Researchers are looking for ways to protect saguaros from climate threats
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At the entrance to Phoenix’s Desert Botanical Garden recently, signs warn visitors to stay hydrated and take caution in the heat. Black shade cloths cover small, round barrel cactuses and leafy agaves to protect them from intense sun. And a row of prickly pear cactuses known for their bright green pads look pale and yellow, as Kevin Hultine, the garden’s director of research, points out.
“This is the impact of excess heat,” Hultine says.
Hultine says he’s never seen heat stress quite this severe among the thousands of plants in the garden before. And these plants are all well watered and meticulously cared for. At the edge of the garden property, where plants aren’t professionally maintained, the effects of heat are even more visible.
Hultine points out a towering saguaro. He guesses it’s at least 75 years old. It has 10 huge arms reaching toward the sky, but one is on the ground. The fallen arm is still green, indicating it dropped recently.
“And that’s probably an indication that perhaps some other of these arms are probably also going to fall,” Hultine says.
There’s perhaps no symbol more strongly associated with Arizona than the saguaro cactus. The famous Sonoran desert succulents can be 40 feet tall and can live more than 150 years. Hultine said they’re even hardier than other cactus species, because they are so well adapted to extreme conditions. But summers have never been this extreme in Phoenix before. And as these desert-adapted plants suffer heat damage or die, scientists are concerned about the future of the species in a warming climate.
Saguaros are adapted to breathe in the carbon dioxide they need for photosynthesis overnight, since they lose less water opening their pores when temperatures are cooler. But overnight lows in July in Phoenix averaged nearly 91 degrees, straining the photosynthetic process for the plants. Meanwhile, there’s been almost no measurable rainfall in Phoenix this summer. When saguaros become severely dehydrated, their firm outer skin shrivels. Since each arm of a saguaro can weigh hundreds of pounds, eventually a heat-stressed plant will lose structural integrity and drop arms or fall over completely.
And Hultine said, in a slow-growing plant like a saguaro, damage from heat or drought can take years to appear. Before this year, the hottest summer on record in Phoenix was 2020. Hultine said the garden has about 1,000 saguaros and typically, eight to 12 per year die. But since 2020, it’s been more like 40 per year.
“When you add the added stress from 2020 to what appears to be perhaps an even warmer summer now, I expect the rates of mortality are probably going to continue to ramp up for the next several years,” Hultine said.
Down south, in the Tucson area, July temperatures were about 8 degrees cooler than in Phoenix. At Saguaro National Park, there’s been no significant damage reported recently among the namesake cactuses.
Still, park biologist Don Swann said, this summer has been hotter and drier than normal. And saguaro seedlings struggle in these conditions.
“They store water in their stems and that water allows them to be resilient in really hot and dry conditions, but when they’re very small they can’t store very much water and so they rely on the soil,” Swann said.
So even if mature saguaros in the park have fared better than their counterparts in Phoenix, the changing climate is still a threat.
“We’ve been in a long-term drought really since the mid-nineties, so we’ve seen a lot fewer saguaros entering the population in the last 25, 30 years,” Swann said.
Saguaros are not an endangered species. But scientists are beginning to think about how to ensure these cactuses can survive into the future.
“We want to be able to plant saguaros that will be the most well adapted to basically our new, hotter environment,” said Helen Rowe, associate research professor with the School of Earth and Sustainability at Northern Arizona University.
Rowe is working to find ways to protect the iconic cactuses along with researchers from the Desert Botanical Garden, University of Arizona, the Audubon Society and National Autonomous University of Mexico.
There are parts of the Sonoran desert even harsher than Phoenix where the cactuses still grow, Rowe said.
“Kofa National Wildlife Refuge is a really hot, dry environment, yet the saguaros there are thriving, they’re doing really well," Rowe said. "So that’s of great interest."
Her team plans to take seeds from cactuses in those regions, along with other saguaro populations, and grow them in northern Mexico, Tucson and Phoenix to compare how they perform.
“So that would give kind of a range of the growing conditions and we could see which is best adapted for each site,” Rowe said.
Specifically, researchers hope the experiment will show which genetic lines can survive the most extreme summer temperatures.
But the project isn’t fully funded yet. Rowe estimates the research will cost $50,000 to $100,000 per year. And since saguaros are so slow-growing — they’re known to take up to 10 years to grow just an inch and a half from seed — it could take years before researchers know if it’s possible to breed a more heat-resilient saguaro.
In the meantime, Kevin Hultine said, hotter temperatures are already taking a toll.
“It’s a bit alarming. Clearly the impacts are pretty dramatic,” Hultine said.
As the Sonoran desert continues to warm, Hultine said it’s hard to guess exactly what’s in store for this species.
But, he said, “Are they going to be more and more difficult to maintain? Absolutely.”
The end of this summer, he said, may be just the beginning of this story.
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