I'm afraid if the zombies knock down that tree the whole thing will come down.
It is really just a glitch in the matrix.
From 300 yards away the texture extends to the ground.
When you get closer the textures disappear and you can see the tree. ( cloaking device to hide the vast mining operation below it? ) I guess one spawned on top of the other.
But it is solid. With a hole going up the center. Would be great if there was a zombie filled shack at the top.
It just looks like the trees are the only thing holding it up.