As many of you probably already know, the "tree ogham", for example,
is not the most archaic form of ogham. The oldest form of ogham did have
kennings for letters, but they weren't exclusively trees, although many
of them were trees. The best book on Ogham is a recent publication by
MacManus, published in 1991 by "An Sagart" press in Ireland, and discusses
Connections between trees, seasons, etc are highly speculative. Remember
that the Celts covered a very big spread of area in Europe, and so the
"seasons" were different for different Celts.
Even the selections of trees were different for different Celts. The oak
seemed to be important to Continental Celts, but not so important to the
A very good article on the Irish calendar appears in "The Celtic
Consciousness" edited by O'Driscoll, although I don't agree on the
interpretation of the Coligny Calendar (which is very controversial
This is very difficult ground. Enter with caution and skepticism.