Minecraft wiki entry for Tree_farming, associated text: Since the player can only harvest 7 blocks above the ground without climbing on something, the most efficient tree farm design limits the height of trees to 9 blocks. This allows 7 blocks of logs as a "trunk" and 2 block of leaves above that. This is accomplished by adding a ceiling at 10th block above the ground, leaving a space 9 blocks high in which trees can grow. This allows all of the wood from the trees to be harvested quickly and with minimal effort. The other option is to grab what you can from the ground and use a flint and steel to burn what you can't reach.