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The city is an ecosystem, pipes and all

What scientists are finding when they treat the urban landscape as an evolving environment of its own

Is a tree trying to survive in the city better off than a tree growing in the forest? The obvious answer would seem to be “no”: City trees face pollution, poor soil, and a root system disrupted by asphalt and pipes.

But when ecologists at Boston University took core samples from trees around Eastern Massachusetts, they found a surprise: Boston street trees grow twice as fast as trees outside the city. Over time, the more development increased around them, the faster they grew.

Why? If you’re a tree, city life also offers a number of advantages. You benefit from the extra nitrogen and carbon dioxide in polluted city air; heat trapped by asphalt and concrete warms you in the cold months. There’s less competition for light and space.

Cities may strike us as the opposite of “the environment”: As we pave streets and erect buildings, nature comes to feel like the thing you find somewhere else. But scientists working in the growing field of urban ecology argue that we’re missing something. A city’s soil collects pollutants, but it also supports a vast system of microscopic life. Water courses beneath roads and buildings, often in long-buried streams and constructed pipes. And city ecosystems aren’t static; they change over time as populations grow, infrastructure ages, and different political structures and social values shape them.

Seen this way, the city is a distinct form of “environment,” and an important one. Truly understanding how it works—and how it affects the millions of people who may live and work there—will mean studying the whole city as a living system, both its organisms and its pipes, roads, and landfills. As cities grow, maintaining clean air and water in a place like Boston may depend on how well urban areas support trees, plants, and microbes.


As researchers delve into this world, they’re uncovering some surprises—like the unusual nitrogen cycles caused by homeowners raking up leaves in the fall, or the fast-growing urban trees. (BU ecologist Lucy Hutyra isn’t sure this is such a good thing: “Trees may be growing faster, but they could be dying faster,” she points out.) They are also, gingerly, suggesting that this gives us a new way to think about the future of cities. Living systems don’t just exist: They evolve, responding to changes in the rules that govern their existence. And as we try to design more sustainable cities for the future, understanding the full picture of urban ecosystems just might give us a smarter way to shape them.


Pamela Templer’s shady backyard in Brookline has been co-opted into a nitrogen collecting site; three funnels attached to PVC pipes buried in the ground are capturing the nitrogen falling in rainwater and debris from the trees above. Templer, a biologist at Boston University, is working with Hutyra to study how this element is absorbed and used by the city’s soils and vegetation. Fourteen other collection sites are scattered across metropolitan Boston.

We often think of human development as destroying ecosystems, but it’s more accurate to say it creates drastically different ones. Templer and Hutyra are using the nitrogen cycle to measure just how different. Nitrogen is critical for life, and a major component of fertilizers, but at high levels it also can pollute waterways in the form of nitrate that leaches into groundwater. Urban areas get an extra dose of nitrogen through car exhaust, industrial pollution, chemical fertilizers, even the waste of pets. Templer and Hutyra have already found that in Boston, more nitrogen moves from the atmosphere into the soil than previously estimated—twice as much as at the Harvard Forest in rural Petersham. Now they’re finding that even within the city there is wide variation, depending on factors like the roads nearby—which could have implications for how well different kinds of urban environments function without becoming oversaturated with pollutants.

What happens to nitrogen after it shows up in cities is also different from what happens in the woods, because humans are constantly grooming the landscape. In a forthcoming study, Templer and Hutyra estimate that up to half of the carbon and nitrogen present in tree leaves in Boston gets moved out to the city’s yard waste collection sites in the fall. This likely creates an artificial “nitrogen hot spot” at the yard waste center, while removing nutrients from backyard soil and forcing homeowners to replace them with chemical fertilizers. As Boston tries to cut down pollution, Templer says, it will be increasingly important to know just how elements cycle through the city’s plants, soil, and air.


In one sense, the discipline of urban ecology has existed for decades; there’s a long tradition of studying the effects of human development on nature. But more recently it has taken on a new emphasis. “In the last 15 years or so, the discipline has grown through this concept of thinking about cities as ecosystems themselves,” says Dan Childers, an ecologist and sustainability scientist at Arizona State University. Many urban ecologists describe the shift as a change from studying ecology in cities to studying the ecology of cities. They’re not only concerned with the state of wildlife or the “brown and green stuff” like trees and soils, Childers says, but the entire system through which elements like nitrogen, carbon, and water move.

In a recent special issue of the journal Biogeochemisty, a group of ecologists points to the ways underground infrastructure like water and sewer pipes affects a city’s environment. “We’ve just been looking at the surface features of the cities,” says William McDowell, an environmental scientist at the University of New Hampshire, who coedited the issue. McDowell studies the health of rivers and streams by measuring their chemistry; typically, these kinds of studies ignore sewers, water pipes, and natural streams that get piped underground to make way for roads and buildings. But all of those are part of the system called a city.


Though urban infrastructure tends to be seen as separate from natural systems, it’s leaky enough to influence them, and vice-versa. A study led by Jody Potter at the University of New Hampshire examined the influence of infrastructure on the above-ground waterways of San Juan, Puerto Rico. The denser the network of underground pipes, the study found, the higher the levels of elements like chloride, ammonium, and phosphate in the water at the surface, suggesting there’s a relationship between the chemistry of underground and above-ground water. McDowell and his colleagues propose that the underground infrastructure of a city has its own geology and hydrology—they call it “urban karst,” borrowing a term used to describe underground limestone layers with water running through them.

In Boston, research published in 2012 by Nathan Phillips, an ecologist at Boston University, shed light on another product of infrastructure: natural gas leaks from underground pipes. He and his colleagues documented more than 3,000 methane leaks in Boston and 6,000 in Washington, D.C. Though not an immediate safety threat, methane contributes to greenhouse gas emissions and ground-level pollution, and is poisonous to plants.


Even the condition of soils underneath cities isn’t entirely known. Soil can serve as a sink for carbon and nitrogen, keeping it from polluting the air and water. Hutyra and her colleagues studied soils underneath New York City pavement that was getting ripped up for tree planting, and found that it was almost entirely devoid of carbon and nitrogen, with no signs of microbial activity to indicate that the soils still functioned as part of an ecosystem. “These soils had been entombed, we were guessing, for 100 years,” she says. Another study by scientists in Leicester, England, however, found abundant carbon in soils under pavement there, which Hutyra says could reflect the areas’ very different histories, one in a longtime industrial powerhouse and the other in formerly agricultural land.

Today, cities are launching initiatives under the goal of sustainability—which in ecological terms, Childers says, means creating a system that gobbles fewer resources from outside and makes better use of what it has. The new urban ecologists may be key to helping with that goal, for instance by tracking how much various elements in the city are being cycled back into the system rather than simply consumed.

This holistic perspective contrasts starkly with the way that cities are currently run. Though cities like Boston have made impressive efforts to transform themselves into more sustainable environments by boosting tree cover, reducing energy use, and encouraging composting and recycling, city officials have little way to know how all of these measures affect the city’s overall environment, and they don’t manage these efforts as an integrated system.

Brian Swett, Boston’s chief of Environment, Energy, and Open Space, says that the divided nature of city management is one major challenge as we attempt to tackle issues like adapting to climate change and becoming more sustainable. “There isn’t a group yet that has a charge to look systemwide,” he says. While ecologists might study the region’s green spaces as a single system, for instance, on a practical level those areas have different managers—the city, private owners, the Department of Conservation and Recreation, the National Park Service, even the MBTA.

If cities truly want to make progress toward sustainability, it may mean moving away from standalone initiatives and toward a model more like that of ecology, with its broader view of how different elements of the city interact—including us.

Part of this awareness is that cities, like all living systems, grow and change. In the recent Biogeochemistry issue, McDowell and his colleagues coined the term “urban evolution” to describe the idea that cities are in flux: Their chemistry and biology change as they grow; they reconstruct themselves physically and accumulate the byproducts of industry and urban life. Much of their evolution is shaped by human policies, maintenance practices, and personal choices. They adapt in response to rules. So rather than managing cities as they are now, we also need to think about where they’re headed. That means rethinking the typical static solutions for urban problems—building now and worrying about tearing down or fixing later—and creating policies that nudge urban systems onto a better trajectory, knowing that like any living system, cities are always in the process of transforming into something new.


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Courtney Humphries is a freelance writer in Boston.