The model will also be much more manageable and the face/edge count will be halved if you substitute 2D face-me components for the trees, flowers and shrubs. (from about 600000 to less than 300000 edges, for instance)
There is also the other problem with the 3D plants you are using. To save disk space, they are divided into many subcomponents that themselves have subcomponents etc (like leaf, branch, etc). This added complexity is also slowing SU down.
Anssi