There are lots you can do with Xmas decorations to reduce your Eco impact and be more green.
Millions of Christmas trees worldwide are 'dumped' or destroyed. This is real waste, as the trees pump out oxygen in return for using climate-changing CO2.
Now you can choose not to buy the tree, but the problem here is that the supplier won't grow the tree's unless there is the demand, so no net gain for the environment. So if you must buy a tree:
Once you are done with the tree, plant it! Although do inquire with the place you bought it from that you haven't bought a tree that will dwarf your backyard in 5 years time!
If you can't plant it - find out if anybody else wants it. Then make sure you dispose of it in an Eco friendly manor, i.e. take to a recycling center to be shredded and composted.
If you don't want to buy a tree, look around outdoors for fallen cones, branches, and ivy, and use this to transform your home into a magical natural wonderland. If you don't have a yard then you can buy a synthetic tree, but only as long as you keep reusing it, using it once and throwing it away is not the right thing to be doing (also do make sure it is fire safe).