When you say prehistoric, which epoch did you mean? Roughly how many millions of years plus or minus fifty (50) million years did you have in mind? bearing in mind that the first known trees in the fossil record lived in the Mississippian Era (360 - 325 million years ago), Flowering Plants are not seen in the fossil record until C.130 Million Years Ago (Jurassic 203-135 million years ago ) with the first grasses appearing in the Oligocene Epoch (Epoch of the Early Tertiary Period 33.7-23.8 million years ago)?
Bearing in mind that depending on the time period you are after and the accuracy you are after will depend greatly on the the plant spices that would go into any image done for you, indeed plant models would probably be need to be researched and made for such an under taking, and would take considerable time to do so, if indeed you could find out what types are known in the fossil record, and form what I understand is not much.
The number of grasses known in the fossil record is considerably smaller than the flowering plants, by how much I can not say, but probably not large enough to reconstruct the environment in which they grow research into other areas would be required for that, and would take considerably longer.
Regards to you.
Cyber-Angel