Roots have a nose.
Many people end up burying the base of trees, thinking that this won't affect their growth, but it does significantly. Understanding tree anatomy is essential to understanding how to plant it, and you should know that the largest area at the base of trees is called the "root collar," which is actually like a nose that draws oxygen to a large part of the roots.
This base should be kept as free as possible, without blockages, so that the maximum amount of oxygen reaches the roots. Furthermore, if buried, that area becomes waterlogged, and because it doesn't tolerate moisture well, the base of the plant can begin to rot.