Everywhere I look, I see volcanoes of mulch around poor defenseless trees. I’ve been known to attack mulch volcanoes in random parking lots, pawing away, much to the dismay of whomever I’m grocery shopping with at the time!
Mulching trees, like volcanoes, buries the area known as the root flare and the very trunk itself. Roots need oxygen to grow. Look at trees in the forest and you will see root flare. If we bury them, with soil or mountains of mulch, they will begin to grow upward in search of air. So, roots that are meant to grow laterally in the soil are now forced to grow vertically and often begin to circle the stem. This is called a "stem-girdling root" and it will eventually interrupt the flow of water and nutrients, making the loss of the tree inevitable. If you're going to apply mulch around your trees, the best practice is to spread a thin layer of mulch (no more than 2").
Whatever your motivation for purchasing trees - whether it be to enhance your landscape, for privacy and shade or you are simply a tree-hugger like me, let the trees breathe and roots flare!