The heat proves too much for even the hardy desert plants, leading to their demise.

LAS VEGAS — On a sun-dappled stretch of West Charleston Boulevard, Norm Schilling parked his truck on the side of the road just to check out his favorite tree.

Schilling, a local horticulturist and owner of a landscape company and garden shop called Mojave Bloom Nursery, rescued the African sumac decades ago after its branches froze and died during an unusually frosty winter. Careful pruning helped the tree survive, but this summer, it faces a new danger: Months of intense heat have dried out the branches, causing the droopy leaves to die in clumps.

This is a seemingly counterintuitive question: the Southwest is accustomed to sweltering heat, and desert plants and trees are drought- and heat-tolerant. Dry, harsh environments are exactly where desert plants and trees thrive.

But as climate change makes heat waves more frequent, intense, and long-lasting, experts say increasingly harsh conditions are testing some iconic desert plants known for their resilience, including saguaro cacti and agaves.

“This summer we’ve seen damage to plants that previously didn’t show heat stress,” Schilling said.

As we drove through Las Vegas, he pointed out the results.

A magnolia shrub in a quiet residential neighborhood was sunburned, its shiny leaf tissue bleached and damaged in places by the sun. On another street, two mulberry trees were dying, likely because they weren’t getting enough water to survive the heat. Around the corner, a large juniper tree was showing signs of “severe decline,” Schilling said, with brown, dead leaves still hanging from its dead branches, evidence that the heat damage was recent.

“That juniper is probably close to 40 or 50 years old. It’s a magnificent tree, but it will soon die,” he said, patting and kissing its rough trunk.

Then, a few blocks away, there was a row of succulents known as gopher spurge, parts of which looked burnt, branches yellowed with dried sap splayed out in all directions.

“This species is very dependable and very common throughout the valley,” Schilling said, “and some of the plants here are getting to the point where they’re not likely to recover.”

Las Vegas has already broken several heat records this summer, including the hottest day on record when temperatures reached 120 degrees Fahrenheit on July 7. Then, seven consecutive days of temperatures above 115 degrees Fahrenheit were recorded. For most of June, July, and August, temperatures remained in the triple digits with little cooling at night.

“The heat we’re seeing right now is a new paradigm. It’s like the ground is shifting beneath our feet,” Schilling said.

Ecologists across the Southwest are studying how different species respond to the annual heatwaves, trying to understand how hot is too hot for desert plants and trees.

Kevin Hultin, director of research at the Desert Botanical Garden in Phoenix, studies the effects of heat stress on ecosystems in the Sonoran Desert. 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 its worst years-long drought and hasn’t slowed down.

“The summer of 2020 was the hottest on record until last year, and we saw a lot of deaths,” Hultin said. “We’ve been seeing deaths ever since, and we’ve seen an accelerated pace of deaths in 2023.”