Thank you for your suggestions.

@sivian: The far trees (for example above the waterfall in pic 1 or pic 6) are indeed low poly ones (with a good skin though :-) )
The trees nearby in picture 6, behind the tents have to be detailed because the player can go up there from the other side. I alway use detailed ones only where the player is able to go. Besides, all detailed trees have 2 lod stages.

@damocles: Thx mate. #2: I know that more trees would also enhance detail, but this is not the look i try to get :-) Not every island has one huge forest. I think adding more bushes and gras will fill these areas.