Hello Travis.
Maybe you want to try my way. This is what i normally would do:
Just do the animation without the trees and save it, then delete everything and just leave the ground objects. Change its material to a matte/shadow material and tick receive shadows and affect alpha. Next get your saved animation and use it as the environment background, unhide trees and render.
Your background animation will get the trees and its shadows printed in.
Sorry for bad English.
Cheers
Tony