A sure fire way to reduce the polygon count is to use alpha planes. Create parts of the scene using the "White Back" atmosphere under Others. Add the items you want, rocks, trees, etc. and then delete the ground plane. Render in Final mode. Save out the resulting, rendered picture as two files: an image file and an alpha file. Load the atmosphere you want and select an Alpha Plane object. Use your rendered picture as the "Color picture" and alpha picture as the "Alpha Picture". Invert the alpha picture to give it a white background. Click OK and render. It's easy to reduce a 100K polygon tree to 4 polygons using this method although it takes a little practice to get lighting and camera angles just right. There's more info. in the manual on page 94. It's a little difficult to explain in words but hope this helps.

Cheers,
Mark