Not sure if this has been posted before, but I just found this over at Avsim. Search for treegen.zip
The default FS9 trees are over-sized and too sparse. As a result, they never create a believable tree-line, or give you the sense of mass and density. I've reduced the size of the trees down by 40%, and increased their numbers. There is no performance hit at all. In fact, you should get better performance because these files are at half the size compared to the original. I've reduced the quality down slightly, but you shouldn't notice any difference once airborn at 200 feet and above. All seasons are included.
Screenshots and download here:
they're excellent, nice one