Hey, LuckyJoe- If you're over here, here's a photo of a finished tree. I realized in the steps shown below, I neglected to actually show a finished tree! So here's a photo to inspire ya! For wild west terrain, I would just use the branches cut off the main stem; trees weren't too big in the scrub of the west (and you'll get a lot more trees for your buck :)
The willow trees use the same stems, but use a different technique: The trunk is actually a bundle of floral wire twisted together and wrapped with white floral tape (touched up with paint to make the birch markings.) As you get higher up the bundle of floral wire (your trunk), you pull out one or two ends of wire to form branches. Thus, the more wire you start with at the bottom of your trunk, the more branches you can have at the top. The branches' floral ends are cut branches from floral stems which are spliced and glued to the floral wire branches. They are then wrapped with more floral tape.
The effect, in my opinion, is more stunning. The construction is also more controlled and using the floral wire allows the branches to be bent into any shape you need. These willows are the only I have ever made- definitely a labor-intensive technique.