I stay awake at night worrying about the trees I see around town with mulch and/or soil mounded around their bases.
From the excellent website of Madison Tree Care and Landscaping—
A mulch volcano occurs when mulch is applied too deep and pushed up against the trunk of the tree.
Mulch volcanoes can kill by:
Girdling Roots. If mulch is piled around the trunk, the roots will start to grow into it. These roots will then grow around the trunk in the mulch. As the root grows thicker, it pushes against the trunk, which is also increasing in size. These roots will eventually strangle the trunk and cause eventual death.
Onset of Disease & Decay. Mulch piled against a tree trunk tramps unnecessary moisture and decaying organic material against the bark. This often leads to the onset of disease and decay around base of trunk which prevents food and energy from traveling up and down the tree. It also creates an ideal environment for fungal and canker development in the trunk, leading to eventual death.
The best thing is to have 2-3″ of mulch around your trees, but without any mulch actually touching the base of the tree. If you have too much mulch, just rake it back from the base of the tree and spread it out.
If there is soil around your tree, carefully remove it until you can see flare of the tree’s roots. The soil should be level all around the tree, or roots will grow up into it and strangle the tree.