Juan,
3D trees - especially at this density and amount - are just too many polygons and too much load for GE. What the 3D Tree Layer in GE does is that they have a certain level of detail (LOD) engine in them which will turn them less detailed when seeing from a distance and even 2D when seeing from a bigger distance. Even the close-up is a highly optimized way of displaying vegetation (not the way SU builds up these trees for sure). Many game engines work the same way.
There is an interest at Google to eventually make this tree layer public and allow contributors to populate their own area with them but until then it is a futile effort to do this as it kills GE as you can also experience.