You shouldn't really use trees or fences to seperate two play areas. Occasionally this happens on hydro and valve have just clipped all the way above them to prevent rocket jumping over.
No, I just have a tree that's on the side of a play area (actually two trees, identical models) that are either side of blue's spawn on the art-pass map. They're not separating anything, but you can get into/onto the branches by double-jumping off the "barn."
The one on the other side you can get to by double (triple?) jumping from the roof over that one room next to the Giant Wall (which takes a triple jump to get onto anyway). That one I made non-solid because the roots were somewhat problematic, and just clip-brushed the root more kindly and added a trunk most of the way up the tree (so you can still stand "inside" a portion of it, but you can still be seen).
I'd like it if you couldn't stand on the tree at all (including the branches) without clipping through it.