As topic says, sometimes trees spawn 1 space ABOVE the ground rather than on the ground. Making the tree look like it's floating. This appears to happen mostly with things like Maple and other smaller single-plot trees.

Running 1.10.2 MC.

I can confirm this. The lower partof the trees seems to be replaced by grass meshes.

Screenshot:
Tigerwood Example

Running 1.10.2 (+ some mods like Biomes O' Plenty and Realistic Terrain Generation)

I noticed this bug too. It happens when there is a bush/flower under the tree. Natura trees generation haven't the priority on bush generation, it's why there is one space at each time a tree generate under a bush.

Also happens with just vanilla grass - the tree will grow on top of it instead of replacing it.

Looks like the tree gen code isn't checking if the top block is a plant instead of dirt/grass, this issue persists in the 1.12 branch currently in FTB Unstable 1.12

The tree generation code for the tree trunks doesn't check if the block is replaceable, only checks if it's air, null or leaves. My fault