Stop #4 - Group gathered, some fuel break pictured, and Superstition Mtn.

Sonoran Fuel Breaks

Introduction

In February 2024, the Southwest Fire Science Consortium led a field trip in central Arizona to learn more about the dynamics between wildfire and invasive species in the Sonoran Desert. The field trip was focused on improving our collective understanding of how invasive grasses and forbs have altered the fire conditions in the Sonoran Desert by increasing the frequency and severity of fire.

Field trip participants, ranging from agency personnel to researchers, learned about techniques that are being tested to reduce wildfire risk to Sonoran Desert ecosystems that are impacted by invasive species, preserve areas of intact desert, and prevent further transition of Sonoran Desert ecosystems to grasslands.

You will learn:

  • Techniques being used to reduce fire risk
  • Expert opinion on the status of fire and invasion in the desert
  • How prescribed fire is being employed in a non-fire adapted ecosystem
  • Why a site-specific approach is needed to reduce fire risk and conserve the native ecosystem

Fire in the Sonoran Desert Credit: Nicholas Castro, Tonto National Forest.

Fire in the Sonoran Desert

The Sonoran Desert is changing. This unique ecosystem is quickly being altered by the introduction of invasive grasses and forbs, and the intensification of wildfire that they cause.

Vegetation in the Sonoran Desert is typically sparse, with large spaces between perennial cacti and plants that are occasionally filled with a colorful display of annual wildflowers during wet years. Water is the limiting factor in the Sonoran Desert, and  to survive here, plants are uniquely adapted to arid conditions and episodic precipitation that largely prevents plants from creating a thick, continuous understory. The native plants evolved to withstand extreme stressors such as drought and intense heat; however, they have not evolved to survive fire.  Historically, severe wildfires in the Sonoran Desert were very infrequent. Most fires were of low severity, with fire-return intervals estimated at 100 to over 1000 years.

Today, wildfires on this landscape look quite different due to the introduction and widespread establishment of non-native invasive plants. Invasives outcompete native vegetation for nutrients and water, fill in the vacant spots between plants across the desert landscape, and often regenerate more quickly than the native vegetation after disturbance (such as fire). As a result, invasive species supply a higher and more continuous fuel load than was previously possible. After each wildfire, these fire-adapted invasives outcompete the native vegetation, transitioning the landscape from desert scrubland into more of a grassland.

Saguaro mortality over time. Photos taken by Jim Malusa.

Invasive species colonization two years after a fire. Photos taken by Benjamin Wilder.

For more information about the grass-fire cycle and the invasive species causing the change, check out  this publication .

Definitions

Fuel Breaks

Fuel breaks are areas where the vegetation is strategically manipulated – often by mechanical removal – for the purposes of wildfire mitigation. For example, to protect homes and property, land managers often clear large strips of land between houses and roads because roadsides are a common ignition location. This concept was developed in montane ecosystems where the main issue concerning wildfires is the build up of fuels from a century of fire suppression. Different approaches are needed to solve the problem of invasive species in the Sonoran Desert.

Invasive Species - AKA non-natives, introduced species, noxious weeds

Invasive species are plants, animals, or other organisms that reside in locations where they did not evolve. They often do very well and can outcompete native species due to various ecological adaptations and a lack of natural predators. Examples in the Sonoran Desert include buffelgrass (Cenchrus ciliaris), fountain grass (Pennisetum setaceum), stinknet (Oncosiphon pilulifer), and red brome (Bromus rubens).

Invasive species of the Sonoran Desert. Illustration by: Kara Gibson

Techniques to Reduce Fire Risk

Click on the images and videos to learn more!

Fuel breaks in the desert

In a nutshell: A 150 foot shaded fuel break was created between roads and the neighborhoods that bordered the wildland-urban interface (WUI). Click to learn more about how this decision was made and the other changes that were instituted to protect this Sonoran Desert landscape.

Invasive perennial grass removal techniques

In a nutshell: A deep dive into the benefits and drawbacks of hand removal and herbicide-use on invasive grasses in the Sonoran Desert.

A case study on stinknet removal

In a nutshell: A Landowner discusses the use of mulch to remove stinknet from his property. Click to learn more about the treatment strategies he employed. 

Methods for creating fuel breaks

In a nutshell: An exploration of different methods for instituting fuel breaks in the Desert. Techniques employed include hand thinning, mechanical thinning, herbicide use, and some burning.

Prescribed fire in the desert

In a nutshell: A description of the use of Rx fire in the desert to help ensure safety of recreationists and firefighters. Click to learn more about the use of fire on desert vegetation at a developed shooting range.

Fuel breaks in the desert

In a nutshell: A 150 foot shaded fuel break was created between roads and the neighborhoods that bordered the wildland-urban interface (WUI). Click to learn more about how this decision was made and the other changes that were instituted to protect this Sonoran Desert landscape.

Summary

The Ghost Fire (2005) burned over 600 acres of the Sonoran Desert near Apache Junction, AZ and the Lost Dutchman Fire (2020) burned approximately 220 acres of the same landscape. These wildfires prompted the affected communities to connect with their local Bureau of Land Management (BLM) office to implement fuel breaks between roads and the neighborhoods bordering public land. BLM officials and local community leaders also decided to institute land-use changes due to the heightened risk of human ignitions in this area.

The BLM created a 150 foot shaded fuel break between roads and the neighborhoods bordering the wildland-urban interface (WUI). A shaded fuel break by nature leaves some of the canopy purposefully on the landscape, while still successfully altering enough undergrown to change the fire behavior when it reaches the break. In the desert, crews aim to remove fine fuels and ladder fuels in the understory, but may leave native vegetation. This method protects natives and targets invasive species – the goal of most Sonoran Desert land managers.

When the  fuels assessment  and  statement of work  were drawn up, BLM officials discovered that state officials were completing a similar project in the same area. To save time and resources, the two organizations worked out a partnership under the  Good Neighbor Authority (GNA) . The decision to use the GNA was especially helpful due to the alternating land ownership across the treatment area.

A culmination of 800 piles were burned from the creation of these fuel breaks. Due to the success of this project, fuel breaks of the same designs were also implemented by BLM officials near the Goldfield Ghost Town.

Land use change

In an effort to reduce the risk of human-caused ignitions, the BLM officials who oversee this area have decided to change the status to day-use only. The Ghost Fire was started by an unsheltered person who was living in the area, and houses bordering public land were threatened as a result. There is also the concern that due to the popularity of this area, increased traffic from people camping may result in more human-caused fires.

Invasives species

Invasive species such as red brome (Bromus rubens) and stinknet (Oncosiphon pilulifer), which were present prior to the fire, are fire-adapted and therefore regrew faster than native species post-fire. The effect of wildfire on invasives and native plants becomes very clear when looking at the understory of the fuel break and the understory of the burn scars.

The next steps of this BLM-led project will include the monitoring and eventual treatment of invasives. However, managers have decided that they will begin after the full land-use conversion has taken place because the transition – which includes road-building – will cause disturbances that could render current treatments ineffective.

Learn more

 Video of Derrick Youngerman from BLM  discussing the 2020 Lost Dutchman Fire, the impacts of invasive species, how the BLM implements fuel breaks within the community, and a land-use change resulting in the relocation of unsheltered individuals and what this means for wildfire.

Invasive perennial grass removal techniques

In a nutshell: A deep dive into the benefits and drawbacks of hand removal and herbicide-use on invasive grasses in the Sonoran Desert.

Summary

Deciding which techniques should be used to install a fuel break in the desert, or choosing another method to directly combat the desert-grassland transformation requires thoughtful consideration. Two of the most popular techniques in use are hand-removal and herbicides. The field trip stopped by Silly Mountain, which has been experiencing colonizations of buffelgrass and fountain grass, to witness the effects of various invasive removal treatments.

Hand-removal

 Friends of the Tonto National Forest  have been digging up and treating buffelgrass and fountain grass on and off-trail  all around Silly Mountain and the Superstition Foothills ,  and numerous groups (including Saguaro National Park) have been doing the same in the Tucson area.

Benefits

  • No chemical use, minor impacts to native plants (mainly trampling).
  • Can be done any time of year, when the weather allows it.
  • Very efficient for small to medium and scattered infestations.
  • Herbicide can treat areas that are difficult to impossible to reach by hand crews.
  • Herbicide has shown to be effective at halting spread of buffelgrass on the landscape-scale.
  • Managers at Saguaro National Park outside Tucson are pioneering treatments of the perennial buffelgrass and have seen success with heeding buffelgrass spread by implementing aerial herbicide spraying. Compared to untreated areas, plots that were sprayed with herbicides containing the active ingredient Glyphosate experienced no further expansion and some loss of buffelgrass. Areas not receiving treatment saw exponential growth of buffelgrass. (Learn more about the science  here .)

Drawbacks

  • Very time-consuming for large, dense patches.
  • Large workforce is required, which is expensive if using paid staff.
  • Soil disturbance from digging can trigger massive buffelgrass seed germination following heavy rains.
  • Aerial spraying has varied negative impacts on native vegetation.
    • The most adverse effects were observed in plants especially sensitive to moisture in the air such as Ocotillo, which are as susceptible to the herbicides' effects as buffelgrass and therefore often suffer mortality.
    • The least adverse effects were observed in Saguaros, most of which were not affected. However, due to some genetic variability throughout populations, some retained small wounds.

Herbicidal treatments

Benefits

  • Herbicide can treat areas difficult to impossible to reach by hand crews.
  • Herbicide can efficiently treat large, dense infestations as well as masses of seedlings.
  • Herbicide has shown to be effective at halting spread of buffelgrass on the landscape-scale.
  • Numerous herbicides exist, which can be used for different invasive plant species and a variety of situations.
  • Aerial herbicide application can reach areas that are difficult to impossible to reach with hand crews.
  • Pioneering treatments of the perennial buffelgrass by mangers at Saguaro National Park outside Tucson, have seen success with impeding buffelgrass spread by implementing aerial herbicide spraying.
  • Compared to untreated areas, plots that were sprayed with herbicides containing the active ingredient Glyphosate experienced no further expansion and some loss of buffelgrass. Areas not receiving treatment saw exponential growth of buffelgrass. (Learn more about the science  here .)

Drawbacks

  • Herbicide use is complicated and requires input from experts.
  • Depending on the herbicide, application is usually restricted to the target plant's active growing season.
  • Aerial spraying had varied negative impacts on native vegetation.
  • The most adverse effects were observed in plants especially sensitive to moisture in the air such as Ocotillo, which are as susceptible to the herbicide’s effects as buffelgrass and therefore often suffer mortality.
  • The least adverse effects were observed in Saguaros, most of which were not affected. However, due to some genetic variability throughout populations, some retained small wounds.
  • Repeated spraying caused increasing damage.

Learn more

Lesson learned highlight:

Cacti and native succulents are resilient to glyphosate-based herbicides, while ocotillo and native plants with “top-absorption” have a similar level of susceptibility to herbicides as invasive grasses such as buffelgrass. Additionally, herbicides that work on annual grasses do not affect perennial grasses.

A case study on stinknet removal

In a nutshell: A Landowner discusses the use of mulch to remove stinknet from his property. Click to learn more about the treatment strategies he employed. 

Summary

Stinknet is an invasive species new to the desert that is rapidly filling in gaps between native vegetation and increasing fire risk. Here, we discussed some methods, like the use of native mulch to reduce the growth of stinknet. The field trip stopped by a property where the landowner was trying ecologically-conscious techniques to treat invasive species without changing the fire risk or natural dynamics of his land. 

Stinknet: 

  • New to the Southwest, originally from South Africa
  • Seeds are the size of a period (.)
  • Creates irritating and pungent smoke when burned
  • Somewhat fire-adapted and quick to grow after fire

Treatment strategy: Mulching native vegetation

  • Creates less green waste than cutting and transporting off-site or burning
  • Can reduce project costs to chip and use mulch on site vs bringing to the dump
  • Less ecosystem implications than herbicides
  • Helps with dust suppression 
  • Helps reduce wind and water erosion
  • Seems to not increase fire risk as mulch burns in a low-intensity, smoldering fire.

Current recommendations for using mulch to suppress stinknet*:

  • Apply at least 4” of mulch 
  • Effectiveness drops off after five years, which calls for reapplication at this point

*More experimentation is needed  to understand how to reduce the spread of stinknet and understand the effectiveness of this treatment method.

Learn more

Lesson learned highlight :

Mulching suppresses dust, and mitigates invasive establishment and growth. It is also a way to repurpose green waste and reduce project costs.

Methods for creating fuel breaks

In a nutshell: An exploration of different methods for instituting fuel breaks in the Desert. Techniques employed include hand thinning, mechanical thinning, herbicide use, and some burning.

Summary

 Siphon Draw Environmental Services  is experimenting with how to best create fuel breaks in the Sonoran Desert, There is an open-call to any researcher interested in studying these techniques.

Techniques employed:

  • Hand-thinning of abundant perennial woody plants (e.g. creosote)
  • Some mechanized equipment (e.g. chainsaws)
  • Some herbicide use
  • Treatments extended into the lot ~70 yards from the road, on all sides of the plot
  • Fuel break ran along roadsides for 70 feet 
  • More aggressive cutting in the first 15 feet from the road
  • Spot-treatment using hand-held propane torches.

There is a current lack of guidance for creating fuel breaks in the desert. The current method includes clear cutting small sections of the vegetation. This method is effective in forested ecosystems, but managers working in the Sonoran Desert are seeing that it promotes the growth of the invasive species while causing harm to sensitive and slow-growing native desert vegetation. At this location, managers are experimenting with clearing around native vegetation with high resource value and low fire risk.

When hand thinning is used to create and maintain a fuel break, it requires periodic upkeep and maintenance to continue to work successfully during a wildfire. Further experimentation is needed in order to understand the best practices, impacts, and effectiveness of the various methods for reducing fire risk in the desert.

Experimental thinning and Rx burning strategies

  • Native vegetation was treated very consciously. Plants that are not fire-adapted were avoided to the best of the crew’s abilities. 
  • Crews started by thinning dead wood from native vegetation and moving to mulch piles or burn piles in the spaces between perennial native plants.
  • Crews then “blanched” the lower branches of other fire-sensitive plants quickly, mimicking a low-intensity fast-moving fire that was present historically in this area.
    • The goal is to return fire to the landscape in the less-severe of the two ways in which fire burned during the evolution of the Sonoran Desert. 
    • Crews aim to use low intensity and fast moving fire, so that the area does not experience a high intensity and stand-replacing fire. 
  • After fire-sensitive plants were treated, plants that could withstand some heat were allowed to burn hotter and with a longer residence time, allowing these treatments to effectively reduce fuel load for as long as possible before maintenance is required.

Learn more

Lesson learned highlight:

Each invasive grass species has its own growth cycle that should be considered, especially in the creation of fuel breaks (natural or man-made). The main issue is not a fire suppressed system but a fire-prone system.

Lesson learned highlight:

Portable propane tanks can be used to torch invasive plants and sterilize seeds. Stinknet drops its seeds, indicating that it is a fire-adapted plant. The plant must be exposed to enough heat so the seeds are no longer viable. Wood chipping using mulch made from non-invasive plants around “polygons” of the study area can help researchers analyze the effects of different treatments on stinknet. In areas where burning treatments have occurred, there is less new growth.

Prescribed fire in the desert

In a nutshell: A description of the use of Rx fire in the desert to help ensure safety of recreationists and firefighters. Click to learn more about the use of fire on desert vegetation at a developed shooting range.

Summary

This field trip stop was at a  shooting range in the Tonto National Forest.  The purpose of prescribed fire at this location is to reduce fuels in a high-ignition area where people recreationally shoot firearms. Firefighters are called to the area multiple times a year to respond to fast moving, dangerous fires and regularly have to employ a full suppression strategy in the intense heat of the summer to protect habitat and recreators. Prescribed fire can be implemented at safer times of year and managed to protect any remaining native vegetation and habitat.

The consensus among land managers on this prescription is that the frequency of fire at this location is not in line with historic conditions; however, matching the historic fire regime is not the motivation for applying fire here. The key objective is public and firefighter safety since the area is now a grassland.

Learn more

Lesson learned highlight:

Burning needs to be done when weather conditions and fuel moisture content are in a range to produce a low to moderate severity burn. Paying attention to the invasive grass life cycles and burning when grasses are not in green-up and have low moisture content will help ensure that the prescribed burn is effective.

Lessons learned

The ecological impacts of the introduction of invasive species and wildfire mitigation techniques in desert ecosystems are not well understood. Entire landscapes that used to be dominated by slow-growing native cactuses are being transitioned into grasslands after only a few wildfires. 

By sharing knowledge, working together, and facilitating research, we can help protect the Sonoran Desert.

Sharing Knowledge

By sharing everything from generational knowledge to specific invasive species tips and tricks and creating a web of connections among land managers and researchers, we are able to understand these disturbance processes better and put more effort towards protection and prevention.

Working Together

If we don’t approach this situation strategically, it is very possible that the work we do to combat invasive species and the growing risk of wildfire may cause invasives to tighten their grip on the Sonoran Desert. Collaboration between researchers and land managers is an integral part of the collective response needed for this unique and challenging situation. This response begins with opportunities, like this field trip, to meet and talk together about what we know and lessons learned.

Continued research is needed

Increased aridity and changes in precipitation patterns caused by climate change will further affect the fire and invasion cycle that is rapidly shifting the Sonoran Desert in unforeseeable ways. During the field trip, the conversations around climate change were met with a high degree of uncertainty as it is hard to imagine how these processes will intensify. We need to help facilitate research on invasive species and the impacts of wildfire and climate on native species and ecosystems. This begins, again, with collaboration between land managers and researchers, and by increasing awareness of the issue of wildfire and invasive species in the Sonoran Desert.

Resources

Presenters

  • Ryan Conway - Fuels Technician on the Tonto National Forest, Mesa and Cave Creek Ranger Districts
  • Leo Chavez - Bureau of Land Management (BLM)
  • Leslie Crabtree - Friends of the Tonto National Forest (FOTNF)
  • Kim Franklin - Sonoran Desert Museum 
  • Perry Grissom - Sonoran Desert Museum, Saguaro National Park herbicide specialist and former fire ecologist
  • Travis Helfrich - Assistant Fire Management Officer on the Tonto National Forest, Mesa and Cave Creek Ranger Districts
  • Carlos Payan - Bureau of Land Management (BLM)
  • Don Pike - Friends of the Tonto National Forest (FOTNF)
  • Alan Sinclair - retired firefighter and IMT lead for Tonto National Forest 
  • Jessi Szopinski - Arizona Department of Forestry and Fire Management (AZDFFM)
  • Derrick Youngerman - Bureau of Land Management (BLM)

Credits

Authors: Shila Simpson, Annie Elko, & Molly McCormick

Reviewed by:  Ariel Marc Leger  and Alex Lauren Gerber

Story map design: Marc Gomez

Additional images provided by: Benjamin Wilder and Jim Malusa

Saguaro mortality over time. Photos taken by Jim Malusa.

Invasive species colonization two years after a fire. Photos taken by Benjamin Wilder.

Invasive species of the Sonoran Desert. Illustration by: Kara Gibson