Boreal means "Northern." The Northern lights, seen near the Arctic circle, are known as Aurora Borealis. The Boreal forest then literally means the northern forest. Thus the forest of the extreme north, near the Arctic Circle yet below the timberline would constitute a Boreal Forest. The present boreal forest is comprised of evergreen conifers such as firs, spruce, pine, etc. It is sometimes known by its Russian name, Tiaga, meaning "little sticks."
During the ice ages, the area of boreal forest was much further south than at present. Much of the present temperate zone was in a boreal zone. Archaeologists have been able to determine the composition of the Boreal Forest by examining pollen left in sedimentary layers which they have dated to the ice ages. Pollen is not only specific to the plant from which it originates; it does not decay; therefore scientists can examine pollen and determine the trees of the Boreal Forest. Their findings are that it was comprised of conifers similar to those I described above in the present far North. These trees are uniquely acclimated to cold climates with long Summer days and short cold Winter days.