Urban Ecology

Planting Trees, Redeeming Lives

An Urban Species

A few years ago, for the first time in our history as a species, Homo sapiens became a predominantly urban species, as people worldwide shook off the dust of rural life and moved to town. We crossed the halfway mark sometime in 2007 or 2008, according to a United Nations estimate, en route from being about 30 percent urban in 1950 to 60 percent urban by 2030. The trend is even more advanced in the United States. About 80 percent of Americans now live in urban areas, which also sprawl across a rapidly expanding share of the American landscape. In less than a lifetime, from 1990 to 2050, the U.S. Forest Service estimates that urban land area will more than triple, from 2.5 percent to 8.1 percent of the lower 48 states. In Connecticut and three other Northeastern states, the land will be more than 60 percent urban by mid-century, up from about 35 percent now.   

So what does it mean to become a city-dwelling species? And how should it change the way we think about the forest in the city?

Until recently, even scientists did not pay much attention to this epochal shift in the way people live. “When I was an undergraduate, if you wanted to do urban ecology and examine the design of cities and the relationship between people and the environment, it was a path to obscurity,” says Morgan Grove, Ph.D. ’96. “It was considered trivial and unimportant. That’s not what ecologists did.” Places with lots of people were the antithesis of nature, an attitude dating back at least to Thomas Jefferson, who once described great cities as “pestilential to the morals, the health and the liberties of man.” Outside the scientific world, planting street trees in cities tended to be little more than an Arbor Day photo opportunity for local politicians and an afterthought in urban budgets. 

But that attitude has changed dramatically over the past few years. City planners and politicians have begun to see trees and “green infrastructure” as a practical alternative to costly technological fixes—not just for traffic calming, but also for controlling air and water pollution, reducing energy demand and urban heat-island effects, preventing floods and adapting to climate change. Likewise, psychologists and social scientists have begun to pay attention to new evidence that trees can make people feel better about where they live, reducing stress, lowering crime rates, improving educational outcomes and helping to minimize asthma hospitalizations and heat-wave deaths. Even ecologists, who once could not see the urban forest for the buildings, have come around. Their recent research suggests that cities are more important to biodiversity than previously thought—and with a little planning could become much more so even as urban areas expand.

The idea that trees can build better cities has recently helped launch “million-tree” initiatives in Los Angeles, New York, Salt Lake City, Indianapolis, Denver, Houston and other cities, generally with a timetable of planting that many new trees within five or 10 years. Corporations and other organizations not normally associated with environmental causes have also signed on to the movement, with the American Bar Association, for instance, committing to planting a million trees by 2015. Even some supporters worry that the current zeal for urban tree planting may be too much, too fast, particularly where the planters neglect to provide for the unglamorous work of maintaining the trees. Such campaigns have sometimes produced little more than favorable publicity in the past. But the new initiatives now often come with highly specific and far-reaching purposes in mind. 

For instance, planting trees might seem at first like an improbable tool for dealing with combined sewer overflows (CSOs)—a common problem in older cities where domestic wastes and stormwater run through the same pipes. Many cities now face federal deadlines—and huge construction costs—to stop such systems from spilling raw sewage into basements and waterways during rainstorms. The conventional engineered, or “gray,” remedy is to build massive deep tunnels capable of retaining overflow, to be released gradually through municipal water treatment plants after the rain stops falling. But cities are now also pushing to prevent the water from getting into the system in the first place by using trees, green roofs and permeable pavements to sponge it up where rain falls. 

In one typical case, the Milwaukee Metropolitan Sewerage District has spent $4 billion over the past two decades on gray remedies, adding 494 million gallons of deep-tunnel storage. But when a major rainstorm hit late on a Saturday afternoon in June 2008, the runoff was on track to fill the entire system in just 56 minutes. District managers had to divert combined wastes into area waterways and reserve the deep tunnels to retain separated household wastes, which also overflowed as the storm continued over the next three days. To prevent that kind of mess in the future, the district is now spending $300 million to preserve open space along local waterways. It also recently paid market prices to remove 75 homes in a downtown neighborhood, adding the land to a city park that doubles as a flood retention pond.

Likewise in Philadelphia, a 2009 study compared gray CSO prevention measures (adding more deep tunnels) with a green infrastructure approach that aimed to halve the runoff from existing impervious surfaces at roughly the same cost. Stratus Consulting, based in Boulder, Colo., estimated that the gray approach would produce $122 million in benefits over 40 years after subtracting construction costs. For the green alternative, the net benefits added up to $2.8 billion—more than 20 times as much—including improved property values, increased recreational opportunities and avoided heat-stress deaths. 

Those kinds of numbers have attracted widespread interest. For instance, an innovative proposal from the Center for Neighborhood Technology in Chicago would fund tree planting and other green infrastructure programs with municipal bonds based on the projected increase in real estate values and tax revenues such programs produce. But there has been only cautious interest so far because it’s unclear whether enforcement officials at the Environmental Protection Agency will accept green infrastructure as a tool for CSO prevention. Even so, Philadelphia recently launched the largest green infrastructure program in the country to transform the city over the next 20 years at a cost of $1.6 billion. (Aquatic biologist Gerald Bright ’08 is coordinating part of the river restoration work.) The city is also adjusting its water and sewer fee structure so that commercial users pay, in part, based on a property’s impervious surface area—a measure of stormwater runoff. Howard Neukrug, director of the city’s Office of Watersheds, described the program as an attempt to “break down some of the barriers against nature and deal with rainwater where it lands.” Despite reservations about the cost, The Philadelphia Inquirer summed up neighborhood response this way: “What’s not to like about cleaner air, cooler houses and prettier streets?”

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Top of Page | Spring 2010 | environment:YALE