Mark, in my opinion trees and other vegetation are the most difficult thing to get looking right. Unfortunately, they are also one of the most noticeable features of a diorama (except a desert or beach scene of course!).
Too many dioramas get it wrong which unfortunately detracts from the overall finish and effectiveness of the model. For example, clumps of lichen look awful!!!
I have difficulty making trees but the best results I've had are using the twisted wire method for the trunk & limbs, then coating these with either thin plaster+PVA mix or liquid paper. I used some small patches of lace (or mosquito net) glued over the outer limbs/twigs of the tree then painted the trunk & limbs. For foliage I used finely ground foam (Woodland Scenics foliage) applied sparingly to the lace/net with PVA and built up into realistic looking clumps.
The process for building trees will, of course, depend upon the type of tree that you are trying to depict. For example, you wouldn't use the above technique for pine trees.
The best advice I can give however, is to spend some time outdoors observing what real trees look like - observe the limb structure, bark texture, leaf distribution, bark & foliage colours, etc. Take some reference photos and try to duplicate the real thing in miniature. My trees looked awful until I started to take more notice of the real thing...