Symbolism, Imagery, Allegory
When the boy is young, he loves the tree so much that he carves initials into the tree's bark as a tribute to their relationship (28). It's a nice gesture (if a little abusive—the tree certainly didn't have a say in that choice), and it shows that the boy is fully dedicated to the tree at a young age.
Later, as the boy grows up, he carves a second set of initials in the tree (32). Again, ouch. This time, he's presumably paired himself with a peer, and the relationship appears romantic. These initials are carved above the original initials, and we know from the fact that the tree is alone a lot more from this point on (35) that this second relationship has, to some extent, replaced the boy's relationship with the tree.
Both sets of initials remain carved in the tree until the boy, as an older man, cuts down the tree's trunk to make his boat (48-49). It's significant here that the boy has severed the second relationship from the stump and roots of the tree. We're pretty sure that those initials—the second set—would be obliterated in the process of building a boat, so we can be fairly certain that relationship is over and done. In fact, it probably has been for a while since the boy is now looking to sail away from whatever is bothering him.
In the last panel, we see that the original initials—the ones celebrating the boy's love for the tree—remain in the stump. They've clearly faded, and the line marking the heart isn't as strong as it once was, but they're still there, and that's got to count for something.