There's also a way in Poseray to add displacement to (if present) parts of a tree. If the trunk/branches are different groups from foliage, you can make the foliage invisible, and import a displacement mask in another group tab, and it's subdivided and displaced by that mask. Then make foliage visible again, and save as.
If there are no groups, only materials you can go to groups and do 'materials to groups'.
If saved thus, you needn't use force displacement.