Forests are restless. Any time a tree dies or a new one sprouts, the forest that includes it has shifted. When new trees sprout in the same direction, the whole forest begins to migrate, sometimes at astonishing rates. Today, however, an array of obstacles—humans felling trees by the billions, invasive pests transported through global trade—threaten to overwhelm these vital movements. Worst of all, the climate is changing faster than ever before, and forests are struggling to keep up.