When growing a 2x2 spruce tree, it is possible for the leaves to forcibly replace dirt or grass.

The best way to replicate this is to put a ceiling 15 blocks or so above the saplings, forcing the growth of a short tree. If you do this, the blocks the saplings were planted on may be replaced by leaves, and the blocks underneath the leaf blocks will be podzol. The net effect being a tree in a small crater sitting on a cushion of leaf blocks.

Without constraining the growth of a tree, leaf blocks replacing dirt/grass blocks is still possible but in most instances the effect is less pronounced. 

Attached is the best example I can provide in a single picture, note how the leaves extend beneath the waterline, this is where they have replaced existing dirt blocks