Ultra Quality for the trees implies you're either not using Raytraced Object rendering, or you're using an older version that doesn't have it available. I'd strongly suggest using it, and upgrading to the latest version if you haven't yet, because it'll save render time and improve quality at the same time.
Also for a scene like this where you really don't see the ground, I'd try Raytrace Everything, with AA set to maybe 4. Give it a shot, might render faster and at as good or better quality.
P.S. Really nice scene though. Welcome back.
- Oshyan