Glad that seems to have solved it. I'll let Matt chime in with explanation of what's going on.
I did forget to mention that for noise in vegetation you'll want to lower your AA noise threshold a bit and/or not use 1/64th adaptive (1/16th is probably OK). Unfortunately this will probably increase render time as well, but it's really going to be necessary for noise-free vegetation rendering. I think if you lower your detail down to say 0.65 you'll still get good results and can probably make up a good portion of the render time increase. There is a light source that has soft shadows and jitter is still at 1 with samples at 8, you could probably turn down samples a bit if you reduce jitter to the 0.2 level you have on other lights. At AA8 which you're using, you can probably be fine with fewer atmosphere samples, maybe 10 or even 8. You also *might* be able to get away with a lower Detail Blending value of 0.5. Also consider using the Voxel Buffer for localized clouds. All that should help on render time.
- Oshyan