If someones interested, here is another Avatar production article, it is not tutorial but is a mindblowing read...:

"It was very interesting. You could actually watch a forest grow in real time with this solution, and any TD could grow just by painting colors on the terrain." With this elegant solution, the big trees would grow first, then the smaller trees would die off as the big trees took away the light, the smaller trees would fight for position, the ground cover would fill in where it could get light."

http://features.cgsociety.org/story_custom.php?story_id=5434