In Shakespeare’s play Macbeth the forest of Birnam Wood fulfills a seemingly impossible prophecy by moving to surround the murderous king (the marching trees are helped, of course, by an army of axe-wielding camouflaged Scots). The Arctic tundra may soon feel much like the doomed Macbeth with an army of trees (and invading species) closing in. In a recent study, researchers found that climate change is likely to push the northern forests of the boreal into the Arctic tundra—a trend that is already being confirmed in Alaska.