Doing some additional research, it appears that FSX handles the model material differently than does FS9. Believe it or not, the material is the solid part of the model (the gray bit you see when a model is missing its texture) and that material can be made transparent if you wish.
Long story short, no texture fix is going to make these FS9 trees work in FSX.

BTW, how bout a big CONGRATS to Miljan on his first AI model, the C-27J.
