Thing everyone is missing is that the trees you speek of are FLAT FOR A VERY SPECIFIC REASON. They are NOT 3d trees, they are a huge flat texture 2d circle like things. This lets the 3d engine draw 50 trees, with the rendering cost of next to nothing. They put 3 layers of these flay textures and we have a pretty impressive forrest.
Making anything like cones or anything else that breaks the 2D FLAT rule will involve huge sacrifices in preformance. There may be a way around this...but I cant think of it.