HÜMMEL, Germany — IN the deep stillness of a forest in winter, the sound of footsteps on a carpet of leaves died away. Peter Wohlleben had found what he was looking for: a pair of towering beeches. “These trees are friends,” he said, craning his neck to look at the leafless crowns, black against a gray sky. “You see how the thick branches point away from each other? That’s so they don’t block their buddy’s light.”