A new study looking across Baltimore City and Baltimore County has found that with few exceptions, the frequency of crimes reported in a particular block or neighborhood goes down as the tree cover gets thicker. Just a 10 percent increase in leaf canopy was associated with a 12 percent drop in crime, it concluded.
The study, published online in the journal Landscape and Urban Planning, supports arguments by advocates that environmental factors, and not just more police, can fight crime. And it challenges the notion that thick vegetation gives cover to car thieves, muggers and other would-be criminals.
"It stands to reason," said Sarah Lord, head of Baltimore's forestry board, "because a shade tree allows you to sit out on your steps and be more neighborly and watch out for the community."
While shrubs may shield bad behavior, mature, well-tended trees do just the opposite, said J. Morgan Grove, a social ecologist with the U.S. Forest Service and one of the study's three authors. That could be important for a city like Baltimore, where trees cover just 27 percent of its landscape and some neighborhoods are practically barren.
The study, underwritten by the Forest Service and the National Science Foundation, looked only at the statistical relationship between trees and crime, without trying to prove cause and effect. But Grove, who's part of a wide-ranging, long-running ecological study of Baltimore, had a couple of theories about why trees might curb criminal activity.
Trees "get people outside," he said in a recent interview while walking around Franklin Square, a West Baltimore neighborhood with both leafy and treeless blocks. Shady streets are cooler in summer, encouraging people to sit or stroll outside. And criminals likely avoid places where their deeds might be spotted and reported, Grove suggested.
It's also likely that a block lined with healthy trees encourages troublemakers to see it as a tight-knit area where people look out for each other, Grove added.
That's a variation on the "broken-window" theory that suggests visible signs of disrepair like a broken window tend to encourage vandalism and escalating criminal activity if not remedied.
"In the tree world, we call it the 'empty tree pit' theory," said Grove, referring to the holes cut in sidewalks to accommodate trees. "If you have trees in the pits ... they're symbols of the fact that the neighborhood is cared about. ... If they see you breaking into someone's car, they're going to call the cops."
Another study, also by Forest Service researchers, of 2,800 homes in Portland, Ore., found fewer crimes around places with many large, mature trees. But the Baltimore study is the first to analyze such a large area, Grove said.
Using Spotcrime, an online crime mapping service, the researchers developed an index of all robberies, burglaries, thefts and shootings reported throughout the city and county from 2007 to 2010. Researchers mapped tree canopy information from satellite imagery and then fed all the data into a computer to match it up, filtering out other factors that might influence the results, such as income.
The link to reduced crime was most apparent on public land, such as parks, schoolyards and other government property with lots of mature trees. But tree-lined streets and avenues also had somewhat lower criminal activity.
Grove said he hoped the study's findings would boost and help guide tree-planting efforts in Baltimore.
Bert F. Shirey, retired deputy city police commissioner, who grew up in Northeast Baltimore, said the study rang true to him.
"The neighborhoods that had more trees and more gardens and things seemed to have less crime," he said. There may have been other factors, he added, and the greenery may just indicate that people took better care of their property and themselves.
John M. MacDonald, chairman of the University of Pennsylvania criminology department, has done research indicating that greening up a neighborhood could help ward off trouble. But he thought this study exaggerated the crime-fighting ability of trees.
"Baltimore is not going to reduce its crime problem on every block 10 percent by planting 10 percent more trees," he said, adding that other studies have shown a similar increase in police has less than half as much impact.
Erik Dihle, the city's arborist, welcomed the report, noting that his budget has been cut the past two years, from $4.4 million to $2.9 million. Even so, with the help of other agencies and nonprofits, he hoped to boost the number of trees planted citywide, from nearly 7,000 last year to 9,000 in the coming year.
The study might help overcome the resistance Dihle encounters to planting trees, he said.
Barbara Rock, a longtime Franklin Square resident, gets it.
In her block, residents recently persuaded the Parks & People Foundation to plant saplings. She and her neighbors look out for their trees and each other, she said. Around the corner on a virtually tree-less street, she said, there's drug dealing and frequent police sirens.
"They make it much nicer," Rock said of the trees. "Once they grow up, they'll be OK."