Since each succeeding ring is laid down on the outside of the wood previously formed, it follows that unless a tree materially increases its production of wood from year to year, the rings must necessarily become thinner.

As a tree reaches maturity its crown becomes more open and the annual wood production is lessened, thereby reducing still more the width of the growth rings.

In the case of forest-grown trees so much depends upon the competition of the trees in their struggle for light and nourishment that periods of rapid and slow growth may alternate.

Some trees, such as southern oaks, maintain the same width of ring for hundreds of years.

Upon the whole, however, as a tree gets larger in diameter the width of the growth rings decreases.
It is evident that there may be decided differences in the grain of heartwood and sapwood cut from a large tree, particularly one that is overmature.