The Summary
- Increasingly frequent and severe heat waves in the Southwest are damaging some desert plants known for thriving in harsh conditions.
- Saguaro cacti and agave have both suffered in sweltering weather this summer and last.
- Ecologists are working to figure out how various species respond to the persistent extreme heat and pinpoint how hot is too hot for them.
LAS VEGAS — On a sun-parched stretch of West Charleston Boulevard, Norm Schilling pulled his truck over to the side of the road just to visit his favorite tree.Schilling, a local horticulturalist who runs a landscaping company and owns a garden shop called Mojave Bloom Nursery, saved this African sumac decades ago after an unusually frosty winter caused its branches to freeze and die off. With careful pruning, the tree pulled through, but this summer, it’s facing a new danger: months of oppressive heat that have dried out branches and caused dieback in clusters of its drooping foliage.
It’s a seemingly counterintuitive problem. The Southwest is no stranger to sweltering conditions, and desert plants and trees are drought-resistant and heat-tolerant. Arid, harsh environments are where they thrive.
But as climate change makes heat waves more frequent, intense and long-lasting, experts say the increasingly severe conditions are testing some iconic desert plants known for their resilience — including saguaro cacti and agave.
“We saw damage to plants this summer that had never showed heat stress before,” Schilling said.
On a drive around Las Vegas, he pointed out the consequences. A mock orange shrub on a quiet, residential block was sunburned, with damaged spots where the sun had bleached the tissue of its shiny leaves. Down another street, a pair of mulberry trees were drying out, likely because they had not gotten enough water to cope with the heat. Around the corner, a large juniper tree showed signs of “severe decline,” Schilling said, with dead, brown leaves still adorning withered branches — evidence that the heat damage was recent.
“That juniper is probably close to 40 or 50 years old. It’s a magnificent specimen, but she’s going to die,” he said, then patted and kissed its rough trunk.
Then there was the array of succulents known as gopher spurge a few blocks away, in which some appeared burnt, their sapped and yellowed branches splayed in all directions. “This species is super dependable and it’s a very common species across the Valley,” Schilling said. “Some of these plants here are just at the point where they’re not going to recover.”
Las Vegas has smashed several heat records already this summer, including its hottest day in recorded history on July 7, when the temperature reached 120 degrees Fahrenheit. A record streak of seven straight days at or above 115 F followed. Temperatures for most of June, July and August stayed in the triple digits, with little relief even at night.
“The heat we’re seeing now is a new paradigm. It’s like the ground is shifting beneath our feet,” Schilling said.
Ecologists across the Southwest are working to figure out how different species respond to the onslaught of heat year after year and just how hot is too hot for desert plants and trees.
Kevin Hultine, director of research at the Desert Botanical Garden in Phoenix, studies the consequences of heat stress on the Sonoran Desert ecosystem. He and his colleagues have been tracking an uptick in saguaro cactus mortality that began in 2020 — when the state was in the midst of a severe, yearslong megadrought — and has not let up.
“Summer of 2020 was our hottest on record until last year, and we saw a major die-off,” Hultine said. “It has been constant ever since, and then obviously 2023 increased the pace even more.”
Last year, when Phoenix endured its hottest summer in recorded history — with a record 31 straight days of temperatures at or above 110 F — stark images emerged of saguaro cacti that had dropped their signature arms or tipped over completely.This year has not brought much relief: July was the city’s second-hottest on record, and Hultine said some cacti and agave have been struggling, though the situation has not been quite as dire.
Hultine said he is still trying to pinpoint the factors to blame for the saguaro die-off, including whether the bigger risk comes from individual heat waves or the cumulative effects of multiple extreme summers. With long-lived plants like cacti, the full scope of the problem is often not immediately apparent.
“It’s possible that we’re looking at this exponential curve of mortality, but it could take years before we know,” he said.
Schilling said a major issue may be repeated exposure to extreme heat: The Southwest is no longer seeing many cooler years in between record-setting ones, and temperatures stay high at night, leaving plants and trees less time to recover and bounce back.
Krista Kemppinen, a senior scientist at the Center for Biological Diversity in Tucson, Arizona, said the double whammy of extreme heat and drought together is what’s really affecting the desert ecosystem.Although it's unlikely that saguaros are headed for extinction, she said, the heat stress observed so far is a serious concern.
“They’re one of the hardiest plants in the desert — they are very robust,” Kemppinen said. “The fact that they seem to be responding so negatively to extreme heat events and to climate change is particularly worrisome.”
Saguaros get a lot of attention because they are a beloved symbol of the Sonoran Desert, but across the region, there are hundreds of lesser-studied species with smaller populations that are likely to be even more negatively affected as climate change intensifies.
The heat stress and die-offs observed in recent years is enough to make horticulturalists like Schilling reconsider what they thought they knew about desert ecosystems.The changing landscape has also motivated Schilling to try to raise awareness about climate change with his clients and in his community, and to work with local nonprofits and government agencies to boost climate resilience across the Las Vegas Valley.
“We need to figure out how we exist in this environment and make it not only a healthy place to live,” he said, “but also one that is beautiful and sustainable and nourishing to our soul.”
