After trying numerous methods to keep our cats from eating the artificial christmas tree, I think I have found an answer. This is finally working for us and we have 4 inside cats ranging from 5 months old to 12 years old.
We took a q-tip and dripped "cinnamon bark" oil onto it and dabbed the lower branches of the tree. The cats hate the smell and we don't mind it at all because it smells like the holidays. Since this is an oil, it stays on much longer than some of the sprays we have tried that have to be reapplied over and over.
Hope this helps those who have fought this problem like we have but don't want to get rid of the tree yet. And our cats stay healthy by not eating the tree too!