Frederick Law Olmsted understood nature’s ability to rejuvenate the mind and body.

One of the principal designers of Central Park in New York, he took pains to replicate the gentle beauty he saw in European parks that blended trees and shrubs, streams and bridges. “The park throughout is a single work of art,” he wrote.

That pastoral artistry can be fleeting in a large metropolis. The relationship between humans and nature improves mental health and promotes prosocial behavior. But as cities grow, green space is squeezed out. Community leaders are finding ways to restore some of the balance.

The opening of the High Line in Manhattan in 2009 brought national attention to the idea of reclaiming underused space for parks. Its success led to the development of a network of projects across the country, including an elevated park in Philadelphia, a deck spanning a freeway in Dallas and a multiuse trail around downtown Atlanta.