LAS VEGAS - On a sun-scorched stretch of West Charleston Boulevard, Norm Schilling pulled his truck over to the side of the road to visit his favorite tree.
Schilling, a local landscaper who runs a landscaping business and garden shop called Mojave Bloom Nursery, saved this African sumac decades ago after an unusually cold winter caused its branches to freeze and die. With careful pruning, the tree survived, but this summer it faces a new threat: months of oppressive heat that have dried out branches and caused its drooping leaves to die in clusters.
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. Dry, harsh environments are where they thrive.
But as climate change makes heat waves more frequent, intense and longer lasting, experts say the increasingly harsh conditions are testing some iconic desert plants known for their resilience, including saguaro cacti and agave.
"We saw damage this summer to plants that had never experienced heat stress before," Schilling said.
During a drive through Las Vegas, he pointed out the consequences.
A pale orange bush on a quiet residential block was sunburned, with damaged areas where the sun had bleached the tissue of its glossy leaves. Down the street, a few mulberry trees were drying out, probably because they hadn't been watered enough to cope with the heat. Around the corner, a large juniper tree showed signs of "serious decline," Schilling said, with dead, brown leaves still adorning its withered branches - evidence that the heat damage was recent.
"That juniper is probably almost 40 or 50 years old. It's a beautiful specimen, but it's dying," he said, stroking and kissing the rough trunk.
Then there was the collection of succulents known as gopherspurge a few blocks away. Some of them looked burnt, their softened and yellowed branches drooping in all directions.
"This species is super reliable and it's a common species in the valley," Schilling said. "Some of these plants here are just at the point where they're not going to recover."
Las Vegas has already broken several heat records this summer, including the hottest day in history on July 7, when temperatures reached 120 degrees Fahrenheit. A record streak of seven consecutive days of 115 degrees Fahrenheit or higher followed. Temperatures remained in the triple digits throughout June, July and August, with little light 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 in the southwestern United States are trying to figure out how different species respond to the heat year after year and how hot it is for desert plants and trees.
Kevin Hultine, director of research at the Desert Botanical Garden in Phoenix, studies the effects of heat stress on the Sonoran Desert ecosystem. He and his colleagues have been tracking a surge in saguaro cactus mortality that began in 2020 - when the state was in the midst of a severe, multi-year megadrought - and has not stopped.
"The summer of 2020 was the warmest on record until last year, and we saw a lot of mortality," Hultine said. "It's been consistent since then, and then 2023 has obviously accelerated the pace even further."
Last year, when Phoenix experienced its hottest summer on record - with a record 31 straight days of temperatures of 110 degrees Fahrenheit or higher - shocking images emerged of saguaro cacti that had lost their signature branches or collapsed completely.
This year hasn't brought much relief: July was the second-hottest July on record in the city, and Hultine said some cacti and agaves were struggling, though the situation wasn't as dire.
Hultine said he is still trying to figure out the factors responsible for saguaro decline, including whether the greater risk comes from individual heat waves or the cumulative effects of multiple extreme summers. With long-lived plants like cacti, the full extent of the problem is often not immediately apparent.
"It is possible that we are dealing with an exponential curve of mortality, but it may be years before we know that," he said.
Schilling said repeated exposure to extreme heat could be a major problem: In the Southwest, there are no cooler years between record years anymore, and temperatures remain high at night, giving plants and trees less time to recover.
Krista Kemppinen, a senior scientist at the Center for Biological Diversity in Tucson, Arizona, says the double whammy of extreme heat and drought is having real consequences for the desert ecosystem.
While it is unlikely that saguaros will be driven to extinction, the heat stress observed so far is a serious concern, she said.
"They're one of the hardiest plants in the desert - they're very robust," Kemppinen said. "The fact that they seem to respond so negatively to extreme heat and climate change is particularly concerning."
Saguaros get a lot of attention because they are a beloved symbol of the Sonoran Desert. However, the region is home to hundreds of less-studied species with smaller populations that are likely to be even more affected by increasing climate change.
The heat stress and deaths observed in recent years are causing horticulturists like Schilling to rethink their knowledge of desert ecosystems.
The changes in the landscape have also prompted Schilling to raise awareness about climate change among his clients and the community, and to work with local nonprofits and government agencies to build climate resilience in the Las Vegas Valley.
"We have to figure out how to survive in this environment and make it not just a healthy place to live," he said, "but also a beautiful, sustainable, nourishing place for our souls."
This article was originally published on NBCNews.com
