The Arctic treeline is the frontline of climate change, where the trees have been creeping towards the pole for fifty years already. These vast swathes of forests, which encircle the north of the globe in an almost unbroken green ring, comprise the world's second largest biome. Scientists are only just beginning to understand the astonishing significance of these northern forests for all life on Earth. Six tree species: Scots pine, birch, larch, spruce, poplar and rowan, form the central protagonists of Ben Rawlence's story.