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Urban Forests Make Cities More Resilient to a Changing Climate

Sandy_night_NYC, Avenue C at East 6th Street, Oct. 29, 2012. Photo: David Shankbone/Flickr

The loss of life and property damage from Superstorm Sandy is still being tallied, but the catastrophe is pointing a spotlight on the need for cities to adapt to more frequent, severe storms (also referred to, in many scientific circles, as "climate change").

"Anyone who says that there's not a dramatic change in weather patterns I think is denying reality," New York Governor Andrew Cuomo said on Tuesday.

Bill Ulfelder, the New York director of The Nature Conservancy, has only lived in New York City for three years, but during that time the city has seen its two most costly storms (Irene and Sandy) over just 14 months. "You're going to see more and more of this," he told me on Tuesday.

Fortunately for all of us who like being outside, one way to make cities more resilient to these storms is to foster large, healthy urban forests.

Have you ever grabbed a low-lying branch of a large tree after a rainstorm and shook it in order to soak your friend or sister or whomever is standing under its canopy? Then you're a jerk. But you're also good at demonstrating the power of a single tree to retain rainwater. Over the course of a year, a single mature tree can intercept several thousands of gallons of stormwater that, in developed areas, could contribute to flooding or at least fall directly into often-overwhelmed sewage systems.

Yes, most post-storm images feature downed trees that snap power lines, but without trees flooding events would worsen, as cities that have become denuded over many decades have learned. Houston, Nashville, New Orleans, and Philadelphia are among the U.S. cities that are reforesting some of their most environmentally sensitive areas in an effort to become more resilient to storms.

"I'm seeing cities beginning to recognize that parks and green infrastructure play a dual role," said Catherine Nagel, executive director of the City Parks Alliance. "They can serve to mitigate against storm events and also provide recreational opportunities."

In 2010, Nashville was hit by 13 inches of rain over 36 hours—the biggest rainfall in its recorded history—that claimed 11 lives and cost $2 billion. In reaction, the city launched a flood control plan that includes a buy-out program to relocate homes in the most vulnerable areas and replace them with parkland. Houston is also cultivating its urban forest as a tool for easing the chronic flooding that plagues the city. Philadelphia is planting its way into compliance with storm-water regulations. Tree-planting is often paired with a transition to porous asphalt, which also absorbs stormwater. The issue is not just the volume of water that storms bring, but also what that water carries: salt, oil, gasoline, hydraulic fluids, and a witches brew of other contaminants that could otherwise move directly into sewers (and, in overwhelmed combined sewage/wastewater systems, directly into streams, lakes or oceans).

This does not account for the other benefits of an urban tree canopy. Buildings shaded by trees require less energy to cool, and one tree can remove up to 26 pounds of carbon a year, the equivalent of 11,000 miles of car emissions.

MORE THAN TREES
As they adapt to a changing climate, cities are also turning to green roofs, wetlands, and even some unexpected helpers, such as oysters, to become more resilient.

Green roofs are taking root and appear to be more than a passing fad. "I visited New York City's largest green roof this summer," said Ulfelder. "It's above a post office building and has reduced its energy bill by $1 million a year, because the building is now cooled more easily and holds its heat better in winter. They're getting an incredible rate of return. When you get these big storms, you can use green roofs to capture rainwater. That's good for water quality, but you can actually use green roofs to restore habitat by planting native species."

But for coastal cities, the brunt of a major storm is generally seen where fresh water meets the sea. Indeed, Central Park's 843 acres could not squelch the record-breaking storm surge than inundated lower Manhattan on Monday. That said, a bivalve mollusc called Crassostrea virginica, or the Atlantic oyster, could have, at least, helped.

That is the point of an opinion piece published in the New York Times on Monday. In it, Four Fish author  Paul Greenberg briefly describes the oyster's long history in the New York Harbor. Before they were over-farmed, their numbers were legion and the massive oyster reefs, deposited over thousands of years, acted as a sort of buffer between the sea and freshwater.

Greenberg wrote: "Just as corals protect tropical islands, these oyster beds created undulation and contour on the harbor bottom that broke up wave action before it could pound the shore with its full force. Beds closer to shore clarified the water through their assiduous filtration (a single oyster can filter as much as 50 gallons of water a day); this allowed marsh grasses to grow, which in turn held the shores together with their extensive root structure."

It won't happen overnight, but sea oyster reefs are slowly being restored in New York Harbor. So too, noted Ulfelder, are the wetlands that were largely displaced as New York (and other coastal cities) were built up. "There are only a small percentage of wetlands left that were here when New York was settled," he said. "But the ones here are important, so The Nature Conservancy is studying how rising sea levels are affecting these wetlands and we're studying ways to move them inland."

Cities are a new focus for the 61-year-old organization. "I think we had an epiphany," said Ulfelder. "With the discussion of our global population growing from seven to nine billion, and the coming shift to 75 percent of the world inhabiting cities, [we said] that we are The Nature Conservancy and we run the risk of becoming quaint and irrelevant if we don't engage in urban conservation."

 —Mary Catherine O'Connor
@mcoc



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