February 4, 2025
By Robert Fitch1 , Carla D’Antonio1, Park Williams2, Max Moritz3,4, Shane Dewees5, and Alex Hall6
1UCSB Dept. of Ecology, Evolution and Marine Biology, 2UCLA Dept. of Geography, 3UCSB Bren School of Environmental Science and Management, 4UC ANR Cooperative Extension, 5UCSB Earth Research Institute, 6UCLA Dept. of Atmospheric and Oceanic Sciences
Key Takeaways:
- Large scale and high intensity wildfires are natural in California’s shrubland ecosystems occuring centuries before suppression efforts occurred. They are not a product of “mismanaged forests.”
- The most destructive wildfires are human caused and driven by extreme Santa Ana winds, a natural phenomenon, and they are not stopped by landscape fuel reduction or prescribed fires.
- Shrubland fuel modification is useful in specific and limited contexts such as the strategic placement of fuel “protection zones” near human infrastructure and in conjunction with active firefighting operations.
- Large wildfires are inevitable across California shrubland ecosystems, and the scientific consensus to mitigate wildfire risk to human communities includes: (1) Home hardening, (2) Maintenance of defensible space, (3) Community & built environment planning, and (4) Ignition reduction.
The wildfires of 2025 in Los Angeles have been devastating, destroying over 10,000 homes and taking the lives of more than two dozen people. However, proposed answers to the “wildfire problem,” such as The Fix Our Forests Act, could open the door to destroying California’s unique biota and ecosystems while not providing real solutions. There are several myths and generalities about California’s shrubland ecosystems, their natural fire regimes, and our ability to control fire in these wildlands. For example, the emphasis on large-scale vegetation clearing to reduce wildfire risk is contrary to the consensus of ecologists, fire fighters, and wildfire experts. Solely focusing on fuels management ignores the complexity of the wildfire crisis in California. Instead, addressing the crisis will require multifaceted, regional, and locally tailored approaches that recognize the nuances of complex ecosystems, and embrace the need for better planning in the built environment (MacDonald et al., 2023). In this article, we scrutinize some common myths regarding California’s Mediterranean shrubland ecosystems (i.e. chaparral and sage scrub) and their relationship with fire, and synthesize established scientific evidence regarding the efficacy of vegetation clearing. Our goal is to shift focus from harmful "solutions" that would likely worsen future fires, to emphasizing the critical roles of ignition reduction, structure hardening, and improved urban planning.
Myth 1: All ecosystems in California share the same relationship with fire
California contains a great variety of fire-influenced ecosystems including mixed conifer forests, oak woodlands, and shrublands. Fire regimes (how often fire occurs, at what time of year, and their size and intensity) differ enormously amongst them and understanding these differences is key to mitigating risk to communities and maintaining ecological integrity. In southern California where recent fires have gained international attention, wildlands are dominated by native shrub ecosystems, termed chaparral and sage scrub (Figure 1). Throughout southern California, shrublands are commonly found abutting densely populated human communities, often making them a focus of vegetation clearing projects, especially after catastrophic events. Shrub dominated ecosystems often form dense closed canopies that burn infrequently (30-100 years) in stand replacing high intensity fires (i.e. fires completely consume all vegetation) (Keeley, 1982; Moritz, 2003). Thousands of years ago, fire was infrequent in shrublands because, prior to human settlement, these lower elevation coastal areas did not experience frequent lightning strikes and have one of the lowest lightning strike densities anywhere in North America (Van Wagtendonk & Cayan, 2008). Plant species in shrublands have numerous adaptations that promote resilience and rapid recovery following high intensity fire, including fire induced seed germination and resprouting. Despite these adaptations, chaparral can be negatively impacted from too much disturbance, including too much fire (see Myth 3). These habitats support numerous ecosystem services such as recreation, hillslope stabilization, groundwater recharge, carbon sequestration, pollinator services, and maintenance of regional biodiversity. The continued provisioning of these ecosystem services is at risk when the disturbance regime in shrublands changes and species are unable to recover under the new conditions.
In contrast, many higher elevation ecosystems, such as mixed conifer forests, are tree dominated. Here, frequent prehistoric ignitions created low to moderate severity burns that spread across the forest floor, rarely entering the crowns (canopies) of trees. Unnatural fuel accumulation from decades of fire suppression, logging, and elimination of indigenous burning are driving forces of the wildfires threatening many conifer forests and the surrounding or embedded human communities (Miller et al., 2012; Steel et al., 2015). These uncharacteristically high fuel loads, along with a warming and drying climate, increase the size and intensity of these wildfires (Batllori et al., 2013; Williams et al., 2019). Therefore, fire activity in these ecoregions has been increasing (Miller et al., 2009). However, in coastal California the annual wildfire area has not significantly increased in the last 40 years (Figure 2a). Fires in shrublands are not fuel limited, so the potential for large fires always exists; instead, the coincidence of extreme wind events and ignitions drives large fires in shrublands (Keeley & Syphard, 2019). As such, the acreage burned in coastal southern California is highly variable. Most years have very little wildfire activity, while some years, when all the necessary conditions for fire line up, have a lot. The most destructive fires for human communities in southern California tend to occur from October to January, during the season of fast, dry, and often warm offshore wind events, and particularly when one of these wind events occurs prior to the arrival of the first major rain event of the cool season (Cayan et al., 2022). As is the case with the annual wildfire area, there has been no trend over the past four decades in the area burned in southern California by wind-driven cool-season fires either (Figure 2b). The lack of increasing wildfire in the unforested areas of southern California is in stark contrast to what has been observed in California’s conifer forests (Williams et al., 2019).
In many conifer forests of the western United States, where over a century of fire exclusion has put a halt to a pre-European pattern of frequent fires; and therefore, led to an unnatural accumulation of fuels, it is common knowledge and logical that the potential for fast-spreading and severe wildfires may be reduced through thinning vegetation, reduction of ladder fuels, and re-introduction of ground/surface fires. However, these actions are inappropriate when applied to shrublands in California’s lower elevation mountains under the pretense that shrublands require the same land management practices as tree dominated forests. California shrublands are ignition, not fuel limited; thus, fuel treatments do not address the core problem: human caused ignitions during extreme weather events. Furthermore, fuel treatments have the potential to convert shrublands to highly ignitable, non-native annual grassland, increasing the likelihood of fast-spreading wildfires.
Myth 2: “Mismanagement” of shrubland ecosystems has created unnatural conditions.
Some propose that aggressive fire suppression starting in the 1930-1950s and the forceful removal of indigenous people and their burning practices have resulted in an unnatural buildup of fuels in California’s shrublands and fueled the large wildfires we see today. In addition to the inappropriate application to shrublands of the lessons learned in forest ecosystems, the idea that California’s chaparral needs more fire is also sometimes motivated by comparison with the chaparral systems in Baja California, Mexico. Based on conjecture, smaller wildfires in the Mexican chaparral are thought to be a sign of healthier ecosystems, where lack of fire suppression has kept fuel loads lower; therefore, the larger fires in southern California were proposed to be due to fire suppression and unnaturally high fuel loads. However, researchers have debunked this concept finding no difference in fire regimes between southern California and Baja Mexico (Keeley & Fotheringham, 2001; Moritz et al., 2004) and no difference in fire size comparing pre and post fire suppression periods (Keeley & Zedler, 2009). Examining wind deposited ash within sediments from the Santa Barbara channel, scientists were able to rebuild the scale of wildfires going back 560 years. They found no change in the occurrence of large fires pre- and post-20th century fire suppression (Mensing et al., 1999). During wildfire, some trees within shrubland systems (such as bigcone Douglas-fir) are burnt but not killed. As the tree recovers, the area of the trunk that was damaged can heal over, creating a distinct fire scar within the annual growth rings which can then be aged. When examining tree rings throughout Santa Barbara County, researchers found evidence of large-scale fire patterns going back centuries, with no significant differences in the occurrence of large fires, pre or post suppression periods (Lombardo et al., 2009). In sum, large-scale wildfires have been—and will continue to be—a natural and inevitable phenomenon in California's shrubland ecosystems.
Myth 3: Landscape fuel treatments or prescribed fire will prevent large fires.
This myth is based on the premise that the fire hazard or flammability of shrublands (specifically chaparral) is directly controlled by the age class of the fuel. After chaparral burns, much of the plant material is consumed or severely charred. As the ecosystem regenerates, shrubs and seedlings produce new tissues. These living tissues and the lack of old dead material creates a less flammable fuel bed; however, as the shrubs grow and age, the myth suggests that shrubs accumulate enough dead material to then become much more flammable than younger stands (Rothermel & Philpot, 1973). Furthermore, this paradigm also assumes that the fine fuels that establish after fire (small bushes and various annual plants) will not accumulate sufficiently to carry a new fire in the years immediately following the previous one. This may depend on the extent of invasion by ignitable grasses, and the occurrence of drought that can cause extensive senescence leading to a flammable fuel bed. Based on this fuel-age paradigm, the USFS in the 1970s and 80s sought to create a mosaic of different aged fuel classes across the landscape. Thus, when a wildfire did occur, the hope was that it would burn into an adjacent, younger, less flammable stand, and then burn itself out. However, when fires are driven by Santa Ana winds, such as the January 2025 LA Fires, fuel age does not matter (Moritz et al., 2004). Several researchers have demonstrated that chaparral fires during extreme wind events can become very large and burn through any stand of fuel regardless of age or dead material present (Cohen & Bradshaw, 1986; Dunn, 1989; Moritz, 1997; Keeley et al., 1999; Moritz et al., 2010).
The USFS recently conducted a review of fuel treatment projects across North America (Jain et al., 2021). They determined, overwhelmingly, that projects which empirically measured fuel treatment effectiveness (meaning effectiveness was measured on the landscape using wildfires and treated areas) occurred in forested ecosystems. They also determined that other ecosystems (e.g., shrublands) were severely underrepresented. They concluded that our understanding of empirically measuring fuel treatment effectiveness is in its “infancy” where terminology and concepts are not consistently defined (Jain et al., 2021). While the efforts of agencies and fire fighters in protecting our communities are invaluable, we currently lack a framework for quantifying success of fuel treatments in shrublands and must instead rely on anecdotal information. However, we do know that fuel treatments have the potential to irreversibly harm shrublands, as described below.
Prescribed fire is touted as a scalable and natural way to control fuel loads in a range of ecosystems; however, prescribed burning in shrublands will not stop intense wind driven wildfire (Dunn, 1989). Furthermore, prescribed fire in chaparral is dangerous, both logistically and ecologically. These ecosystems have evolved to burn as high-intensity crown fires and often occur across rugged topography. This makes containment and control challenging, and the risk of escape high, as seen in prescribed fires turned to wildfire such as the 2000 Cerro Grande Fire, 2006 Sierra Fire, 2013 San Felipe Fire, and 2022 Calf Canyon/Hermit's Peak Fire. Due to these logistical challenges, the cost, and its questionable efficacy, the USFS abandoned using prescribed fire to create a fuel age mosaic in shrublands (Conard & Weise, 1998).
If fires occur too frequently, this can have profound negative impacts on the ecosystem. Repeat fires occurring less than 15-20 years apart can kill shrubs before they can set seed (Zedler et al., 1983; Lippitt et al., 2013), leading to the conversion of entire areas to highly flammable/ignitable invasive grasses (Keeley & Brennan, 2012). If prescribed fire is used to control fuel loads it would be detrimental for the treated area to burn again for at least 15 years; however, given extreme wind events and humans greatly increasing ignition rates, this cannot be guaranteed. Thus, we risk creating less resilient ecosystems or losing them entirely. While reinstating cultural burning to indigenous peoples should be pursued, cultural burning of shrublands is potentially problematic for the same reason as any prescribed fire in chaparral. Landscape scale fuel reduction projects and prescribed fires in wildlands will not solve the destructive wildfire problem in southern California.
CAUTION: Landscape fuel modification and prescribed fire could make wildfires worse in California
In shrublands, fuel-modification often results in type conversion of native woody vegetation to non-native annual grasses and forbs (Merriam et al., 2006; Potts & Stephens, 2009; Grupenhoff & Molinari, 2021; Weinberger & Kaczynski, 2022). Seeds from these weed species spread through the equipment and machinery used to create fuel treatments or during firefighting operations. This in turn results in negative ecological consequences such as habitat fragmentation, and reduced habitat quality for threatened animals (Shinneman et al., 2019). Type converted areas also serve as seed reservoirs for invasive non-native plants (Merriam et al., 2006; Shinneman et al., 2019). Non-native annual grasses grow at high density and senesce in spring long before native species, thereby creating flashy fuels which ignite more easily, bring an earlier fire season, and increase the speed of fire spread. In southern California, most ignitions occur along roadsides and are caused by people. In the Angeles National Forest (where the Eaton Fire started), 69% of all ignitions occur within 500 feet from a roadway. In these roadside environments, ignitions are most common in non-native annual grass vegetation.
Non-native annual grasses have changed fire regimes across the Western United States, including California, creating more frequent and larger fires (Balch et al., 2013; Fusco et al., 2019). These grass species are well adapted to disturbances caused by fuel modification and the subsequent frequent fires from human ignitions. This invasion of annual grass and the associated increase in fire frequency - in addition to drought and human disturbances - can result in loss of native shrubland ecosystems (Syphard et al., 2019). Despite the danger native woody vegetation poses to human structures, woody plants are less ignitable and produce slower moving wildfires. Grass vegetation is highly ignitable and produces extremely fast-moving fires. Fire speed is the leading cause of destructive wildfire (Balch et al., 2024), an unfortunate example being the extremely fast moving, wind-driven and initially grass fueled fire in Lahaina, Hawaii. Thus, the spread of non-native annual grasses after fuel modification could create a landscape conducive to more frequent and faster moving fires.
Agencies and wildfire experts understand the need for unique land management strategies to promote wildfire resilience across the diverse ecoregions of the state (California Wildfire & Forest Resilience). Below, we describe the role of fuel modification and solutions for mitigating wildfire risk in shrublands.
The Strategic Role of Fuel Modification
Fuel modification can be useful in mitigating wildfire risk through use of strategically placed fuel breaks and fuel management zones around the wildland urban interface (WUI) as well as creating defensible space (the clearing of vegetation and other dangerous fuels around homes and infrastructure). Fuel breaks are wide blocks, or strips, where the plant cover has been heavily altered to less dangerous fuels (for example, shrubs are almost completely replaced by low growing non-woody plants) and the area becomes sparsely covered by vegetation (Green, 1977). These features are critical for fire fighting (shorter flame lengths allow for hand crews to fight the fire), allowing firefighters to access rugged terrain, and serving as anchor points for the lighting of back fires (Syphard et al., 2011b). However, fuel breaks are only effective if used in conjunction with active firefighting operations. A study examining the intersection of fuel breaks with wildfires found that only 1% of fuel breaks actually stopped fire if there was no firefighting presence, but 46% the time, fire was stopped with support of fire crews (Syphard et al., 2011b). Many fuel breaks never intersected a wildfire and if fuel breaks were constructed but never maintained, they became overgrown (primarily with non-native species, Fitch et al. work in prep) and then ineffective; thus, care must be taken to plan where to construct and maintain these landscape features to be effective (Syphard et al., 2011a, 2011b). Therefore, fuel breaks can aid in stopping fires if constructed in locations where fire is likely to intersect the fuel break, where firefighters have access and air operations will occur, and where commitment (i.e. funding) exists for their long term maintenance. Their strategic use also requires low enough wind speeds that aerial support can be used and it is safe to put firefighters on the fuel breaks.
Fuel “management” or “defense” or “protection” zones are broader areas that are less intensively managed. Here, the goal is to reduce overall fuel loads but keep the ecosystem in a sparse shrub cover state. Under moderate weather conditions (little wind), this type of fuel reduction can provide further access for fire fighters, locations for emergency contingency lines, be used for aerial retardant drops, and be used for back burning operations (Conard & Weise, 1998). For example, during the 2007 Zaca Fire, firefighters utilized an older burn scar where the lower fuel load allowed them to safely light backfires to stop the advancing flaming front. However, during extreme wind events, billions of embers are blown 100s of meters ahead of the flame front. Most structure loss in the WUI is from these embers, and not from radiant heat from the flame front (Cohen, 2008). Fuel management zones failed to stop this barrage of embers in Paradise, CA during the Camp Fire (Maranghides et al., 2021) pointing to the critical need for home hardening. Regarding wind driven fires in the Santa Monica Mountains, “There is really no means of controlling such fires until the wind dies down” (Radtke et al., 1982).
Fuel reduction also plays a role in creating defensible space. A study examining vegetation clearance and home survival found that clearance only needs to be 30m from structures to be effective and further clearance gained no additional benefit (Syphard et al., 2014). Thus, fuel modification can be useful in localized places with strategic implementation in and around the WU). Moving forward, the approach to risk reduction for humans needs to center on “house out”--that fire protection starts from the structure first (home hardening), progresses to the surroundings (defensible space), and then the strategic use of fuel modification to aid fire fighters in the WUI when conditions allow it.
The Uncertainty of Climate Change Impacts on California Shrubland Fires
The climate is warming, which on its own would generally increase fuel aridity. However, precipitation is projected to change in ways that complicate a straight-forward interpretation of climate change and fire in California shrublands. Projections averaged across many CMIP6 models suggest slight increases in mean annual precipitation in northern California and neutral trends in southern California, but with much variability among models (Cook et al., 2020, 2021; Rahimi et al., 2024). Models are in better agreement regarding future intensification of winter atmospheric river storms, but these are largely compensated by projected reductions in storm frequency and models vary greatly in terms of the timing and magnitude of these trends (Gershunov et al., 2019; Williams et al., 2024). Despite uncertainty among models, interannual precipitation totals are consistently projected to grow increasingly volatile (Swain et al., 2018), which, with warming, could cause both droughts and wet periods to grow more intense. Such trends could in theory cause, in the average year, reductions in vegetation biomass and connectivity, which could inhibit fire activity, while concurrently promoting pulses of enhanced vegetation growth, which could promote fire activity.
Enhancing uncertainty further, it is unclear how offshore wind events such as Santa Anas will be affected by climate change. Recent work suggests a decrease in frequency, but not intensity, of southern California’s Santa Anas in the fall and spring, which on its own may reduce the probability of wind-driven fires in fall (Guzman‐Morales & Gershunov, 2019). However, the effect of changing Santa Ana behavior would be modulated by any changes in fuel moisture related to the precipitation and temperature. Climate models suggest future drying of fuels in fall due to warming and delayed onset of the fall rains (Williams et al., 2019; Goss et al., 2020), though projected precipitation changes in fall appear highly uncertain (Rahimi et al., 2024).
Overall, the large uncertainty in how climatic conditions will change and the complexity of vegetation responses to a changing climate inhibit our ability to predict how anthropogenic emissions and climate change will affect California’s shrubland fire regime in the coming decades. Understanding how chaparral and associated fires respond to a warmer and potentially wetter climate is an outstanding area of research (Molinari et al., 2018).
Looking Ahead Towards Wildfire Resilience
The fire challenge in California shrublands lies at the intersection of human caused ignitions, the increasing prevalence of easily ignitable fuels, and the expansion of the WUI. The most destructive fires are human caused in coastal Southern California (Syphard & Keeley, 2015; Syphard et al., 2022). As we have described, shrubland ecosystems are not fuel limited, and with Santa Ana winds, will inevitably be associated with large scale wildfires that are almost impossible to fight until winds die down. Therefore, preventing ignitions will be critical to preventing fires from even starting. Recognizing this, multiple agencies have formed the Southern California Ignition Reduction Program. Ignitions frequently occur in close proximity to housing and roads where common sources are smoking, vehicles, fireworks, and equipment (Faivre et al., 2014). These areas are often invaded by non-native annual grasses which spread fire quickly; therefore, these areas are subject to numerous recurring ignitions making them “ignition corridors.” While the total number of overall human caused ignitions has markedly decreased over the last decades, the number of powerline ignitions has increased, highlighting the urgent need to develop solutions focused on decreasing this source of wildfires (Keeley & Syphard, 2018). Although powerline arc-ing and downed lines will ignite any type of vegetation due to the large amount of energy released, continued effort is needed towards reducing flashy fuels and improving powerline infrastructure within these areas of risk. Restoring these locations with less ignitable native vegetation can have the dual benefit of enhancing California ecosystems while improving the wildfire resilience of the WUI and surrounding ecosystems (Curran et al., 2017).
Rebuilding and retrofitting housing and infrastructure to be more fire resilient will be critical in mitigating fire risk (Kramer et al., 2019, 2021); for example, having enclosed eaves, vent screens, and multi-pane windows can greatly improve the probability of structures surviving fire (Syphard & Keeley, 2019). However, the LA fires produced extreme levels of damage that defensible space and home hardening alone could not have prevented. Planning for these extreme events, if embraced across multiple levels of responsibility and multiple domains (the landscape, the built environment, and communities) (Moritz et al., 2022), can significantly increase resilience to wildfire. Planners and fire professionals need to rethink how we have traditionally designed our communities in order to minimize exposure to fires (Moritz & Butsic, 2020). Areas where ecological functions have been severely degraded could be turned into wildfire buffers on the outskirts of housing such as: irrigated crops, orchards, or recreational fields, which can be easier to defend and require fewer suppression resources (Moritz & Butsic, 2020; Moritz et al., 2022). Agencies and local governments can provide personalized training and education in wildfire resiliency to homeowners, raise public awareness of wildfire preparedness, and secure funding sources to aid communities in mitigating risk.
Wildfires in California will remain a persistent challenge, so we must develop holistic region-specific solutions to these complex socio-ecological problems. Multiple layers of risks must be mitigated, involving shared responsibility and collaboration across agencies, local governments, and communities. We can adapt to coexist with wildfire (Moritz & Butsic, 2020) while preserving the biodiversity of these unique Mediterranean shrubland ecosystems.
