Thanks for writing. Both of these trees are known to have relatively shallow root systems that like to grow along the ground instead of deep into it. So the roots may have pushed some of the soil as they grew. Or soil may have washed away from the base of the tree from your sprinklers. If the trees seem stable in the ground, I'd guess this isn't hurting them at all. You can probably add a little more soil around the base, if you want.
---Justin, Senior Garden Editor, BHG.com