"So for a new project I decided to go for a greenhouse to grow my saplings in. Since I got my tower installed as central place of my world, I decided that the greenhouse should also be connected to it. So I can up with this nice piece of engineering!
I made the shape of a beehyve haning on the tower. I used a sphere planning tool (wich can be found here: http://www.mediafire.com/?memegmmdnjz) to find out the shape of the top. Then I extended the lowest parts so my trees would fit in.
I've experimented with the distance between the saplings and this way actualy worked pretty good for me. I use to chop down the rows of leafs wich are above the saplings once a tree is fullygrown so they don't block the way of other unfinished saplings.