Climate change may spell disaster for Arizona’s saguaro
Experts say there have been no major regeneration events for the saguaro cactus in at least the last 20 years
For thousands of years, one plant has served as the sentinel of the Sonoran Desert. Thriving through three distinct biomes and migrating north from Mexico over several million years, the prickly giants unique to Mexico and Arizona are perhaps the region’s most distinctive feature.
But for more than 20 years, almost no new saguaros have sprouted from the desert soil.
Researchers at the University of Arizona’s Desert Laboratory on Tumamoc Hill aren’t sure why.
A delicate process
Frank Reichenbacher, a botanist and volunteer research associate at the lab, says saguaros are missing out on reproduction opportunities as summer monsoons come earlier each year.
“The seeds are intended to germinate with the monsoon rains that come right after the seeds are dispersed,” he said. “So, they flower in April, fruits mature through May and June, then fruits are dropping seeds, or animals are picking the seeds immediately after. And then the monsoon hits.”
Less time before summer monsoons means less time for the seeds to germinate, which means lower regeneration rates. Seeds that develop after initial monsoons hit have a lower chance of regeneration because they receive less water. And as monsoons come earlier, saguaro blooms aren’t adjusting.
Peter Breslin, a Desert Laboratory staff researcher, said it’s unclear what indicators, if any, saguaros use to adjust their timing. What is clear is how plentifully they bloom depends on previous winter rains, which have steadily declined for decades.
“The rainfall's changed, and it’s not helping,” said Bill Peachey, an independent ecologist and botanist based near Tucson.
Peachey said the low winter precipitation in 2022 is why nearly no saguaros bloomed last spring. Saguaros have had successful blooming events in only 12 of the last 20 years, he said. On a plot of 139 saguaros on state land in Pima County, 28 miles southeast of the Desert Laboratory on Tumamoc Hill and about 1,000 feet higher, he counted 120 blooms in 2004 and only 50 in 2022.
“Not a fruit was produced,” he said.
Peachey said those low-blooming events coincided with extreme heat. Saguaros produced even fewer blooms in those years than during a freeze in 2011 and the two years of aftermath.
“I can’t say this is climate change,” he said. “These are weather events. But these weather events exactly 100% match two of the big statements about climate change: you’re gonna have more of them than you’ve had before, and you’re gonna have extremes.”
While some of the saguaros on the plot didn’t survive the 2011 freeze, Breslin said he sees no lasting signs of it on Tumamoc Hill, where lower elevation could mean slightly higher temperatures. In the Santa Catalina Mountains, the saguaros “were flowering like crazy” last year while virtually dormant in other areas.
“They’re just as dynamic as daisies,” Peachey said of the variance of saguaros across different temperatures and terrains. “The conditions change, they change.”
Side bloom mystery
One of the most significant changes in saguaro behavior was the side blooms of 2021.
“We’ve never observed that before,” Reichenbacher said.
Rather than blooming only from the tips of the saguaros' stems and arms, flowers sprouted from all sides of the cacti in what the National Park Service called “an extremely rare phenomenon.” So rare, in fact, that nobody really knows why it happened.
Side blooms themselves aren’t rare. When a saguaro areole doesn’t bud in the spring, it still can bloom the following year. Because it’s now lower down the spine and the flower can’t get enough sap to sustain it, the flower aborts and falls off almost immediately. But in 2021, they didn’t all abort. Many of them produced fruits.
“That’s confounding,” said Peachey, who was one of the first to notice the peculiar blooming patterns. “That doesn’t stand up in my experience.”
The small holes along the ribs of saguaros are called spine areoles. From each areole a cluster of spines and stem cell tissue springs that can become either a flower or an arm. Tubules travel from the center of the cactus, called the cortex, through the woody ribs and out to the newest spine areoles, carrying along nutrients and signals from the stem.
As the ribs grow larger from the bottom up, they pinch the tubules off from the areoles, usually after those areoles have bloomed, Peachey explained.
But they don’t pinch every tubule off at once. If an areole doesn’t bloom and the ribs haven’t yet cut it off from its tubule, it can bloom again in later years when it’s further down the side because of the new areoles that grew above it. Because both events have to be synced it isn’t common, and the blooms that do form on the sides of saguaros usually abort right away since the cactus can’t support the side blooms as well as it can support blooms on the tips.
Neither Peachey nor other scientists have a conclusive explanation for why the blooms didn’t abort, and there’s nothing in the past to compare it to.
Because 2020 was the hottest and second driest year in Arizona history, observers hypothesized that the unique bloom in 2021 was a stress response. But researchers in the National Park Service have another theory.
Saguaro productivity is strongly influenced by the level of soil moisture in April, accumulated through winter rains. While April 2021 was particularly dry, April 2020 and April 2019 were wet enough to sustain average vegetation conditions. So, National Park Service researchers hypothesize that prior years had already set the stage for a major seed production they call a “masting event.” Whether the major seed production results in a regeneration event will depend on the wetness of the next few years.
But Peachey points to the past two decades, in which eight of the last 20 springs yielded nearly no blooms. When fewer areoles produce flowers, more are left active the following year, which means more will bloom down the sides in later years and abort before forming flowers.
“But there’s where I fall off because two years ago, it didn't act that way at all,” he said. “They were fully functional, way down.”
Still early in the blooming season, some saguaros in the northern Sonoran Desert seem to be budding down the sides, though it's unclear if they’ll abort as expected or produce fruit as they did two years ago. One cactus on Tumamoc Hill budded slightly down the side in mid-April, but Breslin said the buds looked normal for the most part. Of the 79 saguaros that have bloomed on the plot Peachey studies, none have bloomed down the sides.
Even if all goes well weather-wise, the chances of any seed germinating are still extremely low. Some saguaros can live for 250 years or longer and produce no offspring.
“You could take literally a million saguaro seeds and just throw them out, and five years after you do that, there wouldn’t be any at all,” Breslin said.
A saguaro releases anywhere from 11 million to 39 million seeds in its roughly 175-year life only to produce one more saguaro, Peachey said. Each fruit can contain between 1,000 and 6,000 seeds.
So why is it so difficult for a seed to survive?
“People don’t really know,” Breslin said. “There’s some traits that we do know. Little seedlings are really vulnerable to being eaten, maybe more so than other plants.”
Saguaro seeds are only millimeters long and full of water.
“They’re really attractive to pack rats and other animals because that’s water,” he said.
Once saguaro seeds sprout, they’re still not out of the woods. Seedlings are extremely sensitive to frost events for the first 10 years of their lives, and they’re more sensitive to heat and drought the smaller they are. A seedling can go 10 times longer without water once it doubles in size, but making it there is tough.
Breslin hopes to find more answers after studying the results of a survey conducted last year on more than 4,000 of the roughly 12,000 saguaros on Tumamoc Hill. He and other researchers logged the cacti on each of the hill’s four plots, recording data on the height, number of arms, and indications of damage or problems.
They also looked at microsite characteristics like rock and vegetation cover, the presence of invasive buffelgrass and slope. A microsite is a one-square-meter site in which a saguaro grows, measured by elevation changes over that site.
Saguaros establish a wide variety of terrains. Some prefer sandy soil, while others thrive in rocky terrain. Some only establish facing north, others facing south. Smaller saguaros prefer more plant and rock coverage, as both can help regulate temperature and protect the cacti from being eaten.
Breslin hopes his research will make more sense of this variability and predict how it might be affected by climate change over the next hundreds of years. He hopes to have the first paper from the survey out by September.
“Our results might help people understand how to best restore the cactus population in what is their most preferred environment,” Breslin said.
Breslin acknowledges the available data is limited. The first survey of saguaros on Tumamoc Hill was in 1964, and one has been conducted every decade since. But 60 years is still short-term data, which can mislead.
“At age 60 they don’t even have arms or flowers yet a lot of the time,” he said.
This year’s bloom, which began on Tumamoc Hill in mid-April, will be “average to good” over the next month based on current bud formation, Breslin said. Peachey said about 50% of the saguaros on the plot he studies have bloomed, and this year is already looking far better than last.
“(But) even a big bloom sometimes doesn’t result in a regeneration event,” Breslin added.
The future may seem bleak for saguaros. As the population on Tumamoc Hill ages and few new saguaros grow, Breslin said there could be a major mortality event in the next 60 years or sooner. That may extend across the Sonoran Desert if something doesn’t shift.
That isn’t to say the saguaros will all die out, though. Peachey is confident life will find a way.
“It’s gonna look like saguaros are going extinct because they’re gonna be dying in these places where they’ve been common,” he predicts. “Several hundred years from now, regardless of what we do, (nature is) gonna decide where it wants the temperature to be, and it’ll go back to normal.
“It’ll be magical when they start appearing again.”