The relative "greenness" of your holiday tree may not be the reason you choose whether to get a real tree or use an artificial one (convenience, aesthetics, budget may be other considerations for you), but if it is and you're confused by arguments on both sides about which is "greener" - read on.
Trees are not like flowers. They take a long time to grow especially the 7 ft. (and up) ones many of us bring into our homes for a just a few weeks each year. As much as we love that fresh pine scent, there is something a bit unsettling about asking Mother Nature to grow these trees for such a short, narrow use. On the other hand, we've always felt a bit unsure about just what artificial trees are made from and what chemicals we may be bringing into our home, especially with young kids, and while they can last a very, very long time - that's kind of the problem isn't it - they'll never decompose like a tree.
But, hey, we're not scientists, so we're pretty unqualified to come to any reliable conclusions about which option is better for the environment. However we did come across an article today that sheds some more light on this and even interviews a real live scientist. In the opinion of Clint Springer, a biologist at Saint Joseph's University in Philadelphia, from a strictly environmental perspective real trees are the way to go. Because they're grown on tree farms, deforestation is not the concern it once was in this regard (it's like bonus trees for the earth).
Springer deems real Christmas trees "carbon neutral" since they remove carbon dioxide from the air when they're growing through photosynthesis then release it back when they're chipped for mulch. Fake trees, however, usually contain petroleum (in the form of plastics) and are often shipped far distances which creates a lot of carbon dioxide.
So, there you have it. You can find Springer interviewed about this in an article on Live Science.