above ground, being below ground in unhealthy, claustrophobic, harder to defend from zombies and it just doesn't work with me.
however being in a tree makes for easy picking by raiders- if I can't fend them off they can make me hand over everything i got by simply threatening to burn the tree. also tree houses offer no ballistic protection.
definitely not below ground.
A tree seems appealing, as long as it was isolated enough to offer protection not only from zombies, but also from other (stronger) people.