Imagine the Los Angeles that might exist if it weren’t encased in a concrete crust: Contours of soft earth rather than rigid asphalt. Aromas that evoke the herbal qualities of sage rather than acrid smog. An urban forest of shady sycamores instead of spindly utility poles. Over a career that has spanned more than four decades, landscape designer Mia Lehrer has hammered away at those hard surfaces and, in countless places, replaced them with more hospitable landscapes. On a hillside overlooking downtown, for example, at the site of a former oil field that was once studded with oil wells, Vista Hermosa Natural Park now emerges from the dense urban grid…