Monday, Nov. 25, 2019 | 2 a.m.
The Southern Nevada Water Authority (SNWA) maintains a list of more than 500 species of trees and plants adapted to the region’s harsh desert climate. Drought tolerance, water use, growth rate and other qualities associated with each species are included on the list, helping municipalities and property owners make responsible landscape choices.
Soon, the water authority might add another plant quality to the list: climate resiliency.
Las Vegas has warmed faster than any other place in the country, with average temperatures having risen more than 5.8 degrees Fahrenheit between 1970 and 2018, according to an April report from the nonprofit Climate Central. The projected consequences of that trend are far-reaching, and the region’s plants and trees—important cooling tools themselves—could be affected.
Using information from an American Horticultural Society analysis on plants’ climate resiliency, the water authority has identified six trees and 10 other plants commonly found in the Las Vegas Valley that could be under great heat stress within the next 40 years.
“Nobody is necessarily saying that these plants are certain to perish, but we do know that they could be under increasing stress for a period of time, and it’s definitely something we should be aware of,” said Doug Bennett, conservation manager at the SNWA.
Heat stress happens when plants grow in environments outside of their preferred heat zones, the temperature ranges in which they thrive. The farther they move outside their heat zones, the greater stress they experience, Bennett said.
Some non-native species commonly found in the region, such as the purple leaf plum tree, already operate outside those zones and would be even weaker in a warmer climate. The purple leaf plum fares poorly in the heat and typically doesn’t live long in Southern Nevada, Bennett said.
“It’s a good example of what happens when you put in a plant that’s not really well-adapted to the climate,” he said.
Another type of tree that could suffer in the warming climate within the next 10 years is the ash tree, ubiquitous in the Valley and beloved for their shading capabilities but poorly suited overall for the desert. The same goes for elm trees, such as lacebark elms, Bennett said.
By around 2055, the SNWA expects to see Afghan pines struggle as well. Native to mountainous areas of Afghanistan and the Middle East, these evergreen trees populate housing developments and streetscapes across the Valley, Bennett said.
“It’s really well adapted to desert climates, but obviously it needs help when you move it onto the desert floor,” he said.
While climate change is a factor for plants and trees in Las Vegas, another related phenomenon poses a similar threat: the urban heat island effect, in which heat-absorbing paved surfaces drive up urban temperatures.
This has likely contributed to the dramatic rise in temperatures here, according to climate scientists. UNLV Life Sciences professor Dale Devitt sees it as a more significant stressor than climate change.
“Trees typically like nighttime temperatures to be a lot cooler to reduce rates of respiration relative to what the plant is using and needs for growth,” Devitt said.
Warmer nighttime temperatures caused by the urban heat island effect could speed up trees’ respiration and photosynthesis rates at night, escalating their growth. The resulting taller trees require more water, something Southern Nevada will likely receive less of in the future because of climate change, Devitt explained.
“So the architecture of the trees will change—typically, you will get taller trees with smaller root systems and fairly large canopies—and they’d become more vulnerable to oscillations in water vulnerability,” he said.
The good news is, some trees in the Valley are climate-resilient and will even thrive under climate projections, both Devitt and Bennett noted. Palo verde trees, which produce bright-yellow, flowerlike leaves, as well as the purplish desert willows have high heat tolerances and will likely stick around for generations, Bennett said.
Some plants that haven’t made it through cold Las Vegas winters could also see a resurgence.
“We have certain plants we weren’t able to grow in this Valley that would get frozen back by low winter temperatures,” Bennett said. “There’s some potential to bring plants on at the other end of spectrum.”
Revising the agency’s list to include a climate resiliency metric would help people make informed decisions about which trees and other plants constitute the best long-term investments in a warming world, Bennett said. The SNWA hasn’t decided if and when it will add climate resiliency to the list—there are many stakeholders involved in that process—but officials agree that the information should be shared with the public, Bennett said.
At the same time, the goal is not to scare people, as many of the effects of rising temperatures on trees will be marginal until mid-century, said Bronson Mack, spokesperson for the SNWA.
“We’re just trying to anticipate what’s coming, so our community can continue to stay ahead of the curve,” Mack said.
This story appeared in Las Vegas Weekly.