Minecraft wiki entry for Sapling, associated text: Dirt blocks in the space above the sapling are an exception. Dirt blocks do not stop the sapling from growing. For most tree types, including giant versions, the dirt remains in place and the rest of the tree generates normally around it (it is possible to grow a tree that contains only one log block at the original sapling location, with a column of dirt above where the rest of the trunk should be and leaves spreading out from it). For the large variant of oak trees, the dirt blocks are replaced with log blocks wherever they would normally occur. In Java Edition, all types of log block and wood block and their stripped versions are also treated like dirt blocks (they don't stop growth). In Bedrock Edition, log and wood blocks are treated like other block types and stop the sapling from growing if they are above it.