It's that time of year when many of us put a pine tree up in our living room and decorate with lots of shiny objects and string-like items. It seems that many cats find this combination to be almost irresistible. So... what can I do to keep my cat from getting too festive in the tree?
We used to hang a bell on a low branch that would ring when the cat started to climb the tree. We called it the "cat alarm." When it would ring, we would come and get the cat out of the tree right away.
Whether or not your cat will eventually get discouraged from climbing the tree is up to your cat. :)
The other thing we did was we cut out a three foot circle out of plywood, and screwed the tree stand down to the plywood. That way, even if the cat did climb the tree, there was no way he could knock it over.
We made the bottom of the tree as uninteresting to cats as possible. This involved putting all ornaments up higher on the tree, keeping it away from the chairs, and monitoring the tree when we're around it and immediately spraying the cat with water when they get into the tree.
Our cat did this almost daily when she was a kitten and it was her first Christmas, but after a week or so of pulling her out and scolding, spraying with water, and removing the unexplainably fun ornaments from the bottom 25% of the tree, she stopped.
This may sound a little mean, however it works for Christmas trees, getting on top of the counters, and generally training the cat not to go certain places: quickly mist them with a spray bottle. If your cat doesn't like water as much as most cats don't, then this will work. Eventually you can get to where you don't even have to spray it and just make a spray sound with your teeth, sometimes bringing the bottle out; though the appearance and the remembrance of the event will be enough to train them not to go certain areas.
However, it may take a while for some cats to be trained, and Christmas trees are not usually put up for very long, so the effectiveness of this method will depend on how quickly your cat picks this up and how long you leave your tree up. Also, you don't want to spray too much for both the cat's and the tree's well-being. Last thing you want is a musty, damp tree that drips, or a cat that no longer trusts/likes you. :P
The only way to keep your cat and your Christmas tree from entertaining each other is to physically separate them. This is probably impractical, and likely impossible.
There are some answers here suggesting changes to the bottom of the tree, to prevent tipping, or discourage climbing. Cats (and ferrets) are very unlikely to be permanently discouraged, and increasing the base or stand to prevent tipping, is difficult, and bulky.
Look to the ceiling for your solution. Before erecting the tree, find a joist in the ceiling over where the tree will be placed. Screw a strong hook into the ceiling joist, I would suggest a 25 pound rating on the hook with a threaded end about 2 inches long, creating a strong attachment to the joist. Once the tree is in place, tie the top of the tree to the hook, it should be snug but not lifting the tree. This will keep the tree from tipping over if (when) climbed, and the base will return the tree to proper position once the tree climber (pet or child) is down. While not essential, centering the tree under the hook will provide the best results.
You may also want to consider decorations that are not fragile. Activity in and around the tree are likely to cause some accidents and broken decorations can be harmful to everyone in the home.
The hook can be used in the off season to hang; a plant, dream catcher, etc. I would not recommend removing the hook. It will provide the best service over the years if allowed to remain in place.