Grasses are Changing the Scope of Fire in the Sonoran Desert
By Jonathan Michael
Published October 19, 2023
Grasses are Changing the Scope of Fire in the Sonoran Desert
The Southwest Fire Science Consortium hosted a seminar on September 13th, 2023, Grassification and Fast-Evolving Fire Connectivity and Risk in the Sonoran Desert.[1] The four presenters summarized their research on the 2020 Bighorn Fire, which burned almost 200 square miles and sent the greater Tucson region into a state of emergency. The full scope of their findings, and those of their many co-authors, can be found in their recent article from Frontiers in Ecology and Evolution.[2] The presentation alone gives denizens of the Southwest some key points to consider, primarily the concept of “grassification” and what it means for the future of fire in the desert.
Research on the Sonoran Desert suggests that in its relatively short history, large fires have been uncommon.[3] One clue supporting this conclusion is the dominance of saguaro cacti and palo verde trees.[4] Saguaro do not resprout and have a fire-sensitive outer skin, and the palo verde’s photosynthetic bark is very vulnerable to fire.[5] The large swaths of barren soils that sit between plants provide no fuel for the spread of fire, acting as natural barriers.[6] Research suggests that is why past wildfires often started in the woods of higher elevations and died out when they reached the lowlands.[7]
Recently, in the Mojave and Sonoran Deserts, that trend has shifted to wildfires that rage through the lower desert regions.[8] Scientists speculate that two changes are acting as prime instigators: the invasion of grass species, and the surges of greater rainfall during wet seasons.[9] While more rain leading to more fire might seem counterintuitive, grasses and other short-lived, fast-growing plants can capitalize on a wet season. They quickly consume the water, rapidly grow, spread their seeds, then die when the weather returns to hot and dry.[10] These plants and their remains act as a fuel load for the oncoming fires of the hot and dry season, which is particularly prone to spreading in the high-wind and dry lightning before monsoon season.[11]
Two kinds of grass species have been spreading through the Sonoran Desert over the past century— summer-flowering plants that live many years, namely buffelgrass, and winter plants that tend to live for one season, like red brome and cheatgrass.[12] While both grasses lend themselves to increasing the fuel load and spreading fire, the presence of both types creates a potent combination.[13]
“Grassification” means more than the spreading of grass species— it is this cycle of grass-and-burn that positively reinforces itself in the current climate.[14] Plants that can grow and complete their life-cycle quickly, including many grasses, excel at growing in recently disturbed and burned soils.[15] Their proliferation in periods of heavy rainfall, followed by a hot and fiery dry period, results in conditions growing more favorable for the spread of the same grasses and annuals the following season.[16] This pattern may eventually stabilize into a more fire-resilient form, but it is unclear after how long, and at what cost.
Some researchers believe that longer periods without frequent and large fires can allow for old-growth plants to dominate, leading them to suggest that landscapes with intermittent fires have particularly unique and diverse communities of plants and animals.[17] The ecosystems that can remain stable through frequent and large fires may have adapted into that condition over a long period of time. In contrast, young and formerly fire-resistant regions like the Sonoran and Mojave Deserts could undergo a massive transformation with only a few massive fire events. This may include mass die-offs of iconic keystone species like the saguaro cactus or the joshua tree.[18]
The Bighorn Fire of 2020 was a particularly ominous event in the recorded history of the Sonoran Desert. Though with its ongoing grassification, without intervention, the next event will have a greater magnitude.[19] The most particularly fire-prone climate may occur during the El Niño-Southern Oscillation, which has a prolonged hot and dry season followed by intense, concentrated rainfall in the cool, wet months.[20] As a result, the next two years may bring a more significant wildfire than the Bighorn.[21]
This presents humans living in these sensitive deserts with the task of urgently managing wildfires. Luckily, the past decades have given us a deeper understanding of how to manage and prevent wildfire, while protecting keystone species in the ecosystems. Last year, a study in Scientific Reports suggested that enhancing firefighting effectiveness has the greatest potential to mitigate the effects of wildfire.[22] Catherine Jarnevich of the U.S. Geological Survey in Fort Collins, CO, ran simulations that supported this finding. Saguaro cacti can persevere even among dense populations of grasses like buffelgrass, but they cannot endure fire.[23]
The most effective strategy for preventing the spread of fire is likely a mixed approach of treatment and firefighting.[24] The overall consequences of grassification can be held at bay by the removal of grass species, which can have immediate benefit, because by strategically removing grass species from selected areas, we can create the kinds of fire breaks that historically contained fires in the Sonoran Desert.[25] While there are many ways to go about removing grass on a small scale— on the scale demanded by this job, the most effective means of removing perennial grass species like buffelgrass is routine herbicide treatment.[26]
Until recently, the major roadblock to effectively managing fire in these regions has been funding. However, the Inflation Reduction Act of 2022 has allowed for sufficient federal funding to take action.[27] Since the funding is federal, it can be disbursed efficiently to many of the jurisdictions responsible for vulnerable regions.[28] This is exciting news for advocates who had been sounding the alarm to no avail. However, some important tasks remain ahead of us before the management can begin.
A large, trained, and cost-effective workforce is needed to effectively treat invasive species such as buffelgrass.[29] At the moment, there are few contractors available to do the job. However, local environmentalist organizations like the Tucson Audubon Society have stepped up, now offering invasive species removal services as a way to address the issue.[30] While a much larger workforce is needed, this is a hopeful start.
Finding the labor to complete this task is certainly an obstacle, but we have cause for celebration that there is both adequate funding and a well-informed plan of action to manage fire in the desert. We owe our Sonoran Desert researchers a huge thank-you for representing this beautiful region passionately and getting us this far. We can thank them by engaging with resources they have made available, becoming vocal supporters of the initiatives they recommend, and volunteering to help implement the plans they have thoughtfully designed.
Here are some ways that you can engage right now!
Subscribe to the Southwest Fire Consortium's Webinar Series
Contact City Council With Your Compliments and Recommendations
Volunteer to Remove Buffel Grass at Saguaro National Park
Register As a Volunteer With the Desert Museum
[1]Southwest Fire Science Consortium, Grassification and Fast-Evolving Fire Connectivity and Risk in the Sonoran Desert, https://www.swfireconsortium.org/2023/08/14/grassification_sonoran_desert (Oct. 3, 2023). The presenters were Benjamin Wilder (Next Generation Sonoran Desert Researchers), Catherine S. Jarnevich (U.S. Geological Survey Fort Collins Science Center), Kim A. Franklin (Arizona-Sonora Desert Museum), and Julio L. Betancourt (U.S. Geological Survey Science and Decisions Center).
[2] Benjamin T. Wilder, et. al., Grassification and Fast-Evolving Fire Connectivity and Risk in the Sonoran Desert, United States, Frontiers in Ecology and Evolution Vol. 9 (2021).
[3] Id.
[4] Id.
[5] Id.
[6] Id.
[7] Id.
[8] Wilder, et. al., supra note 2.
[9] Kirk A. Moloney, et. al., Climate Impacts on Fire Risk in Desert Shrublands: A Modeling Study, Frontiers in Ecology and Evolution Vol. 10 (2021).
[10] Thomas J. Rodhouse, et. al., Post-Fire Vegetation Response in a Repeatedly Burned Low-Elevation Sagebrush Steppe Protected Area Provides Insights About Resilience and Invasion Resistance, Frontiers in Ecology and Evolution Vol. 8 (2020).
[11] Eddie J.B. van Etten, et. al., Editorial: Fire Regimes in Desert Ecosystems: Drivers, Impacts and Changes, Frontiers in Ecology and Evolution Vol. 10 (2022).
[12] Wilder, et. al., supra note 2.
[13] Id.
[14] Julio L. Betancourt, Energy Flow and the 'Grassification' of Desert Shrublands, 112 Proceedings of the National Academy of Sciences 31 (August 4, 2015).
[15] Penelope Morgan, et. al., Vegetation Response to Burn Severity, Native Grass Seeding, and Salvage Logging, 11 Fire Ecology 31–58 (2015).
[16] Wilder, et. al., supra note 2.
[17] David M. J. S. Bowman, et. al., Pyrodiversity is the coupling of biodiversity and fire regimes in food webs, 371 Phil. Trans. R. Soc. B. (June 5, 2016); See also Jeremy W. Fox, The Intermediate Disturbance Hypothesis Should Be Abandoned, 28 Trends in Ecology & Evolution 86-92 (2013).
[18] Robert C. Klinger, et. al., Contrasting Geographic Patterns of Ignition Probability and Burn Severity in the Mojave Desert, Frontiers in Ecology and Evolution Vol. 9 (2021); See also Seth B. St. Clair, et. al., Spatio-Temporal Patterns of Joshua Tree Stand Structure and Regeneration Following Mojave Desert Wildfires, Frontiers in Ecology and Evolution Vol. 9 (2022).
[19] Wilder, et. al., supra note 2.
[20] Andrea Brunelle, Interactions Among the Fire, Vegetation, the North American Monsoon, and the El Niño-Southern Oscillation in the North American Desert Southwest, Frontiers in Ecology and Evolution Vol. 10 (2022); See also, Mary K. Schmid & Garry F. Rogers, Trends in Fire Occurrence in the Arizona Upland Subdivision of the Sonoran Desert, 1955 to 1983, 33 The Southwestern Naturalist 437-444 (Nov. 23, 1988).
[21] National Weather Service, El Niño/La Niña Information, https://www.weather.gov/fwd/enso (Oct. 3, 2023); See also Wilder, et. al., supra note 2; and Boyd R. Wright, et. al., Rainfall-Linked Megafires as Innate Fire Regime Elements in Arid Australian Spinifex (Triodia spp.) Grasslands, Frontiers in Ecology and Evolution Vol. 9 (2021).
[22] Farshad Farkhondehmaal & Navid Ghaffarzadegan, A Cyclical Wildfire Pattern as the Outcome of a Coupled Human-Natural System, 12 Sci. Rep. 5280 (2022).
[23] Catherine S. Jarnevich, et. al., Developing an Expert-Elicited Simulation Model to Evaluate Invasive Species and Fire Management Alternatives, Ecosphere 10(5):e02730 (2019), doi: 10.1002/ecs2.2730.
[24] Id.
[25] Emily J. Fusco, et. al., Invasive Grasses Increase Fire Occurrence and Frequency Across US Ecoregions, 116(47) Proceedings of the National Academy of Sciences 23594 (November 19, 2019); See also Schmid & Rogers, supra note 6.
[26] Jarnevich, et. al., supra note 23.
[27] Juliet Grable, How the Inflation Reduction Act Helps Us, SIERRA MAGAZINE (Sept. 21, 2022), https://www.sierraclub.org/sierra/how-inflation-reduction-act-helps-forests-help-us.
[28] Id.
[29] Jarnevich, et. al., supra note 23.
[30] Tucson Audubon Society, Hire Us, https://tucsonaudubon.org/hire-us/ (Oct. 3, 2023).