i suppose not...


if you want the shadows of the trees on the buildings then you put everything onto one render layer (everything that is going to be seen) and turn off the "Primary Visiblity" in the render stats

or if you have lots, you just select them all and go into Window > General Editors > Attribute Spread Sheet and turn their primary visiblity off from there, because it's a lot faster using the spreadsheet than individually selecting each tree and turning them off

if an object doesn't exist in the render layer then the renderer wont know anything about it, so it won't be able to cast the shadows from it, or put it into reflections and all that...