Consider 2-3" of mulch under the trees; it'll help prevent water loss
as you water and kill the grass under the trees (a big source of water loss).

Let me add that you should keep the mulch away from the tree trunk about 6"
or so, and that oaks really, really don't like to have any change in soil
level -- so I wouldn't mulch an oak. But all the species of Prunus (the op's
question) I've mulched have been fine with it.

Kay