Interesting concept. I didn't play it for very long (~5 mins), but I liked the game anyway!
It seems a lot of people find it slow, but I though it was fitting for the "deadland" theme ^^
Only major problem: the player doesn't know when a zombie has spawned, and/or is eating the tree, making game-overs feel random. There could be, for instance, a gauge to show how much the tree is being eaten: if the tree's HP go down, we know there's a problem and must go back immediatly.
Good job nontheless for this submission!