We recently got a copy of this letter co-written by Dr. Alexandra Syphard and Prof. Michael Gollner of the UC Berkeley Fire Research Lab to the Board of Forestry & Fire Protection about the upcoming Zone 0 Regulation.
"Chair O’Brien,
We are writing this letter as a group of concerned scientists who wish to clarify the findings of our research, which has recently been misinterpreted in reaction to several letters to the board. In particular, we wish to clear up some inconsistencies as the board considers regulation on “Zone 0”. As research scientists, we are not advocates, nor do we publicly express opinions. Our responsibility is to interpret the scientific findings and explain their implications.
Our collective research has suggested that defensible space done closest to the house is most effective and does significantly improve the odds of house surviving a fire; but that anything beyond 100 feet does not add significant or substantial additional protection. This supports zone 0 as the most important focal area; but the comments expressed during the April hearing are correct that most previous work by Syphard and Keeley in post-fire studies did not specifically focus on zone 0 and the alternative ways to define it. A recent preprint by Gollner et al. more specifically isolated the role of zone 0 and found a sizable benefit to home survival. Studies have also found that vegetation touching the structure and trees overhanging roofs can create risk, as the trees drop litter. Removing litter and debris is an integral component of creating defensible space. This could also be interpreted as support for zone 0.
Given the debate expressed at the April hearing, it is important to note the scientific evidence of defensible space and home hardening. Post-fire investigations and large-scale experimental testing by NIST, IBHS, UCANR, and others have shown that clearing flammable materials close to structures helps keep flames, that can lead to structure ignition, away from a home. In addition, flammable material next to the house may facilitate ignition by embers that land and accumulate there. Protecting homes in the WUI is a multi-step process: thinning between larger trees and shrubs and moving larger flammable materials away from the home (30-100 foot defensible space), removing all flammable materials within about 5 feet (zone 0, which prevents small flames from igniting the structure), and finally sealing up the “shell” of the house so that embers can’t penetrate or cause ignitions (hardening). Several studies have found home hardening to be the most impactful component of homeowner mitigation, which again, should be done in combination with zone 0.
Not clearing zone 0 of flammable materials invites flames right next to our structures, which have the potential to break through the barriers we’ve tried to create. For example, the recent preprint by Gollner et al. supports this by showing that while clearing zone 0 showed a clear benefit, combining these actions with home hardening, which primarily prevents ember ignition, was significantly more impactful. In a letter circulated by Longcore et al. on April 26, 2025, the authors found criticism in this work, but the authors did not appear to understand the coupled approach of reconstructive fire modeling (which incorporates calculated fuel moisture contents) and machine learning approaches applied to understand past destruction data.
We would also like to note that when combustible woody material in Zone 0 is removed, it
should not be left barren, but rather made impermeable to invasive grasses or weeds (e.g.,
stone, gravel, rocks, etc.), as they will aggressively colonize anything disturbed. Invasive
grasses (that are dry most of the year) could present more hazards than healthy green
vegetation, therefore, continued maintenance is key.
Defensible space is not intended to leave barren landscapes, and vegetation outside of Zone 0 can be modified to reduce flame heights and transmission to the structure. In this 5-30 foot area, more work is needed to understand the role of vegetation moisture content, across diverse ecosystems or plant types under a range of wind (and ember) scenarios. Both laboratory studies and post-event investigations have, however, shown specific hazards from fences, mulch, and other flammable materials in zone 0, leading to structure ignition.
Respectfully submitted,
Alexandra D. Syphard, PhD
Director of Science
Global Wildfire Collective
Senior Research Scientist
Conservation Biology Institute
Michael J. Gollner, Ph.D.
Associate Professor and Deb Faculty Fellow
Department of Mechanical Engineering
University of California, Berkeley
Referenced Literature:
Alexandre, P. M., S. I. Stewart, M. H. Mockrin, N. S. Keuler, A. D. Syphard, A. Bar-Massada, M. K. Clayton, and V. C. Radeloff. 2016. The relative impacts of vegetation, topography and spatial arrangement on building loss to wildfires in case studies of California and Colorado. Landscape Ecology 31:415–430.
Gollner, M., M. Zamanialaei, D. San Martin, M. Theodori, D. Purnomo, A. Tohidi, C. Lautenberger, A. Trouve, and Y. Qin. 2025. Isolating the primary drivers of fire risk to structures in WUI regions in California. Research Square doi: 10.21203/rs.3.rs-
5776626/v1.
Keeley, J. E., A. D. Syphard, and C. J. Fotheringham. 2013. The 2003 and 2007 wildfires in
Southern California. Pages 42–52 in S. Boulter, J. Palutikof, D. J. Karoly, and D. Guitart, editors. Natural Disasters and Adaptation to Climate Change. Cambridge University Press, Cambridge.
Lin, S., Li, C., Conkling, M., Huang, X., Quarles, S. L., & Gollner, M. J. (2024). Smoldering
ignition and transition to flaming in wooden mulch beds exposed to firebrands under wind. Fire Safety Journal, 148, 104226.
Lin, S., Cui, W., Wang, S., Qin, Y., Chen, Y., Zhang, Y., Huang, X., Quarles, S.L., & Gollner, M.
J. (2025). Susceptibility to ignition of landscaping mulches exposed to firebrand piles or
radiation. Fire Safety Journal, 104388.
Maranghides, A., McNamara, D., Vihnanek, R., Restaino, J., & Leland, C. (2015). A Case Study of a Community Affected by the Waldo Fire Event Timeline and Defensive Actions. NIST Technical Note (NIST TN) - 1910
Mockrin, M. H., D. H. Locke, A. D. Syphard, and J. O’Neil-Dunne. 2023. Using high-resolution land cover data to assess structure loss in the 2018 Woolsey Fire in Southern California. Journal of Environmental Management 347:118960.
Suzuki, S., Johnsson, E., Maranghides, A., & Manzello, S. L. (2016). Ignition of wood fencing
assemblies exposed to continuous wind-driven firebrand showers. Fire Technology, 52,
1051-1067.
Syphard, A. D., T. J. Brennan, and J. E. Keeley. 2014. The role of defensible space for
residential structure protection during wildfires. International Journal of Wildland Fire
23:1165–1175.
Syphard, A. D., T. J. Brennan, and J. E. Keeley. 2017. The importance of building construction materials relative to other factors affecting structure survival during wildfire. International Journal of Disaster Risk Reduction 21:140–147.
Syphard, A. D., T. J. Brennan, H. Rustigian‐Romsos, and J. E. Keeley. 2022. Fire‐driven
vegetation type conversion in Southern California. Ecological Applications 32:e2626.
Syphard, A. D., J. E. Keeley, A. B. Massada, T. J. Brennan, and V. C. Radeloff. 2012. Housing
arrangement and location determine the likelihood of housing loss due to wildfire. PLoS ONE 7:e33954."