Keeley Lyons-Letts, a biological technician, collects data on a saguaro that was blown down in Saguaro National Park-West during an Aug. 22 windstorm.

One giant saguaro lies prone, with two huge trunks jutting out from a thick set of roots running parallel to the ground.

Another still stands but it’s been sheared off at the top.

Two big arms are dangling down from a third saguaro, their tips touching the ground.

These are among about 1,200 saguaros that were toppled or sheared during a late August windstorm at Saguaro National Park-West, National Park Service officials have concluded.

On Tuesday the park service finished a count of the fallen saguaros that spanned nearly two weeks, required as many as 12 counters at a time and discovered downed and otherwise damaged saguaros in a large swath of the park, concentrated in the Avra Valley west of the Tucson Mountains.

Of those 1,200 saguaros, about 15% remain standing but had their tops sliced off, said Don Swann, a National Park Service biologist who led the count. A much smaller percentage lost arms but still stand, and a few more are now leaning over when they stood straight before, he said.

This is the second largest known “blowdown” of saguaros in the Tucson area in recent years, behind one 11 years ago that felled about 2,000 in Ironwood Forest National Monument northwest of Tucson.

The latest incident is not by itself a major threat to the saguaro population in the national park, which has about 2 million saguaros total in its two units bookending Tucson, about 1.4 million of them living in the west unit.

And while the lack of propagation of young saguaros, possibly due in part to human-caused warming, is a possible threat to the cactus’ future, the area of Saguaro Park-West where the blowdown occurred has some of the best rates in the park.

But an expert on monsoons, University of Arizona researcher Christopher Castro, says human-caused climate change could have been a factor in the storm that blew down the saguaros. His statement is based on the pattern of growing intensity of monsoon storms in the Southwest over the past 20 years. If it turns out that climate change is helping cause these events, more could happen if the climate continues warming as most climate scientists predict it will, he said.

We will need more data on more events over a longer period to be able to determine climate change’s impact on saguaro blowdowns, however, said Bill Peachey, a geologist who has studied saguaros for more than a quarter-century in the Tucson area.

A group of volunteers and staffers from the National Park Service are collecting data on a saguaros that were blown down in Saguaro National Park-West during an Aug. 22 windstorm.  

It’s known that there have been 10 to 12 saguaro blowdowns in the Avra Valley since 2010, but there’s little or no data on such events going back any farther, he said.

“One of our purposes today is to come out and map this area and get a sense of how big the saguaros area and what direction they fall,” said the park service’s Swann as he stood next to a large fallen saguaro whose thick root structure jutted into open air.

“This is an issue that seeks out saguaros and causes mortality. We don’t now how big of a deal it is because it’s not been very well studied,” he said.

Ben Wilder, former director of Tumamoc Hill’s Desert Laboratory and now a private biological researcher, said he would rank wind-driven blowdowns last on a list of threats to saguaro populations. It would come after the threats from lack of propagation; drought; heat; and fire that’s triggered by buffelgrass and other mostly invasive grasses that readily ignite, he said.

If it turns out climate change is a factor in the blowdowns, that would increase their potential threat to saguaro populations, but they would still rank at the bottom of his list, said Wilder, now founder and director of the nonprofit Next Generation Sonoran Desert Researchers.

While the blowdowns could grow more common with climate change, he still sees them as “infrequent, episodic events,” compared to more existential threats such as drought, heat and invasive grasses.

Soil wet from series of storms

The count started the day after the high winds blew through the park, when, Swann said, “I got a call from a maintenance guy who said he had to pull a couple of saguaros from Kinney Road.” Kinney is Saguaro-West’s main drag, cutting through its core area and leading to its Red Hills Visitor Center.

By the time the tally was finished, the fallen cacti were found in an area whose north boundary lies just north of the intersection of Kinney and Sandario roads and whose east boundary is the Tucson Mountain slopes. Fallen saguaros were seen almost to the park’s west boundary and to Fort Lowell Road on the south. The area covers less than a square mile.

Peachey said he also saw fallen saguaros on private lands west of the park. But the park service lacked permission from landowners to enter those areas.

Nobody has records of how windy it got during the blowdown at the park, but 62 mph winds were recorded at Tucson International Airport at about 4:30 that afternoon.

It’s not known why the blowdown occurred where it did, other than much of the area has soils that are a little softer than normal. Most of it occurred in bajadas, areas that sloped from the mountains above down to the floor of the Avra Valley, Peachey said.

Another contributing factor was that a series of storms preceded and foreshadowed this event. Thunderstorms hit the Tucson metro area on Aug. 16, 18 and 21 as well as on the 22nd when the cacti blew down.

“Every one of these events I’ve seen so far have followed good rain or several days of rain, with the ground damp or soaked,” Peachey said. “All of them were preceded by wet soil or damp soil conditions.”

Can’t replant them; internal damage

On the last day of the saguaro count, Sept. 12, Swann led a reporter around a sloped area east of the park’s Desert Discovery Trail, less than a mile north of the visitor center.

The saguaro counters worked individual transects — specific areas laid out for scientific study — of a little more than 60 feet wide and up to a mile long.

Swann started by walking up a hillside east of Kinney. The soils there were rocky, offering saguaros more stability than finer soils farther downhill, where the vast majority of the downed saguaros landed.

Later that day, several counters, including another Saguaro Park staffer, an intern, several volunteers and Peachey, who served as an adviser to Swann, ventured into flatter ground to the west, containing finer soils that don’t hold roots as securely as rockier soils. Working in that area, one person alone turned up 40 fallen saguaros, far more than they found on the rockier slope areas.

When on the rockier hillside, Swann encountered a prone saguaro, about 20 feet long, its root upended and its top wearing a crack.

This one is at least 100 years old, as are the majority of the fallen saguaros, which are “more vulnerable,” Swann said. While a few of the downed saguaros are less than six feet tall, the majority are 18 feet to 24 feet, he said.

“People ask, ‘Do we replant them?’ They have internal damage. There’s nothing we can do to save them,” he said.

Instead, the saguaros are left to decompose and draw “a large number of different kinds of insects.”

David Rabb, a volunteer, walks further into the desert after collecting data on a saguaro that was toppled in Saguaro National Park-West during the August windstorm.  

“A saguaro is a whole big column of water, mixed in with the plant. The cells are very nutritious.”

When they came upon a saguaro that had toppled or lost arms or its top, the counters stopped, recorded in their cellphone apps its size, the direction it had fallen and whether it had arms.

At one point Tuesday, Swann encountered a “leaner,” a saguaro swaying slightly.

“It’s very likely to fall down,” he said. “There are cracks in the soil. Its roots have been severed, snapped off, from when it swung back and forth in the rain.”

Storm also damaged homes

The park wasn’t the only place west of the Tucson Mountains to feel that storm’s impacts. In the Tucson Estates neighborhood south of the park and just east of Kinney, the storm uprooted two posts on the awning of Nancy Finateri’s house and knocked power out to her neighborhood for several hours, she said.

A house lying two down from hers suffered similar carport damage, “had their car torn up,” and had a saguaro leaning against it once the storm was over, she said. A window blew out of the house across the street from hers, and “all of the glass blew into my driveway,” Finateri said.

“I’ve been in Tucson over 50 years, and never in my life living in Tucson have I ever seen a storm like that,” Finateri said. “I looked out my window and I couldn’t see a darned thing. It was like a whiteout, just like a fog, like a blizzard.

“The awning over my carport rolled up like a rock. The storm pulled out two of the posts that held it down. They were embedded in cement, but they were pulled up and landed on my roof.”

“Tree tossed like a tumbleweed”

The collapse of 2,000 saguaros at the Ironwood monument blowdown occurred 11 years ago this month. It was far more dramatic than the latest saguaro collapse, and its origins have been thoroughly documented.

On Sept. 8, 2011, heavy rains soaked the ground across the Waterman Mountain area of the monument, which lies about 50 minutes by car northwest of Tucson and an hour’s drive south of Phoenix.

At mid-afternoon the next day, a storm cell — an air mass that moves up and down in convex-shaped loops — that was ranked as the heaviest in the state at that time suddenly collapsed near Waterman Mountain, Peachey wrote in his unpublished account of the incident. The storm was essentially dumped onto the ground there.

“It’s like you turned a fountain upside down,” he said. “Instead of going up, it comes down, and hits the valley.”

The collapse of 2,000 saguaros at Ironwood National Monument occurred in a blowdown 11 years ago this month. It was far more dramatic than the latest saguaro collapse, and its origins have been thoroughly documented.

The National Weather Service had classified the winds in the Eloy area from where the storm came as “tornadic,” or tornado-like, recalled Peachey.

The fallen saguaros were discovered lying in ring-like formation two days later by two flight instructors flying overhead, Peachey wrote.

Over the next 15 months, Peachey and several colleagues mapped the fallen saguaros, which occupied a 5.7-square-mile area.

Most fallen saguaros stood 15 to 30 feet high and were single-stemmed, just starting to form arms. Most of the really big saguaros seemed to resist the windstorm — “they might lose an arm or be topped but not blown over,” he said. One exception was a huge 40-footer that fell.

They found “lots” of palo verde trees that had been ripped out of bedrock in the area, “with the whole tree tossed like a tumbleweed into the surrounding greasewood flats,” he said.

Also affected were large stands of chain fruit cholla, from which cholla balls “exploded,” he said. When he and other investigators drove dirt roads in the area two days after the blowdown, they had to bring push brooms to sweep the chollas off the road so they didn’t get flat tires.

“We found saguaros on the slopes that had the sharp tips of palo verde branches driven into them. If you were out there walking around you would have been in shrapnel of all kinds — tree branches, bases of ocotillos, parts of palo verdes.”

Haboob triggered another incident

The only other major saguaro blowdown known by scientists to have occurred in the Tucson area knocked down “many hundreds” of the towering cacti more than 40 years ago on south-facing slopes on each of four desert mountains, including Tumamoc Hill, home of the University of Arizona’s Desert Research Laboratory.

That’s how former Desert Research Lab Director Julio Betancourt remembers the event from August 1982.

The trigger was a haboob, a towering wall of blowing dust coming from high winds that develop from collapsing thunderstorms.

“I was then a beginning graduate student and was working late sorting packrat middens (waste piles) from Chaco Canyon in my office in the main building of the Desert Lab. No one else was at the Desert Lab when the haboob happened,” recalled Betancourt, now a scientist emeritus for the U.S. Geological Survey in Reston, Virginia.

“I heard the roar of the wind and the next thing that happened was a pressure change inside and all of the old windows blowing open, scattering papers everywhere. It was like a cyclone had hit Tumamoc Hill.

The collapse of the 2,000 saguaros at the Ironwood monument blowdown occurred 11 years ago this month. It was far more dramatic than the latest saguaro collapse, and its origins have been thoroughly documented.

“I ran around the offices trying to close the windows but it was impossible. I heard a boom and looked out the front window in my office which faced the desert garden and saw that the giant saguaro next to our driveway had crashed over the greenhouse. It all happened so fast, really in just a few minutes.”

The next day, Betancourt and the late Ray Turner, a longtime Tumamoc Hill researcher, started counting fallen saguaros, and found more than 400 on the hill in about two weeks. Enough more were discovered to bring the total count to probably more than 1,000 on nearby “A” Mountain, as well as on Martinez Hill and Black Mountain in the Tohono O’Odham San Xavier District south and southwest of Tucson, respectively.

Six major thunderstorms had blasted Southern Arizona that month, including four that dropped 2 to 4 inches of rain in the Tucson area, with most accompanied by high winds. Most of the fallen saguaros were larger ones, and most were uprooted, not sheared, Betancourt said.

“Extreme precipitation events”

The possible climate change link that UA’s Castro sees with the saguaro blowdowns stems from an analysis of the monsoon’s behavior that he and four other researchers published in 2019.

Their analysis concluded that in both North America and the Amazon River Basin, total monsoon precipitation had fallen over 20 years as climate change, triggered by fossil fuel burning and other greenhouse gases, pushed up temperatures. But at the same time, the frequency of “extreme precipitation events” had increased, they found. Their study was published in the journal Current Climate Change Reports.

Castro is a professor and interim head of UA’s Hydrology and Atmospheric Sciences Department. He said it’s “a reasonable hypothetical” that climate change could be linked to the recent blowdowns.

“We’ve had one of the hottest summers here in Arizona that we’ve ever had. Phoenix has been the hottest it’s ever been, bar none. It’s going to put stress even on our desert vegetation.

“When you would get an intense monsoon storm, those saguaros are going to be weakened anyway and blow down.” he said.

His 2019 study concluded that even though monsoon storms may not be happening as often as they used to, “when they occur in the right conditions, they’re tending to be more intense.”

That pattern described in the study is consistent with what we see worldwide in storm behavior today, he added.

“This summer, we’ve been a series of really spectacular flooding events all over the world — Spain, Greece, Libya, and in China, just to name a few,” he said.

Castro added, however, that “I would have no reason to disagree” with those such as Swann and Peachey who say more data about saguaro blowdowns from a longer period than now exists is needed to document whether climate change is causing them or making them worse.

“There is in no way I’m saying there’s a published paper or scientific process saying that we’ve explicitly proved this. I’m not saying climate change caused it to happen,” Castro said.

“But I’m saying it seems consistent with the pattern that our monsoon is getting more extreme and variable. That will put more stress on our local ecology, including our saguaro plants. If our plants are more stressed, we may be more prone to a blowdown in a big event.”

Husband-and-wife scientists hand off research after 43 years.


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Contact Tony Davis at 520-349-0350 or tdavis@tucson.com. Follow Davis on Twitter@tonydavis987.