Trees in the city

This article originally appeared in Nexus Media News.

After a series of winter storms hit California this winter, thousands of trees across the state lost their grip on the ground and crashed into power lines, homes and highways. Sacramento alone lost more than 1,000 trees in less than a week. Stressed by years of drought, pests and extreme weather, urban trees are in trouble.

The US Forest Service estimates that cities are losing some 36 million trees every year, decimated by development, disease and increasingly climate stressors such as drought. In a recent study published in Natureresearchers found that more than half of urban trees in 164 cities around the world are already experiencing temperature and precipitation conditions that are beyond their survival limits.

“So many of the trees that we’ve relied on heavily are now falling out of favor as the climate changes,” said Nathan Flack, urban forester for the city of Santa Barbara. Conifers, such as pines and coast redwoods, once widely planted along the coast, are dying in droves, he says. “The intensity of the heat [and] the longer periods [without] rainfall is really forcing us, as urban forestry managers, to rethink what good street trees are.”

Read more: 20 things you didn’t know about trees

What types of trees?

Trees help cool neighborhoods, absorb rainwater, and clean up air pollution. But to provide these critical functions, they must survive under the same conditions. For many cities, this means reconsidering the species that are planted.

Flack says he looks for trees that typically grow further east, such as paloverde, which do better in warmer, drier conditions. “Trees that survive in the desert will be much more useful to us here,” he says.

In Sacramento, species like the ‘Bubba’ desert willow are displacing redwoods, says Jessica Sanders, executive director of the Sacramento Tree Foundation. “It’s sad because it’s an iconic tree,” Sanders says, “but it’s not really suited to the climate of the Sacramento area right now.”

It’s not just California cities that are like this rethinking their canopies.

In Harrisonburg, Virginia, officials brought in willow oak and sweet oak from the coast, trees that are more heat tolerant than many native species. Seattle is planting more Pacific madrone and Garry oaks, which have a better chance of surviving hotter, drier summers.

In Detroit, which was once known as “City of Trees”, because of its expansive canopy, officials are planting hardy trees like eastern redbud, American witch hazel and white oak that can withstand extreme heat and flooding.

Diversity of trees

City officials are also expanding species diversity to prevent disease, aiming to prevent any one species from making up more than 10 percent of the city’s canopy. Detroit lost much of its crown between the 1950s and 1990s to Dutch elm disease and an invasive beetle called the emerald ash borer.

Today, nearly 40 percent of the trees that remain are considered “poor quality,” says Jenny Schockling, senior manager of urban forestry in Detroit for American Forests, a nonprofit organization. “[They] consist of species that are prone to disease and storm damage, cause damage to property and infrastructure, and release large amounts of debris.

Tree cover and climate change

Preserving urban tree cover could mean the difference between life and death on a warming planet. Intense heat kills approximately 12,000 people a year are already in the US; experts say that figure could reach 100,000 by the end of the century. A study published by Lancet in January found that a 30 percent increase in tree cover in the city could reduce heat-related deaths by a third.

Poorer neighborhoods with large nonwhite populations tend to have less tree cover and can get up to 20 degrees warmer than wealthier (and greener) neighborhoods. According to several studies. “A tree map in every city in America is a map of income and a map of race,” says Judd Daley, president and CEO of the nonprofit American Forests.

Cites may soon see some relief. The Inflation Reduction Act, signed into law last year, includes $1.5 billion for the Forest Service’s Urban and Community Forestry Program, amounting to a fivefold increase in the program’s annual budget.

The funding has the potential to transform city canopies, according to experts like Daly. But as Flack and other arborists around the country turn to new species to fill their streets, they face a new problem: supply.

Read more: What makes a tree a tree?

“There’s a bottleneck right now in the traditional kindergarten supply line,” Schockling says. “Growers tend to favor particular species because they grow well in the nursery or grow quickly, but that doesn’t necessarily speak to the standards of species diversity that we’re trying to adhere to.”

American Forests partners with the US Forest Service to invest in and develop nurseries across the country to improve the supply chain. “Nurseries need some assurance that what they’re growing will have a market value, and we have the assurance that what we’re going to buy will have a supply,” Schockling says.

These large-scale investments will be critical to updating the composition of urban canopies, according to David Teuschler, head horticulturist at Devil Mountain, one of California’s largest nurseries.

According to Teuschler, even California’s native trees, such as the coastal live oak, are struggling with the state’s droughts. He would like to invest more in trees like Mesa oak or Silver oak to sell in Northern California and Swamp mallet or Salt Marsh gum to sell in Southern California, but it can take years to grow trees to a size that can be sold, and then he only needs a limited time to sell these seedlings. Unsold trees are usually composted, burned or otherwise destroyed.

He must know that there will be customers who have a clear view of the future.

“You have to remember that there are a lot of old-school people who want to plant redwoods,” he says. “You want to be the nursery that has these drought-adapted species, but if you can’t sell them, it’s a waste.”

One of Devil Mountain’s longtime customers is California arborist Dave Muffley, who stocks all of his projects with drought-tolerant species.

Trees to fight drought

Muffley first began looking for drought-tolerant trees 15 years ago while leading a project to plant 1,000 trees along a two-mile stretch of highway that runs through East Palo Alto. He wanted evergreens to prevent freeway pollution from reaching the low-income community on the other side, and drought-tolerant varieties, but most of the state’s nurseries had few options.

Muffley began scouring the Southwest for acorns from harder oak species; with more than 500 species of oak around the world able to interbreed and create viable hybrids, trees are particularly prone to develop traits that can help them survive rapid climate changes, Muffley says.

With Teuschler’s help, his projects—including a megaproject of 9,000 trees around Apple’s campus—have served as proof-of-concept for cities as they work toward climate-resilient treetops.

By directing federal funding to nurseries like Devil’s Mountain, this kind of holistic system can be replicated across the country to meet the unique needs of each region, Muffley says.

“The truth is, we don’t grow enough trees in the U.S. to spend the money the government just gave,” says Muffley. “So now it’s time to build an arsenal of ecology, and the production lines are the new nurseries that will need to be built to grow the trees.”

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