"My research indicates that spruces have spent winters in places west or southwest of Norway where the climate was not as harsh in order to later quickly spread northerly along the ice-free coastal strip," says Leif Kullman.
But it's also a great time to look back, and that's why we rounded up the most visited places of 2017.
In order to do so, we relied on data compiled by market research provider Euromonitor International.
"In some way they have also successfully found their way to the Swedish mountains." The study has been carried out in cooperation with the County Administrative Boards in Jämtland and Dalarna.
Scientists expect trees will advance upslope as global temperatures increase, shifting the tree line—the mountain zone where trees become smaller and eventually stop growing—to higher elevations.