In oven-like Phoenix, even saguaros struggle
Climate change threatens Arizona’s seemingly heatproof cactuses
With July being declared Earth’s hottest month on record, the grim reality of record-breaking temperatures and extreme arid conditions caused Arizonans to seek refuge inside.
But how did the native plants protect themselves?
Native Arizona flora — most notably saguaro cactuses, succulents, palo verde trees and creosote bushes — have adapted over millennia to accommodate the scorching summer highs of the Sonoran Desert, but growing climate change concerns are raising alarms for how well and at what rate those plants will continue to adapt. Experts say cultivating genetic diversity and mitigating Phoenix’s urban heat island must occur to protect native plants.
Seemingly drought-resistant deciduous plants with sunburned or yellowing leaves have worried nurseries, gardens and plant collectors this summer.
The summer broke Phoenix’s record of most days over 110 degrees, but high temperatures are not new to a desert climate. Summers in the Sonoran Desert — which ranges from central Arizona to Baja California and Mexico — routinely exceed 104 degrees and often reach
118 degrees, according to the National Park Service.
Although hot temperatures are expected, extreme heat anomalies — like 31 straight days of 110-degree highs — are becoming the norm. Most of the warmest years on record fall within the past two decades, according to the National Weather Service.
Chris Martin, a horticulture professor at Arizona State University, said though Phoenix is located in the Sonoran Desert, its metropolitan ecosystem of streets, buildings and parking lots creates an urban heat-island effect. Surfaces soak up heat during the day, which radiates back at night and makes it harder for the city to cool down.
“Phoenix’s urban heat island is really interesting in that it’s largely a nighttime phenomenon,” Martin said. “We’re setting records of nighttimes that have not gotten below 90 degrees, and that’s been the big issue.”
Persistent scorching temperatures with no overnight relief contribute to a new environmental effect to which native plants are struggling to adapt.
Kimberlie McCue, chief science officer at the Desert Botanical Garden, said the extreme temperatures coupled with a significantly dry monsoon are “way beyond what these plants would have normally experienced.”
One way native plants adapt to heat and drought is to do part of their photosynthesis process at night. They’ll store water in their pores all day, and when cooler night temperatures arrive, they will open their pores to take in carbon dioxide. However, with high overnight temperatures, plants may be losing more carbon dioxide and water than they are able to take in, according to McCue and Martin.
In July, the Desert Botanical Garden had two saguaro cactuses collapse, one large and one small. An emblem of the Arizona desert, the drought-resistant saguaro can live up to 200 years and grow up to 60 feet tall. Four saguaros at the gardens also dropped arms within a two-week period.
“That is not normal at all; a healthy saguaro is not going to just drop an arm,” McCue said. “That’s telling us these plants are highly stressed or compromised.”
She said the Desert Botanical Garden has been getting reports from people in the community that arms on their cactuses are falling off. Agave plants at the garden are also “showing these heightened levels of stress,” which is seen by their yellowing and bending toward the ground.
Kyla Daniel, a sales associate at Verde Valley Nursery in Fountain Hills, said multiple mesquite trees, elm and oak trees, agave and cactuses have died, and there were “a lot more deaths this summer than last summer.”
As for the clientele, she said, most people are having issues with sunburns on their plants. Overwatering can harm or kill the plant.
“For most desert native plants, they can get sunburned. And then once they have a little bit of relief, they can start just growing again normally,” Daniel said. “The overwatering is more of an issue than the sunburn is.”
Daniel said she advises a sunshade instead of giving extra water to distressed plants.
There is hope genetic diversity and community efforts can help.
In 2020, teams from the Desert Botanical Garden were deployed outside the urban heat island bubble and into the natural desert landscape to collect seeds from wild populations of saguaros — some thriving in extremely hot natural temperatures — and planted them back at the garden.
“The key to adapting to changes in an environment is to have genetic diversity within your population,” McCue said. “We’re trying to build resiliency into our population of saguaros.”
If extreme summer temperatures return next year, Martin said, “we’re going to have to implement cultural practices in the short term.” That could include shade protection, a little extra water or a change in the urban plant diversity that favors more heat-tolerant plants.
Martin added Phoenix needs to address the heat-island effect. Doing so would provide native plants with a more stable environment and mitigate heat waves like those of 2020 and this year.
“I think most if not all of the people that move here, they really start to embrace the specialness that is the desert, and love their saguaros and agaves,” McCue said. “There’s a whole lot of incentive to take care of what we’ve got.”