It comes from an older tradition that the Germanic peoples would celebrate. The spruce tree festival which was there New year celebration being the last of the short days. The story includes a guy coming from inside the earth arising through a spruce tree, the tradition of leaving presents under the tree is to give to him saint Klaus or in older times Ulgen. When they became Christian but specificaly Arian Christian which is the belief that Jesus was created by God therefore not being God himself (later giving rise to the seperation ofnthe church etc) The story of Jesus's birth was intertwined with this old tradition.