Many people want their xmas trees to spawn on the edges of the base, so that the xmas tree doesn't get in the way of their base.
Contrary to popular belief, you CANNOT set your base such that there is only space for obstacles to spawn on the edges.
Why? Here's the secret: you must leave a 2x2 space for the present as well!
So if there's a particular spot on the edge of your base which is where you want your xmas tree to spawn, you must set your base up such that the only space for obstacles to spawn is a 2x2 space directly in front of the spot where you want your xmas tree to spawn. The xmas tree and the first present comes together, so the Xmas tree will spawn on the spot and the present will spawn on the 2x2 space. Likewise, if you want your xmas tree to spawn anywhere in the middle of your base, set your base up such that the only space for obstacles to spawn is a 2x4 space in the middle of your base (which is essentially two 2x2 spaces side by side, one for the xmas tree and one for the present)
I have proof:
http://oi57.tinypic.com/t8vh8p.jpg
Using my method, I made another xmas tree spawn on the edge directly opposite my first xmas tree!
And if I ever get a 3rd xmas tree,it should spawn at the top corner of my base:
http://oi58.tinypic.com/16icyv9.jpg
Thanks for reading and have fun with your Christmas Trees!
Merry Christmas!