@ porcupine, moodflow, RArcher: many thanks, your comments have been much appreciated!
@Lighting:
Nice guerilla-style, thread hijacking tactics ;-)
... and a nice tree model, too. I would have used this one if I had known about it in time. For now, it's too late. I'm doing a high resolution, high quality render of the same scene at the moment, which is already rendering since 28hrs, and probably needs another 8 hours or more before it finishes. So my patience is up with this scene ;-)
However, in the high quality render, the trees on the little island are looking better than before.
Cheers,
Frank