from wikipedia: I WIN

There is some debate as to whether artificial or real trees are better for the natural environment. Artificial trees are usually made out of PVC, a toxic material which is often stabilised with lead. Some trees have a warning that dust or leaves from the tree should not be eaten or inhaled. A small amount of real-tree material is used in some artificial trees. For instance, the bark of a real tree can be used to surface an artificial trunk. Polyethylene trees are less toxic, though more expensive, than PVC trees [3].

Artificial trees can be used for many years, but are usually non-recyclable, ending up in landfills. Real trees are used only for a short time, but can be recycled and used as mulch or used to prevent erosion [4]. Real trees also help reduce the amount of carbon dioxide in the atmosphere while growing.

Live trees are typically grown as a crop and replanted in rotation after cutting, often providing suitable habitat for wildlife. In some cases management of Christmas tree crops can result in poor habitat since it involves heavy input of pesticides. Organically grown Christmas trees are available in some markets, and as with many other crops, are widely held to be better for the environment. Dr. Patrick Moore, a co-founder and former president of Greenpeace, writes:

Whether you choose a cut or growing tree to enjoy this holiday season, I believe that a sensible environmentalist would opt for renewable over non-renewable every time.