Tree farm has always worked well. The towers have area for trees to grow, then a layer of dirt, with trees on top, making multiple levels. This also limited growth, allowing for specific trees to grow to specific size in certain areas. Now trees are growing and transforming dirt , etc above them into log, and the tree grows into the level above. This will transform anything into log. (confirmed with cobble, dirt, saplings,) We have only been growing oak trees recently, and this has only been confirmed with oak trees. (Picture attached, leaves trimmed to show where tree below has converted cobble and dirt into log)