"In ancient Celtic belief trees had spirits within them and were considered sacred. They were the keepers of memory and lore. The Druids, the men of knowledge, used to record their wisdoms by means of a secret alphabet called Ogham, which is also called the "Tree Alphabet". Ogham is named after the God Ogma, the God of poetry and eloquence. Thus the memory of trees looks into the past.
The future we look at may give us another interpretation of the title, with the destruction of the world’s rainforests it may turn out to be that the memory of trees is nothing more than that for us – a memory."

notes by Roma Ryan on the Enya website