Beneath a vast conifer forest in Canada lies the biggest proven oil reserve outside Saudi Arabia. With the cost of crude sky-high, what price the trees? By Sheila McNulty

It is 3pm on a Thursday, and the carpark at the tiny airport on the edge of Alberta's boreal forest is crammed. Gary Stewart has been coming here for years, and mutters about how busy the area has grown as he circles slowly in his hire car, searching for a space. The 57-year-old has encouraged a photographer and me to come to this western slice of Canada for a bird's-eye view of the area, the world's largest remaining unspoiled forest and wetland ecosystem. But it's not just the beauty of the trees that Stewart will be pointing out. Buried under the canopy lie 140,000 sq km of oil sands - black, grainy tar that is quickly turning into Canada's cash crop.