New research shows urban trees can help fight crime – possibly by signaling to potential criminals that they're in a more conscientious neighborhood.
After nine months of data analysis on 2,813 homes in southeast Portland, researchers with the U.S. Forest Service's Pacific Northwest and Southern research stations found neighborhoods with tree-lined streets and larger yard trees have lower property and violent crime rates.**
It's the first study examining the relationship between trees and crime in Portland, and it's really quite fascinating. But I had to ask Donovan: How does he know it was actually the trees doing the crime-fighting?
To be sure it was really the trees that made the difference in criminal activity, Donovan and his colleague Jeffrey Prestemon accounted for more than two dozen other variables: Things like the size, age, value and condition of each house, the types of barriers around the house, whether each house had a neighborhood watch, a burglar alarm, a dog or bars on the windows.
"You have to control for those things," said Geoffrey Donovan, the research forester who led the study. "Otherwise the trees become a proxy for nice neighborhoods."
All other factors being equal, Donovan said, neighborhoods with trees lining the streets and larger yard trees saw fewer property crimes, such as burglary and vandalism, from 2005 to 2007.
“We believe that large street trees can reduce crime by signaling to a potential criminal that a neighborhood is better cared for and, therefore, a criminal is more likely to be caught,” Donovan said. “Large yard trees also were associated with lower crime rates, most likely because they are less view-obstructing than smaller trees.”
It's the opposite of the broken-window hypothesis, he said, which postulates that neighborhoods with signs of disrepair more are more likely to experience other crimes. His findings show trees are a stronger deterrent of property crimes than violent crimes.
The researchers used crime data from the Portland Police Bureau from 2005 to 2007 to do their analysis.
“We wanted to find out whether trees, which provide a range of other benefits, could improve quality of life in Portland by reducing crime, and it was exciting to see that they did,” said Donovan. “Although a burglar alarm may deter criminals, it won’t provide shade on a hot summer day, and it certainly isn’t as nice to look at as a tree.”
They examined only crimes for which a physical address was given and paired that information with additional data from aerial photographs from Google Earth, onsite visits, and the Multnomah County Tax Assessor’s Office.
Their sample of 2,813 single-family homes around the intersection of SE 82nd Avenue, SE Foster Road, and SE Powell Boulevard experienced 394 property and 37 violent crimes over the two-year window. That neighborhood has a crime rate that's roughly 50 percent higher than the national average, Donovan said.
Of the tree variables analyzed, canopy size of both street and yard trees and the number of trees growing on a lot had the most effect on crime occurrence—large trees were associated with a reduction in crime, while numerous small trees were associated with an increase.
While street trees help fight crime no matter what size they are, smaller trees in yards may increase crime by obstructing views and providing cover for criminals, the study found. Donovan recommends homeowners keep smaller trees pruned and carefully select the location of new trees so they won't block window views.
Donovan has also studied the effect of trees on property values and is looking into the effect of trees on birth outcomes.
"They affect a whole range of other things, too," he said. "If you were to spend money just on crime reduction, you're better off spending it on a burglar alarm. But that won't increase your house or reduce your energy use and storm water runoff. It's difficult to sit in the shade of a burglar alarm and enjoy a cold drink."
**The study shows that for every 1,000 houses, there are 4.5 fewer crimes per 100 square feet of canopy cover. The canopy cover is the span of a tree's top when viewed from above.
Donovan offered two examples to illustrate the crime rate reduction from trees on the street and in the yard:
- If you put a 35-foot tree along the street in front of every house, you'd see 45 fewer crimes per 1,000 homes. If you put it in a yard, however, you'd see five more crimes per 1,000 homes – likely because of obstructed views.
- If you put a 50-foot tree along the street in front of every house, you'd see 90 fewer crimes per 1,000 homes. If you put it in the yard, you'd see 10 fewer crimes per 1,000 homes.