To keep a cat off a Christmas tree, you can use a variety of methods. One common technique is to hang a bell or some other type of noise-making object on the lower branches of the tree. When the cat approaches the tree, the sound will startle it and discourage it from climbing. Another method is to place sticky tape on the lower branches to make it uncomfortable for the cat to climb. Additionally, you can try using natural scents like citrus oil or peppermint, which cats find unpleasant. Finally, you can simply keep the cat out of the room with the tree.