beyond aims to better understand and predict the evolving risks to communities driven by a changing climate.
Watch the full webinar discussion below and stay tuned for the next one in January 2026.
Key Takeaways
The January 2025 LA fires made it abundantly clear that California—and to a great extent, the world—is entering a new era of fire. What started as wildfires quickly became urban conflagrations, wiping out whole neighborhoods in LA County, killing 31 people, displacing thousands more, and still exacting uncertain levels of long-term public health, mental health, and economic tolls.
A key driver of the discussion around Zone 0 and vegetation clearance is how to prevent something like this from happening again at this scale. Future fire is inevitable—these levels of death and destruction are not. The panel’s focus was on what the science says—and what remains inconclusive or requires further study—about the role that vegetation immediately next to homes plays in the spread of fire into urban areas.
The panel of experts presented diverse perspectives on what is known about what drives structure loss during similar fires, distinguished between different methodologies for investigating the complex factors involved in fire spread, the results of small-scale experiments and what they reveal, and large-scale post-fire statistical models.
The discussion highlighted several critical takeaways.
“We don’t currently have broad scientific agreement that complete removal of vegetation within that zone [Zone 0] provides any significant benefit in real world situations,” said Travis Longcore, UCLA’s Institute of the Environment and Sustainability Senior Associate Director.
- There are key areas of consensus regarding the risk to structures posed by certain factors:
- highly combustible materials immediately adjacent to structures, like accumulated dead vegetation in gutters and on roofs, mulch, or attached wooden fences and artificial turf.
- the distance between structures, and where they are built, appears to matter in terms of risk far more than home hardening or defensible space.
- for existing homes, home hardening measures—such as putting screens on vents to prevent embers from penetrating—is paramount to reducing structure ignition vulnerability.
- The role of vegetation is more complicated.
- Different plant species, moisture content, and vegetation maintenance all impact the role vegetation plays during a fire.
- There is some preliminary evidence that high moisture content trees are associated with a reduced risk of structure loss.
- Vegetation is “not a monolith.” Plant species matters, as does maintenance.
- There are real tradeoffs when considering vegetation removal policies, and such policies need to be looked at holistically.
- The loss of tree canopy and vegetation can lead to higher local temperatures, and negatively impact soil permeability, hillside stability, and myriad of other benefits to people and communities.
Overall, the experts noted the need for more information and big-picture thinking.
- "We need much more data and we need much better data to inform policies that are sensible, that are scalable, and that can be enforced if needed," said Alessandro Ossola, Associate Professor, UC Davis Department of Plant Sciences.
- Edith de Guzman, Water Equity and Adaptation Specialist with UC Cooperative Extension, raised the issue of tradeoffs: "I'm not the only one in that space who is very concerned about the impacts that large-scale vegetation removal would have on our urban cooling goals. Loss of shade means higher heat exposure and energy demand.”
- Max Moritz, Statewide Wildfire Specialist with UC Cooperative Extension, echoed that sentiment, calling for independent testing facilities in California to help derive “some synthesis and general principles… to base policy and regulation on.” Moritz also said a phased implementation of vegetation management policy based on “what we already know makes a lot of sense.”
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Please stay tuned for the next webinar in January, which will touch on how policymakers should decide where to implement which fire mitigation strategies.
