I don't think so.... I havn't tried it, do you mean a dropper that places the sapling? Or a dropper that gives a sapling to the player?
A dropper that places it. Might not work with a dropper, but not terribly sure how droppers work. Might need to use a dispenser.
Doesn't work with either one just drops the item the other throws it neither plant a tree from my testing anyway. I may be wrong though.