If you use the unpick axe to create a wall on top of a tree, both the tree and wall will use the same space, and it will use the tree's 'T' character. The tree remains intact after digging through the wall.

EDIT: A similar thing happens when you create a wall on top of a grave, pool, forge, trap, herb bush, staircase, altar, lever, statue, or the chaos gate.
EDIT2: A similar thing happens when you create a wall on top of an anthill or beehive, but in these cases ants and bees will get deleted when they attempt to spawn.