Are trees 'oversold' as cities expand green space?

Trees in the city may not be as green as we think.

They produce rotting leaves that pollute lakes and streams with too much phosphorus. They can actually trap air pollution right at nose level. And what with watering, maintenance and replacement when they die, they are not always an efficient way to store carbon.

In short, often "trees are oversold" as a natural solution for environmental problems in cities, according to Bonnie Keeler, a University of Minnesota assistant professor who studies ways of valuing nature. In fact, she reviewed 1,200 scientific studies on increasingly popular green infrastructures such as urban forests, parks, rain gardens and wetlands and found in a recent paper that it's unclear how well any of them stack up against "gray" solutions like concrete storm sewers and air conditioning.

It's an increasingly urgent question for the Twin Cities. At projects such as the Ford site in St. Paul, the Mississippi River Upper Harbor in Minneapolis and a redesign for sections of Minnehaha Creek, planners face complex choices for managing stormwater and air pollution.

The answers will help define the future for a growing share of the world's population. By 2050, two out of three people will live in urban areas that will affect their health and well-being, Keeler said. Immense social challenges like climate change, public health and public funding will have the greatest impact on people who live in cities.

"There is a huge interest in expanding funding for green infrastructures," she said. "But we don't have a tool to understand their value."

Take trees, for instance. There is no question that they are crucial to global ecosystem health. But in the city it can be a different story.

Keeler's review, published last month in Nature Sustainability, found that most evaluations of urban trees focused on two benefits: filtering air and sequestering carbon. Few considered the costs of maintenance, replacement or public health. Even their estimated ecosystem values ranged widely — from $5 to $402 per tree.

At the same time, there's no widely accepted method to calculate the more ephemeral value that trees provide, such as joy in their beauty, resting places for birds or the coolness of their shade.

"Green is sometimes more expensive, but it can carry other benefits that are not as well-captured in markets," Keeler said.

Nonetheless, urban planners around the Twin Cities area are now incorporating these hard-to-measure benefits into their decisions on managing land and water.

Minnehaha Creek is a showcase for the way different cities along its route increasingly see the waterway as a tool to leverage green solutions for an array of social and environmental problems. As the western Twin Cities suburbs grew around it, the creek became an easy place to get rid of water that ran off streets and parking lots. Along much of its length it was forgotten, hidden by buildings and covered by streets.

But that changed in 2009, starting with Methodist Hospital in St. Louis Park. Like many buildings along its banks, the hospital had turned its back to the creek and a nearby wetland. But the hospital's expansion plans brought officials of the Minnehaha Creek Watershed District into the conversation.

A decade later the entire health care complex has turned around to face the wetland.

The staff "can feel the joy of coming to a workplace where they can enjoy nature," said Duane Spiegel, vice president of real estate for HealthPartners, which merged with the hospital's parent organization, Park Nicollet, in 2013. Patients and their families are no longer confined to the hospital — they can go outside. "A good healing environment, that helps them in recovery," said Dr. Tom Kottke, medical director for health and healing at HealthPartners.

Often, said Keeler, the best opportunity to get the most out of green infrastructure, is a new development — such as the massive project underway at the former Ford plant site in St. Paul's Highland Park neighborhood.

One of the most popular features of the new development is the novel treatment of stormwater. Instead of passing through underground sewers, the water will flow above ground in a stream running through the center of the complex — which will also provide the water to re-create the falls at nearby Hidden Falls park.

It will be aesthetically beautiful — and, on its face, cost about the same as underground pipes, said Wes Saunders-Pearce, St. Paul's water manager. But using a complex computer model, planners were able to quantify the added benefits from the aboveground version, including factors such as water quality improvements and energy- and greenhouse-gas reduction from all the extra trees that could be planted along the stream.

"That was a huge turning point," he said. "It was twice the benefit … compared to the traditional way of planning."

Josephine Marcotty is a Twin Cities science writer and former Star Tribune environment reporter. She can be reached at josephinemarcotty@gmail.com.