Natural Deterrent

If you’re looking to relocate and poring over crime statistics, chances are the number of trees in the potential neighborhood isn’t the first variable that to comes to mind. But a new study co-authored by John MacDonald, professor of criminology, suggests that tree cover may actually play a role in violence prevention.

“I came across some literature on the impact of trees on health,” says MacDonald, the Penny and Robert A. Fox Faculty Director of the Fels Institute of Government. “There were a couple studies on crime, but they weren’t always convincing. If I told you that houses with nicer trees around them can have less crime, you might say, that’s probably just because they’re in nicer neighborhoods.”

But MacDonald says it’s more complex than that. After reading a paper written by Geoffrey Donovan, in which the U.S. Forest Service researcher linked neighborhoods where beetles had killed the commonly-found ash tree with a higher mortality rate, MacDonald approached him, curious about a potential correlation with crime. The resulting study, “The Association Between Urban Trees and Crime: Evidence From the Spread of the Emerald Ash Borer in Cincinnati,” was published in Landscape and Urban Planning.

“I suggested to him that we dig deeper and he loved the idea,” says MacDonald, who has a background in studying how other variables in urban settings—like the prevalence of vacant lots—affect crime. “He knew from his work that Cincinnati was a city that had been hit particularly hard by the beetles, so he contacted their local station and they pointed me in the right direction.”

In order to correlate tree death and crime, MacDonald studied the coordinates provided to him by the Forestry Service and compared them with crime reports. He found that, across the board, most major categories of crime went up after the beetles had attacked.

“There are many hypotheses. For instance, if you have trees with canopy on them, they may block visibility into homes, which is a natural deterrent to possible intruders,” says MacDonald. “This theory is given credence by the fact that property crime turned out to have the highest increase when trees were absent.”

Trees also reinforce the environmental health of a neighborhood, improving air quality and increasing the amount of shade for residents—especially poignant given that there have been numerous studies linking an increase in temperature to an increase in violent crime.

“Tree and foliage death creates a sense of blight, which makes any neighborhood—regardless of its economic standing—less desired over time,” says MacDonald. “If we can offer strong proof that planting more trees would reduce crime, and have other health benefits, then it isn’t hard to imagine spending a couple hundred dollars to deter crime while also adding value to the homes.”

MacDonald says the idea holds special promise because it’s non-inflammatory.

“It’s not like we’re sitting here debating people’s Second Amendment rights,” says MacDonald. “I mean, who’s against having decent trees?”