You can actually build houses on top of trees, or at least you could in earlier alphas. Treetops are a surface like any other, build a ladder up there and it’s walkable.
edit: back in alpha 8 I actually built a treehouse, link here:
http://discourse.stonehearth.net/t/post-your-alpha-8-screenshots-here/9798/26?u=hieronymous
Took a lot of ladders. I forget what happened when I cut the tree down, I think it stayed up in the air, flying through space.