Some legends have it that it was a branch from the tree of life; others from the tree of knowledge. Search, the history of Moses Rod or, the life of adam and eve, or 'wood of the cross' or 'rood-tree' legends:http://www.sacred-texts.com/chr/bb/bb30.htmhttp://watch.pair.com/rat2.html
THE CROSS AS TREE: THE WOOD-OF-THE-CROSS LEGENDS IN
MIDDLE ENGLISH AND LATIN TEXTS IN MEDIEVAL ENGLAND:https://tspace.library.utoronto.ca/bits ... thesis.pdf
The connection with tarot I should think, if any, is that the term tarocch - stump, log, geneological tree came to be used in a figurative sense as blockhead, logger-head, dunce, fool...
By coincidence it is also a symbol of the generations it passes through from first adam to second - 77, or 78 including G-d -- traditionally divided at Abraham, giving a division of 56 and 21/22...
The 77 generation tree was a common, standard feature of princely triumphal processions, btw.
And priests, as I've given examples before, were making puns on the virgin mary, rods, life of christ and penis in there sermons in church, one doesn't even have to go to the taverns for such language.
The tree represents a triumph of love, at the roots is the love of man for fellow man (cupiditas), at the top is the love of God (caritas), the trunk and branches are the virtues by which he climbs to heaven.
(From perfect love, that is love of neighbour and god, comes perfect virtue, from which one makes the climb back to paradise.)