New York City’s Data-Driven Approach to Counting Its Street Trees

At the northwest corner of Washington Square Park in Manhattan’s Greenwich Village there’s a tree, an English elm.

Also known as “Hangman’s Elm,” the tree is said to be over 300 years old, stands about 130 feet tall and has a trunk diameter of more than five feet, according to the New York City Department of Parks and Recreation . As legend goes, traitors were hung from its branches during the Revolutionary War. It’s one of the “ Great Trees of New York City ,” a designation awarded to only a small fraction of the city’s megaflora.

Outside the park, on city blocks surrounding the place where the tree stands, lining sidewalks and thoroughfares from the upper Bronx to outer Brooklyn, are hundreds of thousands of the elm’s less-storied brethren. “Street trees,” as they’re called.

At last count they totalled 592,130.

But that figure, tallied about 10 years ago, prior to two hurricanes and several tornadoes , is now considered outdated. And later this month, the parks department, working with a cadre of volunteers, will kick off a its latest street tree census, dubbed TreesCount! 2015 .

“It’s time for us to reassess our population,” said Jacqueline Lu, director of geographic information systems and analytics for the parks department’s forestry, horticulture and natural resources group. Since the mid-1990s, the census has occurred on a once-per-decade cycle. But, as it stands, Lu notes, “There isn’t a current inventory for street trees in New York City.”

The census does not look at the trees inside parks, only the ones along city streets. This year it will involve a new set of techniques and technology, intended to help the department create an interactive, citywide map meant to show the near-exact location of each tree.

Past efforts were less precise. During the previous census, and one before carried out from 1995 to 1996 , volunteers inventoried approximate tree locations based on nearby addresses.

“It didn’t give you the sort of map that you could take out with you and stand on the corner and say, oh yeah, this tree on this map, is that tree that is standing in front of me,” Lu said.

Volunteers participating in the latest census will use a method developed by a nonprofit group called TreeKIT . It is designed to help people with limited training accurately record the location of street trees. This approach involves both traditional measuring tools, and a new mobile application called Treecorder. The app allows people to use their smartphones or tablet devices to punch in information they collect about the location and condition of trees.

The parks department will add this crowd-sourced information to a database that is part of its newly deployed forestry management system. That system will help guide the department’s future decisions about planning and maintaining the city’s “urban forest.” Where to plant and what to prune, for instance.

“It’s getting that fundamental piece of information, where are there street trees and where aren’t there street trees.” Lu explained. “You can’t manage what you don’t know.”

Volunteers assisting with the census will get some basic gear from the department: a tree identification guide, a tape measure and a loaned tool called a measuring wheel. The measuring wheel is about the size of a walking cane, it has a wheel on one end, and records distance on an odometer as it’s pushed forward.

The method for recording data is fairly straightforward.

Starting at the corner of a block, a volunteer, or “voluntreer” as the parks department calls them, pushes the measuring wheel along the edge of the curb until they reach a tree. They then record this distance. They also find the tree’s trunk circumference using the tape measure, and its species by checking the shape of its leaves against the identification guide.

The volunteer documents a variety of other information as well. For example: Does the tree appear to be healthy with dense foliage? Is there a metal tree guard around the dirt area where it is planted? Has the sidewalk buckled due to the growth of the tree’s roots?

Data about each tree can be entered directly into the Treecorder app as volunteers make their way down the street. Or it can be written on a paper form and entered later using a computer.

After the first tree is catalogued, the volunteer resets the measuring wheel odometer and advances to the next tree on the same side of the block. The process gets repeated until they reach the block’s end. At that point, the trees on one side of one block are recorded.

Philip Silva and Liz Barry are the co-founders and co-directors of TreeKIT. They began to prototype the tree mapping methodology around 2010, according to Silva.

Barry is also the director of urban environment at an organization called the Public Laboratory for Open Technology and Science . “A lot of her work is really around putting the tools of scientific inquiry in the hands of people who don’t identify as scientists,” Silva said during an interview on Thursday. “She’s also got a fantastic background in cartography.”

Silva has a background working on urban environmental projects. The idea for TreeKIT originated with work he was doing in the South Bronx to plant street trees in an area with high childhood asthma rates. In the course of that work, he realized there was not a good way for people to know what types of stewardship others were undertaking to care for various trees.

“No one really knew what anyone else was doing,” he said.

So he and Silva began to discuss the idea of a public map where people who wanted to maintain trees could click on one and see the current status of how it had been cared for.

But there was a problem.

“In order to do it we actually needed to know where the darn trees were,” he said.

They realized quickly that global positioning system technology would not be a simple solution, and that smartphones, which were less ubiquitous at the time, could not always provide accurate enough location information to pinpoint a tree’s location.

Gradually they put together the site surveying method now being used for this year’s tree census. And working with web and mapping technologists, they were able to develop a system to enter data and output files that could be used for digital cartography.

As part of a proof of concept for the parks department, TreeKIT mapped about 500 blocks with roughly 10,000 trees in portions of Queens. The department then found funds to help build the mobile data entry application. The project was never a full time job for Silva or Barry.

“It’s been a little bit of a beg, borrow and steal process,” Silva said.

He gave the parks department credit for embracing TreeKIT. “They were willing to try something innovative with outsiders,” he said, “people who were not part of their agency.”

In Silva’s view, the map that the census will allow the parks department to produce is an important first step toward a better understanding of the city’s treescape. “The spatiality of things has become so important, and a city like New York has invested now, at this point, millions of dollars in this living infrastructure,” he said, referring to the city’s street trees.

“We rely on volunteers to keep them alive,” he added. “Just giving the folks that take care of them the tools to take care of them better is the most important part of this.”

Trees can provide a number of benefits for urban environments. They soak up carbon dioxide , capture rainwater that would otherwise flow into combined sewer systems, and shade buildings, saving energy. As part of the previous tree census, Lu noted that the Parks Department worked with the U.S. Forest Service to quantify some of these ecological gains .

These calculations, she points out, helped convince then-Mayor Michael Bloomberg, a big supporter of data-driven city policymaking, that trees were a good investment and paved the way for MillionTreesNYC .

The objective of that public-private program is to get one million new trees planted in New York over a decade’s time. According to information published by the city online , New York is nearing that goal with 958,728 trees planted as of Friday.

As for this year’s census, counting will begin on May 19, according to Lu, and will run through the fall.

On Thursday, she said that more than 920 people had registered to take part, and that more than 220 had completed online training. In addition to the online training requirements, census volunteers also need to attend a field course that lasts about three hours. But for roughly two hours of that time they’ll be logging actual street tree data.

Data collection is also slated to take place at other events organized by the parks department and by a number of partner organizations that include neighborhood and civic groups. People who want to map independently can do that as well. Over 30 groups are currently signed up, including a chapter of the American Littoral Society , Harlem Pride and NYC Civic Corps .

“The level of interest has been really fantastic,” Lu said.

She also stressed that the data collection process should be “akin to going for a walk around the block with your friend, looking at trees as you go.”

A self-described lifelong New Yorker, Susan Craine, has registered to participate in the census and plans to focus on trees in the Staten Island neighborhood where she lives.

“I’m very passionate about environmental issues,” said Craine, who is also a licensed citizen pruner . “I understand that having an accurate tree census is very helpful for the environment and for the people living in New York City in many ways that not everybody realizes.”

Why does she think trees are important?

“There’s an aesthetic piece in the world that people kind of don’t think about very much,” she said. “There’s lots of studies about how looking at some type of greenery, or experiencing some type of nature, actually has benefits for you mentally.”

TreeKIT’s Silva pointed out that street trees also provide an important place for people to gather, especially for the oldest and youngest New Yorkers, and others who might have trouble traveling to parks or open spaces.

“I really like the way in which they create a place for people to come together outside,” he said of the trees. “I think we have too few of those these days and it’s great that, in addition to all the other stuff trees do for us, they can also create community.”