Floating trees (of various kinds and biomes)

I'm coming across floating trees quite often. They don't seem to be of a certain kind or in a certain biome. They're missing one or two of the tree's bottom blocks, or completely floating over water.

What I expected to happen was...:
To find trees connected to the dirt.

What actually happened was...:
Trees were missing blocks, appearing to 'float' in the air

Steps to Reproduce:
1. Create world.
2. Walk around looking for trees.