I just got an email about a program led by the group Siempre Verde (Always Green), through which you can rent a Christmas tree. Basically, the tree come in a large "flowerpot" so that it doesn't need to be cut and the roots are still intact. You can choose from small, medium or large sizes. It needs to be taken care of, but is then replanted. In certain zones in the DF, delivery and pick-up are included. People residing outside of those zones can pick-up and drop-off their trees at the Distribution Center. They also rent ornaments and flowerpot covers.
Interesting, no? Definitely more ecologically friendly without losing the touch of having a live tree.
For more information (in Spanish), visit http://www.siempreverde.net/