I’m pretty sure the workflow is a bit different from trees to rock piles though as for trees you only want the base verts to be affected but for rock piles all verts should be affected otherwise the mesh gets stretched a lot. For tree as you guys suggested, masking might work. But for rock piles masking doesn’t work and without masking entire mesh gets flattened to terrain surface.
Crytek has this for grass since Cryengine 2, it bends grass patches depending on terrain slopes so you never get a grass cluster aligned poorly on a slope, without requiring vertex paint or masking in general. I’m going to take a look at their code and see if I can figure anything out and will pass the info to @**Deathrey **as usual to see if he’s interested in. ^^