You could try a repellent spray, but they’re not usually very effective. You could place something with a texture that she dislikes around the tree (double sided tape), or you could try scaring her away from it (upside down mouse traps that spring into the air when she brushes against them). Of course, you risk having the tree knocked over if she panics. If you can find one, the most effective option is probably going to be a scat mat. Or, you could make sure that you buy unbreakable ornaments and feel happy that you’ve provided your cat with entertainment during the day.