Surely "O, Tannenbaum" sounds familiar around this time of year, and Tannen makes an excellent and unexpected choice for a boy's Christmas themed baby name. Tannenbaum means "fir tree" from tanne and baum, therefore Tannen as a baby name refers to that tree as the plural of tanne. Tannen in Old English can refer to the occupation of tanning hides and is sometimes seen as a variant of Tanner.
In the song, tannenbaum refers specifically to the Christmas tree, but this is a modern change to the original, non-Christmas themed song. The German "O Tannenbaum" was originally a long song in which the fir tree is thought of as a faithful tree, but when the author changed a few lines as the idea of a Christmas tree got more popular, it wasn't hard for listeners to change the meaning of the song altogether. Later the German title was changed to "O Christmas Tree" in America.
There are many different kinds of fir trees, many of which are still used as Christmas trees today. The Fir Tree by Hans Christian Andersen is a fairy tale about a fir tree who wants to grow up too fast (and in winter fashion was published with The Snow Queen). It would make a great story to read a little boy whose name means "fir tree."
There were only 10 boys named Tannen in 2013. Spell it Tannin and the meaning changes, as tannins are found in green tea once it turns bitter and in tree bark. Tannen can be found as a surname. While boys names ending in the letter N are extremely popular right now, and the name Tanner has had major success, Tannen combines the best of those for a very on-trend name.
Fun fact: Tannen's Magic Shop is New York's oldest operating magic shop, Louis Tannen being the original owner from 1925.