Back in my undergrad days, an upperclassman introduced me to Celtic music other than the Irish Rovers and Chieftains (which I had grown up with). One of the tapes I very carefully copied was a harper and poet (not Patrick Ball) reciting Irish and Welsh stories as well as playing the harp. And one of those stories was a strange tale about a battle of trees. A few years later, I got a tape by a group called Ceredwyn, and one of the songs (my favorite) was a rock setting of the Battle of the Trees. I was lucky enough to find that most of that later recording had been saved and was available on CD, which I snapped up used. I listened to it on my way back from central Texas recently and got to musing.
(NOTE: The introduction is relatively quiet. When the electric guitars kick in, it gets loud in a hurry.)
The Battle of the Trees, or the Cad Goddeu, is on the surface a rather strange and very descriptive account of when Gwydion the enchanter summons the trees to do battle against the forces of Arawn, king of Annwn (the lord of the underworld). Arawn has an unbeatable champion, but Gwydion manages to learn his name and kill him. The trees and Gwydion win the war. It fits into the world of the Mabinogion, the four books of Welsh lore, but it also has elements of the Triads in it, or associated with it. Sometimes it is called the Cad Achren, and adds that Gwydion was on a raid to bring three valuable treasures back from Annwn, secrets held by a dog, a roebuck, and a lapwing.
The text is long: http://www.maryjones.us/ctexts/t08.html
It is an exceedingly neat, and strange, piece of literature, and scholars argue over what exactly it is. Several have dismissed it as either so corrupted by copying errors that what had been a series of unrelated fragments were jammed together into a single piece, or that the errors have rendered it unintelligible, or that the piece is a later parody of Celtic poetry (a bit like some of the 18th and 19th century Romantic forgeries). A few, led by Robert Graves, argue that it contains a representation of ogham script and serves as a key to that writing, and a mnemonic to remember other lore. I’ve also read that it represents the shaman’s journey to the other world, or includes nods to the Welsh stories that became the Fisher King sub-group of the Arthurian stories.
There are a few fantasy stories that I’ve read that draw from the Cad Goddeu. And if you are thinking of when the trees march into battle in Lord of the Rings, yes, it is very likely that Tolkien borrowed the idea, since he tapped so many different mythologies for Middle Earth. There’s a great deal of material to mine from that long, odd poem, and from the Triads, and I suspect someone with more skill than I have could make a heck of a high fantasy tale out of it. I started wondering about Logres, and if you could tell the story as a warped, misinterpretation of when Logres destroyed the other Power that claimed the British Isles, or if there was an even older presence, and Logres was Gwydion, or more correctly, Gwydion served as Guardian for Logres in eh same way that Rada/Rachel does. A landslide that carried a forested area down-slope could well be misunderstood as the trees attacking, moved by magic.
I backed away from Celtic mythology about twenty years ago, after some unsettling moments. There are things that I do not need to mess with, for whatever reason. And I carefully packed my tapes away and couldn’t find the box after three or four moves. So I didn’t think about the poem for quite a while. But it is interesting, how different artists picked it up over the years, and different scholars or neo-pagan writers interpreted it.
Cat Rotator's Quarterly 
I’m pretty sure Elizabeth Moon’s Deeds of Paksennerion draws from this, particularly the third book and the battle at the end of it. But the taig is a “character” in all of the books*.
*Including the sequel series, that I think has a different name, but is just a continuation of the original trilogy with secondary characters as the main characters.