As we are heading ever nearer to Christmas, this question has come up a few times.
‘Can guinea pigs eat Christmas trees?’
Firstly, it depends on what type of tree you have as so many types of Christmas trees are toxic.

Did you know that most Christmas trees are sprayed with a toxic chemical by many stores to protect it from bursting into flames?
‘The fir tree oils can irritating to the mouth and stomach, causing excessive drooling or vomiting. The tree needles are not easily digested either; possibly causing GI irritation, vomiting, gastrointestinal obstruction or puncture.
Even though the question refers to “live” trees, artificial trees, are also dangerous when eaten. The principal things to worry about are toxin release from the artificial material and intestinal obstruction (not digestible). (source: vetmedicine.about.com)
This means that most Christmas trees that are store bought cannot be given to piggies at all. While we’re on the theme, do avoid any mistletoe and holly as they are also toxic for guinea pigs.
So if you have a christmas tree (pine or any other sort) then keep it away from them, and the same goes for fake christmas trees.
What you can do instead.
Personally, I wouldn’t do this at all but if you really want to get your guinea pig a tree, go for non-toxic varieties instead and avoid regular festive trees.
We know that piggies love to nibble on branches and wood so there are many different varieties of tree branch out there that you could use.
Guinea pigs can eat many other woods from trees. Just check out if they have been treated. For example, we have pear and apple trees in our garden. Those branches are fine for them to eat.
Why not dress up some branches of with some veggies that they could enjoy?