10 years ago, looking for ways to render very high quantity of polygons..
To use mesh proxies were the best solution. So in that renders I used only few patches of grass, and 3 trees, just scaling them on some random way.
Also the use of elaborated materials for the translucency of the leaves.
The next render is an insane 4.602.140.070 polygons render!!. Were rendered with an old Intel Quad Q6600, in about 52 minutes. As I said, just few different trees used, and scaled. But good forest result.