Aren't there two kinds of birch? One (regular white birch) that grows in the normal (lower) soil and one that grows in the permafrost (upper) soil? Going down a road in the hills above Fairbanks, the upper side of the road on one side of the freeze line was one kind of tree and the lower road (warmer soil) was a different kind.
I have a bad memory, so it could be a different kind of tree, and my Google-fu is weak this morning.