That's a really good question (about the shadow that is).

There used to be an option to select so that an object would interact in the scene just like normal but it would not be rendered.  In other words, it would cast shadows and provide GI interaction and all that stuff but it would not show up in the render.

I've actually been trying to find that option in the last few versions and it's either been removed or it's gone.  So here are my recommendations.

1.)  If you can find that option (does not appear in render), then add a second plane just like you have it but set the point at behavior to point at your light source.  Then it will cast the shadow the way you're talking about.  Put both these "trees" into a single group and then one of the trees will point at the camera and the other will point at the light and you will get a render of the tree with the correct shadow.  Also, turn off cast shadows for the plane which is pointing at the camera so just the shadow for the shadow tree is working.

2.)  If I'm just nuts and that option can't be found, what you can do is put three to six of these planes in a group and have them rotated by an equivalent amount (90 degrees for two planes, 60 degrees for three, etc.) and sort of make a cheapy tree that might look ok from a distance.  I have no idea how that would look though.  Video cames have been doing it for a long time but it might look really dorky.

3.)  Last but not least, you can actually use the Carrara trees.  If you use the replicator and set up the base tree correctly, it's not too hard and it looks pretty good.

-Kix