Ten thousand years ago, …a great migration was triggered in North America: trees began travelling northward, colonizing newly thawed landscapes at a pace of up to 500 metres a year. …When paleoecologists started piecing this arrival story together in the middle of the 20th century, they were astounded — and confounded — by the speed of its advance. …Today, these theories are getting renewed attention. After 10,000 years of relative stability, the climate is heating up again — Canada’s permafrost line has already shifted 120 kilometres northward in the past 50 years. At those latitudes, however, it isn’t trees that spring up so much as shrubs. And with the Earth now warming 50 times faster than at the dawn of the Holocene, the forest isn’t marching north; it’s imploding from within, pummelled by drought, fire, pests and disease, whose impacts are all exacerbated by a logging industry that levels nearly 750,000 hectares of timber per year.