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There is ongoing debate over whether trees and their maintenance encourage or prevent crime. Some studies have shown a positive correlation between trees and crime, while others find that well maintained outdoor space can enhance social cohesion and reduce the population's sense of fear (Braverman, 2008). Some scholars argue that vegetation will also impact criminal behaviour because ”when looking for a place to commit crime, a perpetrator would move on to a neighborhood where cues suggest a weaker social organization and lesser neighborhood involvement” (Troy, Grove, O'Neil-Dunne, 2012, p. 263), while others link increased vegetation to the mitigation of “mental fatigue” that would otherwise lead to violence (Kuo & Sullivan, 2001, p.347). Others view street trees as a method through which government can influence residents' behaviour and sense of safety (Braverman, 2008).

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North American institutions spend money removing vegetation in an effort to reduce or prevent crime, however some scholars have found that introducing more vegetation can help address crime in poor inner-city neighbourhoods (Kuo & Sullivan, 2001).  In one study, researchers used police reported crimes in a Chicago public housing project with a homogeneous population, variable greenery, and similar buildings. By conducting an Ordinary Least Squares regression, they found that the presence of vegetation and prevalence of crime were negatively correlated (Kuo & Sullivan, 2001).These findings shape the theory that healthy urban trees may drive crime away (Braverman, 2008). There is some existing research about the benefits of city trees for climate and weather adaptation as well as the potential economic benefits for energy use reduction. These features are beneficial to the human experience of the city, including the health and social aspects of a community. As a result, many North American cities spend money to monitor their trees, as seen in the tree surveys of Vancouver, Portland and New York.

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The different findings surrounding vegetation and crime may be a result of analyzing different kinds of vegetation (Troy, Grove, O'Neil-Dunne, 2012). The type of vegetation present is an important consideration, as studies have found that there is a prevalent public fear of dense vegetation and there are links between criminal activities and visibility-blocking vegetation (Kuo & Sullivan, 2001). An article by Kuo & Sullivan suggests that some forms of vegetation can reduce crime by “increasing surveillance and by mitigating some of the psychological precursors to violence” (p. 346), particularly by encouraging the use of outdoor space and subsequently increasing street surveillance (2001). It is also important to consider the context and land use in which the vegetation is located: in residential areas, more vegetation is linked to less crime (or fear of crime) while the opposite is true for nonresidential areas (Kuo & Sullivan, 2001). Throughout history, there has been a trend of trees being inaccessible to the urban poor, therefore the prevalence of crime can be attributed to variables other than vegetation, but vegetation can act as an indicator of the social conditions (Braverman, 2008). One study in Baltimore examines the relationship between crime and vegetation using fine resolution data and incorporates different land use conditions: it looks at the difference between private and public land and finds that crime has an overall negative correlation with vegetation but the impact is greater on public land (Troy, Grove, O'Neil-Dunne, 2012). One study on the topic in Portland examined the relationship between vegetation and crime at single family homes, finding that large trees were negatively correlated with crime while the opposite was true for shorter vegetation (Donovan & Prestemon, 2012).

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There have also been some studies linking lighting with crime, that theorize insufficient lighting will increase the prevalence of crime. One study carried out in Northumberland in England uses GIS and qualitative research to identify regions in need of additional lighting (Pain, MacFarlane, Turner & Gill, 2006). The associations made between the amount of streetlight and presence of crime are possibly attributable to the enhancement of community cohesion by increased usability of outdoor paths and facilities.

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My research project draws from theories in criminology, and  expected relationships between vegetation, lighting and crime to observe city-wide patterns in Portland, Oregon.