My cats don't like tin foil. We had to put it under our sofas because our first cat, Mikey, pulled the underfabric off the couch and climbed inside it! I lined both couchs underneath and they won't go there now.
Perhaps just laying a wide path of tin foil under the tree, maybe even wrapping it around the lower part of the tree.
We haven't picked ours out yet, but I am going to try this tactic. Better than using that nasty spray! (we had to spray the arms of our couchs with it..........yuck)
Oh, and cats don't like sticky. So maybe some double sided tape would help also? I had to wrap that around the top of our guinea pigs cage because kitty #2, Tigerlily, kept climbing into his cage and lounging in there. Poor pig would huddle in the corner!