#3ds
You can turn off the "Cast Shadows" property for the opacity-mapped trees,
if you don't want the square's shadow to show. Gene Bodio has come up with
a way of doing this, plus making matte objects that are the shape of the
shadows to just cast shadows. (I'm not sure of the specifics, but the
results are amazing. Maybe you can find his PPN here on the forum and send
him an email.)
– G