Environment: Ground Clutter
There are five options for the Ground Clutter setting. According to the tooltip, Ground Clutter "controls the density and the distance at which ground clutter items, like grass and foliage, are placed. Decrease to improve performance."
The implications here are pretty obvious. If you want the lushest, densest environments, crank this setting up to Ultra. If you don't mind giving up tufts of grass, stones on the ground, and so on, dial back to suit your performance needs.