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Scientists hope to breed a heat-resistant saguaro as more die in a warming climate

AILSA CHANG, HOST:

There is perhaps no symbol more strongly associated with Arizona than the saguaro cactus, but the record-breaking heat in the Southwest has killed some of these desert-adapted plants. And as Katherine Davis-Young from member station KJZZ reports, scientists are looking for ways to ensure that saguaros can survive in a warming climate.

KATHERINE DAVIS-YOUNG, BYLINE: This summer's intense heat has pushed the thousands of plants at the renowned Desert Botanical Garden in Phoenix to the brink. Small barrel cactuses are draped in black shade cloths. Bright green prickly pears have faded to yellow. And Kevin Hultine, the garden's director of research, points out one of the towering saguaros. It has 10 huge arms reaching toward the sky, but one is on the ground.

KEVIN HULTINE: In fact, let's look at this one over here. You can see there is an arm that actually fell at one point off of this plant.

DAVIS-YOUNG: Saguaros can be 40 feet tall and live 150 years. Hultine says they're even hardier than other cactus species because they're so well adapted to extreme conditions. But summers have never been this extreme in Phoenix before. Hultine says the cactuses can't breathe in the carbon dioxide they need when overnight temperatures don't cool off. And when they're dehydrated, their firm outer skin shrivels. Since saguaros weigh hundreds of pounds...

HULTINE: Eventually, you start to lose structural integrity near the base, and then the whole plant will just fall over.

DAVIS-YOUNG: And that's happening more and more. Before this year, the hottest summer on record in Phoenix was 2020. Hultine says the garden has about a thousand saguaros, and typically 10 die every year, but since 2020, it's been more like 40 per year.

HULTINE: I expect that the rates of mortality are probably going to continue to ramp up for the next several years.

DAVIS-YOUNG: In the Tucson area, a couple hours south, temperatures are cooler than in Phoenix. At Saguaro National Park, there's been no significant damage to the namesake cactuses. Still, park biologist Don Swann says it has been hotter and drier than normal this year. And saguaro seedlings struggle in these conditions.

DON SWANN: When they're very small, they can't store very much water, and so they rely on the soil.

DAVIS-YOUNG: Mature saguaros in the park have fared better than their counterparts in Phoenix, But the changing climate is still a threat.

SWANN: We've been in a long-term drought really now since the mid-1990s, and so we've seen a lot fewer saguaros entering the population in the last 25, 30 years.

DAVIS-YOUNG: Saguaros are not an endangered species, but scientists are beginning to think about how to ensure these cactuses can survive into the future.

HELEN ROWE: We want to be able to plant saguaros that will be the most well adapted to basically our new, hotter environment.

DAVIS-YOUNG: Helen Rowe is with the School of Earth and Sustainability at Northern Arizona University in Flagstaff. She's working with a team of researchers to find ways to protect the iconic cactuses. Rowe says there are parts of the Sonoran Desert even hotter and drier than Phoenix where saguaros grow. Her team plans to take seeds from those cactuses, along with other saguaro populations, and eventually plant them in northern Mexico, Tucson and Phoenix.

ROWE: So that would give kind of a range of the growing conditions, and we could see which is best adapted for each site.

DAVIS-YOUNG: And which genetic lines can survive the harshest summers. But since the plants are so slow growing, it could take many years before researchers know if it's possible to breed a more heat resilient saguaro. And the project isn't fully funded yet. In the meantime, Kevin Hultine says seeing these giants of the Sonoran Desert collapse more frequently is alarming. He says it's hard to guess what's in store for saguaros in a warming climate. But, he says...

HULTINE: Are they going to be more and more difficult to maintain? Absolutely.

DAVIS-YOUNG: The end of this summer, Hultine says, may just be the beginning of this story. For NPR News, I'm Katherine Davis-Young in Phoenix. Transcript provided by NPR, Copyright NPR.

NPR transcripts are created on a rush deadline by an NPR contractor. This text may not be in its final form and may be updated or revised in the future. Accuracy and availability may vary. The authoritative record of NPR’s programming is the audio record.

Katie Davis-Young