In Dallas as in Phoenix, people look to trees for relief from urban heat islands
The Texas Trees Foundation plants trees in a Dallas neighborhood as part of a campaign to increase shade and decrease the effects of urban heat islands. Patrick Breen/azcentral.com
DALLAS — Few neighborhoods in America need trees more than South Oak Cliff, a sun-blasted shade desert in this rapidly warming Texas city.
Like much of Phoenix, the south-side Dallas district lacks sufficient shade for comfort during summer and safety during heat waves, when temperatures can climb as much as 11 degrees higher than parts of the city where the shade cover is greater.
Despite South Oak Cliff's leafy name, big shade trees are relatively scarce along its streets. In this part of Dallas, the canopy, a measure of areas covered by trees, is one-third as dense as it is in the city overall, and neighbors say the trees that do exist are aging and falling over in storms.
Dallas ranks just behind Phoenix on the list of fastest-warming urban heat islands, metropolitan areas where heat-absorbing concrete and asphalt push temperatures higher during the hottest months.
These islands of urban development contribute to a steady rise in temperatures and can intensify heat's deadly effects, focusing the worst of it on inner cities and neighborhoods where residents can least afford to seek relief.
Although it sounds simplistic, the common prescription for such overheated areas is more trees, which create shade to block the heat that city streets and buildings gather and hold well into night.
So when volunteers peeled off their sweatshirts to plant curbside trees a block east of a freeway here on an unseasonably warm 75-degree day in December, Josephine McGee had to know if there were more trees where those came from.
She pulled over on her drive home and asked the crew from the Texas Trees Foundation how she could get a tree in the corner of her yard down the street. They gave her a paper to sign and said they would be back in the spring with a tree for her.
“It’s a filter for good air, and I like the shade,” she said. “If I’ve got good shade, it keeps me from burning up so much energy on the inside.”
Rising temperatures in urban areas
McGee lives smack in the middle of Dallas' urban heat island, where researchers say heat-absorbing concrete and asphalt raise summer temperatures about three-quarters of a degree Fahrenheit every decade.
In a study of urban heat, only two cities posted higher rates of warming compared with surrounding rural areas: Louisville, Kentucky, where the effect is amplified by a much greener countryside, and Phoenix. The study looked at rising temperatures attributed to the urban heat island.
In Phoenix, researchers found that urban temperatures are rising close to a degree per decade, adding to the effects of global warming. That’s an average, meaning some days in summer are much higher. And that means shading streets and sidewalks to cool the air is an ever-increasing imperative.
“Phoenix is warming at three times the rate of the planet,” said Brian Stone, director of the Georgia Institute of Technology’s Urban Climate Lab, where the urban heat study was conducted.
As the heat worsens, more people die. Heat was a factor in at least 150 deaths in Maricopa County during 2016, according to the county health department, and, in 2017, public health officials confirmed 129 heat-associated deaths, with 51 still under investigation. In Dallas, where experts say record-keeping is less robust, about 50 to 60 people are thought to have died in the exceptionally hot summer of 2011.
Trees are an immediate way to address heat at a local level. Stone produced a detailed study of heat by neighborhood for Texas Trees, and he said Phoenix and other cities need similar data to target their tree-planting efforts. It models the precise number of trees a neighborhood needs to maximize the cooling effect. The work costs about $100,000.
While Phoenix’s roughly 11 percent shade canopy is far skimpier than Dallas’, it is a percentage point or two better than the canopy in the South Oak Cliff neighborhood. Dallas benefits from the cooling respiration of more grass and leafier trees, and even its hottest neighborhoods appear greener than Phoenix's.
By blocking heat and preventing its absorption during the day, trees can help keep temperatures lower at night, when a heat island's effects are most profound. After the sun sets, the baked roads and structures are slow to radiate energy back into space, keeping temperatures from falling.
Phoenix now routinely experiences 90 degree summer nights, which were recorded only twice before 1970.
That’s also deadly: Maricopa County statistics show the daily rate of heat-related deaths nearly doubles to 1.8 from 1 when the night warms to the lower 90s from the upper 80s.
Planting trees to temper the heat
Scientists, city planners and neighborhood activists increasingly seek to plant trees as the best way to turn down the dangerous heat. The Texas Trees Foundation is heeding the call in an unprecedented campaign, aerially and precisely mapping suitable locations for 1.8 million new Dallas trees and focusing its efforts in areas that science shows will benefit the most.
The foundation paid Georgia Tech’s Stone to study the city’s heat zones in rare detail this year, using temperature gauges to measure near-ground air temperatures on a grid of 5,000 cells of about a square kilometer each.
This yields clearer information about a neighborhood’s actual feel than do the more common satellite images measuring surface temperatures, Stone said.
The sensors found relatively unshaded pockets of Dallas that are 11 degrees hotter than the city’s average during peak summer heat.
Those areas averaged highs of 101 degrees over five months of the year. The coolest areas averaged 90 degrees.
New trees could help shave up to 15 degrees from these hot zones on their hottest summer days, Stone found. Using established statistics for how much an uptick of 1 degree increases mortality of all kinds, he determined adequate shade could reduce deaths by 22 percent.
Shade cools people as they walk directly under it, but it also cools the neighborhood indirectly as the cooler air moves. Researchers at Arizona State University and elsewhere have determined that cooler air wafting around homes can bring down temperatures inside homes that lack air-conditioning.
Most large cities are warming at least twice as fast as the planet, Stone said.
“We consider that to be actually an encouraging finding,” he said, “because there’s lots that cities can do on their own to combat this.”
Devising plans to spread the shade
Besides planting trees, protecting existing trees is a priority, Stone said. Few places do this better than his school’s home city of Atlanta, which requires property owners to replace trees that they cut on an inch-for-inch basis or pay a fee. Atlanta has a nation-leading canopy shading 48 percent of the city.
Atlanta still experiences a warming heat island, though, because high-growth suburbs have not controlled their heat absorption.
Phoenix requires landscaping plans with certain tree requirements for common areas around housing developments or commercial sites, depending on the zoning. But single-family homeowners are free to remove trees without replacing them.
Dallas has a mix of requirements, but none that boosts shade in private yards like those in South Oak Cliff.
Volunteers planted 200 trees in the lower-income neighborhood on an early-December day, and Texas Trees plans 800 more in the spring. The corporate- and foundation-backed non-profit planted only on lots whose owners responded to messages hung on their doorknobs and agreed to water the trees as scheduled until maturity.
The neighborhood has less than a 10 percent canopy, compared with the city’s overall average of 29 percent. Citywide, Texas Trees wants to bump the shaded zone to 35 percent, which will take about 600,000 trees and, the group hopes, a new ordinance compelling developers to add trees.
Shaded properties save about 30 percent on energy consumption, foundation CEO Janette Monear said.
The need only intensifies as the planet warms, she said, though she has learned that invoking climate change won’t get her far in Texas. Instead, she explains that unshaded urban areas are unnaturally hot and that people who plant trees are helping themselves.
“It’s not a doomsday thing,” she said. “It’s stewardship.”
Without trees, neighborhoods bake
In Phoenix as in Dallas, scientists have determined that the barest, most heat-absorbing neighborhoods are at least 10 degrees hotter than the greenest neighborhoods in summer. The city of Phoenix has a stated goal to more than double its tree canopy to 25 percent, though that would require tremendous cooperation from private landowners.
The city and utilities encourage residents to plant desert- and drought-tolerant trees such as mesquites and palo verdes, or leafier species if they have ample water or live in flood-irrigated parts of the city.
“I don’t know of any city that’s ever tried to double its canopy,” Georgia Tech’s Stone said.
There’s no time to waste, said Aimee Williamson, executive director of the Arizona non-profit Trees Matter. She called trees “preventive care” that takes a decade or more to provide their greatest cooling benefit.
“We need to start getting more serious as the fifth-largest (American) city about how we’re going to make sure people aren’t overheating in the summer.”
Thousands backed an online petition this fall protesting a downtown Phoenix property owner’s plan to remove eight ficus trees that shaded a walkway and outdoor dining area at Renaissance Square, and to plant ornamental palms, likely with less shade.
It turned out several of the trees were dead or dying, but the owner was persuaded to replace them with 12 ash trees that should more than offset the loss.
“There’s not a lot of green space,” Williamson said of Phoenix, “so when there’s a shady space and someone is proposing to cut down trees and replace them with less shade, that’s a big issue.”
One tree at a time, the city of Phoenix is trying to increase its shade canopy to 25 percent by the year 2030. David Wallace/azcentral.com
Encouraging residents to plant trees
It shouldn’t take public outcry to protect such necessary shade on publicly accessible grounds, Williamson said. She is one in a group of advocates asking city planners for new rules protecting the scarce shade that already exists.
City staffers plan to complete a proposal for Phoenix City Council consideration this winter to protect more trees, said Alan Stephenson, the city's planning and development director. It would clarify rules to preserve trees in public rights of way regardless of ownership and would clarify what city code means by saying certain businesses and homeowners associations must “maintain” their public-area landscapes.
It also would permit residents to keep trees that hang out over a street if they provide at least 13 feet of clearance for recycling and garbage trucks and other service vehicles. Some residents who want to do their part in shading streets and sidewalks have complained that the current 18-foot requirement forces them to cut down valuable trees, Stephenson said.
The new rules won’t mandate tree maintenance or replacement at private homes, as Atlanta’s rules do.
Williamson’s organization contracts with Salt River Project to educate thousands of Valley residents who accept free trees from the utility every year. They favor planting in front of west-facing windows, where the potential energy savings are greatest.
Dallas has a tree-protection ordinance that requires a permit to remove mature trees on properties larger than 2 acres, and Texas Trees is pushing for stronger protections this winter.
The city has begun treating mature trees as expensive assets to protect during redevelopment projects, said James McGuire, Dallas’ managing director of environmental sustainability. A commercial development called Pacific Plaza will save millions of dollars by saving mature oaks that anchor tainted soils that would otherwise have to be trucked and dumped, he said, and their shade is part of a plan to ring downtown with a cooling arc of parks.
“We recognize they play an important role,” he said.
'Any tree helps'
The importance is growing.
Across Dallas, the urban heat island already adds several degrees beyond what the rural surroundings experience, University of Texas at Arlington climate scientist Arne Winguth found. During July 2011 — the region’s hot drought year — his team recorded a difference of about 6 degrees Fahrenheit.
Trees and cooler building materials and roofs could shave at least a couple of degrees from that, he said. That would provide significant health benefits if global climate change raises temperatures by more than that by the end of the century, as models project under the world’s current carbon emissions trajectory.
“This is of great importance,” he said. “Any tree helps.”
The Trust for Public Lands and the Nature Conservancy are aiding the Dallas effort, using a JPB Foundation grant that will also help Atlanta, Boston and Denver. The goal is to create safer neighborhoods for walking and biking, said Robert Kent, the trust’s north Texas director.
South Oak Cliff residents need safer walking routes to buses, he said during a break from planting there. He marveled at the “unheard of” warm December weather.
“I don’t think there’s any more denying that (climate change) is bringing hotter summers and hotter winters,” Kent said. “It’s time for our country to get serious.”
Several blocks away, Adriana Leal and her year-old son admired an Oklahoma redbud and an Eve’s necklace, two trees the volunteers had sunk in her yard that morning. The 7-foot redbud, now the lawn’s centerpiece, will double its height in a few years and shade an area about 15 feet across.
She and her husband pledged to water it, following guidelines for up to three times a week at the height of summer. They hope to make the yard safer for their five children and give them something to climb.
“They like to be outside all the time,” she said.
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