A study by the service has found less crime occurs in neighborhoods with big trees in the yards and on the streets, and more crime at homes with smaller trees.
Researchers from the Pacific Northwest Research Station took crime data from the Portland Police Bureau and compared it to information showing neighborhoods' tree coverage.
Forester Geoffrey Donovan says large trees may be a signal to crooks that a neighborhood is well cared for, making it more likely criminals will be caught.
He adds that small trees may aid criminals by providing hiding places for criminals and obstructing views.
The study will appear in the journal Environment and Behavior.