You can get more generalized lighting by using a thin 3D cirrus layer to scatter the sunlight. Did you raytrace the atmosphere? Some noise can be reduced by taking the atmosphere quality default setting higher. 16 is the default (if I remember correctly), try 24.
24 samples is the highest I would try with the atmosphere quality setting. Higher settings just take too long to render. You can also change the AA render quality. Click on the Edit sampling button. Try 24 or 32 samples. Change the First sampling level from 1/4 to 1/16 and the Pixel noise threshold to 0.15.
This is a very good start. Perhaps some other tree types would look good.
