Aspens everywhere, and it boggles her mind that not one of them has grown from seed. All through this part of the West, few aspens have done so in ten thousand years. Long ago, the climate changed, and an aspen’s seeds can no longer thrive here. But they propagate by root; they spread. There are aspen colonies up north where the ice sheets were, older than the sheets themselves. The motionless trees are migrating—immortal stands of aspen retreating before the latest two-mile-thick glaciers, then following them back north again.