Minecraft wiki entry for Tree_farming, associated text: Focusing only on the growth portion of the farm, these designs allow you to quickly grow hundreds of trees by pressing down a mouse button and then ignoring it. A dispenser with bone meal automatically forces the tree to grow, and a column of pistons pushs the trunk into a collection area where it is stacked in a large block for you to later 'mine'. The more complex designs also include leaf breaking, meaning that you end up with a net positive on the saplings and in the case of oak trees get apples without having to do much of anything.