What is the meaning of Tree_lawn?

A strip of unpaved land planted with trees, running alongside a street, usually between the sidewalk and street.

The triumph of slang (dead link) , John A. C. Greppin, 2002-02-01

Full Metal Racket: The perverse thrill of metal detecting, the world's worst hobby., by Emily Yoffe, Slate, 2003-08-18, refers to the area as “the grassy strip that runs along the street side of many D.C. sidewalks”.

Source: wiktionary.org