When Was Ellesmere Island Forested?

The islands of the canadian artic are today covered in ice…but years ago, a geologist found evidence that at least one of them (Ellesmere Island) was once covered in dense forests of coniferous trees. When was this? And, if global warming causes the arctic ice to melt, how far noth might the tree line move? Is it possible that these islands would once gaian be forested?