"The Tree is used as a permanent defender. His name is Treezatoph. 

He is easy to get around, but if you contact him, you will know it as he is solid as a rock (or Tree) :) 

My two kids, 6 and 7 enjoy the rink as do my wife and invited guests. This is my front yard, as the backyard is a hill. Pretty tough to make ice on a hill, besides, the bobsled run is out back. 

I use 2x6 pressure treated lumber for the frame, with plastic underneath. 
It is tricky to seal as I have to make a cut for the tree. This year the cut for the tree did not leak, last year it did. That did cause some problems, but the rink still turned out pretty good. This year I went a bit smaller, and this improved the ice quality quite a lot."

Great job and thanks for the pic Scott.  It takes a lot of guts and patience to put a liner around a tree.