In new mountains, trees are generated with a dirt block at the start. This is not logical, because if you dig out the snow, you will notice how all the trees (especially on a flat surface) are on the dirt block.

 
Repro Steps:

1. Find a Mountain Grove biome.
2. [Optional] Remove Snow blocks around the trees.
3. Observe the trees.

Observed results:
Trees in the Mountain Grove biome generate with Dirt instead of Spruce Logs at the base.

Expected results:
All trees generate with Logs at the base.