Green Firebreaks: Potential to Proactively Complement Wildfire Management

1. Introduction

Climate change, particularly the warming and drying trends, is increasing fire weather and fire season length [1,2,3]. While fires play an important ecological role [4], ecosystem alterations induced by people often increase fire in the landscape [5], and the urban sprawl into fire-prone areas increases the disaster risk [6]. Emergency management seeks to mitigate, prepare for, respond to, and recover from disasters [7], including wildfires, which are considered any unplanned fires in wildland areas [8]. Wildfire management historically relied on proactive approaches, including the cultural and prescribed burns, but reactive firefighting approaches have recently received more attention and investment [9,10,11]. Given the intensification of fire weather and legacy challenges in fuel management, current wildfire response strategies may be inadequate for future large-scale fire events [12,13]. There is growing recognition of the need to shift from a predominantly reactive firefighting model toward more ecological and proactive wildfire management approaches [14,15].

Contemporary wildfire management is about finding a balance between the protection of people and ecosystems [16], where management interventions involve complex trade-offs that are difficult to comprehend [17]. Clearing vegetation in fuel breaks alters ecosystems and affects biodiversity, aesthetics, property values, and privacy [18] while incurring high maintenance costs, disturbing soils and increasing the risk of soil erosion [19,20,21]. The use of controlled burns for fuel reduction has a rich history [22,23] but requires stakeholder coordination and responsiveness to biodiversity and carbon emission concerns [24,25], and is constrained by financial costs and risks [26]. There are socio-political influences on wildfire management, with variable support for prescribed burning and firebreak clearing [27]. Consequently, alternative wildfire management strategies are needed [28], including integrated [29,30], systemic [31,32], and nature-based [33] approaches. Wildfire-resilient landscaping [34], and protection of riparian vegetation and rainforests are alternatives to clearing firebreaks [35], which can reduce wildfire risk.

Promoting bands of low-flammability vegetation as a green firebreak (GFB) may alleviate concerns and increase support, while enhancing landscape heterogeneity and reducing wildfire spread [36,37,38,39]. This paper reviews the literature on GFBs, which aim to proactively reduce wildfire risk by creating fuel discontinuities, and have been implemented in China for over seventy years [40]. The review emphasises the role of GFBs in complementing other approaches to wildfire management, especially in the Wildland Urban Interface (WUI), which are a high priority [41] due to increased vulnerability resulting from poor land use planning that has allowed development in fire-prone areas [42,43]. The goal of the review is to explore the potential for GFBs to complement wildfire management as they may also provide additional benefits, such as protecting food systems [44], improving air quality [45], and enhancing biodiversity, and carbon storage [46]. The review provides an overview of key GFB considerations for wildfire practitioners and decision-makers, particularly those considering proactive options to complement existing wildfire management practices.

2. Materials and Methods

This review of GFBs is based on peer-reviewed papers published between 2004 and 2024, as revealed by Google Scholar and Semantic Scholar. The literature search initially focused on the keywords Green Firebreaks, Green Fire Barriers, and Wildfire, which were combined using AND and OR search combinations. The initial review of titles and abstracts identified a few key publications, so search terms were broadened to include Green Barriers and Green Fuel Breaks. We also cross-checked citations in the identified literature to capture other relevant papers. Given the increased wildfire vulnerability of WUIs around the world, this setting was the primary focus, but, given the limited literature, all settings were included.

3. Results

GFBs are variously defined in the literature, representing a fuel management approach that typically involves the strategic placement of less flammable species [47,48,49,50,51,52], ecosystems [53,54,55] or land uses [44,56,57] to reduce fire spread and increase suppressibility. Rather than removing vegetation entirely, they alter vegetation flammability, thereby producing fuel discontinuities [58].

The 22 identified publications display the diversity of GFB approaches relevant to wildfire management as implemented in eight countries across four continents (Table 1). In general, GFBs are considered a complementary, rather than a stand-alone approach. The literature builds on and extends existing wildfire research, comprising ten literature reviews (L), ten field studies (F), and two studies that used simulation modelling (M). There is substantial overlap in the focus of these studies, with 14 of the 22 focusing on the flammability of different species, and 16 of the 22 focusing on fire behaviour and ecology. Spatially, the country setting is considered, with reference to both rural and urban landscape settings.

GFBs are generally considered a fuel management approach designed to contribute to the development of fire-resistant landscapes [46]. The GFB literature emphasises strips of low-flammability species planted at strategic locations to reduce fire spread, extinguish embers, or block radiant heat [44,45]. There is an emphasis on strategic placement to decrease fire spread and enhance suppressibility [53,55]. GFBs can take diverse forms, including ecosystems dominated by less fire-prone vegetation [48], linear plantings of low-ignitability species, such as Cupressus sempervirens [54,56], low-flammability crops [57,62], and even ornamental gardens [48,52]. China is a global leader, with 364,000 km of GFB planted for low-cost and biodiversity-friendly wildfire management [40]. The GFB literature identifies specific considerations around flammability, fire behaviour and ecology, which are outlined below.

3.1. Flammability

Much of the reviewed GFB literature identifies vegetation and species flammability as key considerations [47,48,49,62], including one variety of Cupressus sempervirens [56], two ornamental species [52], 47 mixed species [62], 60 native species [49], and 66 agricultural crops [57]. The literature draws on a larger pool of existing flammability research, including approaches, such as fuel load [44], rate of spread, forest fuel ignitability [48], flammability values [47], critical heat flux [51] and a combustion index [53]. The flammability terminology includes ignitability, combustibility, sustainability, and consumability [52], with considerations for fuel volume, fire spread (vertical and horizontal), litter production, bark type, and ember production [44]. A leaf flammability index was also used to rank species [45]. Some of the literature also noted a lack of consistency in defining and measuring flammability [49] and a need for standardised measurement protocols to facilitate comparisons across species and regions [44].

Flammability is a highly complex trait influenced by multiple factors, including timing. The literature highlights the role of temporal variables, such as time to ignition [47], ignition duration, and fire frequency [52]. In some cases, ignition timing was prioritised over combustion intensity in flammability classification. For example, two ornamental species were classified as weakly flammable due to their slow ignition times, despite exhibiting high combustion intensity [52]. For agricultural species, flammability predictors include maximum temperature, burn time, biomass, and specific leaf traits [57]. In the context of GFBs, key indicators include both physical properties such as moisture content and total aboveground biomass, and chemical properties such as the emission of volatile organic compounds [51]. A case in point is C. sempervirens, a Mediterranean species noted for its low flammability, high leaf moisture content, compact growth habit, and low biomass [54]. However, the literature also notes trade offs among flammability traits. For example, species with leaves that ignite slowly may sustain combustion for longer periods, whereas those that ignite quickly often burn faster and for shorter durations [49].

Appropriate species for GFBs were described as needing to meet changing ecological, silvicultural, and economic requirements [40], with plantings responsive to reduced flammability traits, topography and changing climatic conditions [46]. Across the literature, fuel moisture content is consistently highlighted as a key factor influencing flammability. It is well established that both live and dead fuels become less flammable as moisture content increases [47]. Species with low moisture content and rapid moisture loss are generally considered unsuitable for GFBs [48,55,62]. However, under extreme conditions, even typically low flammability vegetation can become combustible [40], an observation, which raises questions and is worthy of further empirical research [44].

3.2. Fire Behaviour and Ecology of GFBs

Wildfire spread is reduced by GFBs with the appropriate design, species selection, installation, and management [55]. In the Canary Islands, for example, there is an emphasis on site planning for GFB [53], and a fire-resilient forest landscape design in Portugal incorporates the use of GFBs [60]. GFB effectiveness in slowing or stopping fire spread is also influenced by soil microorganisms and fungi that enhance decomposition, thereby reducing fuel loads [40]. Researchers note that mosaics of less flammable crops may serve as GFBs [62]. Field research on GFBs across 21 WUI sites near Shanghai, China, documents the fire resistance of plant communities [63]. PHOENIX RapidFire simulations from Australia indicate that GFBs can reduce wildfire risks in low to moderate fuel areas, but their effectiveness requires location-specific designs that consider climate, landscape contexts, and asset locations [46].

GFB location should reflect the effects of topography and associated winds on fire behaviour to alter fuel continuity, block wind, absorb heat, and stop flames [40]. Given that fires burn faster uphill than downhill, and that valleys can funnel winds, GFBs may work best on steeper slopes or narrow valleys [55], as well as on the tops and at the bottoms of ravines [53]. For example, GFBs with C. sempervirens in the Mediterranean utilise linear plantations with staggered spacing to create a dense screen effect in strategic locations such as valleys, ravines, and WUIs [54]. Low-flammability species can be planted in multiple rows [56] with variable height strata [48] to build up vegetation structures, mitigate fire spread [45], and promote horizontal and vertical fuel discontinuities [47,54]. Recommended distances between rows of dense vegetation range from 2-25 m [45,48] or 60 m, and evenas far as embers blow [40].

GFBs are reportedly also created through the manipulation of forest stands with cooler and moister understory microclimates, which decrease flammability by reducing surface fuels and understory branches [55]. Other vegetation adjustments can result in reduced wind and evaporation, creating more fire-resistant forests [63]. Researchers in China noted that although closed canopies foster less flammable understory microclimates, some canopy species are more flammable, and burn hotter, for longer, and more completely than some groundcover and shrub species [55]. Importantly, they observed that dense plantings can inadvertently increase wind speeds and thereby enhance ember transport [40], thus presenting a potential trade-off.

While wildfire management is the priority, GFBs can also have numerous other ecological, silvicultural, and economic effects [40], such as non-fire benefits for soil conservation [53], agriculture [57], air quality [45], and biodiversity [46,48,51]. For example, GFBs in the Canary Islands are known to intercept fog, thereby enhancing their effectiveness as firebreaks and contributing to local hydrology [58]. Compared to other fuel treatments in China, GFBs with evergreen broad-leaved trees offer long-term ecological and environmental benefits by maintaining fire-resistant landscapes, which enhance forest health, promote biodiversity, and reduce firefighting costs [55]. Research in Australia, using a Bayesian Network (BN) analysis combined with fire simulations on the PHOENIX RapidFire platform, showed GFBs can increase biodiversity and carbon storage without substantially increasing fire risks; however, these results await field tests [46].

The diverse approaches to GFBs described in the literature indicate substantial complexity but often a lack of empirical data [44] and only limited consideration of trade-offs. A comparison of different ecological techniques for forest fire prevention, which included dense plantings, found no trade-offs [55]. Another paper specifically noted concerns that dense vegetation increases wind speeds and ember dispersal, and also raised concerns that under the right conditions, any vegetation can become fuel [40]. Most GFB research identifies the potential to complement wildfire management approaches, potentially reducing wildfire spread and intensity, while providing co-benefits [46]. While there is potential for trade-offs, GFBs are worth further consideration as a proactive approach to wildfire management.

4. Discussion

Wildfires are natural hazards that are often not considered disasters until they impact people and infrastructure [64]. There has been a reactive bias in disaster management, and the overreliance on suppression may only serve to defer wildfire risks [65], which may render them too intense to suppress when they do occur [66,67]. Within this context, there are growing concerns about the effectiveness of wildfire management and calls for innovation [12], including increased investment in proactive approaches [15]. People are especially vulnerable to wildfires in the WUI [68] where there are constraints on prescribed burning and clearing. In this context, a growing pool of literature identifies GFBs as a proactive approach to wildfire management. This review reveals that existing literature suffers from a lack of empirical evidence [44], and limited consideration of potential trade-offs [40]. While limitations are acknowledged, the existing literature does offer insights into the potential for GFB to complement wildfire mitigation, preparation, response and recovery.

4.1. Wildfire Mitigation:

GFBs utilise less flammable species, ecosystems, and land uses to mitigate wildfires by altering the continuity of fuel [44,47,48,49,50,51,52,53,54,55,56,57,58]. The WUI is noted as being vulnerable to wildfire [69], including a growing risk of infrastructure loss [70]. The strategic siting of GFBs can reduce fire spread and support fire suppression [48,60], providing a living barrier with increased shade and moisture that reduces radiant heat and the likelihood of ember ignition. As an innovative and emerging strategy for wildfire management [5], there is a growing body of guides, reports, and policies about GFBs [71,72,73,74]. Alarmingly, like the academic literature, this grey literature often overlooks the potential trade-offs associated with GFBs. Relying solely on lists of low-flammability species, without accounting for spatial layout, topography, and seasonal fire risk, may create a false sense of security regarding GFB effectiveness. In addition, the literature also highlighted that selecting species for low flammability may carry unintended trade offs, as these traits are also influenced by environmental variables [75]. Under climate change scenarios, droughts are predicted to increase [76], and this may alter flammability [77], so maintaining water for GFBs is an important consideration to ensure they enhance wildfire mitigation and prevent vegetation from becoming fuel [40,78,79,80].

4.2. Wildfire Preparation:

Community preparation, through the establishment and management of GFBs in the WUI, has the potential to mitigate wildfire risks and strengthen engagement in wildfire management, where it is typically limited [81]. While poor urban planning has induced increased wildfire risks in the WUI [42], green and adaptive designs [82], including GFBs, could reduce those risks, helping communities be better prepared. Providing options may better support responsiveness to different preferences for wildfire management between emergency responders and homeowners [83]. For example, while firebreak clearing and controlled burns may receive limited community support [27], GFBs may be preferred by the surrounding community because they promote biodiversity and landscape values that attract people [35], while still complementing the embedded reactive responses [84]. In any event, the implementation of GFBs can build on community engagement in urban revegetation, where revegetation managers work with fire managers [46], promoting shared responsibility for managing landscapes in ways that better prepare for and reduce wildfire risk [85].

4.3. Wildfire Response:

As wildfires become larger and more intense, the effectiveness and timeliness of existing response tools, including evacuation and shelter-in-place strategies, are increasingly being questioned [12,86]. The current emphasis on reactive disaster cycles of response and recovery [84] may be more effective with investments in mitigation [11]. Landscapes with intentional planning, placement, and ongoing maintenance of GFBs can alter fire behaviour and reduce fire spread [45] by establishing networks of firebreaks that reflect species traits, site-specific considerations and protection priorities [54]. In areas already vulnerable to wildfires, the choice often lies between retreating from high-risk zones or investing in effective wildfire management strategies [87]. Although GFBs are not suitable in all settings, they can extend the window for wildfire response by reducing or delaying fire spread, intensity, and ember attack [46]. The FireSmart work in Europe provides a vision for how more integrated wildfire management can complement wildfire response through suppression by building on various strengths and weaknesses [88]. In this context, GFB provides a proactive, nature-based option that, while not a stand alone response, can enhance and support existing wildfire response systems.

4.4. Wildfire Recovery:

Just as anthropogenic drivers such as climate change, land use, and invasive species create novel ecosystems that are more prone to fire [5], GFBs can engage people in the process of modifying ecosystems to become less fire-prone [43], and hazard resilient [89]. The GFB approach represents an opportunity to promote recovery through ecological communities by creating microclimates, supporting wildlife, and delivering soil benefits that contribute to reduced flammability [55]. They may also offer fire refugia [74] and serve as drought refuges, enhancing biodiversity resilience [90] and carbon storage [45,46]. In addition to the ecological benefits, GFBs can support a productive landscape through the integration of low-flammability crops [57,62] and place-based strategies that connect people, land use, and biodiversity [5]. Ecosystem-based GFB approaches to wildfire management may be a fire-regulating ecosystem service, a concept rarely considered in the wildfire literature [91], but one with the potential to enhance wildfire recovery and coexistence.

4.5. Future Research Directions:

Looking ahead, there may also be opportunities to integrate GFB actions with relevant proactive strategies for Cascading Hazards [92] and Urban Heat Islands [93], through Green Infrastructure [94] and adaptive water management [95], enhancing recovery and resilience. As a more integrated approach to wildfire mitigation in the WUI, GFBs can make use of urban wastewater for irrigation [96,97,98], potentially reducing drought intensity and fire risks while promoting water reuse and a multitude of co-benefits. Strategically placed GFBs may also support landscape heterogeneity, which in turn supports wildfire management.

5. Conclusion

Wildfires are complex hazards that call for a range of solutions. In the face of increasingly intense wildfires and reduced capacity to suppress them, it is timely to overcome reactionary biases. This paper reviewed literature on GFBs to consider them as a wildfire management option, and identified their site-specific potential to enhance wildfire mitigation, preparation, response and recovery. Appropriately designed and located GFBs can proactively complement wildfire management, increasing fuel discontinuity to reduce fire spread and intensity in the short term, while enhancing community engagement, biodiversity, and carbon storage for longer-term benefits. The existing literature offers limited discussion of potential trade-offs. For instance, while structural vegetation changes may reduce wind speed, inadequate attention to water availability could undermine GFB performance under future climate conditions. With predictions for hotter, drier conditions, irrigating GFBs may strengthen their effectiveness in complementing wildfire management. For the vulnerable WUI, GFBs may be a strategic wildfire management option, especially where it is possible to consistently irrigate through urban water reuse. We conclude from our literature review that GFB designs will be enhanced by acknowledging and mitigating trade-offs, while reflecting spatial, temporal, and socio-ecological conditions, including societal preferences. While not suitable in all settings, GFBs could play a strategic role in more integrated wildfire management, offering both direct and indirect benefits for the present and the future.