There are no surface roots evident where the trunk meets the root ball. This indicates that the point where the top-most root emerges from the trunk might be too deep in the root ball. The moist, darkened area on the trunk base indicates the portion of the trunk that was buried with soil. Remove soil from on top of the ball so the point where the top-most root emerging from the trunk is within the top 2 inches of soil. In so doing, check for and cut roots that circle, those that are kinked, or those that cross over major roots. This could require a good bit of work and time, especially if the root ball is clay, or if the tree is a maple, birch, crapemyrtle or other fibrous-rooted tree, or if the top-most root is more than a couple inches deep.