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Despite climate threats, landscape may shelter some saguaros

Few new plants, hotter temperatures have scientists worried that desert sentinels could dwindle

A study of saguaros on Tumamoc Hill in Tucson's West Side may help researchers understand how the topography of the land surrounding a desert research laboratory may help some of the Sonoran Desert's iconic giants endure a hotter, drier future. 

Saguaros have long lifespans and that makes it difficult for researchers to understand how shifts in climate affect their population patterns. 

However, researchers at the Desert Laboratory on Tumamoc Hill — an 860-acre ecological reserve owned and operated by the University Arizona College of Science, as part of a partnership with Pima County — were able review a dataset that spans nearly a half-century, helping them to better understand when and where new saguaros begin to grow. 

And, by studying more than 5,800 saguaros, researchers led by Susana Rodríguez-Buriticá discovered that small variations in the hill’s topography might help saguaro populations endure a harsher climate.

The saguaro is uniquely evolved to live in the Sonoran Desert where two rainy seasons are key to their survival, but even a resilient species is vulnerable to extended drought. 

And, a recent study published in the Environmental Research Letters found that Saguaro National Park has warmed about 1.2 degrees Celsius from 1950 to 2010. 

"This was an ambitious but highly valuable undertaking, since these plants can live for over 100 years," said co-author and United States Geological Survey research ecologist Daniel Winkler, in news release from the University of Arizona. "As a result of the long-term data set, we're able to identify how saguaro populations have been maintained over time and predict how they'll likely respond to future climate change."

"We're really trying to understand, what are the implications for saguaros in the future?" said Larry Venable, co-author and director of research at the Desert Laboratory. "We've got landmark data sets from the early days of the laboratory, and we were able to match really great ecologists with these long-term datasets that map every single saguaro." 

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Previous studies have linked changes in saguaro populations to climate, but this new study, published in the journal Ecosphere in August, confirmed that saguaros are more likely to grow and establish themselves during wet periods. But the study also found that the saguaros on Tumamoc are "responding differently" because of subtle variations in the landscape, Venable said. 

"The Desert Laboratory at Tumamoc Hill has the longest record of population dynamics of saguaros," Venable said. "It’s special because the Carnegie Institution founded it in 1903, and it has hosted leading ecologists since the early 20th century," he said. "There’s a deep history of data collection. This research is more valuable because of the long-term nature of the data."

Location, location

For example, saguaros are traditionally favored on south- and east-facing slopes in the winter, when saguaros are most at risk of dying by frost, because that’s where the sun lingers longest, Rodríguez-Buriticá told UA News. 

But this is not strictly the case on Tumamoc Hill. The south and east slopes are steep and rocky, meaning water retention is too low for many saguaros to take hold. Alternatively, the north-facing slopes, which are less steep, receive less sunlight but have deeper, richer soil that retains water for saguaros. As a result, saguaros could establish there too, even in years when the desert is hotter and drier than normal, she said. 

"Ignoring spatial and temporal variability and their interactions can produce incorrect predictions when trying to model species’ past and future population dynamics," said Rodríguez-Buriticá, who was a postdoctoral research fellow at the University of Arizona from 2011 to 2014 when she conducted this research. 

She is currently the lead researcher in the Spatial Ecology Lab at Alexander von Humboldt Biological Resources Research Institute in Colombia. 

The team found that Tumamoc Hill’s saguaro population boomed during wet and cool periods, but that variability in the landscape coupled with changes in climate produced unexpected demographic patterns, including the ability to foster new saguaros during periods of harsh climate conditions.

For more than a century, researchers at the Desert Laboratory have been studying the iconic giant. 

As Diana Wells notes in her book "The Lives of Trees," Dr. George Englemann, a German botanist dubbed the giants as Cereus giganteaus in 1948. However, in the 1920s, two members of the Desert Laboratory of the Carnegie Institute decided it would be better to name the giant cactus after their benefactor, Andrew Carnegie, and so the big cactuses became known as Carnegiea gigantea. 

Venable said researchers looked at data from a saguaro census done in 1964 and compared it to new data collected in 2012. 

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"We had all this data, and we had a smart, young scientist who wanted to bring this data collection to the present," Venable said. 

Venable said that when he arrived in Tucson, the weather in Tucson was "as wet as it had ever been," but that over time, it's been getting drier. But, now, that pattern appears to be "getting outside the historical envelope," and not only is the climate getting drier, he said, " It’s been getting hotter and drier for the last 30 years." 

"We're seeing more extremes, more freezing events, and days that are hotter for long," he said. "So, we want to know, what's going on with the saguaro?"

Long lives make for difficult studies

In previous decades, people have worried about the saguaro's future, and that worry helped in part to fuel the creation Saguaro National Park, the 92,000 acre park that includes a sections east and west of Tucson.  In 1933, UA President Homer Shantz led a campaign to protect the saguaro around the Rincon Mountains, in part mounted because a pulse of saguaros dating back to 1800s meant their apparent decline in the 1930s. 

While an established saguaros can live 175 to 200 years, and rise to 45 feet or more while becoming a two-ton column of water and flesh held up by a thin structure of ribs, the plants are also delicately balanced with their environment. Seeds can be destroyed by drought and freezes, and surviving seeds need cover from "nurse" trees to grow. 

This effort is complicated, Venable said, because of the life-span of the cacti. While researchers can go out a measure individual plants every year, or every few years, and get results, the saguaro's slow growth—it can take 10 years for a saguaro to grow one inch—means that long-term datasets can better help researchers understand the saguaro's life history and its response to climate variability. 

"The cool thing about here is that we have this rich heritage of the long-term data, and we can look at one saguaro's growth rate over variable times, including comparing cool and wet, versus hot and dry periods," Venable said. "And, then we project back in time, with hard weather records, and data from tree-rings to protect weather back to the 1700s." 

With better data, researchers can then pick out trends from every 10 years, and then "pinpoint" weather events in a specific year, and find patterns in how saguaros respond to the climate.

New saguaros rare finds

Tom Orum and Nancy Ferguson, a husband-and-wife team, have monitored the health of some 600 saguaros in Saguaro National Park East, on the other side of the Tucson metro area, for four decades.

They’re the third generation to measure and monitor these symbols of the West since 1941, and the work on 60 acres of the park has become a treasured ritual for them.

“It’s sort of like having roots yourself to get back to the same place and repeat a process year after year,” said Ferguson, who's in her mid-70s.

“The thing about saguaros is they’re noticeable individuals,” she said. “For most of us, plants are like, ‘Oh they’re all like other plants.’ But saguaros are very much individuals, and that’s something that our culture really relates to.”

But since the 1990s, she and her husband have seen what could be troubling changes in their beloved saguaro flatlands.

Saguaros in the park, scientists say, are responding to climate change and prolonged drought by reproducing less frequently. This worrisome downtick could signal the state’s saguaros are in decline.

Orum, a retired plant pathologist who's also in his 70s, remains cautiously optimistic. He believes another decade or so of scientific study is needed before scientists can be certain Arizona’s iconic saguaros are declining.

From 1993 through 2016, Orum and Ferguson found only three new saguaros in the 60 acres they monitor in Saguaro National Park East. (There are two parts to Saguaro National Park. The eastern part was designated a national monument in 1933. An additional 25 square miles in the Tucson Mountains west of the city were added to the monument in 1961, and it was elevated to national park status in 1994.)

But a 2018 study found the problem of fewer young saguaros on both sides of the park.

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Saguaro National Park biologist Don Swann and colleagues found only 70 saguaros younger than age 15 among the 10,000 saguaros surveyed in the park. The study names climate change, prolonged drought and human activity, such as cattle ranching, for the decline in young saguaros.

The results of the study are “broadly applicable to other desert areas for predicting how the saguaro and other long-lived desert species may respond to anticipated climate change,” the authors wrote.

“Some of our biggest concern does have to do with the survival of the younger saguaros with higher temperature and longer dry periods being a potential for the future,” Swann said.

Adult saguaros are well-adapted to dry conditions. Their shallow roots quickly absorb moisture from the soil and their flesh expands to store water.

But saguaros start out just a few inches tall and aren’t able to store much water. Higher temperatures cause water to evaporate more quickly from the soil, which, coupled with drought, has made it hard for new saguaros to survive, scientists say.

Saguaros seem tough, but they’re fragile. Their delicate white blossoms are pollinated by bats, insects and birds, producing fruits rich with tiny seeds. Coyotes and other animals eat the fruit, depositing the seeds in their scat. Most seeds are destroyed by drought, freezing conditions and animals. The few survivor seeds germinate beneath protective “nurse” trees and grow slowly – it can take 10 years for a saguaro to reach 1 inch. But once established, a saguaro can live 175 to 200 years, reach a height of 45 or more feet and weigh more than 2 tons.

Saguaros evolved only in the Sonoran Desert because it offers the two rainy seasons key to their survival. But the challenges posed by climate change and drought show that even a resilient desert species is vulnerable.

Arizona has experienced significant drought since 2000. A recent national study published in the Environmental Research Letters found Saguaro National Park has warmed about 1.2 degrees Celsius from 1950 to 2010.

“That’s the equivalent of moving the park over 150 kilometers (nearly 100 miles) south from Tucson to hotter areas in Mexico,” said Patrick Gonzalez, a climate scientist at the University of California, Berkeley. “Small increments of heating can translate into big changes on the ground.”

Within the urban areas, Tucson and Phoenix were among the fastest-warming cities in the United States over the past 50 years, with average annual temperatures warming more than 4 degrees — and average night-time temperatures in the cities increasing by 8 degrees.

The shift means changes in the saguaro population, which could harm many animals that rely on the cactus for food and shelter. The saguaro is a keystone species, essential to maintaining the delicate balance of the Sonoran Desert ecosystem.

“Species in a community evolve over long periods of time, and not all the species respond equally to those changes in climate,” said Osvaldo Sala, founding director of the Global Drylands Center at Arizona State University. “They’re all very tightly connected one to the other, so that can cause unexpected consequences.”

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While saguaros might be the "most studied plant in the Sonoran Desert," understanding the future of the saguaro and its response to a warming, drier world is important, because saguaros are a keystone species.

"So many plants and animals depend on it, lots of animals live in the saguaro itself, and others depend on the fruit," he said. "Without the saguaro, a whole bunch of species would go kaput."

Not alone

Saguaro National Park isn’t the only national park facing these challenges. Gonzalez and fellow climate researchers conducted the national study and found climate change is causing national parks across the country to warm twice as fast compared with the rest of the United States.

The scientists said location was the main factor causing the disproportionate temperature increases. Most national parks are in areas especially sensitive to human-caused warming, including the arctic, mountainous areas and the Southwest.

Out of Arizona’s 22 national parks, Gonzalez said, 16 have experienced significant warming.

“Our national parks have been exposed to conditions hotter and drier than the U.S. as a whole,” he said. “Climate change is certainly a major driving factor of vulnerability in the future.

Hope in the rocky slopes

Scientists are cautiously hoping saguaros will outsmart a changing climate by reproducing on rocky foothills where precious rainwater better resists evaporation.

Swann and his colleagues found a smaller decline in the number of young saguaros in these slopes compared with the flatlands. In these areas, water can get trapped in cracks or crevices and doesn’t evaporate as quickly, providing slightly better conditions for young saguaros.

“In general what we see over time is that in those rocky areas, the saguaro populations tend to be more stable,” Swann said.

Identifying and protecting these resilient areas is one way national parks can ensure species and ecosystems survive future changes. Gonzalez said Joshua Tree National Park in California already has found some success using this method to protect their namesake species.

Scientists have yet to figure out how, exactly, to protect saguaros in the park from climate change and drought. They say they need to understand more. They need to further monitor the cactuses in the rocky foothills, and they need to determine how much of the current decline is caused by climate change versus natural cycles.

Lifelong learning

Tom Orum and Nancy Ferguson have kept watch over the saguaros on the same 60 acres of the park for nearly 40 years.

Each year, they measure the height, note scars or damages and count the number of arms on each cactus. They also identify new or dead saguaros. Their painstaking, regular monitoring of saguaros has long informed scientific knowledge of the lifespan and population trends of the species.

And they have four favorites. They found these four in 1986, when the saguaros were about 4 years old and just a half-inch tall.

“We found them when they were so small,” Orum said. “When you’ve followed them every year, you get attached to them.”

Now the tallest is 11 feet tall. It grew 8 inches in the past year.

It would sadden the couple if the cactuses were no longer there. They hinge their hopes for the species’ well-being on rain.

“We think saguaros don’t require just a single rainy season,” Ferguson said. “They need rain one summer to get going. They need rain that winter to survive the winter. Then they probably also need a second rainy season to really get established.”

In the drought-plagued 1940s and 1950s, Orum said, there was a similar decline in young saguaros. At the time, scientists weren’t sure whether the drought caused fewer young saguaros or if hungry cattle also were to blame.

But during the especially rainy years of the 1980s, Orum and Ferguson found 30 to 40 new saguaros each year across their acres.

The current decline of young saguaros in the park is not tied to cattle, so scientists figure drought is to blame.

After Orum and Ferguson retired from the University of Arizona in 2000, their love for the desert and saguaros motivated them to keep up the monitoring they’d been doing during their time at the university.

“It’s more important to us to be out here measuring cacti than figuring out how to invest our money, or lots of other things that people do with their time,” Ferguson said.

Besides, their careful scientific observation of the saguaros is key to learning the fate of the cactuses. Only monitoring will show whether saguaros decline or recover.

Orum said he and Ferguson won’t know for sure “until we’re 80 or 85 years old.”

Cronkite News reporter Stephanie Morse contributed to this report.

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A saguaro stands tall along the east slope of Tumamoc Hill on the West Side of Tucson.

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