thank you for the comments, all

It seems you are interested in the render settings. Here they come:
- detail 0.9
- GI 2/2, GI Blur 2, GI Detail = on
- AA 12, AA Mitchell-Netravali, AA Bloom = on
- Soft shadows, default values
- population quality = very high
@ Mohawk: I think you are mistaken. I had removed a few layers of fake stones for the DOF mask render. This has lowered the terrain of course, so the plants were all rendered a bit lower too. Hence, the DOF mask would not fit perfectly. I have re-rendered the DOF mask now. Will apply it probably tomorrow.
@ Walli: Do you have more super high quality models like this? I would *love* to do more close up and macro renders, if only the models I have were more detailed. None of the xfrog ground cover models I have tried yet have that level of detail in leaf texture and displacement. Same for the trees with braod leaves... they all look so flat and CG from close up.
Cheers,
Frank