Story by Karen PetersonPhotos by Cassidy Araiza
March 10, 2021
TUCSON — The giant saguaro, an icon of the American West, is beloved in this state. Arms raised in a perpetual “hello there,” the saguaro covers the desert wilderness and thrives in cities. Its silhouette appears in fine art and on restaurant walls; businesses and schools carry its name. Arizona state law protects the plant, and it is revered by the native Tohono O’odham tribe.
The largest cactus in the United States, the saguaro is distinct, visually and biologically. A mature saguaro can grow to 40 feet and weigh a ton after soaking up rainwater. Supported by its wood skeleton, the saguaro can sprout dozens of arms. Sometimes the arms are curled; if two are growing side by side, they’re often hugging.
The saguaro grows in just one part of the world: in the Sonoran Desert of Arizona; northern Mexico; a smidgen of California; and most prolifically in a mountainous swath that flows west from Tucson to the California border. It’s a landscape of rock, hard sand and open blue sky, and the saguaro has been part of it for 10,000 years.
And now, a changing climate is raising concerns about how the saguaro will survive the 21st century in an environment that’s hot and getting hotter, dry and getting drier. In a climate wake-up call, drought and record-breaking heat in 2020 contributed to wildfires that killed thousands of saguaros.
But the saguaro has friends keeping watch. There is a special affinity between the saguaro and science, a linkage that has made it one of the most studied plants in the world.
“If we’re going to find any communities or ecosystems well suited to these climate stressors, the desert is going to be a pretty good one,” said Benjamin T. Wilder, director of the University of Arizona’s Desert Laboratory on Tumamoc Hill, where botanists began studying the saguaro 118 years ago. “I wouldn’t bet against a desert species.”
The summer of 2020 “fits with what we are going to continue to see with climate change,” says Wilder, beginning with the monsoon.
North America’s only monsoon — and the reason the Sonoran Desert is billed as the world’s “wettest desert”— brings billowing cumulonimbus clouds that drench the land in rain. Nearly half the annual rainfall required to hydrate the Sonoran Desert is delivered by the monsoon.
Last summer, the monsoon never came. A pitiful 1.62 inches of rain fell, compared with the average 6.08 inches — a rare occurrence that in meteorological dark humor is termed a “nonsoon.” As a result, 2020 was Tucson’s driest year on record, according to the National Weather Service. The lack of rain compounded long-term drought conditions.
Michael A. Crimmins, a meteorologist and University of Arizona Climate Science Extension specialist, suspects that the nonsoon resulted from multiple forces, citing La Niña and El Niño, climate patterns in the Pacific Ocean that affect weather around the globe.
Individual weather events don’t inform a trend, Crimmins said. But 2020 was also Tucson’s hottest summer on record. Over the course of 124 uninterrupted days, daytime temperatures never dropped below 100 degrees, with 50 of those days peaking at 105 degrees or higher.
While heat is not necessarily a threat at this point — heat makes the cactus grow — it has contributed to a devastating new risk: fire kindled by invasive buffelgrass, a South African import. Buffelgrass, which has been thriving in hotter, drier conditions, forms a flammable carpet around the cactus.
Thousands of saguaros died on the buffelgrass-laden lower slopes of the Santa Catalina Mountains on the north side of Tucson in June, when a massive lightning-ignited fire erupted and burned for seven weeks.
In response, hundreds of Tucson residents regularly volunteer to hand-pull the grass from city, county, state and national parks. The Arizona-Sonora Desert Museum, a 98-acre outdoor zoo and botanical garden, declared February and March “Save our Saguaros” months, encouraging the public to identify and remove clusters of buffelgrass, which it terms a “menace.”
The museum has partnered with the Desert Laboratory on Tumamoc Hill on a 10-year plan to eradicate buffelgrass from an 80-acre site on the hill. Removing buffelgrass is key to saving native cactus as dry conditions intensify. It involves grueling work by hand and garden hoe, and the occasional use of chemicals.
The effects of climate change are being felt in saguaro country. Just how deeply is under study, with a caveat wrapped in a conundrum: How do you fully assess the impact as it emerges today on a species that lives at least twice as long as the researcher?
The intrepid botanists who began research on the curious armed cactus proliferating on Tumamoc Hill created an extensive baseline of data and observations for generations of scientists to come. The work expanded as the U.S. Forest Service and, later, the University of Arizona took over lab operations.