Are all of your tress and foliage dynamic (ie not static)?
Is your sun (directional light) also dynamic?
Can you post a screen shot of overdraw?
What is the fps when you launch as standalone (not editor)?
What is the texture resolution of each of your assets - rocks, landscape, trees, foliage?
Are there any other settings you are using that affect rendering performance - post process volumes? If so, which settings are active?