We have some new plants, but how can we get LOD for them? of course I can't create a Whole forest without LOD.

Those trees are statics

Create the LODs as the same way you created the plants but with low polys. If I recall there are 4 stages of LOD:

Near

Far

Far away

Distant (used when you see the map ingame)

When you've created the new LOD, edit the static plants in CK and assign the LOD meshes.

Edit: Ah... Sei Italiano?