Ahh, Christmastime. I'm thinking of smells of gingerbread and nutmeg, mulled wine, pine needles, and PVC fixed with lead.
Ah yes. I read an article today saying that most artificial Christmas trees are made of PVC - polyvinyl chloride, which is a known carcinogen at the very least. And lead, which is a developmental and reproductive health hazard, is apparently used to stabilize PVC compounds, so it might be in your fake tree as well.
So if you have an artificial tree, don't chew on it.
And real trees? Well, nice to think that they're out there sucking in some of that excess carbon dioxide while they're growing to be nice and perfectly conical for us - but sadly, they will just release it back into the atmosphere as they decay after you throw it out. Not only that, but depending on where you buy your tree, it might have been treated with insecticides or pesticides.
So if you have a real tree, don't chew on it.
And if you aren't Christian, thank goodness. You get out of all of this bringing-a-tree-inside stuff which is a really weird tradition when you really think about it.