After Sandy: Are Trees Worth It?
It's been over a month since Hurricane Sandy whirled up the eastern seaboard, but its effects remain. Hundreds of thousands of people are still reeling and recovering. I'm from New York City, so for me it was intensely upsetting and scary to see my hometown underwater. On a phone call with my brother shortly after the storm, he pointed out how much of the damage was tree-related. "This is the problem with urban trees," he said. "They're just not worth it." Is he right?
People did die from trees falling on them or their homes, and buildings, power lines, and roads/sidewalks were all damaged, some badly. Looking at the effects of a disaster like Sandy, it's hard to imagine things being worse. The thing about storms like Sandy is that there are no pipes large enough to control and convey the rainwater. There is no mechanism for controlling the incredible winds.
What trees are really, really good at is mitigating small scale storm water events of one to two inches or so. After this amount of water falls in an urban or heavily paved area, the effectiveness of trees and soil for reducing runoff drops. The one and two inch storms of the world storms do add up to a lot of water, but not all at once. The primary cause of the Sandy devastation was a tidal storm surge, not falling rain.
In high wind, unstable trees and trees with poor structure become a part of the problem, not part of the solution: unstable or rotting trees that fell during the storm demonstrated this starkly. Urban trees, which are often planted in narrow beds with inadequate rooting space, are some of the ones that are the likeliest to be unstable.
One popular proposal in response to the storm, as published in the New York Times and designed by Stephen Cassell with Architecture Research Office and dlandstudio, was a border of wetlands around Manhattan's lowest regions, covering the bottom half of the island like a bib. The idea is that the wetlands would serve as a green infrastructure breakwater, like a spongy seawall, absorbing the quantity and force of incoming water.
While they may not have had an impact during a storm of this size, trees and other green infrastructure are an important part of creating a resilient build environment. Trees do manage incredible amounts of water, mostly in the form of rain. Over a year, a single mature tree is capable of managing thousands of gallons of rainwater through interception, evapotranspiration, and absorption in to the soil, thereby diverting it from taxed sewer systems. Most storms are small. Think of the amount of how much flooding and damage would worsen during ordinary rain events like these without the help of trees and soil.
People will try their hardest to quantify the damage from Sandy, but even our best guesses will be off. I don't have a honed, scientific analysis to back up my speculations here. But from a tree and plant standpoint, I think the most important takeaway from Sandy is the most obvious: cities must adapt to more frequent and more severe storms (and other types of weather – like heat). Taking trees out of cities entirely is not the answer – the consequences of that, given what we know about all of their incredible benefits – would be unacceptable and foolhardy. The most sensible thing we can do is to design urban environments where trees and plants have access to the resources they need to grow and to mature, places where they can create stable, healthy root structures, and then to rigorously maintain and monitor them the way we would maintain any other type of infrastructure that can become hazardous in crisis conditions.
Image courtesy of Architecture Research Office and dlandstudio