here is an article from HRS on Bunnies and Holidays… http://www.rabbit.org/care/holidays.html
from that article:
If your Christmas tree has not been treated (with fire retardent, pesticides, etc) or painted, then it should be safe to chew. Note that natural chemical compounds in some evergreens may cause the bunny’s urine to turn more orange than usual, but this is not a health concern.
we don’t do real trees, so i have no first-hand experience with that. i would be concerned if your tree did not come from your own land, b/c there’s no way for you to know what kind of pesticides or sprays might have been used before you brought it home. so to be on the safe side, i’d suggest you block off access to the tree altogether. there are other concerns also (which are addressed in that article…) lights, cords, ornaments, etc…
also tinsel. tinsel is a big bunny no-no and can be very dangerous if ingested!