I did some projects for clients. Large scale jungles.
Couple tips for reducing poligon count.

• Use paint insted ecosystem population, paint tree only on serfices that visible by camera.
• Do not create one universal terrain for all camera paths, create one , and save as different instances for different camera path, adjust your plants to that path.
• Create additional layers and place distant terrain in that layer, make it layer invisible in preview window. this will speed up your prieview.
• Prerender distant terrain and use alpha planes if posible.
• Bake models.
• For distant terrain use very bumpy material, or simple objects, insted complex plants. when render with motion blur they will be out of focus any way.
• Increase quality in atmosphere to reduce flicering in ecosystem.