On a still-hot summer evening, a group of volunteers is digging holes in a Hermosillo cemetery, but far more shallow ones than the graves beside them.
They’re part of an organization called Caminantes del Desierto, which is working to reforest the desert capital of the Mexican state of Sonora, directly south of Arizona, to provide some relief from the scorching heat.
“There’s a lot of stigma about coming to the cemetery," said volunteer Ximena Velasquez. But Velasquez sees these sprawling, quiet acres of white stone mausoleums and crosses as a refuge from the traffic and hubbub of Hermosillo.
At least six people have died from heat stroke in Sonora this year, and experts say the city of Hermosillo needs to find ways to cool down. That’s especially true during the summer, when temperatures reach over 100 degrees Fahrenheit for much of the season. As the climate warms globally, Hermosillo is digging into a relatively simple solution to help solve its urban heat problem: trees.
In addition to city and state tree planting initiatives, those led by community volunteers like Caminantes del Desierto have also taken root in recent years.
“Trees are the most simple way to mitigate heat,” said Sergio Müller, a conservationist and leader of Caminantes del Desierto.
To change the city’s infrastructure — like roadways and buildings — is much more expensive, Müller said. Those investments also degrade over time, whereas a tree gains value as it grows. The group has planted more than 1,000 trees since 2019, including hundreds at this cemetery.
There’s evidence that their strategy is working, Müller said.
“The heat maps for this area used to be in red and orange,” Müller said. “Now, they’re in yellow and orange.”
It’s easy to make an argument that Hermosillo needs the shade trees provide. Vast swathes of heat-absorbing surfaces, like asphalt and concrete, have continued to fan out, as the growing city’s roadways and relatively low buildings have expanded into the desert. That has also expanded the urban heat island effect, experts say.
But it’s not just that trees provide shade, said research professor Luis Alan Navarro with El Colegio de Sonora. They also release water vapor into the air, which cools the environment.
Having vegetation nearby — or even just grass, rather than synthetic turf — can noticeably cool the area, Navarro said.
“You can feel the freshness like a cool towel,” Navarro said.
Public officials have caught on. The state of Sonora recently dedicated a portion of land it owns in the city to urban forestry. The city of Hermosillo says it’s planted more than 113,000 trees since 2021.
Navarro warns it’s not enough to just plant trees. Especially in their infancy, the city’s new saplings need care, including well-functioning irrigation systems. Navarro said it’s especially important that the trees be native to this environment and more likely to withstand the harsh desert.
Native tree renaissance
Over the past two decades, Navarro has seen native plants have a renaissance in Hermosillo.
“I remember when they were totally rejected,” Navarro said. “No one wanted native species. They were even considered weeds.”
Trees like mesquites and palo verdes were once cast aside for tropical species, which require significantly more water, Navarro said.
But as years of drought have put strain on the city’s water resources, public officials and the community have started to embrace plants better adapted to the desert.
“The most significant cost to a city located in an arid climate is water,” Navarro said. “A species that has evolved and grown in an arid environment is adapted to that environment. So its demand for water is much less.”
The next frontier, Navarro and Müller say, is to make more types of native trees available. Many of Hermosillo’s private nurseries tend to stock imported species of plants, rather than focusing on native species, Navarro said.
The lack of options means many of the same few types of native tree species are used over and over again, which Navarro says makes the city’s greenery as a whole less resilient against extreme weather and illness.
“There’s an enormous gap in the domestication of these species,” Navarro said. “There are very few plants available.”
Caminantes del Desierto has been working to fill that gap, by cultivating less readily available native species at a public nursery with Universidad de Sonora.
Back at the cemetery, volunteers use pickaxes to scrape shallow trenches for the thin black tubes that will bring water to each of the 10 new native palo colorado saplings they’re planting. It’s part of an irrigation system that waters the hundreds of trees Caminantes del Desierto has planted in the cemetery over the years.
“Part of the goal is for people to value native plants,” Müller said.
For now, the knee- to waist-high saplings are shorter than most of the grave markers that have become their new neighbors.
Volunteer Ximena Velasquez carefully lowers one of the saplings into the ground.
“It’s really gratifying,” she says as the sun begins to set and the heat of the day starts to dissipate. “To end the day feeling like you’ve helped.”
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