Mitigating wildfire risk by restoring grasslands to the Santa Rita Experimental Range
A multi-year mesquite abatement project aims to reduce the risk of catastrophic wildfires on the Santa Rita Mountains – and improve our understanding of the system-wide ecological impacts of such efforts.
Mitch McClaran
Residents of Green Valley, Sonoita and other communities near the Santa Rita Experimental Range may soon notice an increase of heavy machinery trundling through the landscape – and a significant decrease in the area’s mesquite trees.
The University of Arizona Experiment Station, Arizona Department of Forestry and Fire Management and Arizona Game and Fish Department are launching a new project to mitigate wildfire risk by removing much of the mesquite canopy cover along the northern border of the Santa Rita Mountains. The multi-phase project plans to reduce the standing woody fuel load that contributes to hotter, higher energy wildfires and replace it with cooler-burning native grasses, which are easier to suppress or extinguish.
The project will also provide U of A scientists with a wealth of research opportunities into the impacts this type of wildfire mitigation has on semi-arid ecosystems, including wildlife, plants, soil, hydrology and watershed science.
Returning to its roots
The idea of heavy machinery ripping up native trees may rankle at first, but removing mesquites is actually a step toward restoring the area's original ecosystem.
The base of the Santa Ritas was once a sprawling grassland with only a few mesquites scattered across waterways. But severe drought conditions and overgrazing in the early years of the 20th century slowly transformed the landscape from grassland to mesquite woodland or savannah.
“The grasses became severely denuded, so livestock started browsing mesquites,” explained Brett Blum, interim director of the Arizona Experiment Station and one of the project’s leaders. “They started eating the mesquite beans and then distributing those beans across the range in their manure. Over time, the whole dynamic of the environment shifted.”
By the 1930s, mesquite encroachment had reached problematic levels, Blum said. Land managers spent the following decades trying – unsuccessfully – to eradicate mesquites from the range. Blum and his project partners hope their efforts will fare better.
“The world today is very different than it was 120 years ago,” he said. “We no longer expect to restore the landscape to the steady state of a bygone era; our goal is a balanced ecosystem today. For that, we need to better understand what those dynamics look like.”
Repeat photography at the Santa Rita Experimental Range allows researchers to track changes to the landscape over time. This series shows the same view between 1902 (top left) and 2019 (bottom right).
David Griffith and Mitch McClaran
Balancing priorities
The initial phase of the project will target 1,200 to 1,500 acres between Box Canyon and Madera Canyon. Jessie Warner, the southeast district forester for the Department of Forestry and Fire Management, said that DFFM will lead the project’s implementation, bridging the gap between scientific research and field operations.
“DFFM will create a project plan that balances ecosystem health and wildfire mitigation,” she said. “When planning projects, DFFM project managers utilize historic fire activity, fire risk maps and historical ecological conditions.”
She said DFFM will oversee specialized contractors, who will use a combination of mechanical grubbing and hand-thinning to reduce the woody canopy cover.
“We plan to take the area from roughly 30% mesquite canopy cover down to a target of 14%, give or take,” Blum added. “We want to create a bit of a fuel break, to give firefighters a chance to catch a fire should it start on the range before it moves up into the Santa Ritas.”
Removing that many trees will have significant impact on the current ecosystem, but Blum emphasized that measures are being taken to protect sensitive flora and fauna.
“The Santa Rita Mountains are a biological hotspot, providing habitat for different threatened and endangered species,” he said. “They’re also the primary watershed for the eastern end of the Santa Cruz River Valley. We want to be able to preserve that.”
Blum noted that most of the biomass from the felled trees will be redistributed on the landscape. Smaller pieces may be ground down and redistributed onto the soil surface to prevent the loss of vital nutrients, while larger pieces will serve as erosion breaks to slow down surface water and encourage infiltration during rainstorms. The team also plans to reseed the area with native grasses to slow erosion, promote biodiversity and encourage grassland obligate species like scaled quail to return to the area.
Lathe Evans, DFFM southeast district chief, explained that these restoration measures won’t counteract the project’s fire mitigation goals.
“While native grasses ignite easily, they burn quickly and at a lower intensity that is easier for firefighters to manage,” he said. “Without treatment, dense mesquite creates a high-energy fuel load that produces hotter, more erratic wildfires. By felling these trees and leaving the wood at ground level, we remove the standing vertical fuel that can carry fire into the canopy of trees and spread rapidly through spot fires.”
Decades of attempts to eradicate mesquites from the area have been unsuccessful. The new effort will focus on balance, rather than full eradication.
Photo ca. 1947 by David Griffith
Measuring efficacy and impact
One of the challenges faced by land managers who want to pursue this kind of large-scale abatement project is a lack of data about long-term efficacy and ecological effects.
“Right now, we don’t have the data we’d like to see to make the best management decisions,” Blum said. “This project gives us the mechanism to examine treated and untreated, or pre- and post-treatment effects.”
Researchers from the U of A will gather quantitative data on the treatment’s efficacy, as well as the complex ecological systems that will be affected by reducing the mesquite density.
“We will be altering the habitat with our treatment,” Blum said. “There will be implications for hydrology, infiltration, watershed management, fire, and all the different plant and animal species that rely on that system. We want to use this as an opportunity to learn about the impact of this annual treatment on all these elements of the ecosystem.”
"We embrace adaptive management in these systems," added Amy Ganguli, Marley Endowed Chair for Sustainable Rangeland Stewardship with the U of A School of Natural Resources and the Environment. "In this setting, we are able to use research observations to improve how we do things in the future."
Engaging community stakeholders
The Santa Rita Experimental Range was originally established to help livestock producers develop ecologically sound grazing practices, and that close relationship between researchers and ranchers continues today.
“A lot of this project is driven by a desire from ranchers to develop better management practices for woody species,” Blum said. “This is not a problem unique to the Santa Ritas. The proliferation of woody species due to more dynamic climate patterns is a problem in all sorts of semi-arid systems, from Argentina to sub-Saharan Africa.”
The project team will work with the Santa Rita Ranch to use virtual fences as a means of keeping livestock clear of active work zones and targeting grazing to create natural fire breaks in grassier areas.
Blum said that collaboration with the community is paramount for this kind of large-scale, long-term project.
“We’re working with a coalition of community partners, including Friends of Madera Canyon, Tucson Bird Alliance, Southern Arizona Quail Forever, Pima County 4-H and a number of local K-12 schools,” he said. “This project is a bridge between community organizations whose individual interests may be very different from each other, but we’re all working together for a common cause.”