If I change the "tree size" property (e.g. 5), then all of the trees get placed in a grid pattern. Not natural at all!
Ideally, the trees would be placed in randomly-sized clumps around the map based on the "tree size" value (maybe then it would be called "tree density"). If that's too complicated, I would suggest using the same number of trees that would be placed under the existing script, except placing them all randomly around the map.
thanks,
Phong