Guards around NYC's trees help sewers handle storms: Study

Guards protecting the soil around trees along New York City streets are important to helping cut back pollution from the sewer system, Columbia University researchers found.
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Trees with guards can absorb storm water faster, leading to less sewer pollution in rivers and oceans, experts said. Photo: Rob Elliott, Columbia University

City life isn’t easy, even for trees.

 

Their soil gets stepped on by passersby and their trunks serve as bathrooms for the city’s pets. That’s why some of New York City’s street trees have guards or fences that give the trees some space and keep them safe.

 

Those tree guards affect more than just how healthy your street’s saplings look, though. Trees on a protected patch of soil can absorb stormwater more quickly, which eases the burden on the city’s sewer system, according to a new study out of Columbia University recently published in the journal Ecological Engineering

 

Researchers from Columbia Engineering and the school’s Earth Institute looked at how fast water infiltrates into the soil of trees with and without guards. The study focused on trees lining the streets of Morningside Heights in Manhattan.

 

The soil around trees without guards gets trampled by pedestrians, which affects how quickly it can absorb water. Guarded trees absorbed water an average of six times faster than trees without guards.

But why does it matter how well a tree on the street can absorb water? During heavy storms, the city’s aging sewing system can’t handle all the water flowing from the streets as well as homes.

That means some water flows into nearby rivers, “raw sewage and all,” according to Columbia.

If trees on the street could absorb water more quickly, sewers would overflow less.

New York City’s Parks Department lists about 680,000 trees on city streets and already touts their environmental benefits. The city’s stormwater runoff is a “major source of pollution entering wetlands, streams, lakes and oceans,” according to the department, and “healthy trees can reduce the amount of runoff and pollutants in receiving water.”

Each year, more than 1 billion gallons of stormwater are intercepted by the city’s trees, a benefit that NYC Parks values at more than $10 million.

And yet, "only 14 percent of New York City trees have protective guards," said Robert Elliot, the study’s lead author, in a statement. "Our results suggest street trees could manage six times as much storm water if every tree pit were enclosed.”

You can request a guard for your street’s trees, according to NYC Parks, by reaching out to your local elected officials, donating to the New York Tree Trust or by applying for a tree work permit and hiring a private contractor to build one.

 
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