From Real Simple:
Jackson Galaxy, the host of Animal Planet’s My Cat From Hell and the author of Catification, offers a two-pronged approach to keeping your cats away from the Christmas tree:
Set up a scratching post or cat tower in the same room as the tree. Then “decorate” your tree’s bottom branches with five or so sachets filled with cirus zest (cats hate it), spaced evenly. Add a sprig of holly to each and they’ll look like ornaments. It also helps to position the tree away from a window (a kitty hot spot) and far from furniture they might use as a launching pad. Also, hang glittery decorations out of paw’s reach on high branches and closer to the trunk.