Lisa Ortega / Courtesy
A dying Afghan pine tree rises above the ground at Las Vegas’ Maslow Park. Trees in urban settings have shorter life expectancies, and planting tomorrow’s urban forests in a rapidly warming desert makes it difficult to pick trees that can survive those conditions.
Friday, Nov. 10, 2023 | 2 a.m.
In Nevada, millions of dollars are being spent to plant trees to help cool the concrete jungle, provide shade and help clean the air.
The question is, can the trees survive?
Lisa Ortega, executive director of Nevada Plants, a tree-planting advocacy group, said there already was evidence that some around Las Vegas cannot — at least not without a lot of help.
And rising temperatures in the already-sweltering desert, coupled with the emergence of new varieties of pests, raise even more questions.
As she drove through east Las Vegas last week, Ortega immediately spotted alarming signs of trees in distress: 40-year-old pine trees leaning at 45-degree angles, dying branches and telltale pinholes in dry bark from pests.
A yard near Ferron Elementary School sports a towering, long-dead pine tree. Ortega knocked on the trunk to demonstrate how hollow it was.
She has tried to persuade the homeowner to apply for a grant to cover its costly removal, but the woman has declined.
To some desert dwellers, even a dead tree is better than no tree. “She said, ‘I want to leave it up; it’s the only shade I have,’” Ortega said. “And it’s dead. It’s gone.”
Along with providing shade, living trees filter out carbon, improve air quality and combat the urban heat island effect in which heat builds up where there is little greenery and mostly concrete and buildings.
Earlier this year, Nevada received a $15.7 million federal grant to plant trees. Among the recipients were the city of Las Vegas, the city of North Las Vegas and UNLV’s Las Vegas Urban Forest Center.
The U.S. Department of Agriculture’s I-Tree website lets users calculate how much carbon trees can remove from the air.
They do the best when they grow large and are at the prime of their life, Ortega said.
But trees in urban settings have shorter life expectancies, and planting tomorrow’s urban forests in a rapidly warming desert makes it difficult to pick trees that can survive those conditions.
The U.S. Global Change Research Program’s National Climate Assessment is a report delivered to Congress and the president every four years. The 2018 report (the 2022 report is still in development) projects that the Southwest’s annual average temperature could increase by 8.6 degrees by 2100. Those higher temperatures would drive megadroughts that would persist through periods of heavy rain or snow.
“The pines and the ash are the ones the (Southern Nevada) Water Authority says aren’t going to make it in the hotter temperatures,” Ortega said.
According to a study from the tree-care website Tree Vitalize, Afghan pine and velvet ash make up 18% and 14%, respectively, of Las Vegas trees surveyed.
The 10 most common trees in the Las Vegas sample accounted for 65% of the bunch, which Tree Vitalize founder Fern Berg said was high compared with most cities.
The Southern Nevada Water Authority lists Afghan pines as a “good desert pine” and lists velvet ash as native to the region. Both ranked at two out of five stars for climate resiliency.
“If I was to suggest one thing, it would be increasing species diversity, and the reason for that is because it’s going to create a much more sustainable urban forest,” Berg said.
Berg said tree canopies dominated by only a few species are more susceptible to disease and pests.
For example, the emerald ash borer has torn through Denver’s emerald ash population.
In Las Vegas, a few varieties of pine beetles, including the relatively recent Mediterranean pine engraver beetle,have attacked older trees, Ortega said.
Pests rarely kill trees on their own, but a tree under stress from heat or pollutants can’t defend itself as well, she said.
“They are rampant, and we don’t even know how much,” Ortega said.
The 2022 Tree Vitalize study used a data set of 5 million trees in 63 U.S. cities. For the Las Vegas metro area, the study included about 39,194 individual trees from 199 locations, including city parks, trails and facilities, school properties, road medians and golf courses.
Brad Daseler, urban forester for the city of Las Vegas, said the city was researching which trees grow best in desert climates as it replenishes 45,000 or so trees on city property.
“If we think a tree is adept to our climate, we’ll experiment, but a big focus of ours is having a diverse composition,” he said.
Desert willows, less common varieties of eucalyptus and oak trees now join shoestring acacia and red push pistache in city parks.
“We don’t know what’s going to be successful. We don’t know what kinds of pests to expect,” Daseler said. “We try not to lean too hard on any one species.”
The drip irrigation equipment buried at each city tree’s roots would be expensive to dig up and replace, and any major changes to a tree’s surroundings could hurt it, depending on the species.
Daseler said drought-tolerant species will only remain drought tolerant if they’re properly irrigated.
Irrigation at a tree’s root ball only works for the first few years, Daseler said, but the tree will outgrow it eventually.In most cases, he said, those undersized systems under older trees haven’t been replaced, leaving the mature tree underwatered for the rest of its life.
“For our projects, we’re trying to anticipate the mature size of each tree and use irrigation systems that can support that,” he said. “If we don’t anticipate that full size, we’re going to get stuck in this loop.”
Ortega said there’s also no accounting for what homeowners choose to plant.
Native acacias, mesquites and palo verde trees are all suited to the climate, but they are gnarly, thorny plants a homeowner might not choose if they have children or pets.
“Everybody’s planting Chinese pistache, and I do worry, are we setting ourselves up for the next bug?” Ortega said.
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