When I have toyed with realism in landscapes.  I have found two things that help tremendously: hi-res terrains/material and low angle lighting with soft shadows.  Both are resource and render time hungry.  There are not many times in nature that shadows are as sharp as a default Bryce render.   I have had mixed results with outside HDRI lighting.  I can't predict what the resulting render may look like.  That is probably my failing and not enough practice.

@ dhama

I would agree with ICM that grass doesn't jive with the MOST EXCELLENT motion blurred leaves.  Another pitfall of Bryce trees in shown in your image.  They can't be bent in the wind.  The background tree line is too uniform for my liking.