I think just putting torches in between each tree would keep it well lit enough.oh, just skeletons and zombies, and creepers will spawn on them, even with lights on the floor of the tree-farm. i think, who knows if that's changed by now, was a while ago. but i remember that happening to me. (although less of a problem with creepers destroying blocks off)
i suppose a half-slab would have the same effect.