As climate change continues, trees in cities are struggling

SEATTLE (AP) — If the driest summer ended in the Seattle record books, trees in the city sounded silent alarms.

It was the latest in a string of summers in Seattle over the past decade, including a record-breaking heat dome in 2021, with drier conditions and higher temperatures leaving many trees with premature brown leaves and needles, bare branches and excessive seeding — all signs of stress. .

“You see it in big leaf maples and hemlocks, just loaded with cones or seeds. It’s kind of their last-ditch effort to reproduce,” says Shea Cope, an arborist at the Washington Park Arboretum, a sprawling 230-acre area. acre (93 hectares). park north of downtown.

This summer was fatal for three “significant” trees in the park’s pine collection, including an 85-year-old Japanese red pine that was infected with fungus left behind by beetles.

“We’re losing conifers faster than our broadleaf, deciduous ones,” Cope added, looking at a towering nodule with half the canopy dead.

Cities around the world have pledged to plant more carbon-absorbing trees to combat climate change. Research has shown that shade from mature trees also helps reduce unhealthy “heat islands,” especially in poor neighborhoods. President Joe Biden’s Inflation Reduction Act has pumped $1.5 billion into the Forest Service’s urban trees program — money for cities to do even more planting and maintenance.

CLIMATE THREAT TO URBAN FORESTS

Life in a city can be particularly difficult for a tree, and those challenges are increasing with global warming.

Researchers from France and Australia analyzed the impact of higher temperatures and less rain on more than 3,100 tree and shrub species in 164 cities in 78 countries. They found that about half of the trees were already suffering from climatic conditions that exceeded their limits. They also concluded that by 2050 almost all tree species planted in Australian cities will not be able to survive in urban areas.

“If trends continue, a lot of trees will die,” said Nicholas Johnson, an arborist for Seattle City Parks. “Under heat, trees get weak — just like people do.”

Heat and drought force trees to expend energy on survival that would otherwise go to regeneration, growth or fighting disease and pests, Johnson said. “Everything outside is trying to eat a tree. Tensions are exacerbated.”

Human-induced climate change is also causing more extreme weather events, such as strong winds, rain and freezing temperatures.

“It’s not the gradual change that will be the problem, it’s these extreme fluctuations of too much water, too little water, too much wind and storm intensities that will cause these rapid changes,” said David Nowak, a retired scientist for the U.S. Forest Service.

Hurricane Katrina wiped out about 10% of New Orleans’ trees in 2005, said Michael Karam, director of Parks and Parkways. And in 2021, he added, Hurricane Ida has uprooted many new saplings.

“The need to increase the canopy is greater than in recent years,” he said. “But the benefits in an urban environment remain the same. Sit in the shade on a hot day and you’ll be reminded that trees are such a great benefit to public health and well-being.”

A 2018 study by Nowak found that 25 states had seen significant tree declines earlier that decade.

Residential and commercial construction, compacted soil, pollution and even car accidents contribute to the loss of a city’s canopy.

Cities are familiar with large-scale tree loss, but usually one species of tree is affected, such as birch trees killed by a borer infestation. With climate change, researchers are concerned that canopy loss will outpace the number of newly planted trees reaching maturity, which takes 10 to 20 years.

“Rising tree dieback is coming to a town near you,” said Aaron Ramirez, a tree researcher at Reed College.

Between 2016 and 2021, Seattle lost 1.7% of its canopy, about 255 acres (255 hectares) of trees, according to a city report that partly blames climate change. In the south, Portland, Oregon, saw its first canopy reduction last year since it began keeping records two decades ago.

“We have spent a lot of time talking about the health of our forest in our natural, rural areas as we have seen increased stress from disease, insect infestation and drought, leading to catastrophic wildfires. But the fact is, our urban forest, our urban trees, are just as stressed,” said Hillary Franz, Washington state’s public lands commissioner.

“ASSISTED MIGRATION”

Rows of small black plastic pots bathe in the morning sunlight at a maintenance yard for the city of Bellevue, Washington. They all contain young giant sequoias, just a few inches tall, that the city is growing for climate resilience.

Sequoias aren’t native to the Pacific Northwest, but tree managers in this city east of Seattle are planting more because they can handle drought and pests.

“Once established, these trees grow incredibly fast,” said Rick Bailey, supervisor of the city’s forest management program. Native trees still make up about 70% of newly planted trees.

Non-native trees have long been brought to cities. However, climate change is prompting many arborists to consider expanding into their city’s tree palette — a practice called “assisted migration.”

Arborists look for non-native species with no “invasive tendencies,” says Scott Altenhoff of Oregon’s Urban and Community Forest Program.

Still, a lot of research needs to be done to study resilient trees, said Ramirez of Reed College, whose lab found that an Alaskan cedar fared better in a hot summer than varieties from Oregon and California.

Planting more non-native trees complements something urban arborists have learned from decades of tree dieback: Diversity in the types and ages of trees planted is key to keeping urban forests alive.

RISING COSTS

The small Puget Sound town of Burien, Washington, with about 80 employees, added another in March: their first tree manager. The hiring was part of a larger focus on the city’s canopy.

“We were just having a discussion about ‘Can we get a water truck? Or something like that?’” said Josh Petter, the new arborist. “Because we have these increasing droughts…I’d rather plant one tree and take care of it really well than plant 10 trees and then not take care of it.”

The costs of maintaining urban forests weigh on budgets in various ways. After this dry year, New Orleans is also considering a new water truck. In Bellevue, the city where giant sequoias grow, a large part of tree maintenance goes into removing dead trees.

“We’re not keeping up with the level of maintenance and protection needed to make sure we don’t lose them,” said Georgia Tech’s Evan Mallen, who said more cities need legislation to protect existing trees.

During a recent rainy week, volunteers from Seattle’s parks department planted dozens of trees in a park west of the city. Among them was a western red cedar planted in the shade of the exposed roots of a fallen oak.

“Life always finds a way,” says Johnson, the department’s arborist. “And in Seattle, people help life find a way.”

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Associated Press writer Janet McConnaughey contributed from New Orleans.

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The Associated Press’ climate and environmental coverage is supported by several private foundations. Read more about AP’s climate initiative here. The AP is solely responsible for all content.

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