Urban Forests Make Cities More Resilient to a Changing Climate

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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
, 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

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

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

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
, 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

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

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