Don't remember exactly, but what I did was about: I calculated every potential tree location, also based on soil color (trees only on green), added a slight color variation and shading info. The entire visible forest is then created every frame, using precalculated tree data etc., depending on the distance and angle to the camera, that's a max of 32k tris AFAIR. Individual colors of trees use vertex color, and the entire forest mesh is single surface. Trees are just 1 triangle billboards, but part of a single surface mesh.