Hi Inky,
the trees are some nice models. All in all the scene is good, but I still have a few sugestions for you to try.
1 - use much higher AA, and also I suggest using the smooth AA filter.
2 - the ground would benefit from some height variations. It it's too flat, the grasses look like mown. You can add a little size variation through the populator, but not too much. Instead, add a power fractal to the terrain, which creates little billowry bumps - just slightly. That way, the grasses seem more uneven, without getting out of scale.
3 - The clouds could blend better with the sky I think. It would also add to the realism, it they were less scattered. Reducing the fractal noise can help, but also increasing the gamma, in case you had lowered it before.
4 - Adding a powerfractal to the rocks to displace the fake stones further would help remove the sharp lines on the rocks, hence add to realism again.
Cheers,
Frank
