As climate change progresses, trees in cities struggle

SEATTLE (AP) — Like the drier summer when Seattle’s logs ran out, trees throughout the city sounded silent alarms.

It was the latest in a string of Seattle summers over the past decade, including a record-breaking heat dome in 2021, featuring drier conditions and hotter temperatures that have left many trees with premature brown leaves and needles, bare branches and overseeding. – all signs of stress.

“You see it in the big-leaved maples and hemlocks, just laden with cones or seeds. It’s sort of their last-ditch effort to reproduce,” said Shea Cope, an arborist at the Washington Park Arboretum, a sprawling 230-acre (93-hectare) ) 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 infected with a fungus left behind by beetles.

“We’re losing conifers faster than our hardwoods, hardwoods,” added Cope as he gazed at a towering pine with half its canopy dead.

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


Life in a city can be especially tough on a tree, and these challenges are intensifying with global warming.

Researchers from France and Australia analyzed the impact of warmer 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 experiencing climatic conditions beyond their limits. They also concluded that by 2050, nearly all tree species planted in Australian cities will not be able to survive in urban areas.

“If trends hold, we’re going to have a lot of trees dying,” said Nicholas Johnson, an arborist for Seattle City Parks. “In the heat, trees weaken, just like people.”

Heat and drought force trees to expend energy to survive that would otherwise go to regeneration, growth or disease and pest control, Johnson said. “Everything outside is trying to eat a tree. The stresses get worse.

Human-caused climate change is also fueling more extreme weather conditions such as intense winds, rain and freezing temperatures.

“It’s not the gradual change that’s going to be the problem, it’s these extreme swings of too much water, too little water, too much wind and storm intensity that are going to cause these rapid changes,” said David Nowak, a retired scientist with the US Forest Service. United States.

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

“The need to grow the canopy is greater than in years past,” he said. “But the advantages in an urban context remain the same. On any hot day, step into the shade and it occurs to you that trees are such a boon to public health and welfare.

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

Housing and commercial construction, compacted soil, pollution, and even automobile accidents contribute to a city’s loss of roof.

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

“A growing tree mortality rate is coming to a city near you,” said Aaron Ramirez, a tree researcher at Reed College.

Between 2016 and 2021 Seattle lost 1.7% of its tree canopy, about 255 acres (255 hectares) of trees, according to a city report that blames climate change in part. To 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 and rural areas as we have seen increased stress from disease, insect infestation, drought, leading to catastrophic fires. But the fact is, our urban forest, our urban trees, are equally stressed,” said Hillary Franz, Washington State Public Lands Commissioner.


Rows of small black plastic pots bathe in the morning sunlight on a maintenance lot for the city of Bellevue, Washington. All contain juvenile giant sequoias, just a few inches tall, which the city is cultivating for climate resilience.

Redwoods aren’t native to the Pacific Northwest, but tree managers in this city east of Seattle are planting more because they manage droughts and pests.

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

Non-native trees have been brought into cities for a long time. Climate change, however, is prompting many arborists to consider increasing them in their city’s tree palette, a practice called “assisted migration.”

Arborists are looking for non-native species without “invasive tendencies,” said Scott Altenhoff of Oregon’s Urban and Community Forestry Program.

Still, a lot of research is needed to study resilient trees, said Ramirez of Reed College, whose lab found an Alaskan cedar did better in a hot summer than Oregon and California varieties.

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


The small Puget Sound town of Burien, Washington, with about 80 employees, added another one in March: their first tree handler. The hiring was part of an increased focus on the city canopy.

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

The costs of maintaining urban forests hit budgets in different ways. New Orleans is also considering a new water truck after this dry year. In Bellevue, the city that grows giant sequoias, a big chunk of tree maintenance goes toward an increasing number of dead tree removals.

“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.

On a recent rainy week, Seattle Parks Department volunteers 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 tree.

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


Associated Press writer Janet McConnaughey contributed from New Orleans.


The Associated Press’s climate and environmental coverage receives support from several private foundations. Learn more about AP’s climate initiative here. The AP is solely responsible for all content.


Leave a Reply

%d bloggers like this: