Earth is hotter than it has been in 125,000 years, scientists say, and Las Vegas continues to break temperature records.
The extreme heat claimed more than 500 lives in southern Nevada last year alone, and scientists and city officials are clamoring for solutions. Planting and preserving the city’s street trees is one method that brings many benefits, from the cool air of their shade to their ability to store carbon. Now, a new study takes a deeper look at just how much trees can offer Sin City, as well as the water tradeoffs inherent in growing trees in a desert.
The study, published in Environmental Research: Climate, uses advanced computer simulations of the city to conduct a detailed analysis of how trees impact temperatures. Led by DRI’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 30 degrees Fahrenheit cooler than their surroundings. A large effort to plant drought-tolerant tree species could also cool citywide nighttime air temperatures by around 3 degrees Fahrenheit, while the benefit for daytime temperatures is limited due to the trees’ adaptations to the dry desert air.
«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.”
Las Vegas’ location in the middle of the Mojave Desert means that trees are rarely native away from natural waterways, so identifying trees that offer cooling benefits without requiring significant water is important for the region. Detailed computer simulations like the one used in this study offer a way to examine how different tree species will perform under the local climate and can inform urban forestation efforts.
Research in other cities has shown a larger citywide air temperature decrease resulting from transpiration, or the way that trees release water vapor from leaves (the process is somewhat similar to sweating in mammals). However, in the extremely dry desert air, the study found that many trees will close their stomata, or the pores of their leaves, to conserve water; this limits their air-cooling benefits beyond shading. The finding is consistent with other research that found hot, dry areas receive 40% less evaporative cooling from trees than more temperate environments.
“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 water transpired from leaves is first taken up by the tree’s roots, making tradeoffs for water conservation efforts important to consider. The study examined different tree species and found that Cherry trees offered a more significant air cooling benefit, more than three times that offered by Bur Oak, with a modest daytime cooling of about 0.7 degrees Fahrenheit. However, the Cherry trees also required three times more water to produce this cooling effect.
The study authors note that the study is limited to the computer simulations conducted, which examined citywide forestation of one tree species at a time. The research can offer insight, however, into the benefits and tradeoffs of urban street trees in the hot and arid climate of Las Vegas. Knowing that the largest benefit of urban street trees is their shading can help tree-planting efforts prioritize locations like high-traffic sidewalks, bus stops, and other sunny places with pedestrians.
«Urban trees are not a silver bullet for cooling our cities, particularly for desert cities like Las Vegas,” Henao said. “But they provide significant shade and of course other benefits. 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.”
“Street trees are an important part of the solution to urban overheating,” said John Mejia, climatologist at DRI who co-led the study with Henao. “However, to equip practitioners with a more comprehensive set of tools, our ongoing research is also investigating a wider range of heat mitigation strategies, including reflective materials for rooftops, walls, and pavements; green roofs; and improved energy efficiency in buildings.”
