Silly as it may seem to the public, there's an intense disagreement among scholars about the impact urban trees have on a city's crime rate. Some are convinced urban greenery increases crime — arguing that low trees and shrubs, in particular, create a natural hiding place for criminals. A 2001 case study of auto thieves in Washington, D.C., found that offenders often target areas near dense vegetation because it can "reduce effort and risk by offering concealment."
Others are convinced that urban trees have exactly the opposite effect. This crowd argues that trees actually decrease crime either by attracting more people to public places (Jane Jacobs' "eyes on the street" theory) or by signifying to criminals that people care about their neighborhood (James Q. Wilson's "broken windows" theory). Another 2001 study, this one of public housing in Chicago, found that "the greener a building’s surroundings were, the fewer crimes reported."
Where better to settle the score once and for all than Baltimore? If trees can have a positive impact on crime in the land of The Wire, then it seems safe to assume they can have an impact anywhere.
Well somebody tell Omar the game just changed. In the June issue of Landscape and Urban Planning, a team of environmental researchers led by Austin Troy of the University of Vermont report an inverse relationship between tree canopy and a variety of crimes in the Baltimore city and county regions. All told, Troy and colleagues conclude that "a 10% increase in tree canopy was associated with a roughly 12% decrease in crime."
For the study, the researchers examined crime and canopy rates over a long stretch of area from inner city Baltimore to the exurbs of Baltimore County. The crime rates varied greatly across the region: those in the city were 3.5 times the national average per block, while those in the outskirts were "nearly non-existent." (They limited their study to robberies, burglaries, thefts, and shootings, since assaults typically take place indoors.) So too did the canopy rates: some parts of the city have no greenery; some parts of the county achieve 87 percent coverage.
Troy and his colleagues ran all types of models to analyze the relationship between crime and canopy. Even after controlling for factors known to influence crime statistics — income, race, population density, and the like — they found the aforementioned link between more trees and less crime. While they doubt the connection is "purely causal," the strength of their figures suggest "some genuine relationship between trees and crime."
The results were not entirely straightforward. In some pockets of Baltimore city, for instance, the presence of trees indeed seemed to increase crime rates, just as some of the previous literature suggests. That was especially true in the outer harbor areas of Brooklyn Park, Wagners Point, and Dundalk — places with poorly groomed vegetation where "the concealment value of the vegetation outweighs its deterrent effect":
This last result clarifies much of the confused relationship between urban greenery and urban crime: While low dense brush seems to increase it, tall broad canopies seem to decrease it. That nuanced conclusion harmonizes with another study published earlier this year, in which U.S.D.A. Forest Service researcher Geoffrey Donovan (who has also linked urban tree coverage to home prices) reports the same mixed tree-crime associations in Portland, Oregon. If the evidence tells us anything certain, it's to keep your shrubbery close, but your canopies closer.