I'm back. :-) This is done so that everybody can follow it. click on Edit for the tree, go to material for Branch/ Trunk. Add a transparency map in an empty channel. I used one of Rochr's light transparencies, but you should experiment with various gradients. (Or even with half-white-half-black image, come to think of it.) Click on Blend Transparency. Ok. Ok. Render. Now you've got a trunk that's either completely transparent or has a transparency gradient. Oh, yeah. Parametric mapping doesn't look good. Object Side, as you see, looks better.

-- erlik