Research has shown that urban trees provide a range of benefits to communities, including moderating storm-water runoff, reducing heating and cooling costs, and increasing property values. However, less attention has been focused on the potential of urban trees to affect another important determinant of urban quality of life: crime occurrence. Results from studies that have looked at the effect of trees and other vegetation on crime have been mixed; some have shown that vegetation can increase fear of crime whereas others have shown that it can reduce the fear of crime or crime occurrence. The goal of this study is to evaluate the effects of different types of vegetation on crime in Portland, Oregon, while accounting for other relevant variables.
Primary Research Question
What is the relationship between trees and crime? Specifically, is vegetation on a property associated with higher or lower crime rates?
This research report (a) studies crime occurrence in single-family homes, (b) uses a multivariate description of vegetation, (c) controls for property features besides vegetation that have been shown to affect crime, and (d) assesses the relationship between vegetation and various measures of crime and not just indices of aggregate violent and property crime.
The study was confined to the southeast precinct of Portland, which was chosen for its high proportion of single-family homes. Crime data for 2002 to 2007 were obtained from the Portland Police Bureau. Crimes were divided into seven categories: (a) aggravated assault, (b) burglary, (c) larceny, (d) motor-vehicle theft, (e) robbery, (f) simple assault, and (g) vandalism (murder and forcible rape were excluded for privacy concerns, and their occurrence was probably too low to estimate separate models anyway). Data on a house’s assessed value, age, size, and lot size were obtained from Multnomah County Assessor’s Office. The existence and size of trees were measured from aerial photographs. Because the types of structures surrounding a house may affect crime occurrence, the authors categorized every building as single-family, multiple-family, commercial, or industrial. The authors also categorized any barriers in the front or back of the house (fences, walls, bushes).
Analysis was limited to single-family homes because the relationship between trees and crime may be different for multiple-family homes. The model could not take into account any crime-prevention measures taken by residents not observable by the analysts. The model also did not take into account the presence of street lights or increased foot traffic around some homes.
Variables that decreased the probability of a criminal being observed (high back yard barrier and the number of trees) or increased the probability that a criminal would encounter a house (corner lot) increased crime occurrence. Variables that increased the probability of a criminal being observed (street light, Neighborhood Watch Sticker, and burglar alarm) decreased crime occurrence. The effect of trees was mixed. Lot trees small enough to block the view from a first-floor window increased crime occurrence. However, larger lot trees and street trees (which are farther from a house than lot trees) decreased crime occurrence.
Although the effects of trees on crime were relatively modest, the crime-reduction impact that larger trees and street trees may have may provide a spur to tree planting. However, homeowners may be wise to prune trees to prevent them from obstructing views and select the species and locations of future trees with care. The study also suggests that some crime-prevention measures may reduce crime by giving signals to potential criminals.