I agree with what all the others have said. In the apparent order of things there is no order. There is symmetry in nature but it's in random order. Trees don't grow straight up out of the ground, some turn and swerve and many fall. There is generally many species of vegetation confined in the same area. Every different type of leaf has varying shades of green even within the same plant. Some leaves are more specular than others reflecting more light. This is why you cannot expect one software to handle the whole spectrum of differences and why most artists post-process. Andy Simmons (Hobbit) is a mixed media artist who does extensive 2D painting over his 3D art to help bring them to life. Have you tried following his 2D and 3D tree tutorial? You will also find that Dryfly relies a lot on Xfrog particularly for his underbrush and some trees and even admits to using a little post-processing from time to time. After a while of trying to attempt to get that "natural" feeling just using Bryce, you tend to start looking for other means by which to make your creation attain that look. As an artist, its your job to find the best way for you to achieve the look youre looking for even if it means looking to other software packages to achieve it.