Ogham was an esoteric language known only to Druids.  Ogham characters were notched onto wood or carved into stone.  They are read from bottom to top, the way a tree is climbed; and each character is associated with a type of tree.

To find out more about this subject, I recommend:  The Wisdom of Trees by Jane Gifford (Sterling Publishing Company, 2000); and, The Meaning of Trees by Fred Hagender (Chronicle Books, 2005).