By Ronnie Siegel
The Wildland Urban Interface (WUI) is the area where the built environment meets wildlands. We can reduce the risk of WUI wildfires with a landscape approach. This involves understanding the increased risks of climate change, the condition of wildland ecosystems, and the condition of trees, plants, and buildings in developed areas.
Significant fieldwork, reviews of data mapping, and communication with eye witnesses yield convincing evidence that trees and plants didn’t cause the spread of the Eaton fire in Los Angeles County, California. In fact, they have the potential to reduce the spread of future urban wildfires.
Climate change has increased the risk of WUI wildfires. This finding is explained in much more detail by climate scientist Daniel Swain. He explains that in Southern California, climate models suggest global temperature rise doesn’t lead to a change in mean precipitation, but rather a shift to a more extreme delivery of that precipitation – longer periods with no rain and more intense rainfall when it does arrive.
Dry seasons get hotter, drier, and longer, pulling moisture out of the soils and plants through increased evaporation and transpiration. When the rain arrives, it’s more intense. This extreme hydro-climatic whiplash pushes wildland vegetation to higher drought level conditions well into November to January when offshore Santa Ana winds arrive, often at hurricane force. In the Town of Altadena, an unincorporated area in Los Angeles County, the risk of this deadly combination has increased 36 percent since the 1980’s and will likely increase more as global warming progresses.
To reduce long-term wildfire risk, we obviously need to lower our greenhouse gas emissions and sequester more carbon. And to reduce short-term risk using a landscape approach, we should stop development in the WUI, inhibit wildland fires from reaching urban developments using vegetated buffer zones, and make urban environments more fire resistant.
For fire resistance, we can reduce risk at the micro-scale – individual homeowner’s property – by “home hardening” and using defensible space strategies that act to reduce fire spread. And at the macro-scale – the level of the community – we can use land-use planning to create buffers between wildlands and urban landscapes and provide access for firefighters to reduce the risk of wildfire spread.
The Causes of the Eaton Fire
This winter followed the hottest summer on record, with no significant rainfall since April 2024. The chaparral plants in the San Gabriel hills above the town of Altadena were parched. At the same time, Santa Ana winds arrived in an extreme force gusting up to 100 miles an hour. On January 7, this worst-case wildfire scenario arrived – the Eaton fire had ignited.
The ignition appears to have been caused by a spark from power lines, probably caused by the winds, which then ignited the tinder-dry plants below. Anecdotal accounts by residents found that in the first 15 minutes after the fire started, the wind blew embers almost a mile from the ignition point, starting additional fires.
To complicate matters: during the progression of the fire, the winds generally blew southwest over the mountains and towards the ocean. But locally in the fire area, the winds blew in all directions at times, carrying embers that would ignite homes, the most flammable objects in their path.
Ty Garrison, a local resident and biologist, stayed during the wildfire to try to save his home. He captured on video tall palm trees blowing east while trashcans blew west, with embers flying everywhere. He witnessed some plants defusing embers and others, like bamboo, spreading more. Firefighting from the air was too dangerous in heavy winds, so there were only ground crews deployed in the most dangerous conditions – high winds, toxic smoke, and a water supply that didn’t last.
With thousands of homes burning at once – ultimately over 9,000 structures – the water infrastructure, which was never designed to handle that many home fires at one time, ran dry in many areas. During the first night and into the morning of the Eaton fire, the fire had spread several miles in many directions from the ignition point, primarily from wind-driven embers and in some areas closely-spaced, flammable homes, igniting each other through radiant heat and flame contact.
The Role of Trees and Plants
Post-fire observation of this disaster reveals that among the ashes of entire homes and charred cars, trees, shrubs, and even lawns survived. And today, trees are making a remarkable recovery.
Scientific studies, including one by assistant professor Alessandro Ossola at the University of California at Davis, are documenting tree casualty and recovery using LiDAR imaging. A group of Altadena arborists along with landscape architect Stephanie Landregan, FASLA, fought to preserve trees unnecessarily scheduled for removal by the U.S. Army Corp of Engineers during the home debris removal phase.
It is evident that urban trees and plants didn’t cause the spread of this fire. The homes themselves, along with cars, were the most combustible part of the landscape. This was seen in satellite views of the nighttime city. Even trees that are considered highly flammable, like pines and deodars, were standing unharmed with green needles after the fire scorched their bark. Their fire damage came from being too near burning homes rather than from embers. When trees were found burned, they were usually surrounded by burned homes on all sides or multiple cars.
Deciduous trees were leafless at the time of the fire, possibly making them less flammable. Native sycamores did very well along with other non-native deciduous trees. Evergreen trees – such as the native fire-adapted coast live oak and pine species that were native to other fire-adapted plant communities – and non-native evergreen deodars, did exceptionally well, and in some cases appear to have shielded homes from embers.
Several homes that were not adequately “home hardened” survived because they appeared to be shielded from embers by surrounding native oaks. Eyewitness accounts recall evergreen canopies of deodar trees acting as umbrellas over homes intercepting and diffusing embers.
How did they perform this feat? The answer lies in the plant’s ability to hold water. Native plants and ones that are adapted to withstand drought or fire can retain water very well without extra irrigation. Deep-rooted trees are less affected by soil surface evaporation, and many irrigated plant species perform well with artificially supplied water.
A study of the water holding capacity of various plant species in branches and leaves and needles may be one good way to predict their fire resistance. Native and non-native species survived, and their water holding capacity seemed to protect them from burning.
Potential Solutions for the Next Wildfire
On a micro-scale, “home hardening” – the process of building or renovating homes to create a fire protective shell – should be the focus of rebuilding efforts. It’s the best strategy for protecting homes in the WUI.
Defensible space guidelines are another important strategy. They provide recommendations on landscape treatment in zones surrounding a building – zone 0 is the 5-foot perimeter around the building, zone 1 is the distance of 5-30 feet from the structure, and zone 2 is 30-100 feet. These guidelines currently recommend generous separation between trees and shrubs to prevent fire ladders and canopy fire spread. There appeared to be little of this type of fire spread in the plants of the Eaton fire, and defensible space guidelines may need to be reviewed and amended.
Local fuel modification plan guidelines in Los Angeles County recommend types of plants suitable to each zone and are required to be followed to permit new construction. This plant list should also be updated to correct discrepancies found in post-fire observation where many “flammable” species performed as ember shields and did not burn. Observations indicate plant species that are fire-adapted and hold water during drought are ideal.
However, ensuring even drought-tolerant plants get just enough supplemental water during exceptional dry years is important. Making sure the ground cover and soil has good capacity to hold moisture can keep embers from igniting. Planting native species has the dual benefit of promoting biodiversity.
On a macro-scale: a recent NASA satellite multispectral composite image comparing vegetation before and after the fire seems to indicate that the chaparral vegetation burned more intensely and completely than vegetation in the developed area, including plants around burned homes. This is most likely due to supplemental irrigation.
Also, looking at post-fire aerial maps and satellite images, there are possible indications that the right trees in urban green spaces can act as buffer zones limiting the spread of fire.
An example of this is a 0.7 mile-long double row of 140 year old cedrus deodara street trees on Santa Rosa Avenue, which form an almost continuous canopy.
Against expectations and defying defensible space guidelines – which identify this tree as flammable and a hazard when planted in continuous canopy rows – this double row had no tree loss. All structures under them and on their leeward side were preserved (except for three at the very end of the row in the heart of the burned area). Coincidentally, a golf course and cemetery, mostly lawn and trees each 0.25 mile long by 0.4 mile wide at the perimeter of the fire may have slowed or even stopped the fire’s spread.
Can plants predictably protect buildings from burning? Can vegetative buffer zones make a difference, and, if so, how large should they be and what type of planting works best? What modifications need to be made to the current defensible space guidelines?
It is difficult to reach conclusions. Variables that affected the fire’s spread include:
- Topography
- Wind force and direction that constantly changed
- Wind barriers such as block walls, buildings, or plants that resisted ignition
- Homes that were built to resist embers, radiant heat and flames vs homes that were not designed to be resilient
- Homes that had well irrigated plants vs ones that were dry
- Home landscapes that were well maintained vs those that had a lot of dead vegetation or flammable furniture, cars, or debris
- Embers that by chance fell on flammable buildings or objects
- Efforts by residents hosing down embers during the fire
- And efforts by firefighters battling the fire with water and using other interventions
In post-fire observations, all of these factors complicate conclusions about buffer zones and defensible space guidelines. What is needed is collaboration between CalFire, the Los Angeles County Fire Department, front line firefighters, residents, architects, landscape architects, fire scientists, and planners to pour over maps and discuss the variables. Together, we can reassess the post-fire evidence. With a more in depth assessment and continued fire science research to test plant flammability, we can update recommended plant lists, perfect the defensible space guidelines, and plan vegetated buffer zones. Then we would be better able to reduce our collective wildfire risk.
Ronnie Siegel, ASLA, is founder of Swire Siegel Landscape Architects, author of Towards Zero Emission Business Operations: A Landscape Architect’s Guide to Reducing the Climate Impacts of Offices, and a member of the ASLA Climate and Biodiversity Action Committee.