Terrain trees on a slope

When I place trees using the painter, the ones on slopes sometimes levitates a bit above ground, I guess it is because the collider touches the higher slope and leave the opposite side haning in the air.
Is there a way to adjust the collider geometry of the trunk? It is greyed out when I look at the terrain object.

(I’m a bit new to this so if I missed something obvious, please link me to that)

Workaround: Make the trees in the tree editor, where you can set the offset of the “root”. The standard asset Prefabs is for use on nice smoot ground only…