In the late 1970s, as a Ph.D. researcher in Australia, Meg Lowman took a step that would define her career and the future of forest research—she used a slingshot to launch a rope into the tree canopy. With borrowed caving gear, she ascended and entered a vibrant treetop world, bustling with wildlife and holding scientific information that had been ignored for centuries by tree researchers who made observations only from the ground. She and a handful of others invented a new word: “arbornaut,” an explorer of the tree canopy. Before that, virtually no scientist had studied the treetops, though Lowman later learned a researcher in Costa Rica developed a similar tree-climbing technique in the same year she did. Now 67, Lowman’s science career spans decades, from her start as a botany-obsessed fifth-grader in upstate New York, to her research in forests in Scotland, Australia, Malaysia, India, Ethiopia, and the Amazon. She examined when leaves emerged and how that related to...