You don't even really have to manually replant. I saw a similar system, I'm not going to find it now, but you can design a base grid of dirt for the saplings to be planted on (usually [dirt] [air] [dirt] in a repeating pattern in a line for the most simple design) and use a BC Filler(with 9 brick blocks for the Fill option) to plant the saplings on the dirt (Fillers can work on a line or plane, so you may only need 1 filler for the entire process). When the Filler/s is/are activated, they automatically plant the saplings on the dirt.
EDIT: You can even have any saplings piped to the filler, so you don't have to manually refill it.