I made a batch of birch trees using super tree armatures as well as other more generic trees.
I start by trimming the super tree armatures, removing leaves and slecting the most tree like shapes. I spray these with 2 or 3 different brown and grey rattle cans.
Next I paint the trunk and larger branches an off white colour. The tips of the branches stay the under coat colour. Using a fine brush with black paint I make quick light horizontal marks to mimic the bark. I try to keep it as random as possible and use photos to help with the natural bark pattern.
On the best looking trees I will carefully paint glue on the branches and sprinkle on super tree leaf foliage. I will do this two or three times until I have enough coverage while still keeping an airy look.
These will go on the layout with other more generic trees. These trees will have leaves applied with spray adhesive. I use 2 colours of leaves for this batch of trees, when finished I spray slightly different green colour over a few trees to create some very subtle variation and to tone some of the background trees down to create more of an illusion of depth. Pink styrofoam is great to keep them safe until they are glued in place.