Every autumn a Clark’s nutcracker, a grey crow-like bird of the western American mountains, buries tens of thousands of pine seeds, three or four at a time, in thousands of little caches across the slopes. Months later, under snow, it goes back and finds most of them, apparently by remembering where each one sits against a rock or a stump. The whitebark pine relies on this: its cones don’t open on their own and its seeds have no wings, so almost the only way a new tree starts is in a cache the bird never came back to. A whitebark forest, standing on a ridge, is a map of where one bird’s memory ran out.