Six years ago, Liz Babb and her husband, Angelo Aloisio, retired financial services executives and new empty nesters, sold their place in San Francisco and moved to the woods. They bought a 1970s house on a steep slope in Portola Valley, an enclave of forested canyons minutes from Silicon Valley’s center that boasts a bohemian history, a reputation for moneyed discretion, and jaw-dropping views. It was an iconic—if, by local standards, modest—northern California dream home: wood construction, picture windows, multiple decks, and, everywhere, trees.