Chris was right - O scale trees are scary.
I tried to make a medium size oak tree using Super Sage. You essentially need the whole bag of the super tree material to make the tree look "filled out". My first try ended up looking too small to justify the size of the trunk I chose.
I'll wait to the next Timonium show and ask the Scenic Express guys about their sample (Ed, you know their O scale sample display, right?). I seem to remember it uses some sort of webbing material.
In O scale, trees make you!