Three years after Phoenix and Tucson experienced some of their hottest months on record, the saguaro, symbol of the Sonoran Desert, is still feeling the impacts, biologists say, with elevated saguaro mortality rates observed in parts of Central and Southern Arizona.
This summer’s round of record-breaking heat, as bad or worse than that of three summers ago, has triggered more anecdotal accounts and unofficial reports of damaged and collapsed saguaros in the Phoenix area to the Desert Botanical Garden there.
So far, there have been no reports in Southern Arizona of dying or damaged saguaros specifically attributed to the current heatwave, where temperatures are also setting records but are far less oppressive than those in Phoenix.
But Ben Wilder, director of a group of Sonoran Desert researchers, said he has noticed elevated numbers of older saguaros that have died since summer 2020 in desert lands lying west of Tucson.
In the Phoenix area, where official temperatures soared over 110 degrees 53 times in summer 2020, the more than 1,000 saguaros living on the Desert Botanical Garden’s 146 acres have been dying at “significantly” higher rates since then than before, said Kimberlie McCue, the gardens’ chief science officer. Those saguaros range from an inch to almost 30 feet tall.
Every year, the garden’s scientists take inventory of its saguaros, visiting every one on the property. They rate the cactuses’ conditions ranging from excellent to good to fair to poor. Since 2020, their mortality rate is “definitely elevated above what it was prior to 2020, significantly higher,” McCue said.
This summer, the garden has received some reports from people living across the Phoenix valley of saguaros collapsing, losing arms and leaning over, McCue said. One saguaro at the garden itself had an arm fall off just last week.
But no other damage to saguaro cacti has been noted there, she said. Outside the botanical garden, “how many? At this point, I don’t know,” said McCue of reports of damaged or dying saguaros in the Phoenix area.
Extreme heat, drought effects
Tania Hernandez, a research scientist for the botanical garden, has been conducting a saguaro census in the community for the past two years. In it, she engages people in the urban area to record observations on conditions of saguaros, and to supply the gardens with photos of saguaros and notes on their condition.
And now, Hernandez and her team of scientists have been contacting these people and saying, ‘Hey, can we come out and take samples of your saguaros?’ She’s going out and taking tissue samples with these people’s permission,” McCue said.
“We have seen them suffering the effects of extreme heat and drought,” Hernandez said in a video about the saguaros recorded for the Reuters news service.
“And some people report that saguaros are losing their arms, they are changing their shapes, they are leaning. We don’t know that this is related to extreme heat and drought and to climate change, but we suspect that is the case,” Hernandez said.
During summer 2020, “there were saguaros collapsing across the Phoenix valley. We were getting calls just about every single day,” McCue told the Star Thursday.
“’My saguaro. My neighbor’s saguaro,’” McCue said repeated callers were telling botanical gardens’ employees. “We were getting videos from people’s cameras. i remember watching a video … a saguaro was standing there, completely collapsed,” said McCue, adding that she couldn’t provide any statistics on how many collapsed saguaros were seen in 2020.
In her 13 years working at the garden, and according to what people who have worked there much longer than that have told her, “we’ve never experienced something like this,” McCue said.
Bill Peachey, who monitors a plot of saguaros southeast of Tucson, said he suspects that most of the recent saguaro deaths reported in the Phoenix area will have come from people who had them transplanted to their yards for landscaping purposes.
Ailing columnar cacti
In Southern Arizona, Wilder, director of a group of Sonoran Desert researchers, said the elevated saguaro mortality he’s noticed since summer 2020 has mainly been seen on the Desert Research Laboratory grounds on Tumamoc Hill and on Pima County-owned lands within Tucson Mountain Park. Wilder was director of the desert lab for six years until April 2022.
He and other scientists had similar anecdotal observations of unusually large numbers of ailing columnar cacti in parts of southern Baja California following the summer 2020 period of extreme heat and drought. Saguaros don’t live in Baja, but he particularly noticed unusually high damage to organ pipe cacti, along with sour pitaya, cardon and senita, Wilder said.
“We were seeing the dying back of stems, if not whole plants, in Baja, particularly of organ pipe,” Wilder said.
Overall, “our ability to track change in real time, ecologically speaking, it’s been frustratingly limited,” he said, due to a lack of data on dead or damaged saguaros in many areas of the Sonoran Desert. “It’s frustratingly limited. It’s kind of shocking how few sites we have where we have data.
“It makes sites like the Desert Botanical Garden, Tumamoc Hill and Saguaro National Park so valuable — we’re seeing a real need for increased monitoring,” Wilder said.
Unlike reports from the Phoenix area, Wilder said he hasn’t noticed any unusually large amounts of dead or damaged saguaros in the desert around Tucson. Its temperatures, while at record or near-record levels, haven’t come close to those in the Phoenix area.
Likewise, at Saguaro National Park-East and West, biologists “are not seeing evidence that arms are falling off and saguaros are dying, specifically due to the heat” this summer, said longtime Saguaro Park biologist Don Swann. “That doesn’t mean it’s not happening. It’s that I haven’t heard any cases of it or seen any evidence of it.”
As for whether longer-term damage to saguaros has occurred in the park since the overheated, exceptionally dry summer of 2020, “I can’t answer that question with data,” Swann said. The park has not collected data on saguaros’ conditions since a saguaro census was last collected there in 2020, Swann said.
Higher nighttime temps a factor
Saguaros have a well-deserved reputation of being able to survive and even thrive in extreme weather conditions, including heat and drought, though cold has been their best known nemesis, many scientists have said.
But this summer’s extreme heat in Phoenix is testing that assumption, Reuters first reported in the last week.
McCue of the botanical garden confirmed that.
Typically, “plants can hold on for the dry season, waiting for the wet season to arrive,” Hernandez said on the Reuters video. “But they can just hold on so long, right? And if the rain doesn’t arrive on time, the plants start suffering internal damage.”
One way saguaros adapt to a hot, dry environment is that “they do what’s necessary for photosynthesis at night,” McCue said.
When it’s cooler at night, they open their pores to conduct the essential gas exchange “they need to take in carbon dioxide,” McCue said. “During photosynthesis, the carbon dioxide, along with energy from the sun, is converted into a type of food energy for the plant. Plants make their own food.”
But when plants’ pores open as part of photosynthesis, “they’re not just taking in CO2, their water is going out,” McCue said. “When they open pores at night, cacti lose less water. Ostensibly, it’s cooler, normally.”
However, there is a physiological limit to how much heat that cactus can tolerate, and botanical garden researchers are studying “what are the absolute upper limits in terms of temperature at which these plants can function,” she said.
“We’ve never experienced a summer like this summer. Those low temperatures have stayed so high for so many nights in a row,” she said.
With 18 straight days as of Thursday in which Phoenix’s low temperature didn’t fall below 90, “we’ve gotta start to be concerned.
“If that continues night after night after night, which is exactly what has been happening, it’s pushing these plants toward their physiological limit, their ability to function, like humans. When they (humans) experience heat stroke, and have an internal body temperature of 104-105-106, physiologically, their organs no longer can function,” McCue said.
‘An evolving story’
The botanical garden’s plant physiologist made the point on Thursday that “we may not see full effects of the conditions we are experiencing right now for many months or years. It’s an evolving story,” McCue said.
Indeed, the saguaros showing damage this summer may be feeling the effects of the summer 2020 period of extreme heat and drought, she said.
Every time “a saguaro goes down in the garden, whether it’s winter or summer, I go into the database and look at what was the assessed conditions of the saguaros in the census,” she said. “With two exceptions, every one has been rated in poor condition since 2020.”
These are plants that have been stressed due to heat and lack of water, she said. That also makes these plants more susceptible to bacterial and fungal infections and to insect pests.
“It’s just like a human being, if you are already stressed out, your immune system is compromised. It’s the same thing for plants. It’s easier for them to get sick,” she said. “A really healthy saguaro can fend off infection. One that’s already stressed, there’s a higher probability it can’t.”
Many of the saguaros on the garden that fall over, when they hit the ground, they break apart because they’re so heavy, McCue added.
“That is when you find out they were rotting from the inside out. It’s not the heat that directly killed the saguaro. The heat stressed the saguaro. It made them more susceptible to infection, and when the infection takes hold, it rots from the inside out and it succumbs.
“It’s not a straight line. There’s more to it than just heat or drought,” McCue said.
Skepticism
Peachey, a Tucson geologist who has monitored saguaros for more than 25 years, is more than a little skeptical of others’ assertions that heat is the primary factor that’s killing off saguaros.
“If it was heat killing everything, it would be killing the small saguaros, but you don’t hear about small saguaros dying. It’s killing the ones that are already damaged and close to death. It’s only the old ones — the ones that have been transplanted or weak and don’t get enough water,” Peachey said.
Peachey since 1996 has monitored a two-acre patch of saguaros living near Colossal Cave southeast of Tucson. This year, it holds 141 reproducing saguaros.
“Without roots they can’t get enough liquid. The ones that are leaning will go over anyway. The little one has hundreds of times less mass. This is a natural culling process.”
Peachey did add, however, that heat driven by climate change could have been “the last blow.”
Peachey has had a paper published on the blooming of saguaros in the American Journal of Botany. Last month he published an article explaining the methodology for monitoring and measuring the leaning of saguaros for a newsletter published by the Tucson Cactus and Succulent Society.
He was also the primary investigator of a major “blowdown” of saguaros by wind in the Avra Valley back in 2011. In that event, several thousand saguaros blew down in what Peachey calls “the biggest microburst damage ever recorded,” covering an area of almost six square miles.
“Here’s what going on,” Peachey told the Star Thursday. “Saguaros don’t normally die suddenly unless they’re wind thrown. They’re hit by lightning or they get the rot. The plant will fluff totally, suddenly, if it gets infected. When it gets hot metabolism, rates go up.
“Bacterial growth takes off under hot conditions. It rots the interior, massive storage structure, and the plant can’t stop it. The plants can suddenly develop lesions. Big pieces of it will start falling off and all that.”
He suspects that most of the recent saguaro deaths reported in the Phoenix area will have come from people who had them transplanted to their yards for landscaping purposes.
“When you cut the roots to transplant them, the big roots they cut off never grow back. They grow small roots behind them. It will take decades to replace roots that are lost,” he said. “If you put them in a landscaping situation, if there’s moisture nearby, the cactus will do just fine. But it’s lost its major support in nature.”
At the Desert Botanical Garden, a “good portion” of its saguaros were either planted there after being transplanted or are offspring of transplants, McCue said.
“Some of them it was decades ago, before my time,” that they were transplanted, McCue said. “At this point in time these have been saguaros that have been pretty well established. We’re talking decades and decades.
“I would absolutely take the point” that their transplanted status could be a factor in saguaros’ decline if they’d been put into the ground at the garden “a year or two or three ago. That’s not the case,” said McCue, to emphasize her view that transplanting isn’t to blame.
Peachey, however, said there’s photographic proof of “dozens and dozens of transplanted saguaros” that have fallen over the Tucson area, including some that were there a long time.
Once transplanted, their water gathering capacity is cut down also, Peachey said. “The main thing is they don’t have proper support, so they are subject to being windthrown.”
Saguaro National Park, east and west, shows some of the best of Southern Arizona.
