Why do some northern plant species grow at high elevations in the Great Smoky Mountains?

Some northern plant species grow at high elevations in the Great Smoky Mountains because these mountains served as a refuge for northern life forms during the Pleistocene Epoch.1

As continental ice sheets advanced across North America, they pushed many plant and animal species southward to survive in more temperate climates. When the glaciers eventually retreated, these species moved north again; however, some populations remained in the southern Appalachian Mountains. Today, the higher elevations of the Great Smoky Mountains provide the necessary cooler temperatures and relative isolation that allow these "northern" species—such as red spruce and Fraser fir—to persist as relict forests.123