So much depends on a tree guard

January 23, 2018 by Kim Martineau, Columbia University
So much depends on a tree guard
In a new study, Columbia researchers find that street trees with protective guards soaked up runoff water six times faster than trees without guards. Credit: Lizzie Adkins, Columbia University

In a big city, trees, like people, like their space. In a new study, researchers at Columbia University found that street trees protected by guards that stopped passersby from trampling the surrounding soil absorbed runoff water more quickly than trees in unprotected pits. The results are published online in the journal Ecological Engineering.

Comparing the infiltration rate of street with and without guards in Manhattan's Morningside Heights neighborhood, the researchers found that trees in protected pits absorbed water six times faster on average than tree pits without guards—3 millimeters versus .5 millimeters per minute. The researchers hypothesize that the guards improve infiltration by limiting soil compaction in tree pits.

"Placing guards around tree pits allows urban trees to absorb more , taking pressure off the city sewer system," said the study's senior author, Patricia Culligan, a professor at Columbia Engineering and a member of Columbia's Earth Institute and Data Science Institute.

The researchers were surprised at what a difference the guards made, and how little mulch or additional vegetation improved results. City-recommended tree guards cost about $1,000, depending on their style and size of the tree pit, but an improvised $20 fence can work just as well, said the study's lead author Robert Elliott, a recent graduate of Columbia Engineering and cofounder of Urban Leaf, a New York City startup helping city dwellers grow food at home.

"Only 14 percent of New York City trees have protective guards," he said. "Our results suggest street trees could manage six times as much storm water if every tree pit were enclosed," he said.

New York's 660,000 trees cover about 6 percent of the city. Besides cleaning and cooling the air, trees benefit the city by soaking up rainwater that runs off its impervious roads and buildings. During heavy storms, the aging network of sewers is unable to keep up with the combined flow of wastewater from streets and homes. As a result, heavy flows are often released directly into nearby rivers, raw sewage and all. To reduce these combined sewer overflows the city has turned to 'green infrastructure,' or engineering solutions that harness trees and other vegetation to drain the built landscape.

The push has spurred a related effort to measure the cost-effectiveness of each solution. In a related unpublished study, Elliott and his colleagues compared the relative costs and benefits of guarded tree pits to bioswales, which are pits dug into sidewalks and planted with shrubs to serve as catch basins. They found that bioswales substantially outperformed guarded tree-pits when factoring in their initial cost plus maintenance over 10 years; Bioswales captured 141,886 gallons of water per year, at 20 cents per gallon, compared to a guarded tree pit's extra 1,132 gallons per year, at 49 cents per gallon.

But trees become more competitive, says Elliott, if the cost of guards is reduced or water flows are increased by digging out the tree pit's curb so from the street can flow in (much like a bioswale. Trees also tend to attract less controversy, he points out. Residents have complained that some of the 3,000 bioswales New York City has installed in the last five years in Brooklyn, Queens and the Bronx are ugly.

Explore further: 20 percent more trees in megacities would mean cleaner air and water, lower carbon and energy use

More information: Robert M. Elliott et al, Stormwater infiltration capacity of street tree pits: Quantifying the influence of different design and management strategies in New York City, Ecological Engineering (2017). DOI: 10.1016/j.ecoleng.2017.12.003

Related Stories

Trees can make or break city weather

July 26, 2017

Even a single urban tree can help moderate wind speeds and keep pedestrians comfortable as they walk down the street, according to a new University of British Columbia study that also found losing a single tree can increase ...

California 'street tree' benefits valued at $1 billion

June 14, 2016

Streets lined with gold? Not exactly, but a new report from the U.S. Forest Service's Pacific Southwest Research Station estimates trees lining Californian streets and boulevards provide benefits to municipalities and residents ...

Downed trees not necessarily a lost cause

September 19, 2017

Among the devastation wrought by Hurricane Irma last week, many downed trees and uprooted plants were left in the storm's wake. Those in a rush to get things back to normal have been quick to break out the chainsaws and remove ...

Recommended for you

Sierra snowpack could drop significantly by end of century

December 11, 2018

A future warmer world will almost certainly feature a decline in fresh water from the Sierra Nevada mountain snowpack. Now a new study by the Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) that ...

A glimpse into future oceans

December 11, 2018

Something peculiar is happening in the azure waters off the rocky cliffs of Ischia, Italy. There, streams of gas-filled volcanic bubbles rising up to the surface are radically changing life around them by making seawater ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.