Denver looks to tree-planting to help shade city, add greenspace as heat islands grow
Denver leaders who for two decades have backed densification, paving over greenspace with concrete and asphalt to accommodate more people in the city, now are turning to trees for relief from worsening heat islands that amplify climate warming.
But urban ecologists and city officials say trees alone won’t be enough to keep Denver habitable as temperatures increase. They urge a far more ambitious expansion of greenspace.
“And concrete is definitely getting poured faster than we are planting trees,” city forester Mike Swanson told The Denver Post.
Heat islands — dense urban areas that are much warmer than their surroundings — have widened, data shows, with Denver emerging as one of the nation’s most “impervious,” or paved-over, cities. Older neighborhoods where houses have yards may be more resilient, researchers have found, because compared with redeveloped parts of the city, these landscapes don’t radiate as much heat.
Tree-planting “is going to be our focal point” for beating the rising heat, Denver Parks and Recreation planning director Gordon Robertson said, acknowledging the heat island problem. Doubled funding of $1 million will bolster tree-planting on public-access property and existing parks, concentrated downtown and in “low-equity” neighborhoods, Robertson said.
Creating open greenspace with trees, grasses and plants — instead of heat-radiating pavement — can help cool residents along sun-scorched streets, draw down heat-trapping carbon dioxide and clean the air.
However, Denver since 1998 has veered from its founders’ vision of being “a city within a park,” falling behind most other major cities in greenspace. Roughly 8% of Denver’s 155 square miles has been designated as parkland, compared with 21% in New York, 23% in San Diego, 15% in Minneapolis and 13% in Los Angeles, data from the Trust for Public Land shows.
This means climate warming hits harder here. And Denver has faced rising temperatures, a record 75 days above 90 degrees this year, with projections showing 20 to 35 days a year topping 95 degrees by 2050.
More greenspace needed
Denver climate office director Grace Rink has called for more “connected greenspace” to help mitigate heat islands.
And Denver voters repeatedly have demanded re-greening. In 2017, they passed a green roofs initiative requiring developers to install gardens atop new buildings, which city leaders later relaxed to allow alternatives such as funding off-site solar energy projects. In 2018, voters passed an open space tax to create more and better parks that raises $37 million a year.
Since then, city leaders have purchased a few parcels for new parks, covering less than five acres overall. They say they’ll acquire more land. Robertson said officials this month were negotiating to purchase 10 acres along Sand Creek.
Establishing new greenspace has proved difficult because well-funded developers compete for practically every parcel and City Council members have directed only a portion of the open space tax revenues for land acquisition — $20 million in 2018 and now about a third of that each year.
“And if we acquire one acre downtown, we blow through $10 million to $15 million,” Robertson said.
Denver imposes few fees on developers. There’s no requirement, as in other cities, to fund or install greenspace. While housing and commercial projects covering more than 10 acres in Denver must include some space designated as “open,” it needn’t be green.
The greenspace crunch reflects a worldwide challenge as United Nations demographers project 70% of the planet’s 9.7 billion people in 2050 will be living in cities, up from 55% now — straining habitability as temperatures rise. Denver’s population has increased by 47% since 1998, from 498,402 to 734,134, and density has reached 4,789 residents per square mile, census data shows, with projections for 857,000 people by 2040.
Heat islands jack up daytime temperatures, reduce cooling at night, worsen air pollution and exacerbate heat waves, Environmental Protection Agency scientists have determined. This contributes to heat-related deaths, general discomfort and illness including respiratory ailments, cramps, heat exhaustion and strokes, the EPA says.
Between 2004 and 2018, the Centers for Disease Control and Prevention recorded 10,527 heat-related deaths nationwide. Colorado health data showed 475 heat-related emergency room admissions statewide in 2018. Colorado Health Institute researchers have found people living in poverty, children, the elderly and those with diseases such as asthma suffer disproportionately.
“Tree-planting alone is inexpensive, and often politically palatable, but it is a drop in the bucket. … You’re not going to make a dent in the heat that way,” said Vivek Shandas, a Portland State University ecologist and member of Portland’s forestry commission, who co-authored a national study of heat dynamics in cities that was published this year in the science journal Climate.
“We see a green squeeze in most cities. We want to do dense development, put in more housing units, because we see more people moving in. … so there’s not available space any more. You’re left with this tiny little strip between the sidewalk and the street,” Shandas said.
Politics have pitted urban affordable housing advocates who support new construction in the belief this will lead to lower prices against climate action advocates who prioritize public health, ecological integrity and long-term quality of life in a hotter, drier world.
Meantime “there are all these developers maximizing profits by building over the entirety of lots,” Shandas said. “It’s very easy to take out greenspace and put in gray space. It’s harder to take out gray space and bring back greenspace — not only costly but politically challenging to take out a parking lot or a street,” he said, urging a rethinking of urban design organized around natural space.
The heat islands study measured surface and air temperatures in 108 cities including Denver and has found that radiation of heat from asphalt landscapes amplifies temperature by up to 20 degrees. In the relatively paved-over parts of Denver, summer temperatures measured 12 degrees hotter than in older neighborhoods with trees and lawns.
Average temperatures measured highest in areas once “redlined” by the government to restrict financing for disenfranchised residents, which now have been redeveloped, such as the apartment complexes northwest of Coors Field where trees cover 1.5% of the area and 70% of surfaces are paved. The study found that residents in South Park Hill, with a 25% tree canopy and 25% imperviousness, faced average temperatures more than 10 degrees lower.
Looking for relief
For computer software programmer Mike Lipsitz, who moved from New York to Denver seeking better opportunities to be outdoors cycling and walking his dogs, the Coors Field neighborhood offered luxurious living but he searched for greenspace. Last summer, he moved to Baker, south of downtown, where houses have yards.
“There’s more shade. Just walking from sun to shade, that’s the biggest difference. Now, we get more shade,” Lipsitz said. “This is definitely nicer. And, as for the virus, it’s nicer that we run into fewer people on walks than we did downtown.”
University of Colorado Denver biologist Diana Tomback, who studies forest ecology, walks her dogs in parks on the east side of Denver and has observed more people than ever flocking to parks during the pandemic. Tomback links this with mental health and human evolution as a species tied to nature, and doubted tree-planting alone can ensure habitability as the population grows.
“If we are concerned about heat islands, it is not going to be enough to do things like make people install rooftop greenery. We ought to have more open space,” Tomback said. “Parks are important, not only to counter the heat island effect but psychologically for people.”
At least, last-remaining riparian areas along rivers must be preserved, she said. “It is absolutely essential that we keep the greenspace along the South Platte River. It is a biodiversity corridor for wildlife.”
Developers with city support plan a “River Mile” complex for 15,000 residents with apartment towers up to 59 stories on the 62-acre former Elitch Gardens property in the Platte Valley. However, city parks officials indicated they’re engaged in a complicated challenge of encouraging innovative development while also protecting mental and physical well-being.
“What this challenge may mean is jack-hammering out un-needed parking lots,” Robertson said. “It’s not going to happen overnight.”
Large parks covering more than 150 acres, of the sort city leaders established in the 19th century, have been seen as politically unlikely under pro-development Mayor Michael Hancock, possibly with the exception of the 155-acre former Park Hill Golf Course, where a conservation easement purchased by Denver taxpayers blocks development. Westside Investment Partners paid $24 million for that property and plans development on up to 95 acres. City attorneys say the City Council has the power to change the easement.
“It is our hope that a portion of the Park Hill Golf Course becomes a significant park,” Robertson said.
Another option: expanding greenspace along the newly-established “shared streets” around the city, closed to vehicle thru-traffic during the pandemic, so that residents could walk safely outdoors maintaining physical distance.
Grassroots petitions have surfaced to make these permanent, and Robertson said he has proposed converting shared streets to greenspace by removing asphalt and planting native vegetation. “I would love to go in with water quality folks and really enhance these — plant trees.”
This would address an emerging challenge in the current tree-planting strategy for managing heat islands — simply finding enough space.
Concrete and asphalt is spreading as city crews, with many volunteers, work to establish more trees, said Swanson, the city forester. “This is increasing the heat islands. It eliminates tree-planting sites,” he said.
“It also will increase your storm-water bills. It will increase your heating and cooling bills. It creates more pollution. It makes for a sterile environment.”
Shade will help
Over the past few years, city crews have been planting about 6,700 trees a year (mostly oak, elms and Kentucky coffee trees), achieving survival rates as high as 90%. Their goal is to expand Denver’s overall tree canopy, which currently shades about 13% of the city, to 20%.
By blocking sunlight, trees help reduce the absorption of heat, and trees’ retention of and release of moisture cools surrounding areas, particularly when there’s wind.
Hotter, drier conditions threaten trees. City crews have been looking for heartier species that can endure extremes from traditional winter cold snaps to scorching hot summers.
The emerald ash borer beetle is spreading, threatening the ash trees that comprise 15% of Denver’s current canopy. By 2040, the paved-over surfaces that city records show covering 49% of Denver’s urban landscape may expand to 69%, and pavement covers 90% of newly-developed areas, University of Colorado urban design and land use professor Brian Muller and a team of researchers have determined.
Denver leaders have turned to tree-planting before to try to maintain quality of life. Former Mayor John Hickenlooper, who became Colorado’s governor and recently won a seat in the U.S. Senate, declared a “Mile High Million” goal in 2006 of planting 1 million new trees by 2025. Swanson said 260,000 have been planted.
While more trees won’t entirely offset heat island intensification of climate warming, shade will help, the city forester said.
“But we ought to be planting at least 10,000 trees a year,” Swanson said. “This is a community problem — heat. Everybody needs to be part of the solution. Or else we will have more asthma. More heart disease. It just gets bad. Nobody will want to live in Denver. Yes, Denver needs more trees. And we need to stop pouring concrete.”
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