After an ill-fated expedition to northern Patagonia, in search of one of the oldest living trees on Earth, three explorers reflect on their discoveries and what it means for the future of rainforests.

Peering out her tall bedroom windows to the highway below, Caitlin Davis dreamed of life as an ecologist. When she saw mountains for the first time on a backpacking trip after college, her fate was sealed. Today, in Idaho’s demanding sagebrush country, the raptor biologist spends her days rappell...