Saguaro Cactus: The slow death of a desert giant

On a 105-degree afternoon in Phoenix, Arizona, Dr. Kimberlie McCue stands at the base of an 18-foot-tall saguaro cactus and looks up. She pushes back the brim of her sun hat and squints into the blindingly bright desert sky.

“There!” she says triumphantly, pointing to a spot about seven feet up the trunk of the multi-armed cactus that is an icon of the Sonoran Desert and the American Southwest as a whole. “That is where it was.”

The congenial McCue is chief science officer at the Desert Botanical Garden and the spot she’s pointing at looks like someone had taken a giant melon baller to the colossus of the desert. The spot marks the place where one of the arms detached from the saguaro trunk and crashed to the ground a few weeks earlier. Six other saguaros in the Desert Botanical Garden lost arms this summer, too, a situation that, with a scientist’s aversion to hyperbole, McCue characterizes as “far from normal.”

In fact, since the Desert Botanical Garden’s founding in 1939, there is no record of a loss equal to this one. Among the over 1,000 saguaros on the grounds, perhaps a single arm is lost in a given year, and even then it’s generally from storm damage. This summer, a total of 13 arms dropped, and not one fell during a storm. Within days, some of these mutilated giant cacti had collapsed entirely.

The detached giant arms lying on the ground made for riveting video on national and even international news programs. But saguaros are only the most recognized species suffering, and the problem goes far beyond the 140-acre grounds of the botanical garden. The Sonoran Desert, covering over 100,000 square miles in the U.S. and Mexico, is the most biodiverse desert on Earth. Plants, animals, insects, and countless other organisms have co-evolved over millennia here, developing complex relationships that are key to surviving in an environment of intense heat and little water. But, scientists warn, this intricate web of life is unraveling, the victim of human activity, primarily the burning of fossil fuels.

“This breakdown phase, if not reversed, can lead to ecosystem collapse,” says Tesa Madsen-Hepp, a plant ecologist at the University of California, Riverside. “And if the Sonoran Desert ecosystem collapses, we are all in big trouble.”

That’s because dryland ecosystems, which include deserts, are the largest biome on the planet, covering more than 40% of the terrestrial surface and supporting over 2 billion people.

But the stakes are even higher, says Madsen-Hepp, because dryland ecosystems play an important role in regulating the global cycling of carbon, which traps heat in the atmosphere.

“Dryland soils provide the third-largest global pool of carbon storage potential,” she explains. “And deserts are expected to play an even greater role in carbon storage under rising CO2 levels.”

This potential catastrophe has been unfolding in the American Southwest for decades, largely unnoticed, in part because the rhythms of life here are so much slower than those in less arid ecosystems. It takes a decade for a saguaro seedling to reach the height of just two inches. They don’t begin to flower until they’re 40 to 60 years old, don’t develop their first arm until 70, and only reach full maturity at around 125. This strategy of slow growth has allowed saguaros to survive long dry spells and live for 200 years and up.

Death in the desert can also be slow. “To explain what’s happening to the saguaros here in 2023,” says McCue, “we have to look back to 2020.”

The summer of 2020 shattered weather records: The temperature reached 110 degrees Fahrenheit 53 times and 115 degrees 14 times. It was also the driest summer monsoon season since record-keeping began in 1895.

“All of that heat and aridity takes a heavy toll on cacti,” says McCue. “A lot of saguaros can live years in a compromised state. But, really, it’s more like a slow death.”

With their sharp protective spines and ability to thrive in the harsh desert environment, cacti seem indestructible. “People think cacti should be fine because they’re adapted to the desert,” is how McCue puts it. “To an extent, that’s true. But even they have their limits and we appear to be reaching them.”

The combination of increased global warming and the beginning of an El Niño year caused the summer of 2023 to make 2020 seem benign by comparison. Phoenix had a record-setting string of 31 straight days with high temperatures reaching 110 degrees (the previous record was 18 days). But it was the nighttime low temperatures that were likely even more lethal for saguaros. Though most plants respire during the day (their equivalent of breathing), cactus-like saguaros respire at night to prevent water loss from evaporation. That worked fine when the average low temperature in Phoenix in July was between 80 and 85 degrees. But in July 2023, the nighttime low only dropped to 86 twice, with 19 days at 90 degrees, including a punishing 16-day streak when the temperature never dropped below 90. On July 19, the low temperature was a record-breaking 97 degrees.

“If it’s just too hot at night they don’t even open their pores,” explains McCue. “It’s like they start to suffocate.”

In addition to the record-setting heat, the summer of 2023 was also the driest ever recorded here. Instead of the historical summer average of 2.5 inches of precipitation, only 0.15 inches fell. That’s partly because the desert has been in a megadrought since the year 2000, part of a naturally occurring cycle of years-long wet and dry periods. But the intensity of the current megadrought has been greatly exacerbated by climate change.

“Everything is connected to everything else.”

BARRY COMMONER’S “FIRST LAW OF ECOLOGY”

If saguaros are the superstars of the Sonoran Desert, famous throughout the world, the more than 2,000 other plant species found here are the supporting cast. Although not as recognizable, each one plays a role in the desert ecosystem.

In fact, without plants like creosote bush, cholla cactus, and palo verde trees, saguaros wouldn’t exist, explains ecologist Jonathan Horst, director of conservation and research at the Tucson Audubon Society. These “nurse plants” create microhabitats that are cool and moist, protecting saguaro seedlings from predators and from heat and cold.

“Adult saguaros are incredibly hardy and resilient,” says Horst. “But if you look at their first years, they’re about as vulnerable as any plant I know of.”

Experts have evidence that the changing climate is also causing excess mortalities among these nurse plants, with unprotected young saguaros dying as a result.

The death of creosote bushes is particularly alarming because they are considered the most drought-tolerant perennial on the continent. Creosote plants, which give the Sonoran Desert its distinctive scent after it rains, can live up to 100-200 years. While they produce flowers and seeds, the species depends on clonal reproduction, the plant’s ability to send up new shoots with DNA identical to the original plant. Some of these present-day clonal colonies began life over 10,000 years ago.

“If even creosote is dying,” McCue says, “that really means we’re in trouble.”

While saguaros require nurse plants, they in turn are a keystone species in the Sonoran Desert, with over 100 taxonomic groups of life forms dependent on them.

“Whether it’s multiple species of bats, moths, hummingbirds, bees, wasps, and other insects drinking the nectar and eating pollen from the saguaro flowers,” Horst says, “or mammals like deer, coyotes, and javelina, and reptiles like lizards and tortoises eating their fruit, or the Gila woodpeckers and gilded flickers that create cavities for nesting used by at least 14 species of birds — saguaros are a critical part of the regional community.”

Though the media focused on the record-breaking heat in Phoenix this summer, temperatures don’t have to break records to harm desert ecosystems.

Ladd Keith, an urban heat expert at the University of Arizona, recently called the tendency to focus on record-breaking temperatures “a little bit dangerous.” If that’s all that’s reported, “we tend to lose sight of the fact that chronic heat, or just normally over average heat that we’re experiencing with climate change, is also detrimental.”

From the perspective of an average person, the Earth is warming so slowly that the change may be imperceptible. Since 1880, the average global temperature has increased at a rate of just 0.14 degrees Fahrenheit per decade. Since 1981, the rate of warming has doubled — but that’s still less than a third of a degree per decade.

The pace of change may explain the tendency to focus on days of record-breaking heat. But, as the University of Arizona’s Peter Breslin put it recently, to understand what’s happening in the Sonoran Desert you have to “think in saguaro time, not human time.”

For desert plants with lifetimes measured in centuries, the cumulative effect of years of above-average temperatures and below-normal moisture is challenging their capacity to survive. Sometimes that occurs dramatically, as when saguaros lose their arms. But what you don’t see tells the larger story of the saguaro’s dilemma.

A healthy adult saguaro produces about 100 enormous white blossoms a year, with each resulting fruit containing around 3,500 tiny seeds — each just 1.5 millimeters in diameter. Over its lifetime, an average saguaro will produce between 20-40 million seeds.

That sounds like enough to grow a forest of saguaros, but the Audubon Society’s Jonathan Horst explains that the chances of an individual seed producing a seedling that reaches adulthood are infinitesimal.

Everything has to go just right for the seed to mature into an adult. It has to end up on the right type of soil, beneath a nurse plant, and – perhaps most critically – receive adequate rainfall from the outset.

“In order for a saguaro to have any chance at getting established,” says Horst, “they’ve got to have two wet summer monsoons in a row with a wet winter in between.” Historically, those conditions have existed just once a decade.

All of which explains why an adult saguaro, out of the tens of millions of seeds produced in a lifetime, on average, produces a single mature adult. At least it did before humans began changing the climate by burning fossil fuels. The trifecta of two wet monsoons plus one wet winter has happened just once in the last three decades.

“We’ve moved from once a decade to once every three decades,” says Horst. “And who knows how long it’ll be until the next recruitment event.”

The saguaros seen in Westerns, those archetypal giants with prickly arms reaching for the sky, still appear to be everywhere to the casual observer. But what’s largely missing is the next generation.

“Even though in some places you have what’s called a relic, or ghost population, of large adult saguaros that may still be producing flowers and seeds,” says Michiel Pillet, “those seeds may not result in seedlings that will be recruited into the adult population.”

“The desert species that have been evolving in response to aridity for millions of years are more fragile than generally thought.”

TESA MADSEN-HEPP

Pillet, a doctoral candidate at the University of Arizona, became fascinated by cacti as a teenager growing up in Belgium. It’s a passion he never outgrew. Pillet is now the program officer for the International Union for Conservation of Nature Species Survival Commission Cactus and Succulent Plants Specialist Group, hosted by the Desert Botanical Garden.

Scientists only recently understood how vulnerable the cactus family is, says Pillet, pointing to a 2015 study that found 31% of all cactus species at risk of extinction because of human activity. “That meant cacti were one of the most threatened groups of organisms on the planet — even more so than mammals and birds.”

And that was before factoring in the effects of climate change, which Pillet has since done, as lead author of a 2022 study that determined that 60% of all cactus species could be at elevated risk of extinction by 2050 due to climate change. He sees this summer’s attention to dying saguaros as a mixed bag.

It’s positive insofar as it draws attention to the plight of cactus species, but he adds, “It also shows that we may be severely underestimating the threats facing the cactus family and other desert organisms.”

While species like animals can potentially migrate to better conditions, Pillet cautions, “that’s unlikely to be the case for most cacti.”

Tesa Madsen-Hepp, the UC plant ecologist, was lead author of a recent study that suggests Pillet may be right.

Her article, published in April, examined the change in plant distribution over a 10-mile-long transect line on the western edge of the Sonoran Desert. The line, which was first drawn and analyzed 42 years ago, begins in the hottest and driest part of the desert floor and ends in a coniferous forest 8,000 feet higher up.

What her team found surprised Madsen-Hepp. “The desert species that have been evolving in response to aridity for millions of years are more fragile than generally thought. They can’t tolerate the new, even drier and hotter conditions.”

Some of these desert-dwellers are able to move to a higher elevation, but others are simply dying. In either case, slow-growing plants on the desert floor are being replaced by fast-growing weedy species, primarily grasses. One of the most destructive in the Sonoran Desert is buffelgrass, a non-native species introduced for cattle forage and erosion control in the 1930s. Buffelgrass out-competes many native plants for scarce water resources, but it has another attribute that poses an additional threat, says Kimberlie McCue: “Buffelgrass likes fire.”

Some ecosystems need periodic low-intensity fires to maintain biodiversity and stability, but fire isn’t a major part of the Sonoran Desert’s ecosystem, says McCue. “When buffelgrass burns to the ground it regenerates, increasing the fuel load for the next fire.”

Buffelgrass also burns at temperatures up to 1,400 degrees which makes it capable of killing even very large, centuries-old saguaros. That was the case in 2020 when a wildfire, fueled in part by buffelgrass, swept through the Tonto National Forest in Arizona. An estimated 400,000 saguaros died in the conflagration.

Because saguaros grow so slowly and invasive grasses so quickly, most of the saguaros that died in the 2020 fire will likely be replaced by non-native grass, continuing a vicious circle that ecologists warn could lead to the “grassification” of the Sonoran Desert. The rich biodiversity of the desert is being replaced with a less diverse grassland completely alien to the place.

Micheal Pillet says this loss of biodiversity has reached a crisis point.

“Communities and ecosystems are networks that are intricately intertwined,” he says. “So when those connections come apart, the entire structure can unravel very, very quickly.”

Though science, like most cacti, moves slowly through incremental expansion of knowledge, Pillet argues that programs to conserve desert biodiversity must work speedily if mass extinctions are to be avoided. “The longer we wait, the more species we will lose,” he says. “Already, there’s no doubt in my mind that we’ll lose some species.” But Pillet emphasizes that there are ways to minimize those losses, and scientists and volunteers are already working toward that end.

Back at the Desert Botanical Garden, McCue leads me out of the heat and into the Ahearn Desert Conservation Laboratory, a low-slung smallish building with an even smaller environmental footprint thanks to super-insulation, rainwater collection tanks, and other eco-smart features. In a few steps, we’re inside the laboratory, a long, well-lit space with two enormous freezers spanning nearly the length of the room. There, McCue introduces me to Steve Blackwell, the conservation collections manager for Desert Botanical Garden. He opens one of the stainless-steel freezer doors and out drifts a small wave of extremely cold air. “This,” he says, “is our backstop to extinction.”

Inside are thousands of neatly labeled packets containing seeds of rare and endangered desert plants from throughout the Americas. This seed bank, explains Blackwell, is a treasure chest of biodiversity.

“We have seeds from over 2,500 species and their subspecies,” he tells me. Under rigorously maintained climatic conditions of 20% humidity and zero degrees, the seeds here can remain viable for decades or even centuries.

The seed bank is an insurance policy against an uncertain climate future, but simply storing the seeds isn’t enough, says Blackwell. “We have to be sure we can grow each species,” he says, explaining that some can be extremely finicky. He then leads me over to what looks like a large white refrigerator. It’s a germination chamber, a specialized refrigerator that each day automatically switches from 77 degrees with grow lights on for 12 hours, to 59 degrees in the dark for another 12.

McCue points to a plastic container inside the chamber and Blackwell carefully takes it out. Only as big as a slice of bread, the container is filled with what looks like tiny pebbles and studded with dozens of petite green nubs, none larger than the eraser on a pencil.

“Look at these babies,” Blackwell says, sounding like a proud father as he sets the box on a table.

“They’re saguaro seedlings,” McCue says as I silently gawk. It’s a cliché, but when confronted with such tiny organisms, it’s hard to believe that saguaros, those giants of the desert, could ever be so small.

While I’m still trying to wrap my head around what I’m seeing, McCue points to barely visible white hairs on them. “Those are spines,” she says. “They’re teeny tiny now, but they’re only four months old and they already have spines. I love that.” It’s clear that even after a decade at the garden, McCue’s sense of wonder about saguaros and cacti in general hasn’t dimmed.

Blackwell explains that this box of saguaro seedlings represents one possible future for the species on a warming planet — assuming humans stop burning fossil fuels in time. Different populations of saguaros develop unique genetic makeups in response to conditions where they’re growing. The box of baby saguaros was grown from seeds collected near Kofa National Wildlife Refuge, one of the hottest and driest parts of the Sonoran Desert. The saguaros there thrive even though they receive less than half the average rainfall of Phoenix.

Now that the saguaro seeds from there have germinated, Blackwell plans on transplanting them into soil and moving them outdoors, with the potential to someday use them in a restoration project in the wild. But that next step of transplantation won’t begin until the seedlings are about an inch tall. “So in about a year,” Blackwell says.

One inch in one year.

A reminder of the slow tempo of “saguaro time” in an environment that seems to be heating up and drying out at warp speed.

As I leave the garden, I wonder if there’s time to save saguaros and the myriad plants, animals, insects, and microorganisms to which they’re linked. All the experts I’ve talked with believe that the unraveling of the Sonoran Desert can still be stopped in time to save most species. As prominent climate scientist and author Michael Mann likes to say: “We’re not doomed yet.”

McCue believes that one of the biggest impediments to action is the widespread ignorance about what a desert fundamentally is.

“I have to tell you,” she said when we first talked by phone. “I have my own little personal goal to change the definition of desert in the dictionary.”

She pointed out that if you look up “desert” in most dictionaries, you’ll see words like “barren” and “wasteland.”

“You’d think there’s nothing there,” she said, “and that couldn’t be farther from reality.”

After it rains, the parched desert springs to life, especially in the Sonoran Desert, which is among the most biodiverse ecosystems in the United States. “It’s anything but a wasteland. It’s incredible”

When I asked how she’d redefine the dictionary entry, McCue paused before answering. “Well, I think it should describe deserts as being areas in which there’s minimal rainfall. I mean, that’s a key part of it, right? But then I’d modify it to say that the desert supports plant and animal life found nowhere else on the planet. That’s the true definition of a desert.”

It’s certainly true now. But if the misconception of the desert as a wasteland doesn’t change, funding for conservation efforts here may not rise to the level needed to stop the unraveling. In that case, the image of the desert as a wasteland will become a self-fulfilling prophecy.