At a time when climate change is making heat waves more frequent, trees are stationary superheroes

A tree in the Court Avenue district of central Des Moines, Iowa, on June 16, 2021.
A tree in the Court Avenue district of central Des Moines, Iowa, on June 16, 2021.PHOTO: NYTIMES
A group of trees grow on the Martin Luther King Jr. Parkway in Des Moines, Iowa.
A group of trees grow on the Martin Luther King Jr. Parkway in Des Moines, Iowa.PHOTO: NYTIMES

DES MOINES (NYTIMES) - The trees were supposed to stay. It did not matter that the owners of the squat building alongside were planning to redevelop the property. The four eastern red cedars stood on city land, where they had grown for the better part of a century.

"There's no way these trees are coming down," Mr Shane McQuillan, who manages the city's trees, recalled thinking. "The default position for us is, you don't take out big trees to put in small trees."

Here is why: At a time when climate change is making heat waves more frequent and more severe, trees are stationary superheroes. Research shows that heat already kills more people in the United States than hurricanes, tornadoes and other weather-events, perhaps contributing to 12,000 deaths per year. Extreme heat this week in the Pacific Northwest and Canada has killed hundreds.

Trees can lower air temperature in city neighbourhoods 10 lifesaving degrees, scientists have found. They also reduce electricity demand for air conditioning, not only sparing money and emissions, but helping avoid potentially catastrophic power failures during heat waves.

"Trees are, quite simply, the most effective strategy, technology, we have to guard against heat in cities," said Brian Stone Jr, a professor of environmental planning at the Georgia Institute of Technology.

So, in Des Moines, Mr McQuillan worked with the property owners and city planners to find a way to redevelop while keeping the trees. But one day several months later, he got word that a crew was taking them down.

Mr McQuillan raced to the site, just a couple blocks from his office. One tree had already been cut to a stump, and another was almost down. Mr McQuillan halted the work and fought to stay calm. At first he assumed someone had taken matters into their own hands. But after investigating, he came to believe it was simply a mistake; the property had been leased for a restaurant and the tenants seemed sincerely unaware of the agreement.

"There's a defeated feeling," Mr McQuillan said.

Despite long-standing and ongoing efforts across the country to plant trees, communities in the United States are not adding to their total number or even maintaining it. Research shows that American cities and towns lose the canopy of 36 million trees every year.

'A challenge to get trees to thrive in the city'

Considering the cast of characters in Des Moines, its urban forest should be thriving. The longtime mayor is an environmentalist. The director of public works hails trees as "the only infrastructure that add value over time".

A nonprofit group plants and tends the next generation of trees while giving green jobs and training to local teenagers.

Mr Shane McQuillan worked with the property owners and city planners to find a way to redevelop while keeping the trees. PHOTO: NYTIMES

In recent years, though, the larvae of an iridescent green beetle that arrived from across the ocean, the emerald ash borer, have claimed 6,000 of the city's 8,000 public ash trees. A storm last year took out about 500 more of all kinds. Another big factor is the everyday losses: The tree felled to repair a water line underneath. The homeowner who removed a tree to build an extension or get more sun on the lawn. Countless new developments where trees were in the way. These are often mature trees whose canopy will take decades to replace.

Then, there are the bare-branched victims whose cause of death can only be guessed at: Not enough water? The extra-cold winter combined with all that street salt?

"It's a challenge to get trees to thrive in the city," said Phillip Rodbell, who leads a Forest Service team studying the social, economic and ecological impact of urban trees.

At the same time, American cities are facing a heat crisis: The largest are warming at twice the rate of the planet as a whole.

'Trading one risk for another'

A tree's shade, that sweet relief from solar radiation, is only part of its cooling power. Trees also evaporate water, pulling it from the ground and releasing it into the air through their leaves. That's why walking through a forest, or just sitting in a playground surrounded by several large trees, feels more refreshing than the shade of a lone tree.

Carefully positioned trees can reduce a home's energy costs by 25 per cent, according to the Department of Energy. Nationwide, urban trees offer an estimated US$18.3 billion (S$24.7 billion) in air pollution removal, carbon sequestration, lowered energy use in buildings and reduced emissions from power plants.

Still, across the country many people see trees as a nuisance or liability. They drop nuts, seeds and leaves. They buckle sidewalks. They are accused of destroying pipes - wrongly, according to scientists, who say that pipes crack from age, which only then leads nearby trees to send roots toward the leaking water. Some towns and cities avoid the perceived hassle altogether by not planting on the strip of lawn between the sidewalk and the street.

Occasionally, their limbs break or they blow over, posing real danger. With climate change increasing the intensity of storms, David Nowak, a senior scientist with the Forest Service who studies urban trees, acknowledges the risk. Trees close to houses need to be especially well monitored for weakness. But he points out that trees also block wind, reducing the force of storms.

"You're trading one risk for another," Mr Nowak said. "Branches falling, and having to clean up branches, versus having to clean up broken rooftops."

Summer workers with Trees Forever, an Iowa nonprofit, use a fire hydrant to draw water for newly planted trees in Des Moines, on June 16, 2021. PHOTO: NYTIMES

One major challenge is persuading property owners, who own a large share of the land in cities and towns, to plant and maintain trees in their yards. It is important to choose the species carefully. Large shade trees offer more cooling and carbon storage than small ornamentals. For wildlife, oaks are usually the best bet, according to Doug Tallamy, a professor of entomology at the University of Delaware. They feed more than 900 species of caterpillars, which, in turn, feed birds, whose populations have plummeted.

Incentives can help, but tight budgets often keep them modest. In Louisville, Kentucky, which threw itself into planting more trees after it was found to be the fastest-warming large city in the country, residents can get a US$30 "treebate", up to three per household, for planting certain shade trees.

The director of public works in Des Moines, Jonathan Gano, came up with an idea to give away "tiny trees", seedlings that look like mere sticks with roots. Once a year, residents can pick up five each.

"They're tiny, yes," Mr Gano said. "They're also practically free," costing the city US$1 per seedling.

"You could have 99 per cent mortality and still be in the money 20 years from now on canopy," Mr Gano said. "I planted a bunch on my property and about 50 per cent of them have survived. One of them's 11 feet tall now."