Native Arizona saguaro plants suffer from extreme heat
PHOENIX – With July declared the hottest month on record on Earth, Arizonans faced the harsh reality of record temperatures and extreme arid conditions that had residents seeking shelter indoors to escape the oppressive heat.
But how did the indigenous people, rooted in the desert soil, protect themselves?
Arizona’s native plants — most notably saguaro cacti, succulents, palo verde trees and creosote bushes — have adapted over thousands of years to accommodate the scorching summer highlands of the Sonoran Desert, but growing concerns about climate change are raising alarms about how well these plants will fare and at what rate. You will continue to adapt. Cultivation of genetic diversity and mitigation of Phoenix’s urban heat island must be done to protect native plants, experts say.
Drought-resistant deciduous plants with burnt, yellowing or dying leaves seem to be worrying nurseries, gardeners and personal plant collectors this summer.
The summer of 2023 was undoubtedly hot for Phoenix, with the city’s record breaking for most days at over 110 degrees, but above-average summer temperatures are nothing new to the desert climate. Summer temperatures in the Sonoran Desert — which range from central Arizona to Baja California and Mexico — routinely exceed 104 degrees and often reach 118 degrees, according to the National Park Service.
Although high temperatures are expected, extreme heat-related anomalies — such as 31 straight days of highs of 110 degrees — have become the norm. Most of the warmest years on record occur within the past two decades, according to National Weather Service data.
Although Phoenix is located in the Sonoran Desert, it has its own urban ecosystem with streets, buildings and parking lots that create an urban heat island effect, said Chris Martin, a horticulture professor at Arizona State University. Roofs absorb heat during the day, which radiates back at night and makes it difficult for the city to cool.
“The Phoenix urban heat island is really interesting because it’s largely a nocturnal phenomenon,” Martin said. “We’re setting records for nights where the temperature doesn’t go below 90 degrees, and that’s been the biggest problem.”
These constant scorching temperatures that give no relief overnight contribute to a new environmental impact that local plants have not had time to adapt to.
Kimberly McCue, chief science officer at the Desert Botanical Garden, echoed that sentiment and said the extreme temperatures combined with a significantly dry monsoon this year are “far beyond what these plants would normally experience.”
One adaptation of native plants to help them survive hot and dry conditions is to perform part of their photosynthesis at night. They will store water in their pores throughout the day, and when cold temperatures arrive at night, their pores will open to absorb carbon dioxide to provide sustenance. However, as nighttime temperatures rise, plants may lose more carbon and water than they can absorb, according to McCue and Martin.
“The leaves are yellowing and falling off, the plants are dying, they’re starving to death,” Martin said.
‘Absolutely unnatural’: local plants show distress
In July, two Saguaro cactus trees collapsed at the Desert Botanical Garden, one large and one small. An emblem of the Arizona desert, the well-adapted, drought-tolerant saguaro cactus can live up to 200 years and grow 40-60 feet tall.
Four large garden saguaros also shed their arms over a two-week period.
“This is not normal at all; a healthy saguaro wouldn’t just drop its arm,” McCue said. “This tells us that these plants are under severe stress or are at risk.”
In an attempt to find an explanation, the Mako team looked to the summer of 2020, which was also a record year for heat.
“Some saguaros had already been compromised since the summer of 2020, and then the summer of 2023 hit them, pushing them over the edge,” Maco said.
She said the Desert Botanical Garden has received reports from people in the community that the stems on their cactus plants are falling off. The garden’s cacti also “are showing these high levels of stress,” which is evident by their yellowing and bending toward the ground.
Kayla Daniel, a sales representative at Verde Valley Nursery in Fountain Hills, said many of the mesquite trees, elm trees, oaks, agaves and aloe trees died, and that there were “a lot more deaths this summer than last summer.”
As for customers, she said most people have problems with sunburn on their plants. Overwatering as a means of fixing it can harm or kill the plant.
“For most native desert plants, they can get sunburned. Then, once they get some relief, they can start growing back normally,” Daniel said. “Overwatering is more of a problem than sunburn.”
Danielle said she recommends using an umbrella rather than giving more water to distressed plants.
Aspiring to genetic diversity, alleviating the heat island
After thousands of years of evolution, native plants are now struggling under a new wave of environmental conditions, but there is hope that genetic diversity and community efforts will help.
In 2020, teams at the Desert Botanical Garden were deployed outside the urban heat island bubble and into the desert landscape to collect seeds from different wild populations of saguaros — some of which thrive in naturally hot temperatures — and plant them back in the garden.
“The key to adapting to changes in the environment is having genetic diversity in your population,” McCue said. “We are trying to build resilience in our saguaro population.”
As temperatures start to drop, they’re starting to see green growth pick up, Melissa Cross-Peebles, coordinator of Arizona State University’s Garden Commons program, said in September.
“Desert plants kind of have a boom-and-bust cycle. They benefit when it’s really booming and wet. Then they can survive the bust times,” Cross-Peebles said.
This year is expected to be a wet El Niño year, so Cross-Pebbles said she hopes native plants can recover and regrow deeper roots in preparation for what could be another harsh summer.
If extreme summer temperatures return next year, Martin said evolution or change from plants is highly unlikely this soon, but as a society “we will have to implement cultural practices in the short term.” This could include shade protection, a little extra water, or a change in urban plant diversity that favors more heat-tolerant plants. Using a natural desert as an example and trying to preserve native plants in their original state can begin to mitigate the differences between Phoenix and the surrounding desert.
Phoenix needs to address the heat island effect, which would provide native plants with a more stable environment and mitigate heat waves like those that will occur in 2020 and 2023, Martin added.
“I think most, if not all, of the people who move here are really starting to embrace the exclusivity of the desert, and they love the saguaros and the agave,” McCue said. “There’s a lot of incentive to care about what we have.”
Sign up for CRONKITE DAILY to follow the latest news. Participate