Climate change is forcing cities to rethink their wood mix

The city park

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Cities must plant more trees. But not just any trees.

As communities prepare for a massive influx of federal funding to support urban forestry, their leaders say the tree canopy growing to maturity 50 years from now will have to be painted with a different palette than the one that exists today.

“You need a tree that’s going to survive today’s weather and the climate of the future,” said Pete Smith, urban forestry program manager at the Arbor Day Foundation, a Nebraska-based nonprofit that supports tree planting and care.

Forestry experts say trees are critical infrastructure that can help cities resist the effects of climate change by providing shade, absorbing stormwater and filtering air pollution. But to do that, the trees themselves must be resilient.

“We develop plant lists that are diverse, that look at tolerance to drought, storm events and flooding, heat, changes in elevations and lows,” said Kevin Sayers, urban forestry coordinator at the Michigan Department of Natural Resources. “The extremes in the weather are really going to limit us.”

As arborists look for trees that will thrive in the climate conditions they’re likely to face in the coming decades, scientists say they can’t just rely on a handful of climate “winners.” For example, many cities have lost huge amounts of their tree canopy because they relied too much on a type of tree that was later wiped out by a pathogen or pest, such as Dutch elm disease or the emerald ash borer.

“Unless we start diversifying the urban forest, we’re going to lose quite a bit of it again,” said John Ball, South Dakota State University Extension forestry specialist and South Dakota Department of Agriculture forest health specialist.

Ball encourages cities to plant no more than 5% of one tree genus, but many communities have struggled to meet the diversity goals he and other forest health experts recommend. Foresters say it takes effort to figure out which trees will grow in challenging urban conditions, and nurseries often lack the less common trees they’re looking for.

Amid these challenges, cities and states are preparing to receive $1.5 billion in urban forestry funding approved by Congress earlier this year as part of the Inflation Reduction Act. Forestry executives say the newfound support will be transformative, but turning that money into a healthy tree canopy decades from now will be a complicated task.

“The pressure is on, but in a good way,” said Kesha Braunskill, urban forestry coordinator with the Delaware Forest Service. “This is a once-in-a-career opportunity for all of us in urban forestry, and how we use it is going to affect those in our positions 50 years from now.”

“A bit pickier”

Some cities are already making changes.

Jeremy Harold, green spaces manager for Harrisonburg, Virginia, said the city once took a “cookie cutter” approach to tree planting but is now working to expand the species mix. The city is located in the Shenandoah Valley within the Appalachian mountain range, but it has added trees such as willow oak and sweet gum from Virginia’s Coastal Plain region.

“I’m putting them into our inventory now because as the temperatures rise, these trees will adapt,” Harold said. “We’re looking for species that can tolerate these temperatures and survive.”

In Seattle, many of the city’s big maples and western red cedars are struggling in urbanized areas. Foresters are now careful to plant them in favorable microclimates, with conditions such as good soil moisture and north-facing slopes that remain cooler.

“We’re a little more picky about where we put them on the landscape,” said Michael Yadrick Jr., plant ecologist with Seattle Parks and Recreation.

Meanwhile, the city is planting more Pacific madrones and garry oaks that can withstand warmer, drier conditions. And within individual tree species, it adds trees grown from seed obtained further south in their range, with the goal of adding resilient genotypes to the mix.

Texas state officials operate a genetic improvement program that has produced nine “Texas Tested, Texas Tough” tree species adapted to handle harsh conditions, including Shumard oak and bald cypress.

“They’ve been going through this iterative process for decades and have been proven to perform in this harsh environment that is Texas urban areas,” said Gretchen Riley, manager of the Forest Systems Department at the Texas A&M Forest Service.

The agency supplies seedlings to communities and works to provide seeds to growers who can produce their own supply. It also works with six other states in the region to exchange species and genetic lines and test their viability under different conditions.

Researchers at the University of Florida are working to find out which trees can best withstand strong winds. They hope to expand an existing Florida-based classification system by looking at research from hurricane-prone communities around the world.

“We would like to see this list used to target wind-resistant species in areas where a falling tree could damage property or harm people or infrastructure,” said Allyson Salisbury, a researcher at the university.

Foresters say their preparations will not result in a complete makeover of the trees they plant. They emphasize that such decisions are an inexact science that can have unintended consequences.

“People say we should bring species up from southern places,” said Lydia Scott, director of the Chicago Region Trees Initiative, a partnership of organizations and agencies dedicated to improving the area’s urban canopy. “It’s fine until we get a two-week cold spell in the winter that kills all those trees that aren’t adapted to the cold.”

A need for seeds

Above all, experts say diversity is the best way to ensure that many trees survive the changes to come, rather than pinning all their hopes on which trees might thrive. In most communities, the existing tree canopy is far from that goal.

“Many cities are dominated by a small number of species or genera,” said Mark Ambrose, a research assistant at North Carolina State University’s College of Natural Resources. Ambrose, whose position is funded by the US Forest Service, has been researching the composition of the nation’s existing urban tree canopy.

Elm trees were once among the most prominent trees in America’s urban forests. When the Dutch blight wiped out many of these trees, many towns were replanted with ash. Now they are taking down millions of trees that have been ravaged by the emerald ash borer. Today, maples are spreading in cities, and foresters are wary of any threats to these trees.

“You can plant elm and ash anywhere on any soil and grow them,” said Ball, the South Dakota forestry specialist. “Now we’re done with the simple trees. You should know what the soil is like. You need to understand the microenvironments of your community and fine-tune your plantings.”

The city’s forestry managers say they want to plant a greater variety of trees, but getting the seedlings they need has proven challenging.

“Nurseries lack the species diversity we’re looking for, and that’s hard to crack because it’s the private sector,” said Keith Wood, a contractor with the National Association of State Foresters who staffs the group’s committee on urban and community forestry. .

Arborists cite a feedback loop in which nurseries grow only what sells, and cities buy only what is available. Some have gotten around this loophole by making agreements with nurseries in advance to grow the seedlings they need in the coming years. The Chicago Region Trees Initiative plants 54 tree species, some of which pay off over a five-year period when nurseries grow them.

“We get the species we want, the sizes we want, the numbers we want, all when we want them,” said Scott, the Chicago-area manager.

Some cities are reluctant to contract for trees years in advance, unwilling to make inflexible cost commitments amid unpredictable budget cycles.

But nurseries need some certainty if they’re going to grow less marketable and harder-to-grow species on a large scale, said Nancy Buley, director of communications at J. Frank Schmidt & Son Co., a large Oregon nursery that supplies many urban planting efforts.

“For the cities and nonprofits to get the more unusual trees to meet their species diversity goals,” she said, “they’re really going to need to pull together somehow.”

2022 The Pew Charitable Trusts.

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