Symbolism, Imagery, Allegory
So, what on earth is a grave tree? If you guessed a tree that grows on a grave, you're right. After Mark buries Tristan and Yseut's bodies in a Cornish chapel, two trees with intertwining branches grow up over their graves—no matter how hard Mark tries to cut them down. So let's see: two things that refuse to say no? Yeah, we're pretty sure these trees symbolize the lovers themselves.
And guess what? The text thinks so, too.
And in case that wasn't in-your-face enough, Mark's failed attempts to cut them down are a slap in the face, saying "hey, this is the guy who couldn't separate Tristan and Yseut, either."
Also, if the natural world is going along with the love of Tristan and Yseut like this, does this suggest that their love is somehow natural, or part of the natural order of things? This detail might suggest that their love overrides human laws, or the laws of medieval society. Then again, it could just be the love potion at work again: "Some say it was the power of the love potion that did this" (19.164).