When Rob Grace, 36, wants to remove a branch of an oak or poplar on his property in Takoma Park, Md., he has to consult with the town arborist.

Considered one of the most progressive cities in the United States, Takoma Park, a suburb immediately northeast of Washington, D.C., is not just a nuclear-free zone, a hotbed of former Peace Corps volunteers and a municipality that has extended the local vote to noncitizens and 16- and 17-year-olds; it is also home to an impressive number of healthy, mature trees.

Mr. Grace, who is an analyst at the Government Accountability Office in Washington, said that because of the greenery, you can actually feel the air cool when you cross into Maryland, something he does regularly as a commuter, walking home from the local Metro station on the District side. (Takoma Park is not to be confused with Takoma in Washington, D.C., though the two share a border, a business district and a Fourth of July parade.)