When using bone meal to grow a 2x2 tree (mega spruce, dark oak, jungle), the tree generates on the highest block above the saplings, instead of in their place. If the north-western block of the 2x2 has a dirt block above it, three extra dirt/podzol blocks are generated next to it.

These generated dirt/podzol blocks fully overwrite all other blocks in the 2x2 area.