These are a couple of screenshots from a paper that I presented last year.
What sets this apart is that most of what you see is generated in real-time as
you roam the landscape. This includes all the plants (well there positions), as
well as all the ground cover textures. Even with the current amount of plants,
frame rate is still above 25 on a PIII 500 with a geforce 256 card.
This is done by ecotope modelling ( Ecotope : A particular habitat within a
region with relative uniform climatological and soil conditions. Typically,
specific ecotopes will be associated with specific ecosystems.)
As you move around, the ecotope simulation is done for each new area that
appears on screen, and an appropriate ground cover and set of plants is
generated. Details of this, and the paper can be found on my website.