What has already been done

From what I have seen most forest rendering in computer graphics is based off of rendering many point sprite trees either in order, or with some kind of alpha cut to eliminate z-buffer artifiacts.



 Simple Point Sprite Forests

 http://www.riemers.net/eng/Tutorials/XNA/Csharp/Series4/Billboarding_renderstates.php

Cylindrical Point sprites only look good from ground level
 http://vision.mas.ecp.fr/Personnel/teboul/files/pma06_teboul.pdf

"We would like to show that it is feasible to use three cross
billboards to realize a fast and realistic rendering of a big
forest in our software ForestRoam: Forest Rendering and
Simulation with Image models."




Certainly rendering a large forest scene would have to include a point sprite for every tree- for realistic scenes this would require an astronomical number of sprites- many thousands for a single frame...