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.
The bigger threat
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.
“You’re not going to find longer records in the Southwest than what is up on the Hill,” said Kathryn A. Thomas, a research ecologist with the U.S. Geological Survey. Her work uses models to forecast how the saguaro’s habitat may change as the planet warms.
Broadly speaking, it appears that a suitable climate for the saguaro is “going to be maintained or even improved in a fair number of places,” including the Tucson-to-California corridor, she said.
But those models fail to take into account the impact of the increasingly hotter, drier climate on the plants around the saguaros.
The models do not address the important community of plants — called “nurse plants” — that grow with saguaros and protect and sustain the seedlings. The models are specific to adult saguaros, not to the rejuvenation of the population as a whole, she said.
And they don’t reflect the full impact of invasive species such as buffelgrass.
“The climate might be fine for saguaros, but if they’re being burned up by buffelgrass,” all bets are off, Thomas said.
Saguaro National Park has become an important adjunct to the study of the saguaro. The park, which attracted more than 1 million visitors in 2019, protects 92,000 acres of the cactus in two divisions, on the east and west sides of Tucson, and boasts a vast collection of young and aged specimens.
In one recent study conducted in the east side, Crimmins and National Park Service biologists Don E. Swann and Adam C. Springer examined the assumption that saguaros could do the same as other plants struggling with global warming and expand their range to escape to cooler ground.
Saguaros grow in the foothills, or bajadas, of the mountain ranges rising above the Tucson valley. Movement in the saguaro’s case would be upslope. Until the advent of global warming, upward mobility was a moot point: An elevational shift higher would put saguaros in areas that are prone to severe freezes. Fire is deadly to saguaros; so is 36 hours straight of freezing temperatures.
But as the climate here has warmed, and with a projection of average temperatures rising 1 to 2 degrees Celsius higher in the next 30 years, those elevated habitats are growing warmer, too.
So, would moving up now provide an escape from a traditional habitat that is overheating?
No, the researchers concluded.
Their study, published in the Journal of Arid Environments, found that if the saguaro cactus crept to a higher elevation, it would find itself in desert grassland, not its native desert scrub. Buffelgrass, the saguaro’s nemesis, thrives in grassland. Those grasses, and the fire risk they pose, are “a bigger threat to saguaros than a decrease in freezing events,” said Crimmins, who wrote the study with Swann and Springer.
Every 10 years, Saguaro National Park produces a Saguaro Census, the findings delivered in 2020 by 500 volunteer “citizen scientists” of all ages who mapped, measured and collected data on more than 23,000 plants.
Much of that work requires searching through thickets for dark green gumballs of thorns growing in the rocks — baby saguaros. They sprout from tiny seeds and grow slowly, measuring barely 1.5 inches by age 10. To survive, they live under the canopy of nurse plants, such as creosote bushes and paloverde trees, which protect them from the sun, keep the soil moist, and make it harder for rabbits and other animals to eat them.
Over the past 20 years, there has been a “pretty dramatic” decrease in the number of young saguaros that survive, Swann said. He called the findings “concerning but not alarming,” noting similar conditions have existed before when high temperatures compounded drought conditions.
As for the immediate future, the Tumamoc Hill lab, Saguaro National Park and the Arizona-Sonora Desert Museum are continuing and expanding what they already do: educate the public about the threat facing the iconic cactus.
“There’s no better place than the Hill to communicate science,” said Wilder about the popular hiking spot, with more than 1,000 visitors walking its slopes each day. “We have a remarkable opportunity to help make that connection between people and place.”
Despite all the climate challenges, Swann takes the long view on the cactus. “Saguaros have something to teach us because they live so long,” he said. “With saguaros, you have to look at the big picture and take a long-term approach.”
It’s a view shared by many in the Tohono O’odham nation.
In their cosmology, the saguaro (ha:san) is human. “I don’t look at it as a cactus. I look at it as a person,” said Jacelle E. Ramon-Sauberan of the San Xavier District of the Tohono O’odham nation.
In June, before the monsoon season arrives and with it the rain, the O’odham fashion sticks, or kuipud, from the saguaro’s wood ribs to knock down the sweet ripe fruit, or bahidaj, growing atop the body and arms and use it to make syrup, jam and wine.
The sticks are also used to “touch the clouds and pull them down” for the rain, said Ramon-Sauberan, 32, who teaches history and culture at Tohono O’odham Community College in Sells, Ariz. She has never missed a family harvest celebration, which ushers in the O’odham new year.
“Climate change is worrisome, but the saguaro and the O’odham have been here from time immemorial,” said Ramon-Sauberan. “We’ve survived, we’re resilient.”