However, I have two sources that say trees were attached to ridge polls.
Well, perhaps I can find some sources saying the same.
Any leftover of the past can be considered a source. We don't have those sources here, at least, no-one has found them until now (here).
So it is just said that the tradition of wreath (or 'trees') on houses goes back earlier than the one of Christmas trees.
A couple of years ago, the "Iron Time House" (Eisenzeitliche Haus
) was reconstructed on the island of Amrum.
While a reconstruction of such a building is relatively easy, you don't know a lot of minor details. You 'guess' that using the known traditions.