:) Both your girls will enjoy the Christmas tree allright :)
what I suggest:
0. I would not fence anything: prohibitions only make want things more, is it not so?
What I would do:
1. secure the tree firmly so that in no way it could fall down if pulled by lower branches.
2. do not hang any fragile, easily breakable, and definitely glass ornaments on the lower part of the tree.
If you want to hang some of such up higher, make sure the ornaments are "sitting" there securely : if there is a hook on the ornament, bend it around the twig, if there is a string, tie it up tightly to the twig.
3. this is FUN: make some ornaments with Ella, using colored paper, like chains of colored coils, some origami-like stuff, and cut white-paper snowflakes, and hang these paper ornaments on the bottom of the tree: you win twice: the tree will look nice and will be safe there, and Ella will be proud she participated in decorating it
4. you can already teach Ella to treat the tree well:
show her, how to PET the tree, touch the branches and ornaments gently, like petting, never pulling or grabbing, show her how the tree likes being treated, and she will follow the example as she is in the age of 'monkeying'= repeating all your motions with the greatest pleasure.
Also, you can teach Ella how to teach Ava, as if the elder girl will see Ava pulling on ornaments, she can hug the little sister, and pet her and then pet the tree, and show that this is GOOD way to communicate. Ava won't realize immediately, but there is a big chance she eventually will. Also, the communication between sisters will be growing warmer.
Actually, it is a great time to start making sure your daughters are great friends, trying to avoid any jealousy.
My sons were growing great friends: not a fight, no problems whatsoever, the difference in age like your daughters. now they are in their 20s :) and still great friends, yet you need to make it happen, it does not just fall from heaven, right?!
5. keep the chairs away from the tree, as Ava will not figure it out yet, that she could pull the chair closer to climb and reach the upper part of the tree. Ella will, so think about all the chairs and boxes and whatever she could move to climb...
6. and very important: DO NOT THINK THAT SOMETHING BAD MIGHT HAPPEN!!! Be sure that all is and will be great, and it WILL.
Your thinking process influences the surrounding space, right?! So, MAKE your Christmas space HAPPY!!! :)
Merry Christmas, happy days to you all!!!