If the rings are narrow, more of them are required than where they are wide.

As the tree gets larger, the sapwood must necessarily become thinner or increase materially in volume.

Sapwood is thicker in the upper portion of the trunk of a tree than near the base, because the age and the diameter of the upper sections are less.
When a tree is very young it is covered with limbs almost, if not entirely, to the ground, but as it grows older some or all of them will eventually die and be broken off.

Subsequent growth of wood may completely conceal the stubs which, however, will remain as knots.

No matter how smooth and clear a log is on the outside, it is more or less knotty near the middle.