Hey I was wrong on this.

What I was thinking of doing uses an old trick that won't work for you.  Here's the old trick.

You make a shape, give it no color, set it to 100% transparent, and no light interactions when transparent.  Then you uncheck the light through transparency and sure enough, you get a shadow of an object that you can't see.  The thing is, the way we're working this would dork up the plane that you're using so that's not going to work.

Sorry about that.

The other option is to actually make a 2D shape that is the same as your tree image and map the PNG to it.  Then you could do what I suggested above (two tree shapes, one pointing at the camera and one at the light).  That will work but you would have to draw the tree shape and get a 2D plane setup for it.  It's quite doable but an extra step that takes some time.

-Kix