A study of single-family homes in Portland, Oregon found that trees in the public right of way are associated with lower crime rates. The effect of trees on a house's lot was mixed. Lot trees small enough to block the view from a first-floor window increased crime occurrence, while larger lot trees decreased crime occurrence.
Donovan, Geoffrey H., and Jeffrey P. Prestemon. (2012). The effect of trees on crime in Portland, Oregon. Environment and Behavior, 44(1), 3-30.