From Tolkien Gateway

Tauremorna is one of the names of the Fangorn Forest, used by Treebeard in his song In the Willow-meads of Tasarinan I Walked in the Spring. The other names mentioned by Treebeard are Ambaróna and Aldalómë.[1]

J.R.R. Tolkien notes in his Unfinished Index that tauremorna means "black forest",[2] likely composed of Quenya taure ("forest") + morna ("dark, black").