Written by Kim Franklin for Save Our Saguaros Month: January 31-March 7
This blog post is a condensed version of an article that will appear in the Desert Museum’s annual members’ publication, Sonorensis, to be published Spring 2021. Click here to become a member. You can also read our past issues here.
In the 1960s a new grass species started popping up in the Tucson Metropolitan Area, which then harbored only a quarter of today’s population. This newcomer was buffelgrass (Cenchrus ciliaris), a fast-growing and reproducing bunchgrass native to Africa and southern Asia. Buffelgrass was one of several species declared to be “miracle grasses” by scientists working to restore rangelands degraded by decades of overgrazing at the turn of the twentieth century. Another of these miracle grasses was Lehmann’s lovegrass (Eragrostis lehmanniana), a perennial bunchgrass native to southern Africa. Both are warm-season perennials, persisting on the landscape in a dormant and highly flammable state most of the year.
In addition to warm-season perennials, many cool-season annual grasses have proven to be highly invasive and fire prone. Among the worst of these is red brome (Bromus rubens), which germinates in the spring, especially after wet winters, and then dies back in the arid foresummer. A native to southern Europe, red brome arrived in the Southwest via accidental introduction, well before the arrival of buffelgrass and Lehmann’s lovegrass.
All three of these species have been wildly successful here in the Sonoran Desert, where they outcompete our native plants for limited resources – space, water and nutrients. Where any of these grasses grows thick, native plant diversity is reduced, oftentimes dramatically reduced.
Limited by freezing temperatures, buffelgrass is largely confined to the desert, where it fills in the empty space between shrubs, trees, and cacti. With time, even these woody and succulent species succumb, and what was once a richly diverse desert converts to flammable grassland. Our iconic and long-lived saguaros may persist longer than any other native plant species on the buffelgrass-invaded bajadas in the Tucson Basin, but their days are numbered. As this “grassification” continues, saguaro recruitment and survivorship gradually decline due to competition and wildfires.
Red brome, like buffelgrass, favors desert habitat, but exerts its greatest impact after wet winters. Two consecutive wet winters set the stage for explosive population growth. The seed produced in the first winter germinates in the second winter to produce a bumper crop of green grass, which turns to tinder-dry fuel during the arid foresummer. In contrast, Lehmann’s lovegrass thrives at slightly higher elevation in our desert grasslands. At the transition between desert to grassland, there is a mix of buffelgrass and Lehmann’s lovegrass.
All three of these invasive species are excellent fuel for wildfire. While fire is healthy component of grassland ecosystems, fire is new in the Arizona Upland subdivision of the Sonoran Desert, the favored habitat of buffelgrass and red brome. Historically, fires in Arizona Upland took place only when annual plants boomed after wet periods, and even then fires were mild and patchy. Consequently, many if not most of native plant species in this subdivision of the Sonoran Desert are poorly adapted to withstand fire, especially the high-intensity fires fueled by buffelgrass, which produces fuel loads thousands of times greater than typical Sonoran Desert fuel loads.
Invasive grasses are changing fire regimes in the Sonoran Desert. Buffelgrass, Lehmann’s lovegrass and red brome are all excellent fuel for fire. Moreover, they thrive on fire, creating a positive feedback loop or grass-fire cycle – grass invades, promoting more frequent fire, which in turn promotes further grass invasion, and even more frequent fire.
For what feels like a long time, some in our community have been warning of an impending catastrophe, a wildfire of unprecedented intensity in Tucson’s wildland-urban interface, where housing developments push up against wildlands that have been invaded by fire-loving buffelgrass. Thus when a wildfire ignited on the southwestern flanks of the Santa Catalinas on June 5th 2020, those of us involved in the effort to control buffelgrass feared the worst. The south slopes of the Santa Catalinas Mountains, where buffelgrass took root some forty years ago, now harbor some of the largest, densest stands of buffelgrass in the Sonoran Desert.
The Bighorn fire burned for over seven weeks, engulfing nearly 120,000 acres, but the south slopes of the Santa Catalinas were largely spared, thanks to the tremendous work of our firefighters, cutting fire breaks, conducting back burns, and laying retardant. Their valiant efforts combined with the patchy distribution of buffelgrass at the transition from grassland to desert and the prevailing winds limited the spread of the fire into the desert. The Catalina Foothills wildland-urban interface, with many neighborhoods forced to evacuate for the first time ever, dodged the bullet.
Climate change, decades of fire suppression, an increase in human ignitions and invasive grasses are all contributing to this increase in fire frequency, severity and size. The Bighorn Fire gave Tucson and other Arizona communities a glimpse of a fiery future, a future that has already become the norm in communities across the West.
2020 was a record-setting year for fire across the West. Arizona saw its second largest wildfire season on record with nearly one million acres burned, surpassed only by 2011, the year of the massive Wallow Fire. Moreover, much of this fire took place in desert and semi-desert grasslands, driven by the wet winters of 2018 and 2019, which led to an abundance of fine fuels, including red brome and other invasive grasses. While the Bighorn Fire was burning in Tucson, the Bush Fire was burning through Sonoran Desert up through high elevation forests just north of Phoenix, eventually consuming nearly 200,000 acres. Later that summer the Superstition Fire burned through nearly 10,000 acres of desert and semi-desert grassland threatening Apache Junction and nearby towns.
In August 2020 the Dome Fire, fueled in large part by red brome, burned through 44,000 acres of the world’s largest Joshua tree forest in Mojave National Monument. Substitute saguaro forest for Joshua tree forest to get a sense of what we stand to lose in the Tucson Basin, home to some of the densest stands of saguaros in the world. The toll of such a loss if difficult to quantify. Saguaros are synonymous with Sonoran Desert; they are the reason so many visitors choose to come here, rather than some other warm, sunny winter destination, and thus a pillar of our economy. Saguaros are a keystone species, supporting an exceptional diversity of insects, reptiles, birds, and mammals that depend on them for food and shelter; they are the ancestors of the O’odham people, who have cared for this land since time immemorial.
The Bighorn Fire gave us the opportunity to study the behavior of fire and to learn how we might avoid the catastrophic wildfire predicted decades ago by a few prescient individuals in our community. Perhaps the most important lesson learned is that even relatively small fire breaks can be effective in preventing the spread of the fire. Creating fire breaks and access points for our firefighters could help limit the extent of damage in future fires, which are inevitable. support. Additional restoration and rehabilitation actions are being initiated by the National Forest Foundation through its Southern Arizona Forest Fund, a new campaign which leverages donations on the Coronado National Forest to expand partnerships and fund efforts to map buffelgrass infestations and other invasive species on the south slopes of the Catalinas, and to restore some of the areas hardest hit by the fire.
This type of work takes commitment from the highest levels of government as well as on the part of the local communities most impacted by wildfire, and coordination across jurisdictions, between agencies and levels of government. It also takes a commitment from the public. No group of people have demonstrated more commitment than the Sonoran Desert Weedwackers, who spend several mornings a month digging up buffelgrass in Tucson Mountain Park. Thanks to their tireless effort, a drive over Gates Pass today is just as beautiful as it was 50 years ago. It can be done!