When papers report on heatwaves, they usually accompany the text with a photo of people frolicking in fountains, enjoying the beach or just generally having in fun. The reality is different. For example, it’s estimated that extreme heat claimed more than 500 lives in southern Nevada last year alone, which is why Juan Henao and colleagues have been looking at a possible solution for Las Vegas. They’ve examined how planting street trees may help cool the city, as well as the water tradeoffs inherent in growing trees in a desert.
What Henao and his team discovered challenges some common assumptions about urban trees as a climate solution. Even drought-tolerant species face physiological limitations that restrict their cooling potential in desert environments. This has implications for cities worldwide as temperatures continue to rise.
The study, published June 17 in Environmental Research: Climate, uses advanced computer simulations of the city to conduct a detailed analysis of how trees impact temperatures. Led by the Desert Research Institute‘s Juan Henao and John Mejia, the research found that street trees can create small oases from the desert sun by creating shaded spots that are nearly 17 degrees Celsius cooler than their surroundings. A large effort to plant drought-tolerant tree species could also cool citywide nighttime air temperatures by around 2 degrees Celsius.
Henao and colleagues found the benefit for daytime temperatures is limited due to the trees’ adaptations to the dry desert air. In temperate climates, trees can cool cities through transpiration, drawing water from the soil, through the tree, and releasing it through the leaves. Desert trees, however, are adapted to conserve water which is obviously a much more scarce resource in the desert. A 2023 comparative study between Munich, Germany (temperate) and Beer Sheva, Israel (hot arid) found that transpirational cooling was about 40% less effective in hot, dry climates.
“I think there is some consensus, and we confirmed it here, that in hot, arid climates, shade is the most important benefit of trees, and not necessarily the cooling they provide by transpiration,” Henao said. “Whereas in other regions, the transpiration is a very important factor as well.”
The research is based on computer models with Henao being part of project to create digital twins of cities like Las Vegas, Phoenix and Houston, with goal of modelling climate mitigation strategies. Currently there are limitations to this method, with the modelling limited to single species planting, which would be different in the real world. But this approach has also identified some differences between species. For example, in another experiment, they found that Cherry trees offered a more significant air cooling benefit, more than three times that offered by Bur Oak. The flip side of this is that the Cherry trees also required three times more water to produce this cooling effect. But there’s more to the positive effect of trees than air temperature.
“Trees can really improve our thermal comfort, because when we go under a tree, we can feel the difference,” said Juan Henao, Postdoctoral Researcher in Atmospheric Modeling at DRI. “But this comfort is due to more than temperature — we’re also feeling the difference in the amount of solar radiation that is reaching us. I think one important finding of this research is that air temperature is not the only variable that matters.”
Another benefit of trees is that people like seeing them, something Henao acknowledges. “I know that I prefer to see trees, and they can help store carbon. We just need to remember that in order to cool the air, they need to release water vapor, and we need to give them enough water to do that. Any hot, dry city will need to consider these tradeoffs and really do their research to identify the right species for planting efforts.”
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Henao, J.J., Mejia, J.F., Krayenhoff, E.S., Jiang, T. and Martilli, A. (2025) “Effectiveness of street trees in reducing air temperature and outdoor heat exposure in Las Vegas,” Environmental Research: Climate. Available at: https://doi.org/10.1088/2752-5295/ade17d
