Trees Shed Bad Rap
As Accessories to Crime

By Richard Conniff

Driving the bleak, treeless streets of West Baltimore, through neighborhoods that inspired the NBC series “Homicide” and HBO’s “The Wire,” Morgan Grove recites the evolutionary stages of neighborhood abandonment. First, plywood goes up over the front doors of the two- and three-story brick row houses. When that’s not enough to keep out thieves and addicts, cinder block walls fill in the entryways. Then big spray-painted red “X”s start to appear, meaning the buildings are so dilapidated that even firemen will not enter.

Alumni Profile

Austin Troy '95

Grove, an urban ecologist with the U.S. Forest Service, points out a weedy lot where a house has been demolished in mid-block. It’s like a front tooth knocked out—“The hockey player phenomenon,” he remarks—and a sign that the rest of the block will soon follow.

It’s 90 degrees at mid-afternoon, and the streets are empty of people or any other sign of life. But then Grove turns the corner onto North Carrollton Avenue and, for one small block, it’s an oasis. London plane trees line the sidewalks and lean out toward one another, forming a green archway over the street. The houses appear to be not just occupied, but loved. At mid-block, a stand sells flavored ices, and Justine Bonner, a 74-year-old school teacher, pushes a broom to tidy up in front of the house where she has spent her entire life. The trees, says Bonner, shade the houses and filter the air. “It makes it easier to breathe,” she says, and means it literally.

“It’s just an association. But it’s a very strong association.”

— Austin Troy ’95, University of Vermont

But trees may also help people on this block breathe easier in the sense that they can feel a little less worried about crime. Despite urban folklore that treats all vegetation as a hiding place for muggers, carjackers and drug dealers, new studies in three American cities—Baltimore, Portland, Ore., and Philadelphia—suggest that the right trees in the right place can play a significant role in preventing crime and make even the worst neighborhoods feel safer.

The Baltimore study, co-authored by Grove and published in the journal Landscape and Urban Planning, covers an area of almost 700 square miles, for the most comprehensive investigation yet into the connection between trees and crime. It compares otherwise similar neighborhoods—same income level, same housing stock, same density—and shows that the ones with more trees tend to have a significantly lower crime rate. The researchers are careful not to say that trees cause lower crime rates. “It’s just an association,” says co-author Austin Troy, an associate professor at the University of Vermont. “But it’s a very strong association.” Across the entire study area, neighborhoods with 10 percent more tree canopy cover experienced 11.8 percent less crime than their comparable counterparts after adjusting for numerous socioeconomic and housing factors.

“These studies are not 'research for the sake of research, but research for policy-making'.”

— Morgan Grove, Ph.D. ’96

The results in Baltimore closely match the findings of another 2012 U.S. Forest Service study, using different methodologies, in Portland, Ore. And in Philadelphia, a 2011 study found a substantial reduction in crime, including a 7 percent to 8 percent decrease in gun assaults across most of the city—as the result of a program to clean up and plant trees on 4,300 vacant lots. These studies, says Grove, are not “research for the sake of research, but research for policy-making.” They aim to build common cause with police, public works departments and city planners on the multiple benefits of developing greener cities. They also aim to show public officials “where to strategically target urban forestry investments” for the best payback.

The Baltimore study was the product of a collaboration between Grove and Troy that began when both were at Yale in the early 1990s. (Grove earned his doctorate at F&ES in 1996, and Troy got his masters there in 1995. Both also graduated from Yale College, in 1987 and 1992, respectively.) Together with the third author, Jarlath O’Neil-Dunne, an expert in geospatial analysis at the University of Vermont, they are now principal investigators for the Baltimore Ecosystem Study, a pioneering long-term investigation of urban ecology funded by the National Science Foundation.

Photo by Richard Conniff

Justine Bonner, a 74-year-old school teacher, chats with Morgan Grove, Ph.D. ’96. Bonner said the trees shade the houses and filter the air. “It makes it easier to breathe,” she says, and means it literally.

Their study for Landscape and Urban Planning combined tree-canopy and crime data for the entire city of Baltimore and adjacent Baltimore County, an area where the landscape ranges from crowded downtown streets to cornfields. To detect trees hidden by building shadow and to distinguish canopy trees from shrubbery, the researchers used aerial data obtained with LiDAR, sometimes loosely described as “laser radar.” Spotcrime, which uses police reports to map crimes, provided geo-coded data on robbery, burglary, thefts and shootings—all crimes that are liable to be affected by vegetation and the outdoor environment generally. The study omitted rape and other assaults, because it was too difficult to separate out the large percentage of these crimes that are domestic and happen indoors.

The study was highly data-driven, says Grove, for reasons that go beyond the science. It’s easy for anyone to believe that a tree can hide a criminal or stop a bullet; a lifetime of movies and television shows has indoctrinated people with the idea. So how do you persuade skeptical city cops that trees can also prevent a gun from being fired in the first place? How do you show underfunded urban planners that trees can be a cost-effective tool for stopping crime? How do you persuade people in a crime-ridden neighborhood that the long-term effort of planting and maintaining trees can be a practical way to make things better here and now? In Baltimore, public agencies rely heavily on data to set priorities and make funding decisions. The only way to get trees in the game and get multiple agencies cooperating on city greening, says Grove, is with “really good data.”

AUSTIN TROY '95
The Fate of Cities Tied to Energy

Photo by Susannah Kerest

Over the past few years, the image of the city has begun to shimmer brightly, in both the popular and academic press, as the best hope for sustainable living in a crowded world. Cities are, of course, vastly more efficient than their exurban appendages, with their eternal commutes and zombie subdivisions. But urban enthusiasts may find cause for discouragement—or reason to work harder—in Austin Troy’s new book The Very Hungry City: Urban Energy Efficiency and the Economic Fate of Cities (Yale University Press).

Troy ’95, Yale College 1992, is now an associate professor at the University of Vermont’s Rubenstein School of Environment and Natural Resources. He also served for four years on the Burlington planning commission, and the practical experience seems to have given him insight into the challenges of urban living. At about the same time, the experience of reading his son to sleep with Eric Carle’s The Very Hungry Caterpillar, about the challenges of becoming a butterfly, gave him the title and the beginnings of the idea for this book.

The image of cities as living organisms, each with its own “urban metabolism” of daily inputs and outputs, has been around for a half-century or more. But Troyputs the focus more narrowly on energy metabolism, and he draws a worrisome picture of how cities, as well as their suburbs, over-consume energy sources that are subject, at almost any time, to crippling shortages and price hikes. Los Angeles, for instance, requires such “gigantic amounts” of energy to pump a river of water uphill over the Tehachapi Mountains that its pumping costs spiked by almost $500,000 per hour during the California energy crisis of 2000-2001. Phoenix and Tucson depend on the Central Arizona Project for their water that relies on 14 pumping stations requiring “roughly one quarter of the output of the massive coal-fired Navajo Generating Station.” New Orleans is installing the world’s largest pumping station, “a $500 million monster,” just to keep the water out. The energy costs are not yet known, but in an era of rising seas, coastal cities, like New York, Miami and Venice, should beware.

Minimizing the energy needed to move people efficiently around the urban environment might seem like a relatively straightforward business of reducing sprawl and encouraging high-density development around transit nodes. But “not all density is necessarily good density,” Troy writes. San Diego and Philadelphia, for instance, have roughly the same average density, but people in San Diego log about 25 percent more daily car miles. “Why the big difference? Philadelphia has a real, dominant downtown, where lots of people live and work. In other words, what keeps trip lengths down in a metro area is not just cramming buildings together, but building an urban landscape that maximizes the probability that people will live near where they work, learn, play and shop.”

Stockholm has recently built such a landscape in its Hammarby Sjöstad neighborhood, where centralized and coordinated planning of shared infrastructure ekes the maximum possible benefit from inputs and outputs alike. Mandatory standards for efficient building materials also cut energy use in half, and developers who initially resisted soon discovered that building smarter did not, as one of them put it, add “a krona” to construction costs.

The best hope for making that kind of coordinated effort happen in this country, Troy argues, is through regional cooperation, on the model of Oregon’s Portland Metro, together with changes in federal housing and transportation policy to stop subsidizing sprawl.

Finally, Troy quotes a politician’s description of cities as “the Saudi Arabia of energy efficiency,” with reserves of “potential energy savings that dwarf most of the supply options our country possesses.” So far this potential has gone almost entirely untapped. But Troy makes it clear that we need to get serious about making cities vastly more efficient. Otherwise, the next great economic wave could leave some cities badly stranded, much as our recent economic troubles have done for their exurban counterparts.

The old idea that vegetation causes crime has deep roots, dating back at least 800 years, to when King Edward I required English towns to clear the trees for 200 feet on either side of main roads as a precaution against highwaymen. This line of thinking hadn’t changed all that much by the time Grove first got interested in the topic as a Yale undergraduate in the 1980s. He visited the New Haven Police Department, where someone handed him a thick folder labeled “CPTED.” What had started out in 1971 as the title of a book, Crime Prevention Through Environmental Design, by Florida criminologist C. Ray Jeffery, had become an acronym. The essential idea, incorporating elements of a 1972 book, Defensible Space, by architect Oscar Newman, was to prevent crime by eliminating hiding places and maximizing opportunities for people to see and be seen.

But that often came across as a negative message about trees: Don’t plant conifers and if you must have trees, at least keep the lower branches pruned for better sightlines. Other studies in the 1980s and 90s compounded the problem by arguing that people associate dense vegetation with fear of crime and that dense vegetation can actually encourage crime by providing hiding places and escape routes.

What got lost in the mix was a simple reality: People like trees, and they like neighborhoods with trees even better. Moreover, recent studies have shown that this isn’t just some leafy suburban ideal. The poorest inner-city residents also prefer to live with trees, and they are far more likely to spend time outdoors in areas where trees provide shade and a comfortable space for socializing.

The turnaround in thinking about trees and crime began with a 2001 study of public housing projects in Chicago, comparing buildings that had trees close by with others that were surrounded by pavement. Researchers Frances Kuo and William Sullivan at the University of Illinois in Urbana-Champaign found that buildings with more vegetation had 52 percent fewer total crimes and 56 percent fewer violent crimes. The researchers proposed a straightforward logic: Dense vegetation may perhaps promote crime by facilitating concealment. But that implies the opposite is also true: Widely spaced, high-canopy trees and grassy areas may discourage crime by preserving visibility. Residents who come out to enjoy the shade provide more “eyes on the street,” a phrase Jane Jacobs popularized in The Death and Life of Great American Cities. Being outdoors also builds stronger neighborhood social networks, which tend to make criminals feel unsafe. “One of the classic suspects in environmental criminology,” Kuo and Sullivan concluded, “does not always promote crime.”

All three new studies identify circumstances where vegetation can go wrong. In Baltimore, it’s often in the overgrown border zones between residential and industrial areas. In Philadelphia, weedy vacant lots provide a convenient place for criminals to stash guns and drugs, out of their actual possession when police pass by and yet close enough to be handy at all other times. In Portland, Ore., USFS researcher Geoffrey Donovan was puzzled that, for trees in yards, 42 feet in height seemed to be the critical dividing line. “It was a head scratcher,” says Donovan. There was less crime when a tree was 43 feet tall, and more at 41 feet. “Is this a view-obstruction thing? Finally I sent a student out to measure.” It turned out that when a tree gets to be 42 feet tall, the bottom of its canopy tends to just clear the tops of the first floor windows, meaning clear sight lines from both the house and the street.

“Is it really a placebo effect? That is, is it really the greening?” asks Branas. “Or is it the fact that people show up once a month to do work?”

— Pennsylvania epidemiologist Charles Branas

More important, all three studies also showed how trees can serve as a sort of soft policing tool. Philadelphia’s LandCare program, for instance, doesn’t just clean up vacant lots, but also seeds them and keeps them mowed, plants what are often the only trees in the neighborhood, and installs a knee-high fence as a sort of territorial marker at the perimeter. The result—when University of Pennsylvania epidemiologist Charles Branas compared vacant lots that had been greened with similar lots that hadn’t—was a marked decrease in almost all forms of crime. Moreover, says Branas, the data suggest that crime doesn’t simply move around the corner when green happens. “It's a net decrease.” He plans to follow up next spring with a full-scale experiment, randomly assigning about 200 lots each to the greening treatment, to no treatment at all, or to a treatment in which people remove the trash and then turn up at regular intervals to maintain cleanliness. The aim is to address the persistent doubts of greening skeptics: “Is it really a placebo effect? That is, is it really the greening?” asks Branas. “Or is it the fact that people show up once a month to do work?”

New Haven is also about to take a closer look at the connection between trees and crime, according to Colleen Murphy-Dunning of the Urban Resources Initiative. The new study will look at crime rates around 100 vacant lots that have been cleaned up over the years with help from URI and the New Haven Land Trust. Stacey Maples at Yale and Charles Anyinam at the New Haven Police Department, both specialists in data mapping, are now working together to collect information and sort out variables. “It’s a very debatable issue,” Anyinam told The New Haven Register. “There’s no consensus at all. One group believes this has an impact, another group says no.”

An ample body of research suggests that greening can make a difference. Studies have demonstrated that having trees and grass in the neighborhood reduces stress and anxiety, encourages exercise and generally makes people more civil. But the effect of well-maintained trees also fits the “broken windows theory” proposed in 1982 by social scientist James Q. Wilson. It suggests that broken windows are an invitation to criminals because they convey the message that no one cares about the neighborhood. Grove adds that empty sidewalk tree pits say the same thing.

Photo by Richard Conniff

London plane trees line the sidewalks and lean out toward one another, forming a green archway over North Carrollton Avenue where houses appear to be not just occupied, but loved.

Street trees send the opposite message, and in the Portland study they were always associated with lower crime rates. Both the Portland and Baltimore studies also found that properly maintained trees in public parks have a dramatic effect on crime rates.

All three studies boil down to a few simple rules: 1. Wherever you have a tree, make it look nice, even if it’s just a maple sapling that’s sprung up on a vacant lot. 2. Plant your public parks with tall trees. 3. Plant street trees. 4. Get residents involved in the effort and have them meet their neighbors. 5. Plant yard trees far enough from the house and prune the lower branches, so they don’t block sightlines.

Will it stop crime? “If you’ve got $200 and you want to prevent crime, buy a burglar alarm, not a tree,” says USFS researcher Geoffrey Donovan. “That’s what I always tell people.” But trees are multitaskers, especially on city streets. The list of benefits attributed to them includes moderating rainwater runoff and the attendant flood problems, reducing heating and cooling costs, increasing property values and encouraging people to relax and enjoy their lives.

“Will a burglar alarm shade you on a summer day?” Donovan adds. “Is it going to improve your mental health or even your physical health? It’s not like you can buy a tree and then not lock your doors. But they do provide this wider range of benefits that’s worth considering.”

Trees Shed Bad Rap As Accessories to Crime

Alumni Profile

AUSTIN TROY '95The Fate of Cities Tied to Energy

Trees Shed Bad Rap
As Accessories to Crime

AUSTIN TROY '95
The Fate of Cities Tied to Energy