Hey Spike - I tried what you recommended - it worked!

I'm running into other problems, though. Look at these two screens:
One of the images shows a black border around my tree, but the next image, if I move the camera so the tree is in a certian place, the border disapears.
Why does this happen?
Also, there are varying thicknesses for each of my model. Is this an inherent issue with the shader, and if not, what am I doing incorrectly?
Thanks for helping me! And sorry once again if these are obvious questions.