I had a Taiga full of bo3 trees. But, the leaves were often a bit too low, and I wanted to have variable trunk heights.
I changed these settings
SpawnHeightOffset: 0
SpawnHeightVariance: 0
ExtrudeMode: None
to
SpawnHeightOffset: 3
SpawnHeightVariance: 3
ExtrudeMode: BottomDown
and my taiga was empty! Only the trees and bushes I didn't change spawned...
I don't know what's happening but it's certainly very VERY weird.

@rutgerkok @Timethor any clues about when this will be fixed? :)

you probably have a BlockCheck for ground, the variance happens before the check which is consistent with other properties. Meaning the random location the variance picks is the location that is checked not the bottom extruded block. I talked to Rutger about this and we decided to do it this way to keep things consistent, but I feel like this would be a good place for improvement of either the BlockCheck functionality or the ExtrudeMode/SpawnVariance stuff.

@Timethor okay, I get the issue now... it block checks before extruding.
Well, I'd like to see this issue fixed now, but if there's a workaround, it wouldn't be a problem. But... is there a workaround? I mean, so my trees can spawn only on grass/dirt/whatever block I want my trees to spawn on, and extrude properly.