To start the farm, put any sapling into the chest above the portable storage interface and flick the lever next to the cart assembler. The farm will then grow and cut the trees naturally and sort them into chests based on wood and sapling type. When you want to switch sapling of the farm, flick the lever on the block with a redstone torch and press the button near the chests. Wait a few minutes for the minecart contraption to run out of saplings, then press the button, flick the lever again, and put in the new saplings.