you haven't rendered this with Terragen 2.1, it seems. Do you not have the full license?
If you do, make sure "ray trace objects" is on and that you up AA to 6, or 8.
If you don't have 2.1 yet, you need higher values for AA, such as 10 or 12 (at least

to get rid of the pixelated look of the vegetation. It will render much longer, though.
Other than that, it's a lovely scene, very serene. I like it.
I still the see the end of the population, though

Cheers,
Frank