Wikipedia has a good article on the song "O Tannnbaum." And the stuff
immediately below is drawn from this Wikipedia article.
A Tannenbaum is a fir tree. The lyrics do not refer to Christmas or
describe a decorated Christmas tree. Instead, they refer to the fir's
evergreen qualities as a symbol of constancy and faithfulness.
It started with a 16th-century Silesian folk song by Melchior Franck,
"Ach Tannenbaum". [You can see it at
http://lyrics.wikia.com/wiki/Infamis:Ach_Tannenbaum. — LG]
Joachim August Zarnack (1777–1827) in 1819 wrote a tragic love song
inspired by the Franck song, taking the evergreen, "faithful" fir tree
as contrasting with a faithless lover. [And that one can be found at
Wikisource:
https://de.wikisource.org/wiki/O_Tannenbaum_(Zarnack). — LG]
The modern lyrics were written in 1824, by the Leipzig organist,
teacher and composer Ernst Anschütz, who added two verses of his own to
the first, traditional verse. The custom of the Christmas tree developed
in the course of the 19th century, and the song came to be seen as a
Christmas carol. Anschütz's version still had treu (true, faithful) as
the adjective describing the fir's leaves (needles), harking back to the
contrast to the faithless maiden of the folk song. This was changed to
grün (green) at some point in the 20th century, after the song had come
to be associated with Christmas.
My 1946 thick hardbound Cassell's German-English, English-German
dictionary (a British publisher) defines a "tannenbaum" as a fir tree.
My father had a book of songs he used to play on the piano and sing
(and let us sing along with him) that included "O, Tannenbaum" whose
English version began "The forest fir, its boughs are green forever" and
whose second verse was "The bubbling brook, it lasts but for a season."
Unfortunately, It vanished away sometime over the years. It used to lie
in the piano bench. Google can't find any trace of these lyrics.
--Lee